Construction Archives - Page 2 of 4

Easy building of a moxon antenna with 4NEC2

As we are still on lockdown and my 50MHz yagi is literally too huge to fit in the garden let alone erect on my lockdown lash-up system I decided I needed to make something smaller to use at home. I didn’t have any aluminium tubing at home long enough to make a small yagi so I decided to make a moxon antenna on the recommendation of a friend. These are very compact and easy to make so it seemed like a plan. I decided on a wire based version as although I have some 12mm tube I could cobble together I didn’t have anything I could get today for the corners. Wire it is. I decided easy building of a moxon antenna with 4NEC2 simulations to find the correct starting point would be a cool project.

I’d already looked around the web and compared the various online moxon calculators and the AC6LA Moxgen program (link) and the Moxgen program seemed to be the best fit for the suggested spreader angles. (Even though I’m not using spreaders as such.) It’s dead easy, just put your desired frequency in and the wire size and click calculate:

That’s it, job done. Almost…

I’m using normal insulated wire but the calculator doesn’t cater for the change in velocity factor from insulated wire. So I decided to run it through the free 4NEC2 simulation software (link) to make the required adjustments to the dimensions so it would work with minimal fiddling after. I love building, hate fiddling. Now, before you start backing away from the PC this is quite easy to use and just needs some really simple maths to do this. Stick with me. Look at the image above and see I have selected Format NEC on the right. You just do that and click the Generate Model button and save a file ready to open in 4NEC2. Run 4NEC2 and click Open and load the file you just made.

Click that green calculator looking icon to bring up the next screen, choose frequency sweep and check the start and end frequencies are a useful range and that the step size is not too large, then click generate:

We then get a plot showing us the expected SWR curve of the antenna:

Oddly minimum SWR at 50.1MHz rather than 50.2MHz but looking good. We can click the green calculator button again and this time plot the azimuth plot we are interested in:

Which results in this plot:

We can see the moxon should have about 6dBi gain and the amazing front to back ratio it is known for. Next to nothing off the back.

This is all very well but I’m not make it with bare wire so my antenna will not look like this without some tuning. First of all we need to add in the insulation so we can see what the effect will be. On the main screen, to the left of the green calculator is a red book, for editing the information that defines the wires making up the simulated moxon. Select the Source/Load tab, and tick Show loads. We then add two lines selecting as shown below from the offered selections. For Tag, First & Last we put 0 (zero) which will apply the setting to all parts of all wires. My tri-rated wire has insulation 3mm in diameter so I enter the radius, adding the mm to ensure correct scaling is used:

Once done we can click the green calculator on that screen, select frequency sweep again, but widen the scale. I’ve gone 5MHz either side of 50MHz. You can see the resonant frequency has moved 2.5MHz due to the effect of the insulation’s velocity factor:

So if we had built to the Moxgen dimensions using my insulated wire we’d be looking at an SWR of about 2.2:1. So we need to make some simple adjustments. The dimensions of the wires are on the Geometry tab below. Looks complicated but it’s just a few repeated co-ordinates. Some with a minus ( – ) sign to make the equal around the center of the axis to plot correctly:

What we are going to do is put those numbers into symbols, or what we would call variables in programming. When you look above there is only actually 4 different numbers used so it’s not complex. You can give them any name you like, even Harold but I have gone similar to the Moxgen image further up. Just click the Symbols tab and enter as shown below. You’ll see a 5th value called Vf. I’ve already tuned it by now but pretend I haven’t:

Now we need to flip back to the Geometry tab and put the letters (W, E, DirS, RefS) where the numbers used to be:

If you were to run the SWR plot again now, it should be exactly the same, best SWR on 47.5MHz. But finally we add in the velocity factor. To the end of each of the letter symbols add without spaces
*Vf (star V f):

Now we can run the SWR plot and it will apply a Vf correction to every dimension. I found 0.945 by trial and error. They say Vf for wire is between 0.95 and 0.98. I knew mine would be the lower end as it is quite thick. Now if we run the SWR plot, changing back to the 49 to 51MHz range we get this plot:

Back in business. A point to note here. I’m aiming for the SSB section of the band, but if I wanted to cover more of the band I would still aim at the lower frequency with this design. You can see the SWR curves rises steeply on the LF side but gently on the HF side. Anyway, if we run the far field plot we now get this:

As you can see this has changed. A bit less forward gain because we are chucking some out the back now. Now as I mostly do contesting this doesn’t bother me at all but it is interesting nonetheless.

So now we have redesigned the original dimensions to my real world application of actual wire, we should be shooting for success. We just need to apply the scaling factor of 0.945 to the original sizes with a calculator and a little rounding to sensible numbers:

So for the reflector we need a wire 2051+384.5+384.5 – 2820mm long and for the driven element 2 wires 1025.5+305.5 = 1330.5mm long.

Now we can build the antenna and have a good chance of it working!

I decided to make it from a small length of 20mm boom left over from VHF yagis and find some plastic pipes from the DIY shop to support the wires in the A dimension and just stretch them between the pipes for the E direction:

I bought a terminal block strip with the intention of using the brass inserts with nylon rope to join the ends at the C direction but didn’t after so total cost of parts not lying around £3.87. I used my red yagi elemt plates to mount the pipe clips and snapped the pipes into them after cutting to size. They didn’t really grip the pipes so I just taped them on to stop them sliding sideways.
I fitted a short tail of RG223 to an N-type plug, split the other end into braid and core and soldered the two halves of the driven element on and threaded those into a hole drilled in the centre of one pipe. After testing I sealed this with liquid insulation tape:

I had a better idea for the ends of the elements at dimension C. I quickly modeled up a small plastic part like so and sent two to the 3D printer:
The holes are a snug fit, tight enough to hole the wire until final testing. I marked lines on with a Sharpie 60mm apart and fitted the wires. Once tested I locked off with cable ties:

So, the acid test. What does it measure like?

SWR plot is very close but shifted down in frequency. This is because of the plastic pipes which I can’t (or don’t know how to) model in 4NEC2. Not a problem as I expected this and knew that now the wires would be ‘too long’. So I cut 10mm off the end of each element half (4 ends) and ended up with this:
(just noticed my analyser clock is way out LOL)

Pretty much an exact match to the simulation with 30 seconds of trimming. Just how I like it!

The finished antenna looks like so. It actually looks better than this because this is before cutting the 10mm off each wire end.:

Last updated 17th July, 2024

Lockdown lash-up mast base

The coronavirus outbreak of 2020 put a stop to all portable radio activities so like many people I was forced to adapt and overcome and set up something at home. For non rotating mast systems I strapped a mast to the YL’s parasol base:

This worked for smaller dipoles but I wanted something a bit sturdier and the YL wanted her umbrella back! So I decided to make a new one. I thought about making something from scratch but after asking the local club members for ideas it was suggested to use a tamper that builders would bash sand down before laying slabs or bricks. A quick Google located one in stock in the local Toolstation:
This is a cast iron 10″ base and a fibreglass handle. Cost me about £35 which is cheaper than some of the suitable umbrella bases I was looking at. I click and collected it. It didn’t seem that heavy at all to be honest but I’d already planned to fit it to a paving slab so wasn’t an issue. A quick session with the drills and the tamper was securely fixed to one of those heavy council style paving slabs:

The fibreglass handle feels pretty strong with some give but also felt like it would snap before that slab budged! This would normally only be used as the support until guys are attached anyway.

Before fetching the tamper I liked this design as the 4 webs I thought would be good for locating the bottom of the pole for lashing with a bungee. But in the end I decided to design and 3D print a locating guide that would stop the pole from twisting in relation to the yellow handle. Quick bit of CAD:
The hole on the side is to clip the first end of the karabiner bungee into while I lash it tight. Once a few turns of bungee are overlaid there is little stress on the plastic but it feels pretty strong anyway. This I printed in PLA which is easy to work with though ABS or polycarbonate would probably be better. There’s not a lot of stress on these parts so should be fine.

The biggest problem I had fitting these was getting the rubber handle off! it’s only glued on at the domed top but pesky to remove. Got there in the end and fitted the guides. they were a beautiful fit, close sliding fit. So close in fact the label halfway down stopped the slide so I had to open it up a bit. Couple of small M3 stainless screws and nylocs and job done. Two bungees and we’re ready to rock and roll:

Close up of the top guide:

All fitted a treat.

You may be thinking all well and good if you have a 3D printer of course. You could do the same with bits of wood I’m sure but I do heartily recommend a 3D printer. They are amazingly useful and very affordable now, although the filament at the moment is doubled in price due to the high demand when the 3D printing community was mass producing PPE equipment for the NHS.

Last updated 17th July, 2024

70cms RG179 & RG302 DK7ZB match cutting jig

This DK7ZB match cutting jig will enable you to cut RG179 75ohm PTFE coax accurately and repeatably to the length required to make DK7ZB 28ohm matches. RG179 is chosen as it is easy to work with and is best used for antennas that will only be used as part of an array, or lower power use only. It should be good for about 300W PEP on 432MHz. This jig arrangement gives you enough braid to solder to with a very short length of exposed PTFE dielectric. At 432MHz you want to be keeping your ‘tails’ short and tidy for best results.

Design goal:

Reality:

Building the jig.

You need to print the following from the files supplied:

1x           Part1
1x           Part2
1x           Part3
2x           Part4
2x           Part5

I used PLA at 0.1mm layer height and printed them in the orientation that they are used:

Download the RG179 STL 3D print files

I fitted them to a metal plate so I could clamp them to my bench. Holes for M3 bolts are included in all pieces. Small wood screws could be used to screw to a board.

To set the overall length (finished braid and cut for 2^nd end) I used 2 pieces of tube the closest size to fit RG179 inside I could find which I found in a model shop and is brass tube 4mm diameter x 0.45mm wall thickness. Code BT4 M by Albion Alloys.

One needs to be cut to 120mm (¼wave for RG179) and one to 132mm. The 120mm is a critical dimension. The 132mm wants to be pretty close. Varying by a small amount is not game over but it means the stripping lengths may not be 100% symmetrical so worth looking out for this.
Edit: I have since re-calculated the length for RG179 and other 75ohm PTFE cables and found that it comes up with 121.4mm. I used this to make a 19 element yagi and the match was spot on (bottom of this page).

These tubes need fitting into Part4 & Part5. I ran a 4mm drill through both parts to clear the prints for the tube, and a 1.6mm drill through Part4 only for the coax. This hole needs to take the PFTE dielectric with a nice close fit but not too tight. Dielectric should be 1.55mm so 1.6mm drill should be about right.
NOTE: when running a 4mm drill through Part4 (if required) only drill 7mm deep to clear the blue highlight below. The bore opens up inside after 7mm to ensure there is a flat surface for the brass tube to sit against internally:

Part4, Part5 and the brass tubes then make assemblies like below. I used Loctite 243 sparingly on the tube before ensuring it was pushed completely home into Part4 then on Part5 when that was fitted. Part 5 is just a support and needs to be about 10mm from the tube end.

Using the jig.

Part1, Part2 & Part3 are designed to be used with a craft knife with the type of blade where you can break sections off, like this one:

Extend the blade far enough to easily span the two guide slots. Only light pressure with a sharp blade is required. Do not ‘saw’ with the knife, instead rotate the coax. Start with a length of coax about 150mm long.

Step 1 – strip the outer jacket.

Insert one end of the coax into Part1 until it hits the blind end, insert the knife and rotate the coax. I found the jig worked perfectly as printed, but in case near each guide slot for the blade is a hole that will take an M3 grub screw to fine tune the blade height. This cut is the tightest tolerance one.

The outer jacket should be scored not fully cut through. This is to ensure no braid strands are cut. Gently flexing the coax should snap the jacket easily without damage to the coax.

DO NOT FULLY REMOVE THIS PIECE YET.

Carefully slide the jacket towards the end to reveal about 10-12mm of braid:
Now lightly tin the braid with solder. Enough to wet all the strands all the way round but not too much to increase diameter. I find ‘real’ solder with a lead/tin mixture better than the lead free stuff. Once done the jacket piece can be removed fully. Lightly tin the rest of the braid:

Step 2 – cut the braid to length

Insert the braid into Part2 until jacket stops and again fit knife and rotate the coax at least once full turn.

Again this will only score the braid, in order to ensure the PTFE is not damaged.

Also again, gently flexing the braid at the score mark should fracture the soldered braid cleanly at the score mark. Braid can now be removed easily.

Step 3 – cut the dielectric to length

Insert the coax into part 3 until the braid stops on the inner stop:

As before, fit knife, rotate coax at least one full turn. The PTFE inner will be scored but should easily twist off, shearing at the score line neatly. Lightly tin the stranded inner conductor.

Your coax should now look like this:

Step 5 – trim overall length

Insert the stripped end of the coax carefully into the 132mm tube assembly, rotating it as you go to ensure the PTFE dielectric enters its hole and the braid sits up against the inner stop:

Using flush cut wire cutters cut the coax nicely flush with the brass tube. The neater the better, this will determine the position of the 2^nd jacket strip. You may want to grip the coax with the cutters using the brass tube as a guide then carefully pull out the coax from the tube as you cut it so there is some to grab hold of.

Step 6 – strip 2^nd end jacket

Repeat Step 1 above so you have this:

Step 7 – cut 2^nd end braid

Slide fully stripped end into the 120mm brass tube rotating it as you go to ensure the PTFE dielectric enters its hole and the braid sits up against the inner stop:

Hold your sharp knife blade on an angle to match the blade’s cutting edge ground angle so the cutting edge is flush with the end of the brass tube and using gentle pressure rotate the coax by twisting the bare braid on the left letting the blade edge roll along the circumference of the braid. You are aiming to score it as in Step 2. Remove from the tube and gently snap the braid at the score line and remove as in Step 2. You should now have a braid length of exactly 120mm:

Step 8 – cut the dielectric to length

Repeat step 3.

You should now have a finished cable:

You can now repeat this process as many times as required and should get extremely similar pieces.
I always ensure I cut my matching segments for all antennas I might use together from the same reel of coax in case of any manufacturing differences.

To make the actual DK7ZB match I like to pair the cables with short lengths of adhesive lined heatshrink. One of the few things that tames that slippery FEP jacket. The pics below show how I connected up the short tails.

RG302 Version.
Since making the RG179 matches I have built a version using RG302 for better power handling.
Principal is exactly the same as above except I used 8mm aluminium tube (spare element tube) with 1mm wall thickness.

This is the result I got:

Download the RG302 STL 3D print files

Last updated 17th July, 2024

Yaesu FTDX-5000 transport case

THIS IS NOT A FLIGHT/SHIPPING CASE!!
This is a FTDX-5000 transport case for things like taking it to field day etc.

As the FTDX-5000 is an expensive and heavy beast I designed and made a transportation/storage case to fit it. I’d looked at what you could buy and everything was huge and heavy, increasing the size of the 5000 by a considerable amount. Something simple, low profile and low weight was required.

Images are clickable for larger version.

Below is the resulting case. It’s powder coated aluminium, adorned with protective foam so the radio never gets scratched or dusty/wet (just care fitting the lid is all required) and has latches to quickly but securely lock the lid on. Once in the case it can be carried around without fear of accidental bumps, things falling on it, kids fiddling or rain (to and from car etc).

The box is designed to enable the radio to be operated whilst still in the base, and even offers a handy place to hang the standard microphone:

Rear connector access:

Base showing foam support strips, foam side protection, and rubber grommet foot protection:

Base strengthening and feet protection:

Video showing fitting the lid. (I am extra careful with everything!):

Last updated 17th July, 2024

Plugging yagi dipole ends to waterproof them

When making my yagis I prefer plugging yagi dipole ends so water cannot run down the tube and enter the dipole housing with its corrosive end results. Also, it does stop spiders moving in and making a right mess everywhere.

Many people do not like to add caps to the elements for concerns over altering resonance, so I just cut the cap off and push them in so they are fully inside but water cannot enter. Simples.

These plugs were from nuxcom.de but he has ceased trading now but they are available at various places including Tino’ Funkshop but these days if you are in the UK it’s best to look more local.

Last updated 18th July, 2024

Accurately cutting yagi elements to size

I planned to build more 144MHz & 432MHz DK7ZB yagis for contesting to use in arrays so I wanted to ensure I could make them accurately and repeatedly and also without taking forever doing it! Thus I needed to come up with an efficient cutting jig to make accurately cutting yagi elements to size fast and reliable.

I would use some threaded bar to adjust and set the length and steel angle for the supporting the aluminium tube and cutting the length. First I calculated the range of length between the longest reflector and the shortest director then a quick knock up in Solidworks gave me the length of stud required and ideal places to weld the angle to do the cutting. The studding is M12 because a nut for M12 will take a 10mm ali tube with a little clearance. The long M12 barrel nut (Screwfix, few pence) is locktighted in place. From the left the angle brackets have the following holes: 12.5, M12 tapped, 10.5, 10.5, 10.5:

Some cutting, drilling and tapping later and I have this (the observant will note it’s not exactly the same as the intended design-more on that accidental stroke of luck later!):

Usage is very straightforward. First I G-clamp it to the bench then I used one piece of spare element tube as the setting piece, doing all elements in sequence, longest to shortest. So I set the jig and adjust, cutting and filing the setting piece until spot on then do all elements that length then onto the next element size using the same setting piece. Simple, but more importantly, very accurate and repeatable. I did try calculating the distance a fraction of a turn would give based on the thread pitch but with a simple tapped hole the thread backlash made it too unpredictable so found it easier to measure the amount the cutting piece protruded when setting for the next size down with the small vernier in the photos above and start from that point and fine tune on the thread and lock nuts.

To actually cut the elements (after using the jig to prepare one end of all elements to ensure it was nice and square and deburred) I just slide it through the 10.5 clearance holes into the long nut and hold it pressed against the long nut with one hand. With the hacksaw tilted slightly to start a cut just away from the steel angle to prevent sawing the face of the steel angle I start cutting then as soon as the cut is started square up the hacksaw. Cutting takes a few seconds:

Then it takes a few seconds more to file the element down flush to the steel angle. Over MANY elements the steel angle will gradually file down in thickness but such a large area and so much harder than the aluminium it will not affect the length between a batch (I checked):

Next to just finish off the end. In the picture above you will notice the countersink bit in the screwdriver handle and a pencil sharpener. Both employed very quickly to deburr inside and outside edges:

The only thing left to do is check the length! I am lucky that my place of work happens to have a very large digital vernier. These will do, close enough for G1YBB…

Obviously most people won’t have such a measuring tool but it shows the jig can enable accurate results, more accurate than we can normally measure. I belive working as accurately as possible to follow the simulated design will enable the best possible results to be achieved. I have had fantastic results with my first 9 element DK7ZB working to that ethos.

About that lucky error in placement of the angles with the holes…
One thing I forgot to allow for when designing the jig was the driven dipole halves! But by a stroke of good luck (MOST unusual for me) fitting the middle guide hole angle in the reversed place turned out it was perfectly positioned for the dipole half:

I’ll be able make the 432MHz element cutting jig to cater for dipole halves and full elements easily because the threaded bar covers enough length with the 70cms elements being so small.

Last updated 18th July, 2024

Simple inverted Vee wire dipole for 50MHz

Now it’s towards the end of June and I have been listening to the other club members I chat and contest with reporting on all the times 50MHz has been wide open on sporadic E, I decided today I would knock up a simple antenna for 50MHz for the garden. My location is not suitable for any real antennas due to neighbour issues so I thought I would make a wire delta beam and mount it fairly low as I have a suitable 2m length of plastic pipe I could use for the cross boom. A quick look at the design scuppered those plans as I had no suitable 75ohm coax here.

I then considered a simple aluminium dipole as I have loads of 1.5m lengths of 12mm tube in the garage. Then it came to me in a eureka moment. A good old trusty inverted vee would be easy to make and do the job nicely!

Since I made my first 20m inverted vee dipole I have since butchered it by cutting off the coax to use elsewhere and it has been lying around the garden for a year or so in the grass in the corner of the garden. Rescue that and put new coax on and I am good to go!

Next (as seen already above) I needed a pole. I took the bottom 3 sections from my 8m SOTA fishing pole which gives me about 3m of lightweight but stiff pole. In the garden there was fitted a rotary washing line with a two part stem set in the ground. Amazingly it was the perfect fit for the bottom section of the pole! The two sprung plungers even stopping it flapping around:

Into the HF antennas odds and sods box and I got out one of the SOTAbeams lasered guy rings I bought to hang the dipole from. I slid it down to a reasonably but not excessively snug point and wrapped some tape around below that to stop drifting lower and possibly cracking as the plastic is fairly brittle feeling. I also sealed the feed point with liquid insulation tape which is great stuff and taped the coax down the pole to take the weight of the coax, which is longer than I need and only RG223 but I am just looking for something to get on the air and do some tests:

With my HF dipoles I usually peg the other end to the ground with a length of string to insulate and keep the voltage maximum point off the ground but that really didn’t seem a plan. So that delta beam boom was called into play as a dipole spreader:

It’s literally just lashed on with insulation tape:

At each end I drilled a single hole to thread the wire through which actually retained the wire quite well due to the tension and angle. But I backed that up with the ubiquitous cable tie. Also visible is a blob of the liquid insulation tape on the end of the wire to stop water seeping up the wire via capillary action:

Here is the finished set up ready for tuning. On the left of the image you can see the coax running into my custom wall mount coax connector box:

Talking of tuning…
To ‘design’ the dipole I use done of my favourite sites I use for all my HF dipoles over at sotamaps.org. (Click on the 2nd tab for the calculator). I put in the centre height measurement from my set up and adjusted the end support height to get a little under a metre horizontal distance between mast and end support. You can set the wire type via the settings button. You can see it offers me 1.32m for each side. So I cut mine to 1.42m to start as it’s always easier to trim than add!!

First measurement showed beautiful resonance a little under 50MHz, so I trimmed 10mm off each end. Nearly at 50MHz, so 10mm more. Resonant now more in the CW end so I took off another 5mm. The final cut length is actually 75mm longer than the designer suggested, so it could be my selection of wire should have been for a thinner one or thicker insulation than I chose. (that’s ex red wire with a dose of UV fading applied!):

SWR 1:1, zero reactive component and 49ohms resistive component (50 ohms a tad higher up the band but still in SSB section). I’ll take that!

Plugged into the trusty Yaesu FT-857D and a quick scan showed some Es stations calling. Found an EA station and called him, replied to me first time! As did the next 4 stations. Nice one!

So there you go. It is dead easy to get on 6m even with very awkward neighbours and small gardens. 50MHz truly is the magic band when those Es open up too!

Last updated 18th July, 2024

Lightweight 80m Inverted Vee dipole

My local club Hereford Amateur Radio Society has been embracing the RSGB VHF and UHF activity contests but also some members have been taking part in the RSGB 80m CC series of contests which have SSB, CW and Data sessions. I wanted to join in so needed at make an antenna, which I decided should be a trusty inverted vee dipole.

My first consideration was how was I going to support the centre high enough. I have an 8m and 10m fishing pole but neither are really high enough on their own. So my solution was to attach the 10m fishing pole to the top of my VHF portable mast (a 6m scaffold pole) which will get me the centre of the dipole about 14m high:
Off to the SOTAmaps dipole calculator website (link on my useful links page) to calculate the lengths:

The fishing pole is pretty flexible using the upper sections so I have a pair of guys lower than the dipole to stabilise it a little made from thin but strong cord. These will go at 90° to the dipole, using the dipole itself to steady the very top of the pole:

Typically it was a pretty windy day when I set it up on my favourite test setup hill:

As the wire is quite thin it’s fairly narrow band but I got a reasonable match mid band:

All ready to go.

Apart from the fact that my favourite test site and intended 80m CC portable operating site Westhope Common, that I have been using on and off for about 35 years now has a resident that hassles everyone and anyone who goes up there, and has hassled me each time I have been there lately. So I need to find another site.

Last updated 29th June, 2017

Portable 6 element 50MHz DK7ZB long yagi

Towards the end of 2016 the rules and days for the UKAC series of RSGB contests were changed. The 50MHz and 70MHz UKAC events were moved to the 2nd and 3rd Thursday of the month respectively. This opened up more opportunities for me as working a Tuesday night contest means rescheduling my Tuesday to a Wednesday and means my Wednesday is busy as heck and it’s at least Thursday before I can even look at the tablet to get the log updated. But a Thursday I am usually free so I can get on another band. As I have no 70MHz Tx capabilities I decided 50MHz was the way forward for me. I have an old home made 5 element yagi we used to use but I wanted a newer better performing yagi. I am getting awesome results with my 144MHz 9 element yagi I decided another DK7ZB sounded ideal.

I chose the 6 element 7.2m boom version as I plan to only use it car portable and it’s only 1m each end longer than the 144MHz yagi I’m using. Also it has a great SWR curve. All dimensions are available on Martin DK7ZB’s site:

This design only had figures for 12mm elements. That was OK as I can get 12mm pipe clips like I used in the 144MHz yagi. But it turns out buying 12.0mm tube in lengths greater than 2000mm in the UK is exceedingly difficult. It had been suggested to me to use 12.7mm tube as that would be easy to buy in the UK but would require recalculating the element lengths. Not only that it would prevent me from being able to use the fast fit pipe clip I wanted to use. After much searching and asking in various places I had to admit defeat and arrange with Attila at nuxcom.de to ship (at some considerable expense) some 3000mm lengths of 12mm tube, along with several other antenna parts and also some 3000mm lengths of 10mm element tube.

For this yagi, unlike the 9 element for 144MHz, I had no plans to take it backpacking portable so I decided it only needed a 2 part boom. I was easily able to get 5000mm lengths of 20mm boom for this. 20mm is quite small sized boom for a yagi of this size but my element mounting plates are designed for 20mm boom only. I’ll be using truss supports and side supports if required to stop it flexing too much. The 5.1m long 2m yagi was nice and sturdy with its trusses in high winds on the top of the Black Mountains. Although it’s only 1 metre longer each end, it makes for a big boom!

Once the element positions were marked up the 5 parasitic element clips were fitted in the same manner as the 144MHz 9 element. On this yagi the driven is too big for the element clips so that will need a more conventional box:

For the feed box I decided to go for a beefed up version of that I did with the 144MHz 9 element. I chose an ABS box from Farnell as it is quite thick walled and with a decent lid should be pretty stiff and is also IP65 rated (before I start drilling it). In order to get a suitable height so the driven element 16mm sections could be on the same plane as the parasitic elements it came in quite large at 200 x 150 x 55mm but that is OK as the driven on this 50MHz is quite big:

Here it is marked up for drilling. Marking needs to be spot on as this is what makes the driven element parallel with the parasitic elements in both planes so is crucial!

This is the centre point of the dipole box used for locating it on the boom in exact position:

To enable stability and strength for the 3 metre driven element I am using some 19mm angle. The sighting hole above lines up with the centre of the scribed line:

To mount the two halves of the dipole I got my good friend Paul to 3D print some two part clamps in ABS to my design so the centreline height of the driven element above the boom matches the parasitic elements so all elements are in the same plane. Here are the clamps after drilling and fitting to the box with some 16mm tube to check alignment:

To fit the dipole box I drilled and tapped an M2 hole in the centre point of the boom on the scribed line as seen above and screwed the dipole box in place using the 2mm sighting hole. I then fitted the reflector and first director and ensured they were all parallel and drilled the box to fit the two angle pieces:

When I bought the element materials from nuxcom.de I also bought their dipole centre for 16mm tubing:

Which is very chunky and strong. But the 16mm tubes fit inside the joiner and I couldn’t see how one would get a good secure contact to the elements. So I got another good friend Ed to turn up a piece that would fit inside the elements like the nuxcom one for 10mm elements does. Here is the mechanically finished dipole centre:

The eagle eyed may be wondering what the red things are in each end. Well as I am using jubilee clips to clamp the 16mm tubes down onto the 12mm main driven element parts, once they are removed for transport (this yagi is for portable use remember) the jubilee clips are free to fall off unless clamped down. So my I got some plugs 3D printed to clamp onto to retain the jubilee clips and also prevent any dirt ingress during transport and storage:

Now the dipole needs the DK7ZB match. I’m using the same WF100 75ohm coax I used on my 144MHz DK7ZB, which is fairly low loss for its size, and is not too big or heavy. Its claimed velocity factor is 0.85 so I worked out the length as so:

300/50.150 = 5.982m full wavelength
5982/4 = 1495.5mm for quarter wave
1495.5 x 0.85 = 1271mm

As before I used a section of boom to tame the curling of the coax to allow accurate measurement and cutting of the lengths:

One end of the match fitted:

A picture of the process in operation in the ‘workshop’:

Other end of the DK7ZB match completed:

Apart from the bracing the antenna is pretty much complete except for the fact that the element mounting plates are not really man enough for 3 metre long 12mm elements. They were designed for the 144MHz yagi and only 1m long 10mm elements, which they are perfect for. With the larger elements this is how they were flexing with gentle persuasion:

A quick chat with Paul and he drew up a two part brace that he could 3D print me and soon they were in the post!They utilise the same holes already in the element plates and wrap underneath adding strength without interfering with the element clips at all. I used M4 nylon screws to add extra fixings without adding extra metal, the only metal nut and bolts holding the element clips on:

After fitting the new bracing parts this is the flex with quite boisterous provocation:

Much better!

My next consideration is bracing. Long supports were made and fitted utilising the same fittings as on the 144MHz yagi. In fact as I write this I have started work on a 70MHz yagi. All three will fit on the same fittings on the portable mast, and the 70MHz yagi will re-use one of the 144MHz yagi braces and one of the braces for this yagi. The angle of the supports is quite narrow but they do support the boom and keep it straight. This yagi also will need side guys to protect it from the wind. I have several bottom sections of 4m fishing poles left over from HF antenna projects so I utilised two of those. Again Paul quickly produced some parts for me. Some ‘plugs’ to fit to the mast plate to mount the poles onto:

And some stoppers to go in the end of the poles for the 3mm dacron cord, which is very strong and has very little stretch. The stoppers are a good interference fit:

Side guy poles fitted to the mast. These are a friction fit to the ‘stoppers’ and quickly assembled on site:

The sides guys clip onto a plastic bushed (thanks Paul!) bolt on the boom supports with a single karabiner each end. Very quick to deploy:

Next step was to test it! This was done after work on the Wednesday before the first 50MHz UKAC in Jan 2017 the following day! It was reading 1:15 throughout the SSB portion of the band, which was slightly disappointing. There was no time to look into this as I needed to get back home (testing was done in a mountain’s car park) and pack for the next night’s contest!

On the night of the contest the in radio SWR reading was quite low so I was happy the radio was feeling OK about the match and used the new antenna for the first 50MHz contest I have done in 20 years. The yagi seemed to work pretty good and I even managed to beat G4CLA in the AR section of the RSGB 50MHz UKAC January 2017, which I was delighted with!

Here is the beam on my portable setup. Longer than the mast is…

Last updated 21st January, 2019

DH8BQA voice keyer BX-184 for FT-817, FT857D, FT-897D mic MH-31

I was scrolling down my Facebook feed and spotted a post in I think one of the SOTA Facebook groups mentioning a BX-184 CQ Parrot. It looked interesting at first, then awesome! Within 20 minutes I had it ordered.

This is a modification to the standard MH-31 microphone that comes with the FT-817, FT-857D and FT-879D etc. It will record and playback a message, perfect for calling CQ Contest or CQ SOTA etc, but without the requirement to carry and connect up another gadget as it fits completely inside the microphone body. It was designed by Oliver DH8BQA and he describes it on his website here http://www.dh8bqa.de/bx-184/.

It is available for sale on the German Funk Amateur site here Funk Amateur BX-184. However they also do another kit that includes an MH-31 microphone body if you don’t want to disturb your original mic and that is the option that I took Funk Amateur BX-184M. The website is all in German so if like me you don’t speak German Google Translate will help a lot! There is also now a USA vendor here http://www.box73.com/product/2

It came pretty quickly and this is what you get in the box:

A complete kit with all you need. The PCB is part SMT (surface mount technology) and part through hole components. You just need to fit the through hole components.

The double sided PTH PCB is very nicely built.
Top:

Bottom:

Before it arrived I did some research and found some mods made by DG2IAQ on eHam which sounded worthwhile:

Mod 1:

“I do always replace C8 (4,7µF) by a nonpolarised SMD 1,0µF to fasten up the AGC. With this Change the internal AGC works more as a mic compressor than as a (slightly delayed) mic Level limiter.”

Mod 2:

“And for the first time I changed R4 (82k) into 56k to bring the sample rate from 8kHz up to 12kHz (by a shortened play time of 40s instead of 60s, but that’s still more than enough for my needs). This change gives even more punch for the replayed calls as the sound is a little more high-pitched afterwards. You simply can solder a 100k SMD in parallel to R4 instead of completely replacing it.”

I emailed Oliver DH8BQA for his thoughts on these mods and he confirmed they should be worthwhile.

So I decided an 0805 ceramic chip 1µF capacitor would fit best across the pads for the 4.7µF electrolytic it was replacing:

I should have fitted this first as the 15µF cap next to it made it awkward to get a good solder joint on the GND side of the capacitor due to the ground plane wicking the heat away. Got there in the end though:

R4 is an 82K 1206 sized resistor located on the rear of the PCB (this is easy to locate as the kit comes with build instructions in German with a good circuit and layout supplied. English instructions can be downloaded here, page 5 onwards English Build Guide):

And with a 100K 1206 fitted in parallel (on top of the fitted resistor) as suggested above:

With all parts now fitted (including a 1206 capacitor fitted across the supplied electret insert terminals) all that is left to do is mount the electret insert into the microphone body and solder that to the PCB.

Opening up the supplied microphone and removing the PCB revealed these 2 slabs of steel in the body. The only function of these I could see was to add weight to make it feel more substantial. So I got rid of those. No point in carrying dead weight:

Next I hot melt glued the electret insert into the mic and filled up the void as the instructions said. Actually I filled more than the picture in the build instructions showed by mistake. Then solder the screened cable to the PCB and fit the IC and it’s ready to be assembled:

As the replacement PCB does not have the two position slide switch, there is the unused hole in the back of the mic. For this I just used some good quality sticky label material I have to hand.
One piece inside:

And one on the outside.(not pretty but functional):

Once assembled I compared the standard supplied Yaesu MH-31 mic for weight against the BX-184 CQ parrot. A 40% saving in weight, I’ll take that:

I soon connected it up to the radio ready to go but there was no outgoing audio! What!? With several projects on the go I could do without time spent fault finding. What if I have cooked the electret soldering the capacitor on now it’s well and truly hot glued into place. Hmmm. Hold on, what was that pot trimmer for? I remembered when building it I couldn’t see any mention of it. A check of the circuit confirmed the obvious answer. It’s on the mic output to the radio. A quick check with the meter confirmed it was currently set to ground the mic line going to the radio. A quick tweak and we are back in business.

In fact the worst part was setting up to monitor myself. Eventually I had a reasonable system, FT-857D on 5Watts into a dummy load near the FT-897D with a 4mm banana plus as an antenna feeding into the sound card on the PC, with Audacity dealing with the recording. The recordings are not broadcast quality but are good enough for a comparison.

Here is a CQ call using the standard MH-31 mic:

Here is same CQ call live using the BX-184:

And the replayed recorded CQ call from the BX-184:

I might need some on air radio reports for final setting but it doesn’t seem to be clipping at all though does have a little more punch.

Can’t wait to try this out in a contest or SOTA activation. All in a great little kit well thought out and went together very well. The only small point is no actual mention of the trimmer function or setting which would be good as a reminder if nothing else. I know this kit is aimed at radio amateurs who should be able to look at the circuit and deduce why the pot is there so this is a very minor point in a great kit.

Edit:
I have since used this ‘in anger’ in 144MHz contests. The first contest I got some complaints about over driving and being wide (complainant was also wide to me for that matter!!). But I turned SSB mic gain down in the FT-817 and back at home I did turn down the audio output level on the microphone itself and did some tests between it and the standard MH-31. Subsequent contests have resulted in zero complaints but some good audio reports.

Last updated 3rd July, 2023