Welcome to my ‘Homebrew’ page
From time to time, it’s nice to create a piece of equipment that you’ve made yourself. Below are a few projects that I’ve recently been working on. Over the coming months, I hope to add more as and when.
A ‘Cold-War’ Style Ultra High-Power HF/VHF Antenna Switching Unit.
Here’s a really nifty heavy duty antenna switch that’s better than most you’ll find. Using freely available [NOS] new old stock Russian Vacuum Relays and with a few minor modifications, this will handle KW’s up to and above 70MHz.
Rather than just put them in a junk box, I thought they could be used for some home-brew projects. Given that also I didn’t want to cannibalize a good V-RAT box, I set about building a home-brew switch which would be a little different. As I planned for high power, Rosenberger PTFE SO239 sockets were used.
The switch is based around the Russian B2B-1B W2W-1W (15A 4kv) single pole single throw vacuum relay which is quite tall (115mm), so I had a friend construct an outer housing from 2mm aluminium sheet which was then folded and welded on the sides to make it watertight. Each tube is supplied with the factory spec sheet plus the unique serial number.
In the workshop I cut a 12mm section of PTFE sheet to use as a base to mount the relays. These were then drilled with a hole saw to accommodate the relays. M8 threaded spacers were used to separate and also hold the base to the chassis. Now we have the base and two chassis sections to work with, so let’s look more at the relays.
The relays were built solely for military use and as such are very conservatively rated. I managed to get 6 NOS (New Old Stock) tubes on EBay which were manufactured around 1992 for around £30 each from Bulgaria. Given the amount of stock around, this was a pretty good price especially for new pieces. Expect to pay around £30 used and £40 new each as of 2019.
The published rating is 5kw key down at 30MHz, so lower frequencies will have a higher rating especially 40, 80 and 160m. Additional info from QRO Parts in Russia gave them a rating of 25A at 32MHz and 35A at 3 MHz. Here’s a direct link to purchase 8 brand new ones plus all the technical data you need (and you get a couple of spares for other projects).
I didn’t want to push it too far as destroying the project was not on the agenda. Originally the relays were set up and the switching was tested together with a VNA curve to get some idea of frequency response. As per the published data, the relays saw no issues up to around 33MHz. Going higher in frequency caused quite a step rise in SWR which was to be expected as they are only rated for HF. The relays are spec’d to work at 6-16v DC and work well when switched with a 13.8v dc supply and also seem quite happy all the way down to around 2v although we wouldn’t recommend using voltages this low.
I fitted each relay with an 0.01uf RF bypass capacitor across the coil and a flyback diode (1N4xxx) series. Since an inductor (the relay coil) cannot change its current instantly, the diode provides a path for the current when the coil is switched off. Otherwise, a voltage spike will occur causing arcing on switch contacts. Since I have hundreds of 1N4007 rectifier diodes in the workshop, these were put to good use.
Following on from the original build, I thought it would be a good idea to try and get the system to operate on 6m. Testing with a VNA gave quite a bad SWR on 6m (about 1.6 to 1 – usable but not great) and on 4m the SWR was up to 1.9 to 1. I tested the 2m band but 144MHz was totally unusable at around 3 to 1. After searching on-line, I found another US based home brewer (W6PQL) using capacitors to remove the unwanted reactance created the further up the band you go. He suggested using 5pf on the input and 5pf on each output. However, the project shown was using relays that were much friendlier at the lower VHF frequencies than my chunky Russian comrades.
After an hour or so of trial and error with some small junk-box capacitors, I had settled on 10pf on the each output and 10pf on the input. The return loss on each port was excellent and on two ports the SWR at 144MHZ was below 1.4 to 1. Not perfect but usable. On 70MHz all ports were below 1.2 to 1 and at 6m no reactance was present with a nice flat 1 to 1 SWR. Looking on-line again, I found someone from Bulgaria selling some pretty hefty 10pf doorknob capacitors. These again were NOS Russian military items rated at 10kv. I purchased 8 for around £23.
It was great to see that the figures with the Russian caps were pretty well much the same when compared to the junk box test caps. Whilst testing a super QRO Vortex commercial antenna for a customer, we had the chance to use a 20kw high-power wideband HF/VHF amplifier so making good use of the time available to us; we put the switch into action for some testing during lunch.
On 20m the relays were handling 8kw key down carrier and at 30MHz 6kw key down carrier for short periods without any issues. However, this was way over and above the ratings of the SO239 connectors. I decide to play on the safe side at VHF but the system didn’t break a sweat with 5kw key down at 50 and 70MHz. Even at 2m on the two best ports, 3kw was passed through without issues. To be honest, the use of N-Type sockets [or even 7/16ths connectors] would be preferable as the ratings are much higher and of course are rated at VHF – something most SO239’s can’t hack. Saying that, it’s testament to the Rosenberger units fitted as I was pushing them way above their design criteria.
The Shack ‘Switch Box’
The switching control for the box was constructed using a simple ‘Black Box’ enclosure purchased from ‘Rapid Electronics’. The box was the drilled on the base to accommodate four rubber feet purchased from Farnell which we also use for the VRAT boxes.
A six way rotary switch also from Farnell (I used a hefty 2.5A version) and knob was fitted together with six green LED’s (one for each port) and a red LED for the power on/off indicator. A small heavy duty 5A toggle switch turned on the power which fed via a small 5A DC connector on the rear.
To prevent a mistake of connecting the device to the wrong DC terminal, a hefty polarity protection diode (SB340 – Schottky Rectifier) was used in series with the main positive DC feed. There’s a minor price to pay in a small voltage drop (about 0.3v) but this is better than frying everything in your new project.
Small quarter watt current limiting resistors were used in line with each LED. The resistance will vary depending on the value of resistor used but I used 470k ohm resistors when using a supply of 13.8v and 2.2volt LED’s. You can work out other values here. After a few days, I added extra resistance to the on/off LED as it was a little too bright. Power was supplied via 13.8v DC power supply.
The main switching connector used was a ‘SP17’ series 7-Way by Weipu. Don’t use the SP13 smaller connectors as soldering these is really fiddly. These are Chinese connectors but are very robust. Being rated at IP68, they are fully waterproof which makes for a good outdoor solution. There’s plenty on EBay to find and a plug/socket set will set you back around £6.00 (You’ll need two, one for the relay box and one for the switch box).
Although I have been busy over the winter, it was nice to make time for something different but it did make a great winter project. If you add up the cost of the components and time, it’s not a cheap and cheerful project. It uses military grade hardware throughout and takes quite a while to construct but it’s worth it. In all I spent around £500 on components, hardware and fabrication and a few hours every now and then over a three week period to get everything as I wanted it. I also found that with the additional capacitors that it also works well on VHF. Not bad for a unit that handles these really high power levels.
As a PS, N-Type connectors [or 7/16ths DIN] would make a better solution certainly at VHF as the good old SO239 can easily give sloppy impedance readings the further up the band you go. Of course putting this kind of power through SO239’s for any length of time is not recommended but as I’m only using UK legal limit:) – then there shouldn’t be any issues.
A Pro-Grade HF/VHF/UHF 300w
6-Port Antenna Switch
A Cheap and cheerful but professional MIL-SPEC antenna switch that’s good for 300w at HF/VHF and UHF using a professional switching relay which is good up to 18GHz and costs only around £65 to build.
As I’m doing quite a lot of work outside on the tower and antennas, one recent project was to provide switching capability for a variety of VHF and UHF antennas. Higher frequency switches tend to be more expensive because the higher up the frequency you go, the more margin of error from relays and from split feed lines. Everything in the tx loop wants to see 50 ohms but splitting your coax for whatever reason causes a ‘bump’ in the 50 ohm feed line. Sometimes a split of a few centimetres will cause a 600 ohm+ bump. It’s not so critical at HF but start drifting over 100MHz or so and the bump can cause quite an impedance shift. At 70 cms it can be quite dramatic.
An EBay search revealed a variety of different ‘Transco (Dowkey) Switches’. One in particular was a 6 port switch with SMA connectors which was an ‘MO’ type relay switching six outputs (The model was 146C70600). The unit was MIL-Spec and rated at 300w key down so certainly adequate for what I required. It had a great low return loss with the VSWR curve going right into the GHZ range. In fact it’s rated so well, even at 18GHz the maximum SWR was 1.5 to 1. The relay worked off a 20-30v DC supply and with superb isolation (better than 100db below 10MHz and better than 80db below 500MHz this seemed to fit the bill nicely. All this for only £25.00 and in great condition.
First item on the agenda was to obtain an enclosure. On this occasion I chose a rugged plastic ABS enclosure from Rapid Electronics. I had also decided that with six outputs, I’d use a combination of N-type connectors (on four of them) and SO-239 sockets on the remaining two. The input was also an N-Type. It’s worth noting that at VHF and above, many SO239 sockets are not 50ohms and struggle to retain a good level 50ohm impedance. I’ve heard stories of quite hefty impedance bumps using SO239’s which doesn’t seem to be the case when using ‘N-type’ connectors.
To power the relay, I sourced a 24v DC 3.75amp laptop power supply again on EBay for £12.00. With the relay wired purely for testing, I applied 24v DC to each terminal and everything switched without any fuss. I used a small 50ohm SMA dummy load which was good up to 5GHz and a wideband scan with the spectrum analyser backed up the manufacturer’s claims. Pin 7 on the relay is the common negative feed and each port is positively switched so pretty straightforward stuff so far.
OK time to do some drilling. Six holes were drilled on one side of the enclosure and one for each end for the N-type socket input feed and for the SP17 Weipu connector. I then soldered the terminal on the relay to the SP17 connector and left it loose in order to fully tighten later.
Each one was measured and spliced and then soldered to the appropriate output socket. It took quite a bit of time to get things right and to also tighten up each SMA connector accordingly.
As I had wired the switching on this unit exactly the same as the QRO switch, this means I could also use the very same 6 way shack switch box that I had made for the HF/VHF switch. Well at least test the functionality before making a duplicate. I then put the analyser back on and took readings. The splicing of the coax had caused some slight impedance bumps but nothing to worry about below about 600MHz. So from 70cms downwards everything looked pretty well flat. Even up to the 23cms band the SWR was pretty reasonable (below 1.5 to 1) but if you plan to use this on 23cms then you’d need to look closely at any coax splices and be very precise in connecting them to remove any potential impedance bumps.
All-in-all for about £65, I had a quite neat HF/VHF/UHF 6 port antenna switch. OK it won’t do high power, but at 300w it’s a good all-round unit especially for the great frequency range it covers.