Project X: Power Splitter / Combiner – Moxon anntenna

Its conception came way before Project Y but the actual building was done later. The idea was born in CQ WW SSB 2009 (link). I was doing SB80 but I jumped to 15m in the afternoon. That was fun. Imagine being able to work in two directions at once? Power splitting and RX combining, like the big boys do! A second tower or multiple yagis on the existing tower is out of reach. But some form of antenna might be possible. And of course I would need some form of power splitter/combiner a/k/a the stackmatch.

Soon after that I started homebrewing. I went to a sporting megastore to buy cheap fishing rods for the antenna. I ran to the hardware store for clamps and small parts. I had big ferrite cores in stock to make the 25-50Ω impedance transformer. I had relays for the switching. Go go go!

Initial testing of the transformer proved disappointing. It didn’t do what it was expected to do. Maybe I was using the wrong wire? I used ordinary 3-conductor flat cable on a pair of toroids I had bought in Germany. Problem: where to find the right cores? And where to find enamelled copper wire in Belgium?  So I ordered a CWS-Bytemark off the shelf 25:50 UN-UN transformer and I would provide the switching myself to bypass it for A or B or select it for A+B. Needless to say project X never materialized and remained just another item on my list of projects, filed under ‘started but never finished’.

Fast forward one year. Winter 2010-2011. It seems cycle 24 is slowly providing better propagation on the higher bands. Time to make Project X reality. After all: contests like WPX SSB or CQ WW SSB might be fun doing single band with a second antenna. But I needed to get it right this time. So back to the drawing board Google.

CT1ILT has built a splitter based on SJ2W‘s PCB. Mike just ran a new batch of PCB’s so I was glad to buy one. Mike, you happen to know where I can find the right toroids for the un-un? Yes?
♦ SM2WMV told me SpiderBeam sells the FT-240-61 cores. That’s great! I ordered four, two for the UN-UN and two spares. Now I’m sure I have the right material for the transformer.
♦ I had the right relays in stock to fit the PCB. I had an enclosure ready from the first trial. All small stuff was at hand.
♦ But where to find enamelled copper wire? Online sources force me to buy a big spool. A local company specialized in rewinding electromotors wanted to sell me and charge per weight but before going there, I checked two local electronic parts stores. The second had small spools of enamelled wire so I was all set.

Soldering the components on the PCB and winding the transformer was a non-event. I was anxious to test and hooked up two 50Ω dummy loads. The splitter made it back to almost 50Ω purely resistive again. Unhooking one of both made the SWR rise to 2.x:1 so it was indeed working!

Now what about the second antenna? The purpose is to cover local EU (tactically important in WPX!) while the yagi is high and pointing to the USA. A positive side result might be to keep loud UA’s off my back. A low antenna in their direction might temper their urge to creep too close to me. I could go for a sure shot and provide a simple dipole. A complete triband moxon would be very prestigious. So I settled in between for a single band moxon antenna. I will place it about 7m high on my old trusty telescopic aluminum mast. First I made a nice aluminum X-frame but … (long story cut so short it remains untold) … but in the end I used the boom of my old monoband 10m yagi and mounted the fishing poles like normal yagi elements.

I used the calculator found on for a 15m moxon. The first test showed I was in the ballpark but it resonated too low so I missed the upper part of the SSB subband. I had read that spacing/coupling between the elements was very critical so I fiddled with that distance. I also learned by experience that the resonance and bandwidth change dramatically in function of the height above ground. With the antenna at about 6 to 7m high, it resonated in band and the SWR seemed acceptable (1.6:1 or so).

Wednesday before the contest I soldered together a simple control box for A/A+B/B switching and indication. That worked almost perfect. I only had to move a wire on the DB-25 connector that I soldered to the wrong pin. But the switching actually worked. I checked SWR as indicated by the K3 in the shack and that was OK too. And no problem with 1kW in either of the three positions. Bonus: I don’t need to retune the amp in any of the three positions. All systems go! Along the way I tested last week’s coax cable with the solderless plug which passed the acceptance test.

Off to the real test then. I figured it might be easier to get A/B test reports in SSB. I called CQ and logged a bunch of Americans and some DX (ZF and HP1), along with some local EU. Test result: most K/VE only notice a small difference between only the yagi and the A+B position. When they TX and I listen on the moxon, they’re gone. EU’s tell me I am loudest on A+B compared to the big and high yagi only, but when I switch to the low moxon only, they add a few dB’s to the report. The same holds true from my side when I RX only. The only difference is that USA completely disappears when listening on the moxon only, whereas EU only drops 20-30dB on the high yagi only. It wasn’t a good day for this kind of tests. The band was open but there was strong QSB boosting or attenuating the signal with a few S-points difference.

Conclusion: the hardware works and the concept is proven. The question now is: what is the optimum height for the moxon to cover a 1500-2000 km range? It was about 7m high, but maybe a little lower would be better?

I’m really curious to see how this plays in the WPX SSB contest this weekend. Important factor is the solar condition dictating the propagation. The emergency exit strategy for SB15 is to get rid of the moxon and stick a 20m dipole to the fishing rod and do SB20…

** A few pictures will follow later  >> done 31/03/2011**

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