DIY HF Antenna Analyzer

some time ago i got a nice deal on a buddipole antenna. I already had an alexloop which i was very happy with, but I could not resist the temptation of a “real” dipole that could take the full 100W from my rig. so i went online and ordered one.

after i got the package  i went on and setup the rig and antenna, and it worked ok. but i wasn’t happy and wished for more… it took a lot of time to assemble and result were on par of the alexloop.

At the last hamvention i had a chance to talk to the buddipole engineers at their booth. the first question they had was “did you checked it with an antenna analyzer” ? “no”, i’ve replied. “so you should” they said. and it sounded like a solid advice. now i had a plan…

so I’ve headed to the MFJ booth next door and asked for one, and how crashed my spirit was when I noticed the price tag. it was $200-$400 for a very basic model. i guess it’s not a lot to pay for such an instrument, but my budget was already exhausted.

I tried to look online for a cheap analyzer, but it looked like real garbage. but then i’ve noticed some links to DIY analyzers. I could not resist the temptation.

i started by looking at the design by Beric Dunn K6BEZ. while not exactly a SWR meter, but an impedance meter, it was good enough for my need.

the basic design is simple enough and modifiable, it contains a AD9850 signal generator (about $8 on eBay), which drives an H-Bridge. Vx is always half the input voltage.  if the antenna is 50 Ohm, the voltage between points X and Y should be zero. if the impedance is low, Vy will drop to zero, and so on. so both points are driven to an envelope detector comprised of a germanium diode and a capacitor, which is then sampled by an  arduino and results are shown on the display.

there were a couple of things i want to change to make it more portable and appliance like. first I wanted a graphic screen, so i can display a frequency graph without connecting it to a PC. and I also wanted to use a rechargeable battery for ease of use.

I found a display module that is used in prusa 3D printers for $10. it got a 128*64 display, a rotary encoder and a push button. I had a LiPo charger/Regulator which I once bought (and regrettably is no longer available). so I designed a PCB to host all the components, modified the software to add a graphic UI and 3D printer an enclosure.

the result look as good as any professional analyzer I saw.

to test the device I connected it to the alexloop. mag loops are notorious for their narrow Q, but here it was an advantage as I could see the freq response very clearly. since I had no reference analyzer, I simply tuned my antenna to lowest SWR at the transceiver, and then measured the same configuration with my analyzer. results were surprisingly good.

Freq on RXTX SWR Freq on Analyzer SWR
7.040  2.2  7.060  3.4
14.074  2.2 14.080 2.0
21.200 1.1 21.230 1.6

I then finally tried the buddipole. i was still very labor intensive to set up. and performance wise i did not get the desired “WOW” feelings. I guess I still need to improve my antenna tuning skills….

source files can be found here

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