Strange experience with SARK-110

[racingt...@gmail.com]

I have been using my SARK-110 for several years for HF measurements but just recently got some 6m capability so I built a moxon antenna for rover use and was using the SARK-110 to adjust it.  Now I have modified my instrument with an adapter to SMA which has been fine, I also often use an external battery which has been fine for HF work but something is up at higher frequencies.  I had made a preliminary measurement on the moxon adjusted it for about 40+j7 at the feed point and then went back to computer modelling, found that I could raise the real part and lower the reactance by increasing the end gap on the moxon, did that and went back out for more measurements.  This time I ran the instrument with the external USB battery connected by a USB cable folded up (see photos).  The antenna appeared to be reonant about 1MHz lower and the impedance had changed to something like 25+j10 which was the opposite of what I'd expectd.  I wasted a lot of time making repeated adjustments to the antenna and scratching my head.  When I was just about to give up I removed the USB power cable and voila the measurement changed significantly.  I lowered the antenna one more time, adjusted everything back to where the NEC modeling suggested with the increased end gap and got the feedpoint impedance of 44+j2.4 which is a nice 1.15 to 1 match right on the expected frequency at 50.3 MHz and that was gratifying to see the real world follow the NEC modelling but now I had to scratch my head again wondering how a little USB cable could affect the measurement?  Any ideas from this group would be very welcome. The images below show how the USB cable was while this issue was happening.  I should add that I have included a good CM choke at the feedpoint of the moxon array with 6 turns of RG-58 would on an FT-240-61 core and was seated inside the cab of my truck which should afford additional faraday shielding from the antenna.  Why would the instrument be sensitive to RF on its DC input like this?

Joe ve3vxo

[John G3PQA]

Hi Joe,  At LF I can get away with just holding the Sark with internal battery in the air, but the shunting effect or proximity effect of the Sark and cables may be significant, and the CM choke at the feedpoint might not be enough or possibly being pulled off resonance.

Try an isolation transformer with separated windings and core material suitable for 50MHz.(others please advise if 61 core would be suitable). Recalibrate after the transformer of course.

John

[Joe Street]

Hi John

Thanks for your response.   The 61 material should be right for 6m CM choke with 5-6 turns according to my homework.  I guess I should try choking the USB cable but didn't think of that before putting everything away.  Still though, shouldn't the DC supply be well decoupled internally?

Joe ve3vxo

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[racingt...@gmail.com]

Today I got a chance to check another antenna which was an inverted vee, again for 6m this time up 10m and fed with a combination of a short section of LMR-100 to keep the weight off the top of the fiberglass pole and then the remainder RG-8.  This time I was ready with a couple of different USB cables.  I pruned the antenna with just the internal battery on the SARK-110 and got it how I liked and then I plugged in the external USB power pack with the same cable I used the other day while adjusting the Moxon array.   Sure enough the measurement wen crazy with the external power.  I then tried a cable with a molded ferrite choke and the measurement went back to the same as it was with only the internal battery.  I tried a second USB cable which was one like the original but I had added my own clamp on ferrite.  Again it was indistinguishable from the measurement with just the SARK-110 internal battery.  There was some question as to how effective the common mode choke was on the moxon and whether feedline radiation was coupling to the USB cable.  This inverted vee was meant to be a very simple and lightweight 6m compromise antenna for portable use and did not include a balun or choke at the feedpoint so most definitely the feedline was participating in the radiation in this case but the issue seems to be completely related to RF coupling on the DC USB power cable.  So a note to anyone using external power, make sure you use a choke on the power cable if using external power during a measurement.

I hope this information saves someone else some confusion!

Best regards and strength to all.

Joe ve3vxo