[LarrysList] After Breakfast activity...and an antenna mystery

Laurance Staples larrystaples at mac.com
Sat Sep 15 15:41:19 CDT 2018


Well, another fun Saturday morning today, with 5 participants on deck...George, KD4RUU, Dennis, K0DGB, Dan, KC0VYT, Bob, W0AO, and John, AB0O. 

Dan and George set up Dan's end-fed 1/2 wave as a vertical on an MFJ 43ft telescoping mast, with a radial plate. They worked several 20 meter contacts with Dan’s TS-480.

Bob had his KX3 on a 40 meter dipole equipped with an NVIS ground wire in the hopes of snagging Iowa QSO Party stations.
 
John had his 2-week-old home-brew 40 meter mag loop set up, this week with a commercially-made variable capacitor. Tuning was much improved over last week's cardboard capacitor. The loop was easily tunable to an SWR of 1.3 to 1.5 

Weather was perfect with warm temperatures and a light breeze. 

Now for the antenna mystery…

During the morning, Bob and John tried working 40 meters...signals were present, but usually weak...but generally, in spite of some strong signals, it proved difficult to work stations with either antenna. It was as if the band conditions were poor, and few stations could hear us. Just the band conditions, right?…

...around 11AM, John proceeded to first disconnect the coax from his 817, and then shortly after, lay the mag loop on the ground for disassembly. Well, as those two operations were carried out, it was like a switch was turned on at Bob's radio...40m signals immediately became strong and plentiful…Bob worked several stations in quick succession within a few minutes!

It was as if the mag loop was "sucking" the RF out of the surrounding area, to the detriment of Bob's antenna. Would it have been the same if Bob had taken down his antenna first...we don't know, and didn't try it. (John had the mag loop up and running before Bob had set up his dipole, but signals on John’s station didn't seem to change once Bob put the dipole on the air.)

We had never experienced an instance of such pronounced mutual antenna interference. 

For those who are intrigued, the feedpoints of the dipole and mag loop were a little less than 1/2 wavelength apart. The plane of the mag loop and of the dipole were roughly perpendicular to each other. The mag loop design was optimized for 40m with a circumference of 25 feet and a design Q=330. The apex of both the dipole and the loop were at approx 12 feet. 

Very interesting observations. We had operated many stations in our respective experience where multiple dipoles or yagis were close or overlapping, even on same bands, but without such extreme interference. Does the mag loop's high Q and larger dimensions cause it to create an RF "black hole" around itself?

The only explanation that comes to mind (over lunch) is that the configuration, proximity, and design/tuned frequency of the two antennas happened to be at the exact optimal spacing so as to create an phase difference of EXACTLY 180 degrees that somehow caused the two antennas to act as a hi-Q notch filter at exactly 7.200+/- Mhz. It occurs with passive components on a PC board, so why not with tuned components the size of antennas? I dunno...
It's one of those things that we couldn't cause on purpose, but that occurred by accident!!??

So, Larry's List now has a mystery puzzle column...enjoy!

73,

John Ford,  AB0O
jsf106 at gmail.com


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