[TowerTalk] Symptoms of antenna interaction?

[Al Kozakiewicz]

Thanks to all for the help!

To make a long story short, this setup was what I could do without a zoning variance, which I'm saving for a time in the near future when I have more land and will put something up in the 70-80' range.

Originally I was going to just put 6M and 2M beams on this shorty (35') tower as I got bit by the VHF bug last summer. Ray NM5O was selling his QTH which was just a couple miles from me and I bought a disassembled old C3 for a couple hundred bucks from him about the same time I was ordering parts and getting the building permit for the tower, so I decided to throw that up there as well. I'm glad I did.

I tuned all the antennas either sitting on sawhorses or elevated a few feet and pointing skyward.  This was done late last October, with the project completed after the first small snowfall in November (I needed frozen ground to run the man lift over the yard without sinking - a very long horror story).

So yes, this is an all new installation for me.

It does sound like my symptoms are interaction. Extending the mast is probably beyond what I can do with climbing alone. I'm using some 1.5" EMT (I think that is the size) as it was a) what I could get locally and b) was what would fit through the top section guide after trying and returning water pipe and rigid conduit.

I would not have thought that turning the 6M beam 180 degrees would have any effect except to repeat the problem, but I will try that first as it makes the azimuth math easier.

Al
AB2ZY

From: Mike Smith VE9AA <ve9aa at nbnet.nb.ca>
Sent: Wednesday, March 24, 2021 7:53 PM
To: towertalk at contesting.com
Cc: Al Kozakiewicz <akozak at hourglass.com>
Subject: re: [TowerTalk] Symptoms of antenna interaction?

Hi Al,

Was this a sudden change on your 6m yagi?

I've always tried to put my yagi's reflector some distance above ground, pointed at the sky, to do rough SWR adjustments before
erecting them on a tower.  I haven't had a real tower here in about 11 yrs, so I now forget the formula but there are also rough calcs available out there in Internet land which will tell you how much your resonant frequency is likely to move once you move it XX feet above ground.

Interacting with other antennas is sometimes a little bit of guesswork unless you're good with the modeling programs using NEC etc.(I'm not)

On my 6m & HF beams, I put the reflector 1' off the ground on a "milk crate" and aiming it skyward, tied off with rope to the leaning away from the nearest tree etc. This always puts me very close.  I've done an A3S this way as well.  Also close.

Eight feet is not a lot of separation for 3 antennas in your case, but as an example, I have a 17B2 (2m beam) and an A50-6S (6m beam) on a 24' mast behind my house separated by maybe 3.5' and I have essentially no SWR issues, pattern degradation or RF interaction. (can transmit on either antenna w/ 500w and can barely detect RF in the other radio).  I don't have an HF antenna in the mix however.

Thanks for all the work you do with N1MM+ logger.

GL !

Mike VE9AA

Mike, Coreen & Corey
Keswick Ridge, NB
till have a foot of snow on the ground, so I'm not quite ready to climb yet.

See photos on QRZ page.

I have 3 antennas on a 10' mast sticking a total of 8' out the top of the
tower. From top to bottom, 2M7, 6M3 and a C3, all equally spaced.  All were
tuned to resonance on the ground.

Now up in the air, the C3 and 2M7 are fine, but the 6M3 is useless. The SWR is
about 3:1 and Z=~70 ohms at the minimum for both.

Is this symptomatic of interaction, i.e. feed point characteristics change
while resonance frequency remains the same? If so, what is a recommended
solution? The one possibility I haven't checked yet as it requires climbing is
a bad rotor loop cable.

Al
AB2ZY