[TowerTalk] Symptoms of antenna interaction?

[Mike Smith VE9AA]

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