[TowerTalk] TowerTalk Digest, Vol 126, Issue 5 - VE7RF comments on ant interaction

[Pfizenmayer]

  AHAAAAAAA ___ I just put up a Msquared  40M1L with the 17 meter "stinger" 
about 8 feet above a KT34 - on the ground and at 26 feet on my roof - 
everything looked normal (usual shift lower in freq  close to ground ) - but 
when I put it up at 50 feet ( in line with the KT34 boom since my old 40M 
rotary dipole screwqed up the KT34 F/B on 15 meters badly), 17 meters 
became almost 2:1 and the SWR curve looked totally different from on the 
ground - was now about 1.8:1  from almost 17 mhz to 18350 !!!! . Before it 
had the typical SWR dip.  (all measurements with VNA2180). While the crane 
was here , I started clipping off ends of the stingers and not much was 
happening , on the roof it followed the lengths as it should have.  I quit 
because it was not moving appreciably and  it was about  $35crane time per 
clip . 40M followed factory dmensions closely .

Msquared says they have no data , not even a  freq sweep at height by 
itself.

I did a simple EZNEC model of a 16 foot long  3 inch diameter boom with 15 
foot elements at end and depending on how much capacitance (about 15 pfd and 
less) you put from the element center to the boom , you can move the boom 
resonance from around 15 mhz to about 19 mhz . So I suspected that was what 
was happening -- but all I got was a ten yard stare from M2 . (When I was 
working on the KT34 15 meter F/B , I ran  hundreds of patterns with a source 
a mile away and looking at front to side - , F/S was a tad better on 20 and 
15 with the  ref and dir floating so I left it that way. Next time a crane 
is here I think I will put the straps on to connect the KT34 ref and dir to 
the boom and see what happens to 17M.

BTW the old 40 M KLM rotary dipole  was 35 feet above and about 10 feet 
horizontally away from the KT34 while I was working on it , and it would 
move 15 M F/B from 19 dB to 6 dB or so and at different freq for best F/B as 
I rotated the dipole broadside or at end on to the signal source . I chased 
that one for several days until it dawned on me I was leaving the dipole at 
various positions as I tried to set the KT34 reflector  resonances. I did 
not expect that much interaction at 35 feet away .

Sounds familiar -- Thanks VE7RF .

73 Hank K7HP


> ##  You also have to be careful when 2 yagis are at right angles to each 
> other.
> years ago, folks would try and use a KLM  KT34XA    with a KLM 3 el 
> yagi.... it doesn?t work.  40 + 15m
> would interact badly..with both pointed in the same direction.   If one 
> yagi rotated  90 deg, the  40 + 15m  interaction
> is now zero,  but a new problem arises.    The 32 foot long boom of the 
> KT-34XA  will screw up 20M band real bad !
>
> ##  So the Fix  was not to use the 3 el klm 40m yagi on the 32 foot 
> boom,..... but to instead use the klm 4 el  40m yagi on the
> 42 foot boom.   Then when at right angles, no interaction on any band.
>