[CQ-Contest] Is better antenna isolation also diminishing antenna azimutal pattern distorsion?

[SM0HRP Kari Gustafsson]

Hi contesters!

I am suspecting that I have distorted antenna patterns due to three towers
close to each other. All are in 65 feet range from each other and they all
are 70 feet high (at triangle corners). The terrain elevation is 60 feet
high and sloping in all directions (with good take off - premium) towards
the water sea line. Tower 1 - 4 el 20+5 el 15, Tower 2 - 3 el 40 m linear
loaded (Hy Gain) and tower 3  - 1 el 80 dipole Create. 

 

Issue: Antenna isolation measures reveal that my antennas couple strongly
about - 30-40 dB without any filtering. But I have invested in high power
bandpass filters to get down the harmonics under S7 on all bands.  Comparing
my RBN spots with nearby "smaller antennas" close to my QTH (some 150 km
away) I have experienced worse antenna strength signals into many
directions.

 

Some say that filters and stubs do not absorb power but reflect. It makes
sense since filters are low loss.  So it is that my antennas couple so
strongly with each other that their radiation patterns get distorted
despite the low net S7 harmonics on most bands? I am thinking of measuring
antenna azimuthal  plot from the antennas some 200 feet away (should be more
though) on a 20 m dipole and see how the patterns look like when rotation
the 20/15 yagi and the 40 m yagi. Another remedy would be to put suitable
stubs on the antenna elements feed points. I know some wellknown antenna
yagi manufacturers do so to avoid unwanted resonances on adjacent bands (40
m yagis and 20 m mutual coupling). 

 

How do you contesters perceive these kind of antenna isolations issues on
big Christmas Tree stacks? Anyone been trying to solve antenna patten
distortion issues like this and what are the lessons learned?

 

Thanking in advance, 

Kari SM0HRP