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

K3ZJ David Siddall davek3zj at gmail.com
Fri Jun 3 11:06:31 EDT 2022

Hi Kari,

I would start with the comprehensive analyses published in 8 parts in the
NCJ by Eric Scace, K3NA.  All are available at ncjweb.com, beginning with
part 1 here:

73, Dave K3ZJ

On Fri, Jun 3, 2022 at 10:45 AM SM0HRP Kari Gustafsson <kari at sm0hrp.se>

> 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
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