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

George Fremin III geoiii at kkn.net
Fri Jun 3 12:41:54 EDT 2022

On Fri, Jun 03, 2022 at 02:30:52PM +0200, SM0HRP Kari Gustafsson wrote:
> 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.

RBN data with stations 150 km (or even closer) will likley not produce
very good data.

At any distance from your station - and even different height antennas
at your location can produce very differnt signal levels second to
second.  The ionosphere changes all the time and signals generated an
different locations or even at diffrent arrival / takeoff angles will
vary in signal level a great deal over time.

You would need to run these tests for some much longer periods of time
with a lot more data than any RBN system will give you.

I have seen wild variations even on what I would think is a short line
of sight path.  I have a nearby ham that is about 5 miles or so away.
This path is line of sight.  So I thought I could compaire my two 40
meter yagis.  During the day I tired to do this with him.  But just a
steady carrier out of one of my antennas had fairly large QSB at his
reciever.  I was very surprised. I am guessing that there were
multiple modes of propagation between us that was causing this QSB.

> 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?

I am sure you could try to measure all of the interactions between
antennas but I would think that in many cases it would be hard.
Sometimes they are obvious and I suspect that many times it would be
very hard.  I know some station owners try to do all sorts of little
things to get the last little bit of perfromace out of their systems
(ie. K3LR).  It would be interesting to know if he has tried to figure
out the things you are talking about at all or even a little bit at
this station.

For me - I try to do some things - like not have antennas / towers
directly in front of other antennas for key beam headings.  The tower
locations were thought about a long time before holes were dug.  For
most of us - you can not solve this for all beam headings unless you
were to put up arrays that all point away from eachother for most or
all beam headings.

I guess you could take it all down - including the towers and put up
one system at a time and figure out how to do signal measurements that
you can prove you can repeat excatly every time you do them and then
redo them wiht each change in your station.

I think for most if not all of us this is impractal. 

Computer models have gotten good but I suspect that modeling an entire
station such as mine or enen yours might be very hard to do.  The
variables get out of hand quickly.

As a result, I and many others make choices that are compromises. 

For me, they are things I know could or might be better - but due to
cost constrants or space constrants the only way I could have the
'perfect' is not have the 'good'.

I recall years ago a ham posting on Tower Talk or maybe here on
CQ-Contest something to the effect of:

"I have a 2 element 40m yagi, but I can only get it up 50 ft.  Since I
know that is too low to be any good on 40 meters I have not put it up
and it is in the box in my garage. And, no it is not for sale."

And all I could think is - that is silly.  It will work at 50 ft.
Maybe not as well as say 150 ft but it will work much better at 50 ft
than it will sitting in the box in the garage.


George Fremin III - K5TR
geoiii at kkn.net

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