Hi Ed,
I appreciate your efforts. Unfortunately the conclusions only apply
to the antennas mounted at you QTH, and nowhere else.
> Both of the verticals were at DC ground potential unlike yours. The
> impedance measurements were taken to get a relative likeness between
> the antennas due to the closeness to all the utility infrastructure
> and house wiring in the near field of both antennas. My QTH is a
> typical city lot for a low band antenna. You might as well toss out
> all the text book standards when trying to take local noise
> measurements in this environment with a vertical antenna that listens
> well in all directions.
In that environment, you could have induction field (either electric or
magnetic dominant) coupling as a primary source of noise
coupling. It's tough to say what is going on.
One thing for sure, it is very unlikely noise levels are the same at
each antenna. One partial correct would have been to swap
antenna locations frequently in time compared to the overall length
of the test.
> You are lucky you can see the interaction between your antenna
> 500'-600' from a tower at your location. There are not any towers
> around here, only half a dozen power line poles with ground wires
> running down to a earth ground. I would believe both antennas couple
> to these structures in a like manner in the space they are located.
> Not much choice for me in this environment. I would think enough noise
> to create a receive problem is probably ground wave anyway.
The odds are actually almost 100% they do not couple to the
environment the same, and they certainly would not share the
same distance to noise sources!
> at 75 feet apart from any or all directions. I don't believe you would
> see much difference in the local noise between the two antennas with
> only 75' separation on this lot for a single vertical antenna.
You see less difference in wide open environments, and much
more difference with small changes in distance (as a general rule)
when the environment is cluttered. 75 feet is more than 1/2 wl on
7MHz.
There can be no difference in S/N ratio for a like-pattern if only the
feed method is changed. If there is a noise change, than one of the
feed systems is modifying the antenna pattern.
Noise is RF, **exactly** like the radio signals on the same
frequency! Distance and direction are the only possible differences.
73, Tom W8JI
w8ji@contesting.com
--
FAQ on WWW: http://www.contesting.com/FAQ/towertalk
Submissions: towertalk@contesting.com
Administrative requests: towertalk-REQUEST@contesting.com
Problems: owner-towertalk@contesting.com
|