Guys, I have to chime in on this one.
In many of the responses to the guy asking for advice about what 160M
antenna to use with his 90-foot tower, I'm reading over and over
snippets of "common knowledge" and "known fact" being chanted like
some kind of mantra. Sometimes long-held beliefs and theory conflict
with actual practice.
Specifically, I'm talking about the many ops who have chimed in
telling the guy that an inverted-L won't work very well on his tower,
and that any vertical antenna close to his tower will not perform
well. I beg to differ!
Let me explain: My 160M TX antenna is a six-wire cage vertical
monopole with an effective diameter of 33 inches. It is 83 feet tall
and suspended alongside my 90-foot Rohn 25 tower with KT-34XA
tribander on top, just as the vertical section of an inverted-L would
be. Center-to-center spacing between the vertical and the tower is 42
inches. 106 radials, ranging in length from 72 to 155 feet complete
the installation. And, let me tell you, IT WORKS!
A good way to test the effectiveness of any antenna is to turn the
power down and see who can hear you. A couple of years ago in the
ARRL 160M contest, I cranked the power down to 5 watts output. In a
few hours of search-and-pounce I worked 46 states and 7 DXCC
countries. I never had to call a station more than twice to be heard,
95% or better came back on the first call.
Now, I know you're going to say that a vertical that close to a tower
simply forms a complex method of feeding RF into the tower. But, even
if this were true, it would surely be an inefficient way of doing
things, and would hardly produce the level of performance I've seen
from my antenna. Further, I can discount the "feeding the tower"
theory in several ways.
(1) When I model the vertical with EZNEC WITHOUT the tower present in
the model, I get a certain set of values for Z and X, and these
values are born out to be correct by the values of the components
that achieve a proper match at the antenna base. But, here's the
catch: the tower is THERE in the real world. The L-network component
values that match the antenna correspond right-on with the values
calculated for the vertical alone, with no tower present. When I
model the set up with the tower in the picture, I get values that
don't even come close to what I'm actually seeing.
(2) I've done two different blind A/B tests to try to determine
interaction with the tower. In the first test, condition A has the
tower grounded, but not attached to the vertical's radial system,
condition B has the tower grounded AND attached to the center of the
vertical's radial system. A vacuum relay was used to remotely and
instantly switch between the two positions. SWR on the matched
vertical goes from perfect
(A) to only 1.3:1 (B), not the wild change predicted by modeling.
Also, my helper, located 9 miles away can detect no difference in
signal strength, either by ear or S-meter between A and B. (Tests are
done near mid-day.)
The other A/B test I've done was with de-tuning the tower. I added
two 4-foot long aluminum arms to the tower about 25 feet apart and
connected them with #12 wire through a combination of fixed HV caps
and a large air variable cap. I tuned this loop for resonance at
1.83Mhz (lowest Z, in this case about 6 ohms). In theory, this would
de-tune the tower, making the 25-foot section between the arms
"invisible" at 1.83Mhz. (If you do this, you need some high quality
transmitting caps and/or wide spaced air-variables or even a vacuum
variable, because the RF voltage at this point will be VERY high).
The vacuum relay was placed in the wire at the connection to the
caps. Again, the SWR varied only slightly between the two positions,
and again my helper could detect no change in signal strength.
So, what does all this tell me? It tells me that the dreaded "tower
interaction" is not always the "boogie man" everyone says it is.
Every person's situation is different, and you won't know how any
particular antenna "plays" at your QTH until you try it.
YMMV
73,
Charles - K5ZK
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