Towertalk
[Top] [All Lists]

Re: [TowerTalk] Half Sloper recommendations - Results!

To: "'Towertalk'" <towertalk@contesting.com>
Subject: Re: [TowerTalk] Half Sloper recommendations - Results!
From: Charles Bibb <zedkay@telepak.net>
Date: Sat, 02 Jan 2010 11:50:45 -0600
List-post: <towertalk@contesting.com">mailto:towertalk@contesting.com>

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


_______________________________________________



_______________________________________________
TowerTalk mailing list
TowerTalk@contesting.com
http://lists.contesting.com/mailman/listinfo/towertalk

<Prev in Thread] Current Thread [Next in Thread>