[TowerTalk] Half Sloper recommendations - Results!

[Chet]

Charles, 

I don't doubt any of your findings. On the other hand I started with the
inverted L on my 90 footer with a C31 on top and a cushcraft shorty 40 on a
gate at 75 feet and went to shunt feeding the tower.  Neither are great
receiving antennas but the shunt fed tower here appears to be An s unit or
better in all directions.  I have 60 radials attached.

Using 1/2 half the amount of wire he would need for an inverted L he could
Use that other 60 feet for an extra buried radial.If my Results hold true he
stands to gain signal strength.  He would have to Climb the tower between 50
& 70 feet to install the shunt but he would Probably have to climb to 90
feet to get the inverted L up there too.

He has already stated that he has a vacuum variable cap so tuning either
Antenna would not be an issue, as long as he cuts the inverted L a bit Long
and shortens it with the cap.  All in all having tried both the L and the
Shunt fed tower I am staying with the shunt fed tower.  In last months arrl
160m in  7 hours I had 330 q's and 86 Mults running 80 watts and calling cq.
My amp was down and I Was just trying to gets some points for PVRC and was
blown away to have About a dozen EU stations call me. I was not able to get
on the second evening. I have never tried any extended dxing with the L or
the shunt feed at 5 watts. In my case the other disadvantage to the inverted
L  is that I can only bring the tail of the inverted L out to the front Of
the house, something the neighbors didn't particularly care for.  The Shunt
feed remains in the back yard and is invisible from the front Yard and does
not require the 2nd antenna support and insulators  that the "L" does. I
have never tried to Detune the antenna but it is on my list of things to do.


Sounds like your "cage" is working, I would guess that it is more
broadbanded Than my shunt fed tower. I do have my VVC motorized so it only
takes a second or 2 to retune at 1900 or higher.  You will never know if a
shunt feed will Be better at your qth unless you try it.  I think that both
the L and the Shunt feed are "a sure thing"  the sloper is a "possible
maybe" if he gets Lucky. I think the cage would be much more of a "project"
One of the locals here tried one and the wind destroyed it. He was thinking
of rebuilding but then he found a motorized VVC and has not revisited the
Cage. While it was up his opinion was that he did get a bit better bandwidth
on xmit but no discernable increase in Performance or rx or tx.  I only want
to climb my tower once so give me a sure thing every time.

Press on regardless

73

Chet N4FX

-----Original Message-----
From: towertalk-bounces at contesting.com
[mailto:towertalk-bounces at contesting.com] On Behalf Of Charles Bibb
Sent: Saturday, January 02, 2010 12:51 PM
To: 'Towertalk'
Subject: Re: [TowerTalk] Half Sloper recommendations - Results!



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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