R: [TowerTalk] Tall verticals
K0FF
K0FF@ARRL.NET
Wed, 10 May 2000 15:26:37 -0500
Pete, the tower must be guyed? Are the guys broken up with insulators, or
did you use Phillystran?
If so the best test method but hard to actually accomplish is to dip it with
a grid-dipper. A more practical rule of thumb is to measure the height of
the tower, add that to 1/2 the boom length of the bottom beam (what goes on
above the bottom beam is of little consequence), and add that to 1/2 of the
longest element on the bottom beam.
In otherwords, draw a line up the tower, to the longest end of the bottom
boom and to the end of the element on that end.
IF all that adds up to 125 feet or so, you have a natural for 160.
Again assuming the guy wires are not a factor, you can shunt feed the tower
with Gamma (single series capacitor)_ or Omega (2 caps. on series, one
shunt).
All you have then is the vertical part of a Marconi. It will require a
radial field below it to work.
Another approach is the K0FF 1/4 sloper. No radials needed.
You can only use one sloper per tower though, in case you need also 80 M on
the same tower, stick to the shunt feeds.
This would be dyno-mite without the added beam at 20M. Try loading her up
first, but chances are, all you'll wind up with when you put the other
antenna on, is an 80 meter radiator. The bottom antenna is of course acting
like a capacity hat, and really nullifies the effect of whatever is above
it.
Also if none of that seems like it's worth the effort, you can run 4 raised
radials out from the tower, not attached the tower, but to a ring of wire or
copper strap going completely around the tower at the 10 foot height.
Connect the braid of the feed line to the RING and the center conductor to
the TOWER. 4 Radials on the ground completes this FB 160 transmit antenna.
PS these are transmit antennas, and don't perform well as DX receive
antennas, but that's another issue.
Good luck, Geo>K0FF
-----Original Message-----
From: Pete Smith <n4zr@contesting.com>
To: towertalk@contesting.com <towertalk@contesting.com>
Date: Wednesday, May 10, 2000 3:43 PM
Subject: Re: R: [TowerTalk] Tall verticals
>
>At 09:59 PM 5/10/00 +0200, Maurizio Panicara wrote:
>>
>>It's practically impossible to prevent the beam from (top) loading the
>>tower.
>>Even isolating the yagi antenna from the tower, its coaxial cable will run
>>along the tower and will consequeltly follow the current distribution of
the
>>antenna.
>>The yagi, isolated or not from the tower, will consistently load the
>>vertical structure as much as the horizontal antenna is approaching (or
>>close) to an high voltage point of the vertical radiator.
>
>Interesting. My tower is 30m of Rohn 25 with a Force 12 C-3 and an EF-240S
>(total about 1 square meter of wind area) at the top. I'm planning to add
>another C-3 (.5 sq. meter) side-mounted at the 20-meter height point on the
>tower to stack on 20-10.
>
>I would also like to shunt feed the tower on 160. Is there any way to
>ascertain analytically whether the electrical length of the tower or the
>loading at 20 meters above ground will cause serious complications so far
>as the impedance to be matched, and the height of the shunt-feed point?
>I've looked at ON4UN's book for guidance on estimating the loading effect
>of antennas on a tower, and have followed this discussion, but don't know
>whether the bottom line is that the shunt feed is just a bad idea, or
>whether it can be done practically. I lack the engineering skills to do
>the analysis.
>
>73, Pete Smith N4ZR
>n4zr@contesting.com
>
>
>
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