[TowerTalk] Tower resonant frequency

[K9MA]

I once had a 2 element small receiving loop array with my 70 foot tower 
in between them. I knew that re-radiation from the tower would be an 
issue. The tower was shunt fed on 80 and 160. I was able to detune the 
tower on 160 and get a good f/b with the loops. On 80, however, that 
just didn't work. NEC modeling confirmed that the tower was just too big 
to detune that way on 80. (It was probably about an electrical 3/8 
wavelength on 80.) I suppose it's possible your tower might be enough 
bigger that it might work. First, try modeling your tower, and the 
antennas on it to get an idea of its electrical length. Then model it as 
a base-insulated tower of that electrical length, and see if some 
terminating reactance allows the 4-square to work. If so, there's at 
least a possibility that a gamma match up the tower terminated in a 
reactance might detune it enough.

If that looks like it might work. listen on the four-square and adjust 
the terminating reactance for minimum noise or best f/b. I mounted a 
short vertical on my car, parked it off the back side of the loop array, 
fed it with a little oscillator, and adjusted the termination for a 
null. Minimum noise seemed to work about as well.

All that said, re-radiation from the tower might not be such a big issue 
on transmit, as it mainly affects the f/b and other nulls. If you use 
other antennas for receive, it's probably not going to be a problem 
unless the tower happens to be resonant at exactly the operating 
frequency. Still, it would probably be useful to determine the 
electrical length of the tower, and include it in your model.

73,
Scott K9MA

On 9/17/2020 7:27 PM, Bill via TowerTalk wrote:
> Plans are VERY slowly progressing for my new station in Belize.  Plans are for a 106 foot tower with lots of goodies on it.
> Here's what is planned:106 ft of Rohn 55 guyed with Phillystran.  Three swing armed  Skyhawks at 32, 64 and 96.  Two A3WS WARCs at around 76 and 42 ft.  I plan on running four ropes to hold a wire 4 square surrounding the tower.  This is similar to what I had in KH6 and the 4 square worked well although the F/B was down to around 10-12 dB.
> My question is this:  I know the tower will have some effect on the 4 square surrounding it.  Obviously I don't want the tower to be anywhere near resonant on 80.  My guess is the top loading of the tower will lower the resonant frequency quite a bit, and the effective height will be around 145-150 ft, but I am not sure.  Obviously if it is in the range of around 130 it would be a big issue.
> There is not enough room to put up a 4 square by itself so I'm stuck with this set up.  I do have some fiberglass poles that I could attach to the tower vertically to give it another 13 feet or so by running a wire up the pole and attaching it to the tower, but I really don't want to do that if I don't have to.
> Thoughts on this?
> 73  Bill K4XS/KH7XS
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-- 
Scott  K9MA

k9ma at sdellington.us