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[Towertalk] Detuning of phased verticals

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Subject: [Towertalk] Detuning of phased verticals
From: (Clive Whelan)
Date: Fri, 02 Aug 2002 20:43:00 -0000
Hi, Newbie poster

For about 18 months now I have been using a pair of Butternut 
HF9Vs spaced at 33ft, and fed either in phase or quadrature, on 
7Mhz, but also of value on 14Mhz where the broadside pattern has 
good gain.

The antennas are joined by an electrical 33ft of coax, with 
equal length pigtails to each vertical to reach. Feeding is then 
at the centre of this line, or at either end. Results have been 
very good, and on 7Mhz achieved world number 5 in CQWPX CW 2001.

Of course the SWR is now never brilliant, but is manageable. It 
was only after all the above that I read that I could not do 
this, since the essentially non identical nature of each antenna 
would upset the phasing. Believe me this was not the case, and 
the F/B was probably approaching 20dB in the endfire position.

I decided therefore that I could expand to a triangular array, 
and recently added a third HF9V, and altered the spacing to 40ft 
in an equilateral triangle. This change was really to avoid a 
tree at the 33ft position. In fact this is effectively a W1CF 
sort of array- see chapter 11 of the ON4UN book. However I do 
now have a serious problem, which the pundits again tell me is 
due to the non identical nature of the antennas, but I am not 

The problem is that when the third antenna is added to the 
array, either directly in parallel (just experimentally), or 
more seriously when phased at say -135 degrees, there seems to 
be serious detuning of the composite array. For example on 7Mhz, 
the "resonant" frequency ( really min. SWR) is about 6.4Mhz. 
Similar effects are noted on all other bands, and the detuning 
is generally LF. When tuning around with the MFJ259B, I now find 
all sorts of stray resonances which are not there in an 
individual antenna, or in any pair. For the present therefore I 
have put the project on ice, and am simply selecting any pair 
from the three antennas and feeding these in phase i.e. 
broadside, using three coaxial relays in a switch box at the 
base. This is something, but not what I want. I accept that I 
may have to do some work in balancing the antennas to achieve 
optimum results, but I sense there is something more serious 
going on at present, and am presently stumped.

I wonder if anyone else has seen this effect.

All input welcome.


Contest Cambria-

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