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
convinced.
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.
tia
Clive
GW3NJW
gw3njw@gw7x.org
Contest Cambria-http://www.gw7x.org
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