Using EZNEC, I have designed a 40M reduced size delta loop that uses
450ohm ladder line in the lower corners to give the required inductance
for the reduced size I need at my QTH. Now the question: EZNEC indicates
that the feed impedance for this loaded loop runs from around 35ohms
at a lower corner to about 60ohms at the apex. EZNEC also "suggests"
that the radiation pattern is fairly constant for just about any feed
point along the sloping vertical section. I know that full size loops
are fed 1/4 wave down from the vertical. However, with this reduced
loop, 1/4 wave from the apex wraps around the lower corner and starts
out along the horizontal lower section. Where should I feed this thing?
A second question arises as to the feed line. With my previous full size
delta loop, I used 1/4 wave 72ohm coax to bring the impedance to around
50ohms at the shack end of the feed line. This antenna performed well. I
worked Heard Isl. barefoot with one call on 40M from WNY. Also, on the
full size loop, I put a ferrite choke balun on the coax at the feed
point. In retrospect, I don't think that such a choke is needed and may
even be contraindicated. After all, the delta loop is not a symmetrical
antenna when fed 1/4 wave from the apex. Any thoughts on this point?
Thanks much for any and all comments.
Robert G. Strickland PhD ABPH - KE2WY
Syracuse, New York, USA
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