Notes on the dual-feed K9AY Loop....
"Dual feed" is the equivalent of a remote R +/-jX termination, plus the
ability to compensate for asymmetry and systems losses such as wire loss,
hence the extremely deep nulls that Andy reports. I suspect that the huge
gain increases shown in the dual-feed model are artifacts of the modeling
mathematics - the free-space model shows the same increase, yet common sense
says "no way." A little delving into models with multiple sources is in
order.
A few clarifications to N4GG's notes...
>>1. The far field pattern is ALL vertically polarized - consistent with
>>the antenna behaving as two verticals with a phasing line connecting them.
Close, but not quite, Hal. It's 100% vertical in line with the loop, but
there is a 10 db down horizontal component broadside to the loop. The "two
verticals" analogy is useful, but you can't achieve exactly the same pattern
with two infinitesmal verticals. I prefer to describe it as a DF loop and
sense antenna in one structure, i.e., the antenna is simultaneously a loop
and a short (bent) vertical, with outputs balanced by the resistor.
>> 2. The connection to ground is 100% common mode to the two verticals.
>> All my K9AY modeling includes what I call Rg, which is a resistance in
>> the ground leg. Front-to-back is very sensitive and to some lesser
>> degree gain is sensitive to Rg, exactly as you would expect.
To expand ... Although the ground connection is sensitive in the models,
there is no such abiguity when using a mirror image of the loop to create a
free-space model. For this structure, the terminating resistance is approx.
1000 ohms, which is 2x (Rterm + Rgnd) where Rterm is 390 ohms and Rgnd is
100 ohms. It also says that Rterm is actually 500 ohms minus half the ground
loss. Feed impedance is affected similarly. I considered this the best
verification of my "Mininec Ground + Loss" model.
73, Gary
K9AY
_______________________________________________
Antennaware mailing list
Antennaware@contesting.com
http://lists.contesting.com/mailman/listinfo/antennaware
|