[Antennaware] Dual fed K9AY

K9AY k9ay at k9ay.com
Wed May 27 11:35:03 PDT 2009

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 

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

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