[Antennaware] Dual fed K9AY

[Hal Kennedy]

Agree with Gary on all points - thanks for jumping in Gary.  I have only
ever viewed the two vertical analogy as a rough approximation of the
antenna - useful for explaining it simplistically.  It is as you say,
not a precise representation.

I have some modeling runs Ill share in a day or two when they are done.
Gary, Pete and I all see the big gain jump for dual-feed.  I need to
check a few things before I feel comfortable with it being real or a
modeling artifact.  After two hours of looking at models this morning
however, I moved from where Gary is "doesn't pass the common sense test"
to not being too sure one way or the other.

Open for comments...
Hal
N4GG
 

-----Original Message-----
From: antennaware-bounces at contesting.com
[mailto:antennaware-bounces at contesting.com] On Behalf Of K9AY
Sent: Wednesday, May 27, 2009 6:35 PM
To: antennaware at contesting.com
Subject: Re: [Antennaware] Dual fed K9AY

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




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