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@contesting.com
[mailto:antennaware-bounces@contesting.com] On Behalf Of K9AY
Sent: Wednesday, May 27, 2009 6:35 PM
To: antennaware@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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