I already tried myself using the transmission line feature in EZNEC for that
"stub", but found that it did not at all accurately model the real-world
antenna. The "stub", "trap", "phaser", or whatever you wish to call it
affects the phasing of the signal and it's impedance back at the feedpoint.
In reality, my measurements on a finished antenna at 120ft (top wire) using
a 4:1 Guanella design current balun to 50ohm coax showed SWR of 1.3 on 80M,
1.3 on 40M, 1.4 on 20M, 1.6 on 17M, and 1.7 on 15M...this after adjusting
lengths of wires and stub in the old trim-measure-trim-measure method.
Putting those dimensions back into the three inexpensive modeling programs I
was using resulted in completely different SWRs/impedances from
real-world...only the expensive pro program showed accurate results, and the
antenna engineer I worked with on that model says it appears to be the
phaser/trap/stub requiring at least a very high number of segments to better
model the field between the closed-spaced ladder wires.
The final dimensions when using #12ga stranded THNN insulated wire is 52'6"
for each of the 2 top wires, which are of course joined with the 450ohm
ladder line "stub" which wound up for me at 30'11" (I use 14ga stranded
ladder line) and that makes the antenna a total of 105ft across the top,
64'2" for each of the side wires. Note that wire insulation has an affect
on the velocity factor (measured at abt 97%)and therefore wire lengths for
this antenna; also, wire lengths are unusually senstive on this antenna for
some reason I don't quite understand (perhaps the affect they have in the
pattern against the hanging "stub"). The stub's physical position seems to
have an affect on antenna metrics, as I watched my analyzer readings change
in real-time on a breezy day from the stub blowing around...I minimized this
by putting a 24" acrylic plastic bracket at the bottom so that I could
better secure the stub in place while preventing it from twisting (which
also has an affect). An interesting point: when constructing the antenna, I
first used the analyzer to cut the ladder line "stub" to resonance at
7.175MHz and with VF measured at 90.9% it wound up at abt 34ft...I soon
learned that it had to be shorted in the final antenna, so there are some
affects with the rest of the antenna system that changes its
characteristics. I'm intrigued by Terry's idea below of a 1/4 wl 80M open
"stub"...I'll have to try that.
The balun is not a mystical trade secret. A good Guanella current design is
all that is needed...see W2FMI's balun book for examples. You will get a
bit more ERP if you run 40M length ladder line from the antenna feedpoint to
the balun and if needed a short length of coax into your shack.
FWIW, I found that the performance of the Carolina Windom antenna is
comparable to the Super Delta Loop...but it costs more since it requires a
2:1 Ruthroff voltage unun at the top and a 1:1 current balun at the bottom
(I used a Guanella for that one). But it models nicely, thus is quite
predictable. It also has an interesting vertically polarized component in
the antenna plane, which makes it sort-of an omni pattern (horiz gain
perpendicular and vertical gain in-line). That vertical componenet somehow
brings the horiz take-off angle down to abt 7deg. The Super Loop saves
money and is a few db quieter for receive, but without an accurate model
you'll futz a *lot* with it getting it to play nice.
-----Original Message-----
From: antennaware-bounces@contesting.com
[mailto:antennaware-bounces@contesting.com] On Behalf Of Terry Conboy
Sent: Thursday, September 24, 2009 11:42 AM
To: Robert Kirkland
Cc: antennaware@contesting.com
Subject: Re: [Antennaware] SuperLoop 80 (was Help! Which One)
It's no problem modeling the stub in EZNEC using the built-in "virtual"
transmission line feature. From the Radioworks description, it appears that
it's a shorted 1/4 wl stub on 40m, which looks like a open circuit on 40m
and an inductance on 80m. Since the antenna is symmetrical, the coupling
between the physical stub and the rest of the antenna cancels out, so the
mathematical model of a virtual stub should work fine. In the real world,
the position of the stub shouldn't make any difference as long as it is
perpendicular to the top wire.
What I'd like to know is what's inside the RadioWorks "DMU" (dedicated
matching unit) at the feedpoint? The antenna looks like a 1 wl quad loop on
80m, with a Z of 110-220 ohms, depending on height. On 40m, it's a 2 wl
loop open at the top (similar to a bisquare) and the impedance is about
1300-1600 ohms. I guess it depends on how bad the match actually is on
40m... with a 4:1 balun, the SWR could be 8:1 on 40m. Radioworks is pretty
elusive on the actual match specs.
I found the match with a 4:1 balun was better on 80 and 40 when the top wire
was only 106 feet long and the top to bottom dimension was 27.4 feet. With
the top wire at 60 feet above ground, the resonance on 80m is near 3.85 MHz
(200 ohm SWR=1.23) and near 7.165 MHz (200 ohm SWR=1.45). The stub was
assumed to be 28.6 feet of 450 ohm line with VF=91% and loss of 0.1 dB/100
feet at 14 MHz.
Another configuration could be to use an open stub that is 1/4 wl on 80m.
Then the stub looks like a short on 80m and an open on 40m. This moves the
resonance on 80m from near 3.5 MHz (with the inductive shorted
stub) to around 3.9 MHz with the longer open stub. It appears that the
match is a bit better on 20m and 15m with the longer stub. Of course, then
you have to deal with the extra stub length in the real world.
73, Terry N6RY
On 2009-09-23 4:59 PM, Robert Kirkland wrote:
> Just a thought, If someone desires to model a Super Loop and has
> trouble with the 40 Meter stub try to model the stub up, away from the
> Balun instead of down. It should not make any difference on how the
> model operates. Also consider a self resonant coil (half the turns
> wound in one direction, the other half wound in the opposite direction),
like a stub wound into a coil.
> I may have to go to work on this, it's getting interesting.
> 73,
> Bob,w5pvr
>
>
>
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