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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