email@example.com (Roderick M. Fitz-Randolph) wrote:
> Envision 2 vertical antennas for 75/80 meters separated by
> whatever is the best compromise for the situation (1/8 Wave?
> 3/16? 1/4?). The forward one (in the direction of the desired
> signal strength) would be cut for 3795 kHz or thereabouts; the
> rearward vertical cut for 3525 kHz. When using the rear one
> (for CW) the forward one would act as a directive parasitic
> element. When using the forward one (for SSB) the rearward one
> would act as a reflective parasitic element. No phasing harnesses,
> nothing exotic.... just a relay at the base of each that would
> "make" when using that frequency and be "open" when using the
> other element.
For 1/4 wave verticals, you should ground the unused element
to the radial system. If you left the unused element "open", the
unused parasitic element would have little effect on the pattern
and you would essentially have a single vertical.
> Variations on the theme of W5HVV would be to ground
> the parasitic element to the counterpoise, which might have a very
> beneficial effect
Yes, gain and directivity!
> Anyone game to try their EZNEC or whatever software on this one?
I built a 40 meter version of what you are describing using two
1/4 wave verticals at 3/16 spacing. One was tuned as a reflector
via a base loading coil. We used it at J6DX during the 1996 CQWW
CW. It seemed to work quite well though it was difficult to judge
since 40 meter conditions were excellent at our location.
Feed point impedance drops substantially thus, good radial systems
under both verticals is more important compared to a single
vertical, especially when ground conductivity is poor.
> Surely this has been done before, or has it? Don't think I've
> ever run across this combination, to my best recollection.
Yes, besides the 40m J6DX version, K3LR has a similar system with
4 parasitic vertical elements on 160. I am sure there are others.
The more difficult aspects of this approach are properly tuning
the parasitic elements and the lower feedpoint impedence which
requires a better radials system. We used 24+ 1/4 wavelength
radials under each vertical and should have used more.
I do not recall the exact gain figures but, I believe the 40
meter array would have 4 to 4.5 dB gain compared toa single
vertical when fed against an ideal conducting ground plane.
At J6DX, the ground loss was approximately 1.5 to 2 dB based
on feedpoint impedance measurements compared to the computer
Hope this helps,
Steve Miller N8SM firstname.lastname@example.org http://www.dma.org/~millersg
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