In a message dated 10/14/01 6:41:00 AM Pacific Daylight Time,
realex@flash.net writes <<
Hi Pete,
Losses and feedpoint Z add so they are effectively in series. Thats why we
quite often see a 1:1 SWR at the base of a vertical over poor ground that
has no radials at all or not enough radials. Z should be around 34 ohms
(1.4:1) over a perfect ground. The ground losses add to it bringing the
value closer to 50 ohms.
ADD more radials and the SWR should go up. (without a matching device at the
feedpoint)
73, Bob, W5AH >>
Another example of the above is in a 75M mobile whip. The Rr is typically 1
ohm but in certain 75m loaded whips with small wire the Rtotal can be as high
as 36 ohms (35 ohms loss resistance--about 3% efficiency) where a Screwdriver
whip can measure 10 ohms Rtotal (9 ohms loss resistance--about 10%
efficiency). Shunt feeding with an inductor is widely used mobile to get 50
ohms Rtotal. Fortunately on 40m the Rr increases and the amount of shunt
loading increases in the right ratio to match the new Rtotal to 50 ohms again
and likewise on 20-10.
Don Johnsons Screwdriver Whip (Box 595 Esparta,CA 95627) has this shunt XL
and is cleverly built right in the whip where others have it external. He
was the original inventor of the Screwdriver Whip and sells it at a very good
price.
I would like to run a test of using 2 of them back to back on 160-40M on a
100' + tower horizontally polarized, a sloper or an inverted vee. It can be
controlled in the shack for resonant frequency over the whole band. Then I'll
extend the tips and bottom rods say 10' at a time and see how it perks up. I
would like to think that at about 40' on each side of (center loaded), this
rotatable dipole, sloper or inverted vee would be a fair DX antenna above
100' and not be a mechanical monster. It would be a great antenna for small
lots also. I think I'd tip it up say 30-45 degrees to keep water out of it.
It could blow up the skirt, however. It would be an "Inverted Inverted Vee"
up away from the tower. I've done this with 2 HyGain trapped verticals
80-10M that were about 35' long on each side. The bandwidth was a bit
narrower in the Inverted Inverted Vee configuration.
I'll have to run some patterns on center loaded each side dipoles compared to
full size and add 9 ohms at the coil on each side on 75M. I'm not sure what
the final matching technique would be but there is no doubt 50 ohms Rtotal
could be obtained in a couple of ways. 100 ohm Rtotal would be easy to
obtain also and could be fed with 100 ohms balanced coax. I'll do the
preliminary test at say 30' for a high angle radiator and it would serve a
very useful function even at that height.
Using 2 Screwdriver whips back to back is a great concept with all kinds of
possibilities. To simplify the mechanical aspects for one band, use a fixed
mobile coil in the center of each side and 2 variable Xc's selsyn driven at
the feedpoint to change the resonant frequency. I and others have done this
so I can assure you this technique does work. It doesn't work only for those
who never try it.
The feedpoint R should be above 50 ohms so by making the antenna longer than
a 1/2 wave and using the Series Xc's, 100 ohms Rtotal can easily be obtained
for the 100 ohm balanced coax. Or use a 1/4 wave of 75 ohm to jack up a 50
ohm feedline to 100 ohms and use a balun. This has been done before. The
longer the overall length the less coil loading needed for resonance and the
lower the Rtotal. Depending on coil size, the coil Rloss can be fairly
large. Lengthening antenna beyond 1/2 wave and using the series Xc's to
cancel the XL gives a progressively higher feedpoint Z allowing the use of
higher Z feedlines. More coil loading can increase the Rtotal also. Higher
Z feedlines and feedpoints means lower currents and greater bandwidths.
Instead of shunt feed in mobile I use this very technique with the
Screwdriver whip. It's 10 ohms Rtotal on 75m. I resonate it at about 3.6
MHz, add series Xc and presto 50 ohms at 3.85 MHz--and a great signal. Since
it works great there it has to work equally well with 2 of them back to back
as a dipole. Perhaps these suggestions can stimulate some creative work by
those who like to advance the state of the art and not set it backwards with
all the things that some feel just can't be done. Lets start a new trend on
TT. I and some others would like to hear any new ideas k7gco
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