February 4th 07, 03:06 AM
posted to rec.radio.amateur.antenna
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First recorded activity by RadioBanter: Jul 2006
Posts: 296
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help- Mobile 10meters on a jeep
"KE5MBX" wrote in message
ups.com...
On Feb 3, 2:02 pm, Cecil Moore wrote:
KE5MBX wrote:
I'm still really interested in the
cophasing thing. I have mounted a CB whip on the roll bar before, but
it has two disadvantages: Extreme height of the antenna and the
inability to close the soft top while the antenna's up. Could I not
cophase diagonally and therefore center the patten ?
First a few thoughts. Co-phasing doubles the cost of
the antenna without outperforming a single whip (unless
the elements are separated by 1/4WL or more). So
why not a magmount on the hood with a single whip?
That would be my solution but I'm not very "cool". :-)
My contacts could not tell the difference between the
magmount on the roof of my pickup Vs on my hood.
Phasing would work best front-to-back. With 135 degree
phasing in elements separated by 1/8WL, a whopping 4 dB
gain is possible switchable from front to back. If 1/4WL
spacing is possible, bidirectional front and back gain
of 3.5 dB is possible with 180 degree phasing. If I were
driving an 18 wheeler, this is what I would do. Changing
the phasing to 90 degrees would give me a front or back
vertical beam.
There's no reason why your co-phasing idea will not work.
The feedpoint impedance of the two elements in parallel
will be 1/2 of the impedance of each element transformed
by the length of coax chosen. If you put the coax 'T' 1/4WL
back from each element, the impedance at that point should
be close to the feedpoint impedance of each element (assuming
35 ohms).
--
73, Cecil http://www.w5dxp.com
Excellent.. If I put my antennas on opposite corners of the Jeep as I
was thinking, I find that they will be 4.2 meters apart. I promise to
go home and see what the ARRL handbook says about cophasing, but I
only understood what you just told me partially. Are you describing a
way to get better front/back gain from a system with one antenna in
front and one in back, as opposed to the conventional "trucker"
setup? I have no particular use for directional gain on 10 meters, by
the way.
I know a lot of people are reluctant to do the superior front-to-back
cophasing because it looks goofy, but I can assure I'm not interested
in being "cool" either! I want the most efficient 10-m operation I
can get out of a Jeep Wrangler.
Just go with one antenna.
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