The ARRL Antenna Handbook has a number ways to broadband a dipole,
several probably difficult as noted.
One not in the current handbook is the Witt/Leeson tuned coaxial
resonator feed method, see
http://www.robkalmeijer.nl/techniek/electronica/radiotechniek/hambladen/qst/1993/09/page27/index.html
At QRO on 80m, small coax such a Davis RG8X-LL will be ok and RG6 is QRO
capable for the 75 ohm section per W8JI evaluations. There are no L, C
or relays in the air, it's a plain wire dipole and the coax is needed
anyway to feed it, so it is cheap, light, and easy to maintain.
I've built one with RG8X-LL and Belden 8281 double shield video cable
(often cheap on ebay) for next year Field Day but not run the in the air
swr curves yet. As with any dipole there is height sensitivity so for
this application modeling with the tower is a good idea. I modeled it
for my application with EZNEC/4 and optimized it with AutoEZ and the swr
is < 1.6:1 from 3.5 to 3.92Mhz, rising to 2:1 at 4.0MHz.
Grant KZ1W
On 8/18/2017 0:56 AM, Roger (K8RI) on TT wrote:
I'd need 5 auto tuners and the configuration would require that the be
close to in sync.
These are center fed, half wave dipoles. I've had excellent results
on 40 and 75. Unfortunately with the common mode choke and BuryFlex
hanging from the center. It requires a LOT of tension to keep the
antennas straight...About 200# worth. There's a lot of 5/16ths double
braid rope already on the tower through pulleys, making working on the
antennas easy, but I don't think the 5 auto tuners would be practical.
73, Roger (K8RI) .
On 8/17/2017 Thursday 3:20 PM, Rod Greene wrote:
Or use an auto tuner at the feedpoint to match the slopers on
whatever band/freq you want to use. That's what I do.
73, Rod/w7zrc
------------------------------------------------------------------------
*From:* Roger (K8RI) on TT <K8RI-on-TowerTalk@tm.net>
*To:* "towertalk@contesting.com" <towertalk@contesting.com>
*Sent:* Thursday, August 17, 2017 12:33 PM
*Subject:* [TowerTalk] Broadbanding 80 meter dipole
I've seen a few possibilities in the past, but I'd like to add a dew
slopers to come up with the 5 used for some directivity, gain and front
to back. Switched coils at the feedpoint with frequency sensing would
likely work. I've had excellent results with just a couple switched
slopers on 40.
To complicate things I'd like to do this and maintain the dual band 75 /
40 meter operation, which would likely require separate feedlines. The
unused line would / could end up as a stub. I could possibly use the
frequency sensing to disconnect the coax for the unused band.
Thoughts?
--
73
Roger (K8RI)
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