Thanks for that link Grant. I think I'll try it. I have a couple of
spools of RG8X-LL which will handle the legal limit if the SWR is low.
I tune the amp into a dummy load, so I want an SWR limit of 1.5:1 so it
requires no tuning when switching to the antenna. I currently use a
line tuner on 40 as the SWR is low across (roughly) the bottom 3/4 of
the band at the antenna. I typically operate below 7.2 and 3.8. I may
still need to resort to remote switching to keep the dual band
capability. If I can't then I won't be able to go with the 5 switched
slopers.
The actual SWR (on 40) at the feed point is low enough that the 8X-LL
easily handles the legal limit. Not so on 75 where I've used either
LMR400, or BuryFlex with one common mode choke at the antenna and
another at the tower. I'm going to wind a dual choke with 10T of RG 400
on 2 cores and 7T on another 2 cores in series at the feed point in
hopes of negating the need for the current choke back at the tower. A
75 meter, center fed, sloping, half wave dipole from roughly 90' to 10'
is far from being balanced. I'd like to use the RG400 for feedline, but
that stuff is expensive!
I'm currently using LMR-400 for feedline with the chokes wound as part
of the feed line, but will go to 8X-LL if I can broadband the 75 meter
section. Even as shown in the article, I can use the line tuner so the
rig, or amp sees 50 ohms, simplifying the amp tuning.
Using 8X-LL would reduce both the weight and wind resistance substantially.
73, Roger (K8RI)
On 8/18/2017 Friday 10:34 AM, Grant Saviers wrote:
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)
---
This email has been checked for viruses by Avast antivirus software.
https://www.avast.com/antivirus
_______________________________________________
_______________________________________________
TowerTalk mailing list
TowerTalk@contesting.com <mailto:TowerTalk@contesting.com>
http://lists.contesting.com/mailman/listinfo/towertalk
_______________________________________________
_______________________________________________
TowerTalk mailing list
TowerTalk@contesting.com
http://lists.contesting.com/mailman/listinfo/towertalk
--
73
Roger (K8RI)
_______________________________________________
_______________________________________________
TowerTalk mailing list
TowerTalk@contesting.com
http://lists.contesting.com/mailman/listinfo/towertalk
|