I can't comment authoritatively regarding Jim's concerns, but I have used the
antenna you describe for decades with excellent results on multiple bands.
Whether a doublet or a horizontal loop (for me, best results always came with
the HL), I run open-wire line (not ladder line) straight down from the
antenna's elevated feed point to an SGC autocoupler (or an AH-4) mounted on a
short non-grounded post (and underneath an inverted wastebasket weather
shield). The open-wire line connects directly to the autocoupler's hot and
ground terminals. The coupler is floating. I do not ground the tuner at this
point (although I may install high-value static-bleeding resistors to ground).
I run RG-6 or RG-11 coax back to the shack (you can use 50-ohm if you want). At
the tuner's coaxial input I install a beefy ferrite-core choke. When fed with
RG-6 I simply wind 13 turns of the stuff through an FT-240-43 ferrite core (on
hand, Jim can advise on whether there's a slightly better choice). For the
tuner's DC feed, I do the same thing: wind 10-15 turns through a smaller Type
43 ferrite core.
The coax is grounded to the bonded external lightning ground just before it
enters the basement wall. If I was in a super noisy environment I might place
another ferrite choke at the radio end of the feed line.
Some installations require me to adjust the open-wire length a bit to avoid
"weird impedances" on some frequencies, but generally, this antenna works well
on all bands.
BONUS: If you add a radial field centered around the mounting post you can
short the open wire feeders and use the feeders as a "top-loaded vertical type
antenna" worked against the radials. If you do this (I use a pair of beefy
relays for remote switching) you must switch the antenna leads AND the tuner's
connection to the radials. You do not want the radials attached when working
the system as a doublet or loop, as the radials will become part of the
antenna, which isn't generally useful in that configuration.
All multiband antennas are compromises to some extent, but I have had fantastic
results with this arrangement for many a moon. As long as your tuner can handle
the RF power you're running, it's a good arrangement for crazed band-hoppers
such as myself. :)
I also use this arrangement with a "double-headed" inverted-L for the low
bands. One wire is 104 feet for 160 and 80, the other is 50 feet for the higher
bands (for the rare occasions that the ground-mounted inverted-L outperforms
the loop or the doublet, which isn't often).
Oh -- as far as I can tell, with such an arrangement using SGC/AH-4 type
autocouplers there's no need for a balun on the antenna side of the tuner.
Regards,
--Kirk, NT0Z Rochester, MN
My book, "Stealth Amateur Radio," is now available from www.stealthamateur.com
and on the Amazon Kindle (soon)
On Tuesday, July 17, 2018 1:32 PM, Jim Brown <jim@audiosystemsgroup.com>
wrote:
On 7/17/2018 11:00 AM, Wes Stewart wrote:
> If your open wire is short, why bother? Replace it with low loss
> coax, put a decent common-mode choke at the feedpoint and tune it.
> Sometimes the effort to use open wire simply isn't worth it.
Caution on this. See JUN 2015 - QST (PG. 30) "Don't Blow Up Your Balun"
by Dean Straw, N6BV
Dean's work clearly shows why it simply is not practical to choke a
non-resonant antenna. And, because receive noise is such a major issue
for most of us, it's why I don't recommend antennas like this. The
random wire and off-center fed wires were great ideas for 30 years ago,
but terrible ideas today.
Fan dipoles are a FAR better choice.
73, Jim K9YC
_______________________________________________
_______________________________________________
TowerTalk mailing list
TowerTalk@contesting.com
http://lists.contesting.com/mailman/listinfo/towertalk
_______________________________________________
_______________________________________________
TowerTalk mailing list
TowerTalk@contesting.com
http://lists.contesting.com/mailman/listinfo/towertalk
|