"I am not sure how a 40' tall vertical with 94' of top loading, models in
NEC, but it seems like it might be close-ish(?) to resonating on 160m. (will
I be in the ballpark?) What kind of matching will I need if any?"
Mike,
I modeled a 40' vertical with 92' of top loading on 1.825MZ using EZNEC+
5.0.66. EZNEC+ 5 uses NEC-2. NEC-2 does not like connections to ground with
Real/High Accuracy ground. So I modeled two (2) 65' radials at 6 inches
above Real/High Accuracy ground.
The pattern looks very good with a deep overhead null.
As for feeding the antenna, 40' of vertical and 92 ft of top loading is too
short for 160M.
The calculated feedpoint impedance is 16.22 - J 365.6 ohms. You can add a
series inductor and use a 1:2 balun (50 ohm source to 25 ohm load) and get
the SWR to about 1.5:1.
The actual feedpoint impedance will almost surely be different than
calculated. Before you order/wind a balun and wind an inductor, it might be
a good idea to build the antenna and measure the actual feedpoint impedance.
Then you will know for sure what you need to match. I would not count on an
internal ATU to do the job.
I hope this helps at least some.
73,
Marsh, KA5M
From: "Mike Smith VE9AA" <ve9aa@nbnet.nb.ca>
To: topband@contesting.com
Sent: Wednesday, March 25, 2015 8:57:18 PM
Subject: Topband: ZS6BKW(G5RV) on 160m?
I might have a chance to operate from a portable location this summer for a
contest or two but at the location they have no antennas, nor is there room
for any huge vertical, inverted L, guys ropes etc. etc. and no permission to
climb roofs, chimneys, or put a bolt in the building, nor touch the trees.
All very restrictive.
I might be able to erect a 35' freestanding fiberglass mast with a ZS6BKW.
Maybe. The ZS6BKW is basically a 94' dipole with a 40' downlead of 450ohm
ladeer line..which will be sloped slightly, as I don't see a way to get the
peak above 35'
Suffice to say, the best I MIGHT be able to do is sneak in this low
ZS6BKW(G5RV) as an inv. vee to run 10m-80m but I am looking for a way to
ALSO be on 160m to make a few dozen contacts in a contest or two.
I am thinking the peak of the ZS6BKW (G5RV like antenna) would be at max
35'.
For 160m, could I short the 2 leads of the 450ohm ladder line together where
they meet the coax run and sneak out 2 x 65' (bent) ground radials and
attach them to the braid side of the coax?
(maybe I'll even make up a switchbox so I don't have to be running around in
the dark a few times a night reconfiguring bare wires in an unfamiliar
location.)
I've read 3830 contest reports where folks have done this type of thing, but
of all the antennas I have ever had (many!_) I have never EVER tried this
shorting of the ladder line or coax and feeding what is a G5RV as a T-Top
vertical.
with a couple of radials.
I am not sure how a 40' tall vertical with 94' of toploading, models in NEC,
but it seems like it might be close-ish(?) to resonating on 160m. (will I be
in the ballpark?) What kind of matching will I need if any?
I could maybe even add more coax or ladder line to the "vertical" section
when on 160m, however it will also have to be sloped/bent and on or very
near the ground (probably not good, eh?)
I'll be using a modern transceiver with internal ATU.
I am not looking to run pileups on 160m.(where I am going is not rare, just
a nice summer place)..I just want to pick up a few 160m mults in a contest
or two.
Ideas? Remember, I am very restricted. Gotta have the ZS6BKW for my
antenna.and I may not be permitted to have more than 1 coax run out to the
antenna. I doubt adding a couple extra 65' legs is possible either. (no
square footage for that.
Thanks
Mike VE9AA
Mike, Coreen & Corey
Keswick Ridge, NB
_________________
Topband Reflector Archives - http://www.contesting.com/_topband
_________________
Topband Reflector Archives - http://www.contesting.com/_topband
|