Don pretty much described my low band antenna. Full Disclosure: I live
on 5 acres and have a 70' tower. That said, it is a Sloping V [I'd call
it an inverted V except is isn't resonant on any band], about 210' on a
side from the top of the tower. 450 ohm window line to the bottom of
the tower, DXE 4:1 balun, and coax into the house. I have chokes on the
coax at the balun and at the weatherhead entrance, but I've never had
any problems with RFITS [RF In The Shack] with or without the chokes.
It works well on 80-40-30, requires a tuner of course [KAT500]. Works
on 160 but warms the clouds and worms, I use an Inv-L for top band. It
also works on all the bands up from 30 but the pattern gets fairly
complex and squirts my RF in a lot of non-productive directions because
it's so big.
My experience is that an 88 ft doublet, center-fed, works really well on
40 and up in frequency, often used by those activating summits in
Summits On The Air. Shorter doublets are also effective, and not being
resonant doesn't really matter [in some cases, it helps]. Neither does
what you do with the ends. Most of the radiation comes from the center,
high current sections.
450 ohm window line is sensitive to moisture ... if you set up your
tuner for dry conditions and it's now raining, things will need retuning.
Keep in mind the wisdom of Tom, N6BT, "Anything conductive will radiate
if you get power into it."
73,
Fred K6DGW
- Northern California Contest Club
- CU in the 2015 Cal QSO Party 3-4 Oct 2015
- www.cqp.org
On 10/17/2014 3:54 PM, Don Wilhelm wrote:
Make it a balanced dipole antenna (equal lengths on either side of the
feedpoint) for best efforts in keeping RF off the feedline. The actual
length does not matter a lot, but it should be greater than 80% of the
half wavelength for the lowest band of interest.
Use open wire line or 450 ohm ladder line to feed it down to the point
where it enters the shack - hopefully you can run the feedline
perpendicular from the radiator for at least 1/4 wavelength on the
lowest frequency of interest for lowest radiator to feedline pickup. Put
a good 1:1 current mod choke at that point. See page 29 of K9YC's RFI
tutorial http://www.audiosystemsgroup.com/RFI-Ham.pdf
for instructions on how to construct a very effective current mode choke
- note: a good balun *is* a current mode choke, but many fail to perform
as well as the ones tested by K9YC.
You will need a tuner, and any Elecraft tuner should do the job nicely.
If it does not, then you may have to make some adjustments in the length
of the parallel feedline to see if you can achieve success on all bands
of interest.
You may want to take a look at the Antenna and Transmission Line article
on my website www.w3fpr.com for a bit on non-math theory on antennas.
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