Hi Erik,
It is certainly possible to use a separate filter in the feedline to
each antenna. For example, here's an article I wrote many years ago on
how to homebrew your own:
http://p1k.arrl.org/pubs_archive/89595
Those filters are only good for 100W or so, but there is no fundamental
reason they couldn't be scaled up to 1500W. I haven't researched it,
but I'm thinking there must be commercial equivalents as well.
Alan N1AL
On 02/28/2017 10:41 PM, Erik Basilier wrote:
Alan,
Thanks for your reply. Your approach would provide 40m capability to replace
the 40m capability that my (Sommer) beam loses through detuning.
However, I wonder how hard your tuner must work on 40? Enough to create
substantial feedline losses when the tuner is in the shack?
Anyway, I have another reason not to choose your approach:
I am rebuilding my station to support SO2R, and it is tough to avoid
interference between the two radios operating on different bands, especially
40 - 20 and with the antennas close together.
I found to my surprise, before the QST review came out, that the Low Band
Systems multiplexer and band pass filters (my setup includes 40m) eliminates
the interference problem. However, this scheme requires that the antennas be
on a shared feedline. With your approach the 40m antenna would no longer be
on the same feedline as the higher bands.
-----Original Message-----
From: Elecraft [mailto:elecraft-boun...@mailman.qth.net] On Behalf Of Alan
Bloom
Sent: Tuesday, February 28, 2017 11:09 PM
To: elecraft@mailman.qth.net
Subject: Re: [Elecraft] Using your tower as a vertical - 160 or 80
I use two of the top guy wires as an inverted vee. There are insulators
near the top of the guys and about 50 feet or so down. The vee is brought
to resonance on 80 meters with a center-tapped loading coil, which also acts
as a balun. The best match was with the coax tapped right about at the end
of one side of the coil (and of course the coax shield to the grounded
center tap).
Works great on 80 meters without a tuner and on 40 meters with a tuner.
Alan N1AL
On 02/28/2017 09:42 PM, Erik Basilier wrote:
Years ago, after putting up a heavy-duty crank-up tower with several
antennas on a tall mast at the top, I was interested in loading up the
whole metal tree for 160 and/or 80. However, I didn't like the
prospect of transmitted power getting back into the shack via the
existing feedlines, causing all kinds of problems including losses.
From a surplus vendor I obtained four square slabs of ferrite (no
spec's) and taped them together to form a box-shaped common mode choke
around the existing feedlines (and rotor control cable etc). I didn't
have radials at the tower base, but a couple of long and wide copper
strips buried and connected to ground rods to which the tower was
grounded. I opened up the tower ground connection and I had a feed
point. Finding resonance was not as easy as using my antenna analyzer.
The signal from the analyzer was overwhelmed by picked-up broadcast
signals, rendering the analyzer unuseable. I got by using a custom
measurement setup.
It turned out that the entire metal tree resonated in the broadcast
band and was inductive at 160. I tuned it with a serial variable
capacitor, and found the antenna worked very well on 160. However, I
didn't continue using it, as I didn't feel safe not to have the tower
grounded. I pondered schemes to add some kind of gamma-like matching
device, but never got around to it. Also I never tried it on 80, but I
suspect it would not been ideal for low angle radiation.
Later I added more antennas to the mast, and with the added cables,
the whole bundle would no longer fit in the makeshift ferrite choke.
Out of curiosity I once again tried ungrounding the tower to check on
its characteristics as a vertical antenna. I could no longer find the
resonance I had seen and used before. Apparently, the ferrite choke
had been a crucial part of the scheme.
At this point I still don't have an antenna for 160 or 80. (I did try
an inverted vee off the tower for 80, but it caused terrible de-tuning
of the 40m part of my beam on the tower, so I gave up on that. Maybe I
should try a
sloper.) I am thinking of putting up a dedicated vertical, but on my
small lot it would couple to the tower. Perhaps it would be better to
give the tower another look as my low-band vertical? My source of
ferrite slabs dried up years ago. I wonder if anyone else on the list
has used a similar approach and found a good way to choke off RF on a
bundle of feedlines?
Individual chokes don't seem very attractive to me as I have many
cables, but if one has to go that route then it would make sense to
look very carefully at the choice of chokes. I would also be
interested in knowing about others' experiences with feed systems that
leave the tower grounded.
Thanks in advance for any useful ideas!
73,
Erik K7TV
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