Regarding using a tower as a vertical:
Some years ago I had a 50-foot mast with a tribander on it next to my
house. I ran two wires out my window, one down to the base of the mast
and one to a point on it that produced a reasonable SWR on 80 meters.
The rotor cable and feedline for the beam ran down to the ground, and
then back up to the shack. No ferrites or anything on the feedline. I
wrapped the rotor cable around a ferrite rod at the rotor controller. I
had a system of 16 radials, each about 20' long, and in a half-circle.
I ran about 600 watts to this arrangement and didn't notice RF issues in
the shack. It worked surprisingly well, producing contacts from here
into the US as far west as Illinois.
73,
Vic, 4X6GP
Rehovot, Israel
Formerly K2VCO
http://www.qsl.net/k2vco/
On 2 Mar 2017 05:21, Erik Basilier wrote:
Hi Alan,
Your link didn't work for me, but I assume your article describes how to
make a good bandpass filter. I do in fact recall seeing such an article in
QST years ago. Assuming that you were just pointing out that I can put a
bandpass filter on each feedline, as is common practice in SO2R operations,
I need to state my case in more detail:
Since I have second antenna covering 20-15-10 (a vertical), I started out
doing just that: putting a bandpass filter on each. I was in a hurry and
bought the LBS commercial filters rather than building based on the old
article. The antennas are close together, and even after I upgraded so as to
have two K3 radios, I was unhappy with the isolation with one K3
transmitting on 40 on the beam and the other K3 receiving on 20 on the
vertical. Just out of curiosity I plan to dig deeper into this situation, as
I think somewhat better results could be achieved. However, seeing the big
difference in performance on a given band between the vertical and the beam,
I really wanted both radios to have a beam. That is where I decided to get
the multiplexer (a model that includes 40m). Performance wise, this was
going to be like having one multiband beam, including 40m, for each radio.
Much better than using the vertical for one radio, assuming of course that
the physical sharing of one beam would aggravate the interference situation
I had when using separate antennas and bandpass filters. With the
multiplexer, per manufacturer's recommendation, each band still uses its
separate bandpass filter, so that total attenuation between bands is much
greater than what can be expected with bandpass filters alone. Still, I was
apprehensive of a possible increase in interference. The outcome was nothing
short of stunning: No interference at all. I see a possiblity that I could
have somehow reduced the interference experienced with separate antennas and
using bandpass filters, but I can't see that approach competing with the
superior results using a single antenna + multiplexer + bandpass filters.
The QST review backs up my assumption that the great results with that
configuration was no fluke. Of course, I cannot include 80m in the same
approach as long as I don't have a single antenna that includes that band
(and I can't quickly get a multiplexer that includes all 5 bands). So, for
80 I will have a separate feedline and just a bandpass filter. BTW I am very
happy to not have yielded to the temptation to upgrade my beam to a Steppir,
as the multiplexer approach requires the antenna to be tuned simultaneously
to multiple bands, not to tune to one band at a time.
If I had been able to achieve really good isolation with just a bandpass
filter for each band, your approach with an 80m antenna that also covers 40
would make good sense to my situation. As it is, keeping 40m within the
beam, even with no gain over running 40m on the second antenna, makes sense
as it allows me to route 40m not just through the bandpass filter, but also
through the multiplexer. Since I already tried an 80m inverted vee on the
tower, and it ruined the 40m performance of the beam, I am looking for other
approaches for 80m. It might be possible to change the beam to bring it back
to resonance on 40, but this particular beam is a complicated design
already. A sloper is one possiblity, but it is not likely to be my first
attempt, as it would be close to the beam and it would be somewhat similar
to the vee. At this point I am leaning toward either a separate shortened
vertical such as a Butternut model for 80 and 40 (not likely to use the 40
part) or using the tower itself as a vertical. For the latter approach I
would need to prevent the tower's feedline bundle (the part going into the
house) from forming part of the radiating element. I remember an old Antenna
Book discussing methods of feeding the tower as a vertical, but the author
seemed to ignore the issue of the cables coming off the tower. Maybe it was
assumed that the cables would be disconnected at the bulkhead whenever the
tower would be used as a vertical. Nobody has replied to me about
experiences dealing with that issue. Also as the tower with its antennas is
resonant in the BC band, I would model it to find out if it has a resonable
vertical radiation pattern if fed as a whole on 80.
73,
Erik K7TV
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