On Sat, 14 Mar 2015 10:12:20 EDT, Tom Horne wrote:

I am looking for recommendations for a dual band antenna that will serve tw
o separate radios. The reason that I want to use a single antenna is that
I have a limited number of mounting points for antennas.

Many years ago I had a similar problem on a commercial tower. I was
paying for tower space essentially by the antenna. My boss at the
time did not want to pay for two antennas. I had to devise a scheme
that would share a single mount point. I asked if two coax cables
were acceptable, to which he agreed. So, I built a dual 5/8 wave VHF
antenna, with one antenna mounted above the mounting arm, and the
other mounted below the arm, pointing downward. Getting the isolation
right and camouflaging the fact that it was really two antennas, was
tricky. I wish I had antenna modeling software, but resorted to
optimizing by cut-n-try. Sorry, no NEC antenna models, but I may have
some photos buried somewhere.

One useful trick was to make the two coax cables the same length. The
leakage between antennas was about 1/2 wave apart, resulting in
cancellation in the respective receivers. As I vaguely recall, I had
about 12-15 dB isolation, which I consider to be quite good for such a
derangement. I don't know how dual band will work, but I suspect it
will be usable. If you have a line stretcher (or huge collection of
barrel connectors), you can test the effects of coax line length
differences.

Also, be advised that commercial antennas are designed to drain water
only in their normal mounting position. After filling the lower
antenna with water in a garden hose spray test, I had to drill a hole
in the end of the antenna to provide an exit path.

Of course, I was generating considerable intermod if both transmitters
were on at the same time, but I didn't know much about intermod at the
time making it a non-problem.

Within a few months, some of my competitors on the same mountain top
discovered my trick and cloned it (with varying degrees of success).
That worked for a few years, until the tower/building owner figured
out what was happening and began to charge by the coax cable. After
that, common receive antennas with multicouplers became the cost
saving method of choice.

Good luck and remember that with antennas, the uglier it looks, the
better it works.


--
Jeff Liebermann
150 Felker St #D http://www.LearnByDestroying.com
Santa Cruz CA 95060 http://802.11junk.com
Skype: JeffLiebermann AE6KS 831-336-2558