On Mon, 1 May 2017 20:52:04 -0700 (PDT), wrote:

On Monday, May 1, 2017 at 8:45:45 AM UTC-7, Jeff Liebermann wrote:
On Mon, 1 May 2017 01:42:17 -0700 (PDT), wrote:

My point was he might be able to use multiple antennas with diplexers
and get one overall feed that could be treated like the cable company
- all channels available all the time with no adjusting.
G²

Nope. A few problems:

1. A diplexer splits the frequencies between VHF and UHF channels. If
one antenna is VHF and the other is UHF, it will work, but only if the
stations in one direction are all VHF and the other direction are UHF.
Methinks that's unlikely to happen.

2. If you replace the diplexer with a combiner/splitter, you
theoretically can get both VHF and UHF signals from both antennas at
the same time without switching. I think that's what you're
suggesting. However, that doesn't work because the same signals are
picked up by both antenna at the same time. If the signal are in
phase, then the signals combine and you get good reception. If
they're 180 degrees out of phase, you get cancellation and no signal.
However, that's an over-simplification. What really happens is that
the signal is 6 MHz wide and the phase cancellation varies with
frequency. Some of the 6 MHz wide frequency range gets added, but
other frequencies in this range get cancelled. The result is a very
rocky and erratic frequency response which makes an ugly mess of the
signal. Bad idea.

3. You can make it work with two antennas in two directions using a
coax switch. Only one antenna is connected at a time so there is no
interaction. You might need two coax cables from the mast to the TV
where the switch is located. Or, you can setup a remote antenna
switch. Or, you can setup a cross-over switch and two receivers,
where you can record on one receiver/antenna combination, while
viewing on the other.

It's not a diplexer but these guys claim it can be done without a rotator.
http://downloads.channelmaster.com/S...spec+sheet.pdf
I presume they know what they're doing.
G²

Amazing. From the specs, it looks like there's a simple passive
splitter/combiner inside the box. $39 is quite a bit to pay for a $2
combiner.

When I tried the same thing, I ran into problems with antenna
interaction as I previously described. It was easy enough to
demonstrate the problem to the customer. I disconnected and
terminated one port on the combiner. The picture quality dramatically
improved. I repeated the exercise on the other port and had the same
effect with stations on the other antenna.

I also ran into one installation that had three antennas (two UHF
only, and one VHF/UHF). I again could demonstrate that it worked
better with just one antenna at a time. However, the customer did not
want to run additional coax cables to his TV, and I couldn't find an
affordable 3 or 4 port remote antenna relay. So, I built one using
magnetically latched relays. It didn't look very good at the high
channels when swept, but the FCC saved me by auctioning off the 700MHz
channels.

I suspect that a passive combiner might work if the two antennas were
isolated from each other and positioned so that the antenna side lobes
are not pointed in the direction of the "wrong" station. Looking at
typical TV antenna patterns:
http://www.hdtvprimer.com/ANTENNAS/comparing.html
I think it could be done if the antennas were 90 degrees from each
other.

Of course, you're welcome to verify my analysis and tests. It's easy
enough to do with a $2 combiner. I would be interested in your
results.


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