In sci.electronics.repair, on Thu, 27 Aug 2015 13:29:15 -0700, Jeff
Liebermann wrote:

On Wed, 26 Aug 2015 02:50:55 -0400, micky
wrote:

To use one splitter as a combiner, for two antennas,
and the other splitter as a splitter, to feed the signal to a DVDR and
to a digital-to-analog set-top box that feeds a VCR.

You have a possible problem. When you take two antennas and combine
them with a splitter/combiner, you run the risk of creating a
situation where the two signals cancel. For example, if you have two
antennas, and both pickup the same station, with roughly the same
signal level, and roughly 180 degrees out of phase, the signals will
cancel in the combiner. Worse, you will probably have different group
delays (phase shifts) between the two antennas at different
frequencies and even on different parts of the 6 MHz wide signal. The
result is a "hole" in the frequency respons (gain) of the combined
antennas. What that looks like on a TV is a tolerably strong signal,
but with a truely rotten picture. With digital TV, it can also be the
equivalent of the old "ghosts" problem, where you have two signals
arriving at different times, resulting in 2 pictures. However, with
digital TV, it just results in a poor quality picture.

Combiner schemes work if:
1. The two antennas can't "see" each other and cover different TV
stations.
2. The two antennas are for different frequency bands, such as VHF
and UHF. You'll need a diplexer to do that, not a combiner.
https://www.google.com/search?q=vhf+uhf+tv+diplexer&tbm=isch

I've been thinking about this. Don't the inputs for a diplexer have to
be in different frequency bands? Since I have two TV antennas,
they'll be the same frequencies.

And if the "satellite" input filters out some frequencies, what
expectation do I have that they will be the frequencies from Baltimore
(nearby)? Instead it might be filtering out the very DC stations I
wanted the big attic antenna to get?

Also, after some frequencies are filtered out and some are left, will
my DVDR, which expects input from an antenna, not a satellite, be able
to interpret what does pass through the "satellite" half of the
diplexer?

3. The two antennas are identical and are pointed in the same
direction. That will give you 3dB more gain, but you'll either a
phasing harness, or two amplifiers and resistive combiner to make it
work.

I suggest you get rid of the two antenna scheme and combiner for now
and see if one antenna works better. Also, you can make it work with
an RF switch between antennas, which admitttedly isn't convenient.

Right. It won't work because I watch mostly that which I record, so
there's no one at home to flip the switch from one antenna to another.


I'll address the rest later. Customer bearing checkbook just
arrived...