In alt.tv.tech.hdtv szilagyic wrote:

| I have two antennas that I am trying to combine, and have a question
| in regards to the best way to do this. The first antenna is a
| Winegard HD7084P (FM/VHF/UHF lpda) that is hooked to a preamp and
| tuned in perfectly. I want to add my other antenna (Winegard PR-8800
| UHF bowtie) that is also tuned in and points in the opposite direction
| (almost 180 degrees) to pick up some UHF stations. The two antennas
| are side by side, but the ends of the elements are about 4 feet
| apart. When I disconnect the HD7084P from the preamp and connect the
| combiner and hook the HD7084P to the combiner, the signal strength for
| HD stations maintains about the same. But when I hook up the second
| antenna to the combiner, the HD signal strength from the HD7084P drops
| by 10-40% (depending on the channel). I tried two combiners/splitters
| and got the same results, one is a RCA brand splitter, the other is a
| Radio Shack hybrid splitter/combiner. Would something like the
| Channel Master 0538 or the Winegard CC-7870 work better for this?? I
| read up and these models mention "high isolation" which I can't seem
| to find any information on. It seems like a powered combiner would be
| ideal so that there is no loss at all, but I haven't seen anything
| like that. I am looking for any suggestions or ideas that anybody
| might have.

The simple resistive combiner, which can also be a splitter when wired
in reverse, intentionally loses 3db of signal as part of its simple
circuitry to be sure the impedances are matched. If the impedances are
not matched, things get worse because the joint becomes a point where
signals can be reflected, resulting in a number of signal abberations.
In addition, a combiner should also isolate the branches from each other.
The importance of having this depends on other factors in your design,
such as how well the antenna feedpoints match across the frequencies you
want to receive. You can get away with a mismatch on the source end if
the target end (amplifier) is well matched. But unless the combiner has
very high isolation, the two antennas form "ends" opposing each other on
the wire, and can result in signal reflections between them. A combiner
with 6 db isolation (very simple) will at least reduce those reflections
between antennas somewhat.

The proximity of the antennas to each other may also warp the pattern
they receive in, depending on their design, the exact distance, etc.

Even with a combiner of high isolation, when one antenna is picking up
the signal only the other antenna should (e.g. the "back lobe" in such
a case with antennas almost 180 degrees apart), that can result in two
sources of the same signal being merged. Depending on phase difference,
that can combine or cancel-out. Both can happen at different frequencies
depending on the exact length.

The ideal combiner system involves frequency isolation. That is, channel
filters only allow each channel to come from one or the other antenna.
This is common in well engineered cable headends. But these usually also
involve lots of large single channel antennas.

If one direction were VHF and the other were UHF, this would be easy as
the frequency isolation could be done via a VHF band filter and a UHF band
filter. I take it your case does not involve that.

Losing signal is almost certainly going to be a part of combining two
antennas. Larger antennas can compensate for it. Amplifiers per each
antenna may help as well (especially with reflections along the coax).
If your channels are intermingled, frequency isolation is going to be
very expensive.

Your last resort for full signal is two separate feeds to a switch.
That can be a nearby switch manually operated, or a switch on the mast
that is remotely controlled. But either way, your TV will never see
both sets of signals at the same time, and apparently most modern
digital tuner systems need to see the whole channel lineup at once on
the one RF feed. If an STB is part of your setup, you might consider
a 2nd STB for the 2nd feedline, and switch between them.

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| Phil Howard KA9WGN (ka9wgn.ham.org) / Do not send to the address below |
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