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Old 4-May-2015, 5:33 PM   #3
rabbit73
Retired A/V Tech
 
Join Date: Aug 2012
Location: S.E. VA
Posts: 2,753
When the coax lines need to be the same length

If you have two identical antennas, aimed in the same direction, and are using a splitter reversed as a combiner, the coax lines must be the same length for maximum gain. You will be able to get up to 2.5 dB more, 3 dB because of doubling the signal minus the 0.5 dB internal loss of the combiner.

When the antennas are aimed in the same direction, the incoming wave front arrives at both antennas at the same time, and the signals arrive at the combiner at the same time, so they add in phase.

This only works if the wave front is uniform across both antennas. If the wave front is not uniform across both antennas (like thru trees), you don't get the gain you expected. This explains why a 4-bay bowtie antenna sometimes works better than an 8-bay bowtie, like 4221 VS a 4228, because it has a smaller capture area.

And when they don't need to be the same length

If the two antennas are NOT aimed in the same direction, the incoming signals do not reach each antenna at the same time, so it is not necessary to have the coax lines the same length, because the same signals aren't going to arrive at the combiner at the same time anyway. This means that they might interfere with each other because they aren't in phase.

It is possible to adjust the lengths of the coax lines to different lengths so that one desired signal arrives at the combiner in phase, but that often harms the other signals that might have been OK before adjusting the lengths.

Quote:
Originally Posted by somerset View Post
I know that combining two antennas is very hit or miss, but I want to see if anyone out there can help me out.

I currently have an 8 bay bowtie pointed NW towards Pittsburgh, and a 4 bay bowtie pointed NE towards Altoona, PA . Both have a Channel Master 7778 preamp, and run 75’ to a Channel Master DVR+. One antenna is mounted on the front of the house and the other is mounted on the side of the house, I think that the only channels picked up by both antennas are WJAC, WPCW and WWCP.
When either antenna is hooked up separately, most channels have a good signal (close to 100% strength and 100% quality). When I combine the antennas, the signal drops a little bit on some channels but the signal quality on WATM and WTAJ drops down to 0% (with 100% signal strength).
Does anybody have any tips or trick that I can try to pick up WATM and WTAJ?
An Alternative to Rotators and Antenna Combiners
http://forum.tvfool.com/showthread.php?t=820

Another Alternative to an Antenna Rotator or Combiner
http://forum.tvfool.com/showthread.php?t=2882

Quote:
Originally Posted by rabbit73 View Post
In spite of some glowing reviews, the antenna with two panels, each aimed in a different direction, often doesn't work. When the same signals from each antenna panel reach the combining point, they can interfere with each other if they are not in phase. It's bad enough for two directions, impossible with three. Three separate antennas combined into one coax, as previously suggested by stvcmty, sounds more complicated, but stvcmty and I feel that it would cause you less grief in the long run.

I am always amazed when someone wants a simple system that duplicates what a CATV system headend does with multiple antennas, amplifiers, and modulators costing thousands of dollars. You will need to cut back on your expectations and settle for improving one direction or be prepared to spend a lot of time and money building a system that does what you want.

The simple system that I favor, if you are not prepared to go down the road suggested by stvcmty, is to use two converter boxes or tuners to add the channels from the second and third antennas to the signals from the primary antenna, using CH 3 and 4 analog outputs, a splitter reversed as a combiner, and a HLSJ to add to the system with a second combiner.

It is important to be aware that the CH 3 and CH 4 RF outputs of the the converter boxes are mini transmitters, and must have their outputs attenuated as much as possible before combining. The port to port isolation of a splitter used as a combiner is not sufficient to guarantee that their analog signals will not reach the primary antenna and be radiated, unless you take measures to attenuate their signals and provide additional isolation. The isolation provide by two combiners and a HLSJ is probably sufficient.
It is also possible to use the A/V output of a separate tuner and connect it to the A/V input of the TV. You then use the input switch of the TV to switch antennas.
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Last edited by rabbit73; 6-May-2015 at 2:17 PM.
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