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Old 15-Sep-2015, 11:44 PM   #1
rabbit73
Retired A/V Tech
 
Join Date: Aug 2012
Location: S.E. VA
Posts: 2,738
My Answer to "Need Help with Combining Antennas" Post by 4.6 Explorer

Quote:
Originally Posted by 4.6 Explorer View Post
I have two 8-bay collinear arrays stacked on top of each other aimed east to capture the Buffalo and Grand Island stations about 50 miles away. Decent signals for the most part with occasional drops outs due to weather time of day (or whatever). Those two are also using a preamp CM 7778;

one simple 4bay to Capture Toronto towards the north and another Yagi that aims west for a single station.

Presently the single 4bay is not hooked up, yet

Because I'm below an escarpment the western transmitter is hard to capture and it's only about 4 miles away. I do get it but a very weak signal.

When I bought a used 8 bay a weeks ago the mechanical properties of the used 8 bay were in terrific condition but the simple combiner that came with had fried due to a light lightning strike or eddy current. No burn marks anywhere but the coax wires inside the combiner popped like fuses.
I simply rewired as original and put it back in action. I do have more gain and adding the preamp certainly helps even more so.

I copied that concept to make a second combiner by stripping an existing two-way splitter and wiring the same as the original one.
So, right now, I have the western facing Yagi into one side of the combiner and the two eastern facing 8bays with a preamp to the other side of the combiner down by the TV set it self.

That's all that's active so far. My plan is run a third coax line from the 4-bay and create a 3 way combiner and hopefully capture an almost 180 degree signal pattern.

yes? No?

What would you advise?

Thanks thus far to all you guys.
We need some more information to make a good analysis.

Please tell us the town or city where you are located and give us a tvfool report using your exact address (which will not show) or coordinates (which will be shortened) using this:
http://www.tvfool.com/index.php?opti...pper&Itemid=29

If you have a problem with that page determining your location, use the interactive map browser with exact coordinates, or move the cursor to you antenna location and generate a report by clicking on Make Radar Plot >> at the upper right corner of the map and give us the URL link in bold type near the top:
http://www.tvfool.com/index.php?opti...pper&Itemid=90

Quote:
I have two 8-bay collinear arrays stacked on top of each other aimed east to capture the Buffalo and Grand Island stations about 50 miles away. Decent signals for the most part with occasional drops outs due to weather time of day (or whatever). Those two are also using a preamp CM 7778;
Does that mean you have combined the two 8-bay antennas into a 16-bay that is connected to the 7778? Is it the old 7778 with separate VHF and UHF inputs or the new 7778 with only one antenna input?
Quote:
and another Yagi that aims west for a single station.
You have more than one yagi? Is it a VHF or UHF yagi? What is the callsign of that station?
Quote:
Because I'm below an escarpment the western transmitter is hard to capture and it's only about 4 miles away. I do get it but a very weak signal.
The transmitter is 4 miles away or the escarpment is 4 miles away?
Quote:
So, right now, I have the western facing Yagi into one side of the combiner and the two eastern facing 8bays with a preamp to the other side of the combiner down by the TV set it self.

That's all that's active so far. My plan is run a third coax line from the 4-bay and create a 3 way combiner and hopefully capture an almost 180 degree signal pattern.... yes? No?
Combining two antennas aimed in different directions, using a splitter in reverse as a combiner, doesn't always work because when the same signals arrive at the combining point they will interfere with each other if they are not in phase.

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.

My previous post sending you here to solve your reception problem:
Quote:
Originally Posted by rabbit73 View Post
What started out as an antenna theory question has turned into a reception problem question. I quoted your question and have given my answer on the Help With Reception thread:
http://forum.tvfool.com/forumdisplay.php?f=7

Please go to:

My Answer to "Need Help with Combining Antennas" Post by 4.6 Explorer
http://forum.tvfool.com/showthread.php?t=15747

You will get more help there, and ADTech might have a better answer that I have overlooked. He works for Antennas Direct and has a lot of experience with 8-bay antennas because they make the DB8 and the newer DB8E.
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Last edited by rabbit73; 16-Sep-2015 at 10:20 AM.
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