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Old 4-May-2015, 5:37 PM   #1
eggman531916
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Joining 2 UHF antennas in different directions?

Hello everyone. Since having built my own DBGH and being able to reliably get nearly every station possible in my location (stations about 90 degrees north are blocked by a hill), a friend has asked me about his location. I'm fairly sure he doesn't want to bother with a rotor and he doesn't have to contend with the hill. So, what I'm asking is what would the pros and cons be of joing two UHF antennas at approximately 90 degrees apart? I know I need equal lenghts of coax from each one and (like myself) he'll need a preamp. I also understand I need to seperate them by 3-4 feet. I was just wondering how it affects performance.
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Old 4-May-2015, 5:39 PM   #2
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It's generally impossible to predict what will happen. It's an experiment, so give it a shot and see how it turns out. Sometimes it works fine, other times, not so much.
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Old 4-May-2015, 6:33 PM   #3
rabbit73
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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 3:17 PM.
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Old 5-May-2015, 6:48 AM   #4
eggman531916
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Thanks to both of you for replying. I thought about just experimenting with them. He might be good with it, but I'm not so sure his wife will be. Also, I'd never considered using seperate tuner boxes and using the different input ports on a TV before. That's an awesome idea. I'll have to remember that. It's funny that you used a quote mentioning WTAJ and WATM. They're my two strongest signals followed by WJAC on RF 34 (which I'm not supposed to get). And thank you also for correcting me about needing equal lengths of coax. I hadn't thought about the fact that the signals aren't going to be in phase anyways.
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Old 6-May-2015, 5:14 AM   #5
eggman531916
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Another thought about this?

I was thinking today about this and I started to wonder about something. Every thing I've read says you shouldn't try to join non-identical UHF antennas. But my question is, Does it matter if the antennas are identical since the the signal wouldn't be in phase anyways if they're not pointing in the same direction? I was wrong on needing equal lengths of coax, so I thought maybe I was wrong about needing identical antennas as well. Thanks again in advance.
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Old 6-May-2015, 11:51 AM   #6
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It's not that one "shouldn't", it's a matter of it's impossible to predict with reliability whether or not it will work satisfactorily. Therefore, it's not recommended if you expect it to work perfectly as the odds of disappointment are substantial.

If the antennas are aimed different directions, it doesn't matter if they're the same or different.
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Old 6-May-2015, 2:03 PM   #7
eggman531916
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Thanks ADTech. This idea was for a different location from the first one I mentioned. I'll have a little more room to experiment at this one. An A/B switch is a possibility at this second location as well. Thanks again!
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Old 6-May-2015, 5:47 PM   #8
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Quote:
Originally Posted by eggman531916 View Post
Thanks ADTech. This idea was for a different location from the first one I mentioned. I'll have a little more room to experiment at this one. An A/B switch is a possibility at this second location as well. Thanks again!
An A/B switch will have less insertion loss than combined antennas.
You may however, cause conflicts, if the tuners require complete scans, for each direction. They also cause conflicts w/ multiple viewers and recording.
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Old 7-May-2015, 5:06 AM   #9
eggman531916
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Quote:
Originally Posted by Stereocraig View Post
An A/B switch will have less insertion loss than combined antennas.
You may however, cause conflicts, if the tuners require complete scans, for each direction. They also cause conflicts w/ multiple viewers and recording.
Thank you Stereocraig. I won't know for a few days yet if the TV requires a complete scan. I do know, however, there's only one TV for the house. It's a small vacation home that's only inhabited a few weeks a year, so an antenna makes perfect sense for TV service.
Thanks again to everyone for being patient and answering my questions. Most likely, you've heard them all before. Seems like everytime I think I understand something completely, I think of more questions.
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Old 10-May-2015, 7:16 PM   #10
eden
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I'm actually combining two Antennas Direct antennas (a C2 and a C2V) pointed about 70 apart with excellent results. The VHF element is meaningless as all the stations I receive are UHF here. The two antennas are about six feet apart. It's worked really well for me, as in producing MUCH better results that a DB8e I had here. I can't explain why it works. It just does.

The disclaimer, of course, is that this was an experiment using antennas I had on hand. I would never have gone out and planned this configuration. I was at my wits end with the DB8e and decided to try whatever I could with what was in the house. I lucked into this configuration. As ADTech points out, you have no way of knowing if combining antennas will enhance signals or the two will have a canceling effect on one another. In my case it enhanced 8 out of nine stations and added a tenth. I receive a total of 33 channels of programming this way.

Last edited by eden; 10-May-2015 at 8:12 PM.
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Old 10-May-2015, 7:49 PM   #11
eggman531916
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Glad it worked for you. Are you in an area with strong signals? I took a look at my friend's situation again and it looks like two antennas isn't going to help much anyway as there are other obstructions in the way of the weaker signal. However, it's always good to obtain new information for future use as a few others have asked me about looking at their locations.
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Old 10-May-2015, 7:59 PM   #12
eden
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I'm in an area with mostly weak signals and lots of hills. Here is my TV Fool report: http://www.tvfool.com/?option=com_wr...2c1573a1a67363 The one thing I don't understand is that channel 17, down in the pink, is strong enough to receive with an indoor antenna. Anyway, I receive almost everything on that report down to real channel 30 (requires turning one antenna) plus channel 32. I don't get channel 3 since I don't have a low VHF antenna, or channel 25, which is blocked by a hill. The use of two antennas was a desperate attempt to get channel 32 (virtual channel 26) consistently as I really like PBS. It worked!

Here is the thread with my experiences: http://forum.tvfool.com/showthread.php?t=15269 After two weeks of being able to watch PBS Newshour every night plus a Justin Hayward concert I consider the result a real success.

Last edited by eden; 10-May-2015 at 8:04 PM.
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Old 10-May-2015, 8:53 PM   #13
eden
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I should at that the three Roanoke stations to my north (17, 18 and 30) do not come in with my current configuration unless I climb a ladder and turn one antenna. Those are the stations I refer to when I talk about adding a third antenna and second feedline on the other side of the house. The only real reason I would do that is because channel 35, the FOX station for my market, is the one station that's hit or miss still. It's the one that wasn't enhanced. So, to be more precise about that report, from top to bottom:

5 bars: channels 14, 19, 51, 29, 33, 31, 43
3 bars: channel 47 (no reception before)
2 bars: channel 32
1 bar: channel 35

If I point one antenna at 345
5 bars: channel 17
2 bars: channel 30
1 bar: channel 18
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Old 10-May-2015, 10:26 PM   #14
eggman531916
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I can understand about having one channel that comes in strong that, according to the Tvfool report, shouldn't be really be there.
Here is mine: http://www.tvfool.com/?option=com_wr...f1f031d13592fb
Channel 34 shouldn't come in, but, there it is. Also, Channel 50 doesn't exist and from what I understand, hasn't been there for a very long time. I'm using a Double Bay Gray Hoverman that allows me to get a total of 9 channels, all of which are 2 edge. Channel 15 is blocked by a hill north of me, which stinks as I'm a PBS fan as well.
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