Stacking, Ganging, Combining TV Antennas - Page 6 - Canadian TV, Computing and Home Theatre Forums

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post #76 of 1264 (permalink) Old 2009-01-26, 09:38 AM
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Question Combining 2 Antenna Feeds? Is it possible?

Hello,

I am living in east GTA and having a hell of a time capturing everthing from Buffalo to CN Tower and beyond. Kind of like those people living in Hamilton. I was wondering if I could use two antennas, of the same type and point one to Toronto and one to Buffalo and then use a multiplexer to combine the signals. If that would work. I really do not want to install a rotor.

Has anyone tried this?

Thanks.
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post #77 of 1264 (permalink) Old 2009-01-26, 10:37 AM
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Please read through the OTA FAQ and especially Post #16 which is all about how to use 2 or more antennas together. ------------------------------------->

Last edited by stampeder; 2009-01-27 at 08:39 PM.
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post #78 of 1264 (permalink) Old 2009-02-18, 01:05 PM
 
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Combining two identical antennas

My quest to improve reception of WVNY-DT on VHF 13 continues...

Is-it better to put a balun on each antenna, then join them together by identical coax length to a 2-way splitter used in reverse or to connect their feed point with 300 ohm twin lead and use a single balun in the middle?

- Antenna 1: homemade Yagi, 10 elements for VHF 13.
- Antenna 2: 10y13s

I tested each antenna individually, each one gives about 42% signal on WVNY-DT

I connected them with the first method, putting them side by side, pointing to the same direction and spread about 2m apart, the signal seems to improve to 44% when combined. I also tried reversing the polarity on one of the balun but it did not change anything.

I also tried stacking them one above another, with about 1m apart (space limited in the attic), the combined signal is worse, even if I reverse one of the polarity.

So before I go out and buy twinlead wires, I would like to get some advice

thank you.
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post #79 of 1264 (permalink) Old 2009-02-18, 01:32 PM
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Have you read Post #16 in the OTA FAQ? Here's a small part of that post:
Quote:
And here's an experimental alternative to using a coax splitter when stacking identical antennas, but the problem is that the "rated" 300ohm impedence of each antenna (as broadband antennas their real world impedence is variable depending on the frequency of each channel) will combine to result in only 150ohms, which will cause a typical 4:1 balun to yield only 37.5ohm impedence on the coax downlead on some channels. This means that some channels may be weaker than the coax splitter technique while some may be stronger.

1. get about 8 to 10 feet of 300ohm twinlead (the old fashioned stuff)
2. connect both antennas with it, leaving about 2 inches of extra length
3. measure the length of that twinlead connection as exactly as possible
4. mark the exact middle of the length
5. cut and strip the 300ohm twinlead at that middle point
6. temporarily attach the balun to those twinlead ends (as the unison point of the output from both antennas)
7. connect your coax downlead to the balun
8. test by watching TV on a distant analogue station
9. if the reception is great your antennas are in phase and you can solder, twist, or crimp the twinlead ends to the balun and cover it with roofing tar or heavy duty electrical tape
10. if the antennas are out of phase your reception will be horrible
11. fix that by simply switching the two conductors on just one of the twin leads
12. test again, and it should be fine to permanently attach all the connectors
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post #80 of 1264 (permalink) Old 2009-02-19, 09:07 AM
 
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update

I went to buy some twinleads yesterday, they are harder to find than coax and more expensive too.

Here are the results:
- using twinleads, I connected both antennas feedpoint using two identical lengths and a balun in the middle that feeds the preamp. I tested on VHF12 analog, the image was very bad the first time. I then reversed the polarity on one of the antenna and the image was almost perfect.

- now with WVNY-DT: using twinleads, the signal seems much more unstable than the with the coax. The signal indicator on my TV shows large fluctuations from 30% to 60% constantly whereas with the coax method, the signal is pretty steady around 42-45%.

Does that mean the coax is catching noise, reflection etc.? I tried to move the twinleads away from objects (in the attic) but it did not help. By the way, the store where I bought twinleads had only the less expensive type available. Does it exist shielded twinleads and should I expect to see an improvement on the fluctuations? A steady reception around 50% will make me happy
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post #81 of 1264 (permalink) Old 2009-02-19, 11:41 AM
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Yeah, shielded foam core twin lead existed, I used to use it.

Twin lead can act as an antenna. Twisting it, say 3 to 4 times a foot can minimize that, as well as keeping it vertical.


Quote:
I went to buy some twinleads yesterday, they are harder to find than coax and more expensive too.
For short sections, youre probably better off brewing your own low loss open wire. Guide here : http://www.qsl.net/co8tw/openline.htm

The commercial super low loss open wire 300 ohm twin lead, that I still have a little bit left of on the spool, varies from that chart a little.

My builds/plans (not the latest models) are located here.
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post #82 of 1264 (permalink) Old 2009-02-22, 11:01 PM
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Combining Two Identical Antennas and Using a Pre-amp

The latest lab results posted on this site show pretty respectable numbers for the CM 4221HD. When two identical antennas are stacked, it has been suggested that an improvement of 2.5 dbi would result. To further benefit the two stacked antennas, how would the use of a CM 7777 pre-amp (gain of 26, noise of 2.0) fare with the stacked 4221 HDs? The use of the pre-amp seems compatible since there are two inputs in the mast pre-amp. Two exact equal lengths of coax from each antenna would feed into the pre-amp then a single exit/output coax would run to the tv or distribution amp. How's that sound?

91XG (47'), YA-1713 (42'), CPA-19, DB4e (37'); Denon AVR-1312
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post #83 of 1264 (permalink) Old 2009-02-23, 01:21 AM
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Quote:
How's that sound?
Quote:
Models 7777 and 7778 may be configured for either a single UHF/VHF input or separate UHF and VHF inputs. It is factory set to "single UHF/VHF input".
It doesnt sound like it has two separate uhf inputs.

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post #84 of 1264 (permalink) Old 2009-02-23, 10:30 AM
 
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twinlead

the local source by circuit city was clearing out 300ohm twinlead a few months back and I managed to get a pack for $2
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post #85 of 1264 (permalink) Old 2009-02-23, 01:07 PM
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300ohm:

I stand corrected, it's 1 input for vhf and 1 for uhf. So, where would the pre-amp be best placed when combining two antennas- before or after the splitter, which I assume would have power pass?

91XG (47'), YA-1713 (42'), CPA-19, DB4e (37'); Denon AVR-1312
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post #86 of 1264 (permalink) Old 2009-02-23, 05:56 PM
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Quote:
So, where would the pre-amp be best placed when combining two antennas- before or after the splitter, which I assume would have power pass?
Thats the tricky part. If before the CM7777, youll get a 3.5 dbi loss from the splitter/coupler on a good one, negating the gain increase from ganging the antennas. If after, then youll need two CM7777s really.

Ive never really looked for one, so there might be a preamp with two 75 ohm uhf inputs and one out somewhere.

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post #87 of 1264 (permalink) Old 2009-02-23, 06:12 PM
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Originally Posted by 300ohm View Post
Thats the tricky part. If before the CM7777, youll get a 3.5 dbi loss from the splitter/coupler on a good one, negating the gain increase from ganging the antennas. If after, then youll need two CM7777s really.

Ive never really looked for one, so there might be a preamp with two 75 ohm uhf inputs and one out somewhere.
I might be better off just pre-amping one antenna then?

Also, since the splitter/coupler gives signal loss, then how can stacking antennas give more signal?

91XG (47'), YA-1713 (42'), CPA-19, DB4e (37'); Denon AVR-1312
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post #88 of 1264 (permalink) Old 2009-02-23, 06:53 PM
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Quote:
then how can stacking antennas give more signal?
Stacking using custom phasing lines, making in essense one antenna. But that requires careful modeling.

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post #89 of 1264 (permalink) Old 2009-02-23, 07:31 PM
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Originally Posted by 300ohm View Post
Stacking using custom phasing lines, making in essense one antenna. But that requires careful modeling.
so, I just can't stack two antennas together, facing the exact same direction, exact lengths of coax into a quality splitter to get the 2.5 dbi increase in signal?

91XG (47'), YA-1713 (42'), CPA-19, DB4e (37'); Denon AVR-1312
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post #90 of 1264 (permalink) Old 2009-02-23, 09:52 PM
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Quote:
so, I just can't stack two antennas together, facing the exact same direction, exact lengths of coax into a quality splitter to get the 2.5 dbi increase in signal?
2.5 dbi increase minus 3.5 dbi decrease (from a good marked splitter, the unmarked ones can be about 4.5 dbi or more) leaves minus 1 dbi.

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