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Rotor vs. Multiple antennas

1320 Views 8 Replies 7 Participants Last post by plasmamaniac, Jan 24, 2004

eric.krieg

Discussion Starter · Jan 22, 2004

Question: What are the drawbacks to combining multiple antennas?

Background: I'm new to the antenna world, having just dumped Comcast for the "Big Ass" Phillips Magnavox Broadband antenna on the roof.

I'm in the NW 'burbs of Chicago. I do not yet have a HDTV, but I am trying to buy the new 32" Zenith with the built in tuner (very hard to come by).

I can get some South Bend stations in addition to the Chicago stations (SB is at 111 degrees, Hancock at 129, Sears at 133, my antenna is at around 130).

Ch 4 in Milwaukee is picked up in the channel scan, but I get neither a picture nor sound.

This intrigues me. I could probably put up a rotor and get tons of different channels. Antennaweb lists over 30.

OR I could get multiple antennas. Keep my broadband aimed as it is, and add a couple of yagis, one at Milwaukee, one at Rockford.

What are the drawbacks to multiple antennas?

The only drawback I see to a rotor is complexity and the fact that you have to ROTATE THE ANTENNA to change stations (duh!). I bet that a Chicago winter would be pretty tough on a rotor.

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rjbrooks

· #2 · Jan 22, 2004

If you combine antennas, don't just hook them up in parallel, or the impedance match won't be right, and signal will be greatly reduced. You can use splitters, but there would still be signal loss. If none of the

stations are particularly weak, this might not be a problem. Amplifiers

(separate ones near the antennas, before the splitter) could work. Or

separate cabling, plus a switch at the receiver.

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JVanderwalker

· #3 · Jan 23, 2004

I would suggest a rotor since the cost is less than buying 2 more antennas. I have a VHF and a 7 foot parabolic on a rotor with no wind problems. The key is to use a thrust bearing which takes the side load off the rotor.

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sregener

· #4 · Jan 23, 2004

The only advantage to multiple antennas is that you don't have "the kids want to watch affliate X out of Chicago upstairs and you want to watch the sports event on channel X out of Milwaukee downstairs" conflicts.

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eric.krieg

Discussion Starter · #5 · Jan 23, 2004

The rotor is such an... inelegant solution. It is kind of a downer to have something mechanical in anotherwise electronic system. Maybe I'm just insane, stuborn, whatever, but I think no more so than the other "enthusiasts" on this site.

But I am pretty sure that there is a good reason that they sell rotors, and it isn't because our wives will be twice as pissed at us if there are twice as many antennas on the roof!

There isn't a lot of info on this on the web, but from what I can tell, there is serious signal loss (~3.5 dB) when you combine in parallel two different antennas in two different directions. The best solution would be to run each antenna on its own RG-6 to an A/B switch. To me, that is a little better than a rotor, but not much!

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eric.krieg

Discussion Starter · #6 · Jan 23, 2004

I found this at hdtvprimer.com:

When two identical antennas are mounted together (ganged) and pointed in the same direction and wired together properly, there is a theoretical possibility of a 3 dB improvement. That is, twice the signal power is delivered to the TV compared to what a single antenna would do. In practice, 2.5 dB is readily achieved, 0.5 dB being the typical loss in the combining device. But if the two antennas are pointed in different directions (towards different stations) a 3.5 dB loss for each antenna is the likely result.

The above statements are true regardless of whether the antennas have shared or separate amplifiers. For a shared amplifier, if the antennas point in different directions, half the power each antenna takes in reflects off the combiner and is rebroadcast out the antennas. Why this doesnâ€™t happen when they are pointed the same way is harder to explain.

For dual amplifiers, when the antennas are pointed the same way, this signal is increased by 6 dB but the noise is increased by 3 dB, so the overall improvement is still 3 dB. When they are pointed differently, the 3 dB noise increase causes a signal/noise ratio loss of 3 dB for both stations. Dual amplifiers will eliminate the combiner loss, but only if the amplifiers are closely gain-matched.

(Ganging non-identical antennas is not recommended. They would need to produce equal voltages, and adjusting out the phase difference might not be possible for all stations.)

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DAP

· #7 · Jan 24, 2004

The exception is when you can get a combiner that lets one frequency range pass from one antenna, and a different frequency range from the other. This works for VHF & UHF antennas (so they can be pointed in different directions). Unfortunatly, it is difficult to do this with adjacent stations as the sharp edge of the filter too close to the channel of interest introduces phase errors & other distortions. If you had a dead channel between each of the channels on different antennnas, you could theoretically build a combiner that only let certain channels from each antena through.

The problem of course is that every instalation would be custom & therefore expensive.

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arxaw

· #8 · Jan 24, 2004

Quote:

Originally posted by eric.krieg
... run each antenna on its own RG-6 to an A/B switch. To me, that is a little better than a rotor, but not much!

Not bad when you connect both antennas to one of these and teach the A/B commands to a learning remote.
http://www.radioshack.com/product.a..._name=CTLG_003_010_011_000&product_id=15-1968 http://www.radioshack.com/images/Pro...15/15-1968.jpg

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plasmamaniac

· #9 · Jan 24, 2004

When I put my antenna up after it was put in my attic and didn't work very well my antenna guy said "You don't need an amp or a rotator." I said well I want one and Boy Oh Boy am I glad that I spent the extra money and got it. I can tweak settings and they are locked in so when you want to go to channel 2 you enter a 2 on the rotator remote and the antenna automatically swings to that position. When you want channel 5, you enter a 5 and it swings to the position for channel 5. Then you can move it back and forth to tweak the channel because it changes from time to time. Trees get in the way and I can fine tune it to get around that until I can have them topped off. I would definitely go with the rotator. Get one with a remote.

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