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huge attic type antenna w/ "booster" attached; problems

post #1 of 6
Thread Starter 
Hi guys, I've been using a very simple, unamplified rabbit ears antenna for my HD reception (to a pcHDTV card running on MythTV) for most of this year and have been more or less satisfied with it. Except for Fox, all of the channels come in w/ probably 99% reliability, and even Fox mostly works, but will occasionally drop out. I think it's safe to say that I've got pretty strong signal where this setup is located (antenna was sitting on top of the TV on the main floor of a one story house).

A few weeks back, I finally bought an LCD TV, and now there's really no good place to locate that antenna, and I've been meaning to get it hidden somewhere else anyway (attic or roof). I found an unused antenna up in the attic of the house I'm renting and set about trying to use that (on the main floor first, to see how it works). I assume it's a Gemini brand antenna, b/c that's what's stamped on the "booster" box that's attached to the antenna. The model number on it says "DA900" and I've been able to find a few scattered references to that model on the web (also branded Philips since the buyout), but nothing with enough detail to help me with my problem. The antenna is very large, about 8 feet long and has a couple of small side pieces that fold out.

When I first hooked up my coax line to the booster box's F-type jack, I got no signal on my LCD's ATSC tuner, as in no difference from having nothing plugged in. While fiddling with that, I did notice that if I carefully inserted the cable so that just the signal part of the wire touched the jack (and the outer metal part of the connector did *not* touch anything else), I immediately got great signal. Even looks good on the one problem channel, probably better than w/ the little rabbit ears I've been using for some time. As soon as the outside then touches, the signal completely disappears on the TV again.

After messing around with that for some time, and moving from annoyed to perplexed to agitated, I started checking some of the various contact points of the antenna and booster with an Ohm meter, but I didn't really know what to look for, as I'm not very knowledgeable re. antennas. It appears there is (correct) 75 Ohms of resistance between the signal and ground part of the F-type jack. The box actually switches between 75 and 300 Ohms, but I'm sure I want 75 and regardless it seems to make no difference on either setting.

So, if you made it through all of the above exposition, does anyone have any idea what might be going on in this situation? Is there anything I could check on the antenna or perhaps more likely the booster box? I've also determined that just touching the signal part of the coax anywhere on the metal of the antenna yields me a strong signal.

If any of these details could need some firming up, or if pictures might help, let me know. I'd like to be able to use this antenna rather than buying a new one for the attic, but as it is I'm confused and concerned about using it.

Thanks.
post #2 of 6
Quote:
Originally Posted by tgice View Post

I found an unused antenna up in the attic of the house I'm renting and set about trying to use that (on the main floor first, to see how it works). I assume it's a Gemini brand antenna, b/c that's what's stamped on the "booster" box that's attached to the antenna. The model number on it says "DA900" and I've been able to find a few scattered references to that model on the web (also branded Philips since the buyout), but nothing with enough detail to help me with my problem. The antenna is very large, about 8 feet long and has a couple of small side pieces that fold out.

When I first hooked up my coax line to the booster box's F-type jack, I got no signal on my LCD's ATSC tuner, as in no difference from having nothing plugged in. While fiddling with that, I did notice that if I carefully inserted the cable so that just the signal part of the wire touched the jack (and the outer metal part of the connector did *not* touch anything else), I immediately got great signal. Even looks good on the one problem channel, probably better than w/ the little rabbit ears I've been using for some time. As soon as the outside then touches, the signal completely disappears on the TV again.

Thanks.

I have a few thoughts. One is that the booster box you mention appears to be a preamplifier near the antenna; there should also be a power source that goes at the other end of the coax, near the tv or pc. The coax is connected to the power unit, which is plugged into an outlet, and another piece of coax runs to the tv or pc. (See the link posted below to a da900 for a picture of the two units.) Do you have the power unit (maybe hidden in whatever room the previous occupants watched tv)? If you don't, my experience is that the preamp without the power unit will kill the signal.

But that doesn't seem to explain why you get a signal when only the center conductor of the coax is in contact with the preamp, and not when the shield is also in contact (did I understand this correctly?). If you get or have a 300 to 75 ohm adapter (4:1) balun, you could bypass the preamp and connect the coax directly to the antenna (I'm guessing that the antenna has terminals for 300ohm twinlead; that's why the balun might be needed). If you don't have the power unit for the preamp I think this is what you will want to do anyway.

Finally, can you test the coax itself by connecting it to something else, maybe to the rabbit ears? I had an antenna outage in the spring which turned out to result from a mashed center conductor in the coax at the end at which it attached to the tv, but I wasted all kinds of time looking elsewhere for the problem.

I hope that I've understood the situation and that these comments will help. A picture of the antenna, by the way, would help forum members identify it and give opinions on how well it should work for your situation. But I would not look askance at a free antenna.

http://electrical.hardwarestore.com/...r--666580.aspx
post #3 of 6
Thread Starter 
Quote:
Originally Posted by lemmalone View Post

I have a few thoughts. One is that the booster box you mention appears to be a preamplifier near the antenna; there should also be a power source that goes at the other end of the coax, near the tv or pc. The coax is connected to the power unit, which is plugged into an outlet, and another piece of coax runs to the tv or pc. (See the link posted below to a da900 for a picture of the two units.) Do you have the power unit (maybe hidden in whatever room the previous occupants watched tv)? If you don't, my experience is that the preamp without the power unit will kill the signal.

But that doesn't seem to explain why you get a signal when only the center conductor of the coax is in contact with the preamp, and not when the shield is also in contact (did I understand this correctly?). If you get or have a 300 to 75 ohm adapter (4:1) balun, you could bypass the preamp and connect the coax directly to the antenna (I'm guessing that the antenna has terminals for 300ohm twinlead; that's why the balun might be needed). If you don't have the power unit for the preamp I think this is what you will want to do anyway.

Finally, can you test the coax itself by connecting it to something else, maybe to the rabbit ears? I had an antenna outage in the spring which turned out to result from a mashed center conductor in the coax at the end at which it attached to the tv, but I wasted all kinds of time looking elsewhere for the problem.

I hope that I've understood the situation and that these comments will help. A picture of the antenna, by the way, would help forum members identify it and give opinions on how well it should work for your situation. But I would not look askance at a free antenna.

http://electrical.hardwarestore.com/...r--666580.aspx

Hey, thanks a lot for your reply. I believe you completely understood my rambling description and your input is helpful. That picture represents what I've got, but only the box on the right, not the power supply, as you suspected.

Like I said, this was up in the attic, and I haven't seen any other module around, plus I'd rather not have to use one if I don't need it (which I think I don't based on my experience w/ the other antenna). However, this may completely answer the question as to why the booster doesn't provide signal, I guess. I did consider getting one of those simple baluns, but I wasn't sure where to attach it.

The box that I do have is bolted to the main mast of the antenna, and then has two wires coming out of it which were attached via wingnuts to two very small (compared w/ the rest of the antenna) sections of the antenna that seem to be completely isolated from each other and the main body of the antenna by plastic mounts. That confused me; it almost seemed like the booster box itself was not electrically attached to the main body of the antenna at all (unless the metal bracket carries the connection). As you can see, I'm not very skilled at antenna diagnosis, and it really helps to be able to draw on the experience of people here. I'll try to get a couple of photos of the antenna soon, and maybe someone could suggest how I should attach a small balun to bypass this preamp or whatever it is.
post #4 of 6
Quote:
Originally Posted by tgice View Post

Hey, thanks a lot for your reply. I believe you completely understood my rambling description and your input is helpful.

The box that I do have is bolted to the main mast of the antenna, and then has two wires coming out of it which were attached via wingnuts to two very small (compared w/ the rest of the antenna) sections of the antenna that seem to be completely isolated from each other and the main body of the antenna by plastic mounts. That confused me; it almost seemed like the booster box itself was not electrically attached to the main body of the antenna at all (unless the metal bracket carries the connection).

You're welcome. Assuming that the preamp is connected correctly, I believe that you would just attach the two contacts of the balun (the twinlead ends, not the coax end) in the same place via the wingnuts. The plastic is a bit confusing, but some antennas have a piece of plastic that is meant to hold the twinlead wire securely. As you suggest, a picture might help people figure out where to connect, but given what you have said so far, I would just a attach a balun where the preamp is currently attached to the antenna.
post #5 of 6
The plastic pieces insulate the active element (what you want your balun/cable attached to) of the antenna from the inactive (or "director") elements, as this tyro understands it. If the preamp lacks its power supply so interferes with a good signal from the antenna, I'd disconnect its two wires from the wingnuts and tape them back out of the way where they won't touch the balun's wires or the active element. Then attach the balun's wires to those wingnuts, the coax cable to the balun, then to your TV. See what you get. If the rabbit ears worked pretty well and this attic antenna is huge, I suppose the reception will be even better. FWIW I had EXACTLY the same problem with a preamp recently when I switched televisions. It used to work fine (had its power insertion unit and all), then on a new TV set, it seemed to short out or something (the pilot light went out on the Winegard hdp-269) not when I inserted the center wire in the femal F-connector, but just when the outside sleeve touched it. I monkeyed with it for days and it seemed to depend on WHAT I plugged it into! It was fine on a couple of sets, but "shorted out" or whatever on the new set and on the Samsung STB. Beat the heck out of me. Took it off an installed it on someone else's system and so far the unamplified signal seems to be enough. Good luck.
post #6 of 6
Thread Starter 
Quote:
Originally Posted by tyromark View Post

The plastic pieces insulate the active element (what you want your balun/cable attached to) of the antenna from the inactive (or "director") elements, as this tyro understands it. If the preamp lacks its power supply so interferes with a good signal from the antenna, I'd disconnect its two wires from the wingnuts and tape them back out of the way where they won't touch the balun's wires or the active element. Then attach the balun's wires to those wingnuts, the coax cable to the balun, then to your TV. See what you get. If the rabbit ears worked pretty well and this attic antenna is huge, I suppose the reception will be even better. FWIW I had EXACTLY the same problem with a preamp recently when I switched televisions. It used to work fine (had its power insertion unit and all), then on a new TV set, it seemed to short out or something (the pilot light went out on the Winegard hdp-269) not when I inserted the center wire in the femal F-connector, but just when the outside sleeve touched it. I monkeyed with it for days and it seemed to depend on WHAT I plugged it into! It was fine on a couple of sets, but "shorted out" or whatever on the new set and on the Samsung STB. Beat the heck out of me. Took it off an installed it on someone else's system and so far the unamplified signal seems to be enough. Good luck.

Thanks for the input. Your explanation of the "active" vs. the "inactive" elements was what I was missing, I guess. If anyone has even more exposition on that (perhaps a document somewhere) I might be interested in checking that out.

Anyway, to update the thread, I went and bought a balun at Radio Shack for about $5:

Radio Shack balun

And I hooked it up to the wingnuts just as you suggested, w/ the pre-amp disconnected (not sure if the latter part is necessary... haven't tried it w/ the pre-amp connected).

And the signal seems fine w/ the coax fully connected to the balun, so I think this may've solved my problem. I'll update the thread if anything else changes, but I think I might be good for now.

Thanks guys.
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