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Discussion Starter · #1 ·
Well, I'm in need of you antenna gurus out there...


As of a week ago, I only had a dual band (VHF & UHF) yagi in the attic. VHF reception was fine, but UHF performance did not allow me to reliably receive all Wash DC and Baltimore DTV stations.


My solution was to add a CM 3021 on the roof and combine its output with the attic VHF/UHF. I used a RS UHF/VHF combiner to merge the UHF from the 3021 and the VHF from the attic dual. So far, so good. This setup worked, but I needed more gain on UHF due to lots of downstream splitting in my house.


So I added an old RS 15-20 dB bullet amplifier (yes, it's not the best, but it was free in my parts box!) at the 3021. And I plugged in the power injector separate from the VHF/UHF combiner just to test the 3021's amplified performance alone. Great results! Lots of signal.


BUT....when I then combined the amplified 3021 with the attic VHF/UHF, my UHF reception was filled with very bad interference, as in completely unwatchable on analog, and no signal on digital. The combination of the attic dual, the RS combiner, and the amplifier is somehow creating great interference. This combiner is supposed to pass DC (amplifier power) thru the UHF side only, and I verified this with a multitester. I also tried injecting the power upstream of the combiner with the same bad results. When I disconnect the attic dual from the setup (leaving the combiner inline with power to the amp) all is good again. Bottomline, of the three system components (attic dual antenna, roof UHF, amplifier), you can pick any two and it works great.


Next, I inserted a 12dB attenuator in the UHF before the combiner, and this made the bad interference go away, but does not leave me with enough signal strength (although it's better than without any amplification.)


Does anyone have any thoughts on what is happening here? In principal, this setup should work, yes? Should I try a better quality amp? Better quality UHF/VHF combiner? A variable attenuator inline?


Thanks, Gerald C.


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"In principal, this setup should work, yes?"

Sorry to say, no.


Combining RF signals from two different antennas will allow them to cancel each other out. You'll need to filter out the UHF from the previous antenna, or filter out specific channels.


Radio Shack has a good VHF/UHF splitter/filter. VHF & UHF in on one coax, VHF & UHF out on separate coax. This is part of the Gold series, some stores may not carry it, it's not real cheap either, I think over $10, but it works.


Another way is to use the Channel Master products. Check out this link for more details: http://www.avsforum.com/ubb/Forum11/HTML/014710.html



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Sounds like the same problem I had when setting up my antenna and amp. What worked for me (and might also help you) was to put the attenuator on the output of the UHF amp. I found that without an attenuator it was overloading the front end of my TV but attenuating the input didn't amplify the signal enough. Putting it on the output amplified the signal but reduced it to a level where it wasn't causing a problem anymore. I used a 6dB attenuator in my situation - you may want to try a variable until you find the value that works for you.
 

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What you have going on is "MULTIPATH" from UHF reception on both antennas (I'm fighting this one myself - and my "other" antenna is not even a UHF).


What you can try (at least what I'm trying to do) - get one or more Low-pass filters (I've got 2 in place and a 3rd on the way) (see http://www.smarthome.com/7822.html ) and put them on the dual-band. You could also try hacking off the UHF part of the dual-band. Finally, if it is just 1 or 2 channels that are causing problems, look at using the Channelmaster Jointenna's as bandpass filters on the dual band.


The whole goal here is to effectively negate the UHF reception on the dual-band, or make it so small that your 3021/pre-amp overwhelms it.


Another solution would be to use an A/B coax switch to switch between antennas. However, this doesn't work too well when you are feeding multiple TV's.


Other Ideas I've considered/tried - use split feeds, but put the 3021 directly to your DTV tuner.


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I've tried both the Channelmaster 0549 and the RadioShack $10 Gold series - same results each way. I HAVEN'T tried the attenuator on the output of my amp (the Radio Shack 30dB pre-amp) - will try that tonight.



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It occurs to me that you are actually transmitting out of your attic antenna. You might have created a feedback loop. You are pushing an amplified signal into your attic antenna, which might be transmitting that signal back to the rooftop UHF antenna.


Why don't you just remove the UHF elements from your dual-band attic antenna, and then put the amp downstream of your combiner?
 

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Well, the CM0549 does do a slightly better job of combining for me as compared to the Radio Shack $10 Gold series combiner. Using the 6dB attenuator "helped" my strong stations, but hurt my weaker stations (that I can't get on Dish). YMMV.


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Discussion Starter · #8 ·
Thanks Ken and Scooper-- I guess my long-winded post wasn't all that clear. The VHF/UHF combiner I am using is exactly the "gold series" splitter/combiner you mention. I realize that you cannot directly combine two UHF sources without generating multipath, so this combiner should do the trick as long as its bandpass function is well isolated. In fact, it seems to work fine as long as the amp is not on.


But when I turn the amp on, all UHF freqs become obliterated, not just subtle interference. VHF remains unaffected. There is something more bizarre going on here.


--Gerald C


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Discussion Starter · #9 ·
dlsynder--


I hear what you are saying about overloading. In fact, using an attentuator seemed to solve some of the problem when combining the VHF and UHF. But, when using the UHF antenna alone with full amplification, there is no overloading, and UHF reception is perfect on all channels. The mystery happens when I combine the other antenna into the setup.


Maybe the RS combiner can't handle the strong signal UHF and the unamplified VHF without choking the UHF. I'm not a filter designer. Anyone else? Channel Master makes a similar combiner (Model 0549). Anyone try it?


--Gerald C


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Discussion Starter · #10 ·
Quote:
Originally posted by GuyGordon:
It occurs to me that you are actually transmitting out of your attic antenna.
Now, this is an interesting idea. I'm not exactly sure how to disable the UHF section of my dual antenna, but I'll give it a look. Thanks.


As far as putting the amp downstream, yes this is possible, but I wanted to maximize the signal amplification before it goes through about 70' of RG-6.


--Gerald C


[This message has been edited by Gerald C (edited 07-18-2001).]
 

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It's 70' from your outdoor UHF antenna to your attic? Wow. I was thinking it would be more like 10', so that it wouldn't make much difference.


I don't know exactly what your attic antenna looks like, but the VHF elements will be much wider than the UHF elements. The UHF elements should be about 12" wide (6" on either side of the boom), while the VHF elements will be up to 6' wide. The UHF elements are almost certainly mounted in front of the VHF, forming a little antenna inside a big one. Then the two will be tied together where your 300 ohm leads attach.


Look for some wires or straps that connect the large elements to the array of small elements. Disconnect them and attach the 300 ohm line to the larger VHF array only.


You should be able to do this without permanently damaging the antenna, just in case you need to reverse the process later.


BTW, on my Channel Master 3018 you can dismount the UHF antenna completely.
 
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