The great jointenna exchange - Page 3 - AVS Forum
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post #61 of 68 Old 10-03-2012, 10:32 AM
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There's an insertion loss of the bandpass filter that might be dropping your signal strength below an acceptable level. It varies from one unit to another. It has been a long time since I seriously experimented with UHF jointennas on a bench, but I think the insertion loss of the bandpass path commonly varied from about 2 to 5 dB.

The other thing is, the band reject filters aren't very narrow, and the channel 23 band reject circuit could attenuate channel 20 on the thru path, and the channel 20 filter's reject circuit could reject the channel 23 on its thru path, so it is important to couple the weaker, problematic signal last in a string of jointennas. You might also benefit from lightly amplifying the channel 23 lead by 10 to 12 dB before inserting it into a filter, but whether you can get away with doing that will depend on the relative strength of the other signals coming off that line. If you only have a 15 to 20dB amplifier available, you can use it to preamplify the channel 23 line, but if you do, you might need to "double up" your two channel 23 Jointennas, using the first one on the preamplifier output just as a bandpass filter and using the second one for coupling.

Do you have a TV fool plot we can look at?
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post #62 of 68 Old 10-07-2012, 08:49 AM
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AntAltMike

Thanks again for taking an interest in my problem. I will try to address the issues you raised in your last post. I do have a TVFool plot for my location but I do not know how to send it to you. I can tell you the results. Most of the stations I watch are located to the east which I receive by diffraction due to a ridge between me and the transmitters. Two of these are VHF stations which I receive on an upper band VHF antenna. The signals from the UHF antenna and the VHF antenna are combined through a CM-7777 amplifier. However, I also have two UHF stations located to the north which I receive by line of sight. One of these, channel 20, is so strong that I can receive it on my UHF antenna pointed east. The other, channel 23, is strong but needs an antenna pointed north.

In my last test of the channel 23 jointenna I used an antenna pointed north connected to the CM-7777 amplifier. The channel 20 jointenna was not connected. I tuned my TV to channel 23 and had a good signal. I then inserted the channel 23 jointenna right at the TV set, and got no signal. Does this information give you any ideas as to why it doesn't work?
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post #63 of 68 Old 10-07-2012, 12:17 PM
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Quote:
Originally Posted by rptr View Post

I do have a TVFool plot for my location but I do not know how to send it to you.

I think this site requires that you have made five posts before it will let you post the hyperlink to your TV fool plot, but your next post will be your fifth.
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post #64 of 68 Old 10-07-2012, 12:21 PM
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You might try using one channel 23 Jointenna as just a bandpass filter, between the antenna and the preamplifier input, and then, if that preamp output is tunable by your TV, then couple the preamp output into your other antenna line using your other channel 23 jointenna.

What is the published gain of your CM7777? It has been mentioned here that newer Channelmaster preamps have different electronics and different gain figures than do earlier units bearing the same model or part numbers.
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post #65 of 68 Old 10-16-2012, 08:17 AM
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AntAltMike

I have the old model of the CM-7777 which has separate inputs for VHF and UHF. It has a gain of 26dB on UHF. I did not understand why you are suggesting to use both channel 23 jointennas. Neither one will pass the channel 23 signal on the channel 23 input. Can you explain why that is happening? Thanks again for for your interest and suggestions.
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post #66 of 68 Old 10-16-2012, 11:16 AM
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Actually, both of them "pass" the channel 23 signal if they did so when Channel Master tested them and if no one has subsequently attempted to retune the,

It is possible for a TV to work when connected to a signal that has passed through a working filter when insertion loss drops that signal below the minimal level needed by the tuner, or when preamplification has degraded it. It is actually easier to deal with the probnlem of an anemic digital signal because, since preamplifiers all have lower noise figures than do digital TV tuners, a weakened digital signal can be "rescued by "post preamplification" after it comes out of a bandpass filter.

Since you have the two filters and a preamp on hand, you need to just try this and see how it does. If it doesn't work, then the next step would be to try pre and post amplification combinations using a lower gain amplifier.
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post #67 of 68 Old 11-08-2012, 05:46 PM
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Good news. I solved my channel 23 Jointenna problem by buying an Antenna Signal Injector from Tin Lee. It does the same job as the Jointenna. I just connected it to the antennas and it worked. I can now get all the channels from the two antennas.


AntAltMike

Thanks for your interest and all your advice.
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post #68 of 68 Old 11-09-2012, 09:05 AM
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rptr,

If you're interested, I can check out the ch23 Jointenna to see what was wrong with it. Sorry, missed your previous posts or I'd have offered to check it sooner. Shoot me aPM if you're interested.

BTW, how much did the TinLee injector cost? Just curious since we often run into oddball channel locations when talking to customers.

Cheers!

Tech support for Antennas Direct
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