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Radio Road Which tuner to get and getting the most from it. Thank God, for the radio! |
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Radio Road Which tuner to get and getting the most from it. Thank God, for the radio! |
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In Reply to: Yes, that is what I'd like to do. posted by Timbo in Oz on August 17, 2019 at 23:20:45:
It seems to me there should be a commercial solution for this, but I haven't looked. Let me just throw out a few thoughts.
The most significant obstacle is that a balanced FM antenna requires a balun to drive unbalanced 75 ohm coax. Most commercial baluns are designed to work over the range of FM and TV frequencies. The lowest frequency of interest is therefore mid-50 MHz, so the balun - essentially a transformer - will have an inductive reactance suitable for that lower frequency limit.
Regardless of the type of AM antenna in use (balanced or unbalanced), the output cannot be connected directly across either side of the FM balun. At AM frequencies, the inductive reactance of the balun is virtually a short circuit. So, even though AM and TV signals can be carried simultaneously on the coax, they must be combined in a way that provides isolation.
One technique I can envision, just off the top of my head, would be to add a small coupling capacitor in series at the output of the FM balun. The AM antenna (either inherently unbalanced or using a balun) could then be connected to the coax. The coupling cap would present a high impedance looking back into the FM balun, so AM signal loss would be minimized. Likewise, it might be necessary to isolate the FM signals from the AM antenna/balun. This is because the internal capacitance and/or self resonance of a low frequency antenna (such as a loop) or low frequency balun can degrade the FM/TV signal strength. The simplest solution would be a small series choke to keep high frequencies out of the low frequency network.
Unfortunately, all of this only addresses one side of the issue. In order to work properly, isolation techniques need to be used at both ends of the coax. The coax carrying the AM signals cannot be fed directly to a VHF/UHF transformer-type coaxial splitter. Nor can it be fed directly to the input of a TV or FM tuner if an internal high frequency balun is connected to that point. In either case, the AM signal will be effectively shunted to ground throughout the system. Standing waves will also be created on AM frequencies, but the effect of that is a topic for a different day. :)
Perhaps there's a more practical means to accomplish this, but it hasn't occurred to me as of this writing.
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Follow Ups
- RE: Yes, that is what I'd like to do. - Triode_Kingdom 11:51:33 08/18/19 (0)
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