Narrow Band Coax Antenna and Filter 1

[Eclectic3]

I need to couple a 369.5 MHZ UHF remote control signal into RG-6 cable while blocking radiation of a single channel.
A quarter wave antenna on one leg of a splitter/combiner works to couple the signal into the line, but local UHF channels are also introduced, and I suspect my TV signal is being radiated to some extent. Is there a simple arrangement of tuned stub and exposed center conductor that would act as a narrowband antenna with a notch bandstop between 500-800 MHZ? Or just a narrow bandpass to the stub antenna?

 

[VEBill]

Simple answer - no.

To achieve the required response would probably take a set of what are sometimes called 'duplexers' or 'cans' (Hi-Q tuned cavities). At these frequencies they'd be about the same size as a breadbox and would cost (used) about $500.

I've got several probably-similar UHF remote controls around the house and I thought that the point was that they didn't need anything extra to send the signal all over the house. I can change channels from halfway out my (very long) driveway.

So why are you doing what you're doing?

 

[Eclectic3]

L-shaped brick house with receiver at end of one leg and tv at the other. The brick walls stop the signal like, well, a brick wall. Rather than tuned cavity filters, how about tuned 1/4 or 1/2 wave coax sections? Will a 1/4 wave rod on the end of a 1/4 wave shorted stub radiate?

 

[VEBill]

Are you doing this because you're trying to share one existing run of coax cable - to provide the RF video signal to the distant room, and also to send the UHF remote control signal back?

My Satellite TV receivers (that ones that use UHF R/C) have a separate remote control (R/C) antenna input. If I had bad R/C reception, then I could add an external antenna (at the end of a long cable if required) _JUST_ for the R/C. The inputs are all totally separate.

I don't think that you'd get enough isolation using the sort of crude filtering that you're describing. In other words, the problem would be more the dB than the MHz. If it is a Cable TV set-top box that you're referring to, then my concern is primarily related to leakage of Cable TV signals to the outside world. Not good.

 

[Eclectic3]

I'm just looking to avoid pulling another cable (really a pain in this situation). It's a Dish receiver setup. The home distribution channel is a single UHF frequency above 500 MHZ, and could be chosen to be the 2nd harmonic of the R/C freq. at 369.5 MHZ (channel 72-73?) The splitter/combiner on a single cable works just fine, I just got curious about how to pass the R/C and notch out the UHF channel for a cleaner setup. I've found some stuff on 1/2 wave transmission line narrow bandpass filters, stripline bandpass the size of a cigarette pack for this freq., helical resonators, stub notch filters.

Dish makes a "Diplexer/seperator" about 1 cubic inch, but the crossover is between 1.2 and 1.3 GHZ. I need one for .5 GHZ.

Seems like the proper length of coax stub feeding a 1/4 wave monopole should do the job. Anyone ever see anything like this?

 

[VEBill]

Considering how easy your proposal is (basically just some coax), why not just try it and see?

I guess the shorted stub would correspond to the notch frequency, and the monopole would be for the R/C frequency. One will affect the other. I'm not sure how you would tune them without test equipment (and most test equipment is for 50-ohm vice 75-ohm anyway). You will need to know and account for the velocity factor of the cable.

It sounds like you have a Satellite TV receiver similar to one of mine. My Bell ExpressVu 9200 is pretty much the same as the Dish 942. The satellite receiver can use either 'over-the-air' channels or 'cable' channels for the 2nd room TV RF output. The cable-channels might be on frequencies used locally for other purposes.

The leakage, if significant, would just be on the single TV channel that you're selected. I'd spend some time choosing that channel carefully to avoid causing interference. For example, an over-the-air UHF TV channel (locally unused) would be much better than a frequency used on an Air Force base next door (for example). I'd recommend sticking to a locally-unused UHF TV channel.

If I were in your shoes, I'd find a way to address just the RF path from the R/C to the receiver without involving the existing coax. It's a classic case of trying to improve the signal (ham radio DX-ing). Good luck.

 

[Eclectic3]

Thanks for the input. At this point , I'm more obsessed with the engineering problem of duplexing the signals on one cable than solving the original situation that led me to it.

For a test device, I'm thinking center conductor, brass sleeve, drilled solid rod for monopole, and sleeve on the monopole rod to tune it's length.

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