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GMRS Bandpass Filters



Anyone have an experience with the front end on the Retevis RT97?  


Does anyone make a GMRS Bandpass filter with SMA connectors. Right now I have a high pass filter added in after the duplexer that starts attenuation in the low 400mhz range. I would love to try a bandpass for the heck of it.


While the unit works fine when set to "high power" I can always hear it further than I can get into it unless I am pushing significantly more power from my PM400 mobile radio.

When set to "low power" it about matches a 5 watt HT. A 5 watt HT can get back to the RT97 with about the same signal clarity that the RT97 can get to the HT. No idea what actual power the RT97 puts out on High and Low power.


Just wondering if I am getting some desense from strong RF in the area. I have multiple FM radio and TV broadcast stations with in a few miles. Would be nice to get further improvement if possible...or should I leave well enough alone?


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52 minutes ago, WRNA710AZ said:

If you can find an old (1970s) mobile radio telephone transceiver, they had very nice compact duplexers in them. You'd need a service monitor with spectrum analyzer or a signal generator and spectrum analyzer to tune it for you needs.

Likely something like these Chinese duplexers?


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ETA:  I do have a flat-pack, 4 cavity pre-selector that would do what you are looking for.  It has 5MHz bandwidth in the GMRS frequency range.  It has a female N, and female BNC connector.  I think I have cables  for it to get you to PL-259 at minimum, MAYBE SMA (I'd need to look).  Send me a PM if interrested. 


------------------------------- original message below ----------------------------

The RT97 is spec'd. as 10W on high power, but on mine, I measured a little over 5W into a dummy load right off of the SO-239.  I contacted Retevis about this.  The duplexer that they are using is designed for a minimum 10MHz split.  Though it can be tuned for a 5MHz split, that does increase losses.  It looks like they're losing about 3db in the duplexer (my bet is on both ends, TX and RX).  I was thinking of adding one of those cheap 30db pre-amps to the RX side, after the duplexer.

How can they rate it as 10W you ask.... the output of the TX chip is 10W (before the duplexer).




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According to the FCC grant the radio is rated 5 watts.


As far as filters, doesn't the RT97 already have a cavity duplexer in it?  Have you verified that it's tuned?  Being off center could be giving you several tens of dB of attenuation and cavity filters aren't wide bandwidths.  Just temperature fluctuations can cause them to drift.

You shouldn't need a preselector with a cavity filter unless you have a very strong source interfering.  It's possible but you need to verify that overload is actually your problem.  It's not likely to be out of band with cavity filters, which are going to give you 80, 90, 100dB of notch.

You do sometimes see sensitivity issues when the transmitter isn't clean enough and the radios are interfering with each other despite the filters.  It's possible and easy to test.  Disconnect the transmitter from the duplexer and connect to it either a dummy load or another antenna and see if the reception improves. 

If this helps then it's not really easy to filter without incurring a lot of insertion loss that negates the steeper notch.  Usually means you need a better transmit radio when this happens.  When additional bandpass filters are used in repeaters there's usually an accompanying amplifier to offset and the preselectors are also cavities and not discretes.

I'd really make sure the duplexer is tuned before anything else. 



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