Batboard

The GTX mobile radio is spec'd to transmit in the 896-902 MHz range and also 936-942 MHz "talk-around" range. It should only receive in the 936-942 MHz range. I have one that's working fine, transmitting on 902 MHz and 927 MHz equally well, and receiving decently in the 927 MHz band.

I connected a DMM to the RSSI output on the accessory jack and measured the voltage for various input signal strenghts. I have the data in an Excel spread sheet (PM me if you would like further details or a copy of that file). I also programmed 939 MHz as a simplex frequency. I recorded the RSSI voltage with varying signal levels at 927.5 MHz and 939 MHz.

Basically, the receiver is 1-2 dB poorer at 927.5 MHz than it is at 939 MHz. This is almost trivial and not worth worrying about. Due to the 936 MHz bandpass filters in the receiver's front end, I didn't even try receiving in the 896-902 MHz range, and from what I've heard, the radio is quite deaf at those frequencies.

I just thought the group could use some actual empirical data regarding this series of radios.

Bob M.

I reprogrammed my GTX mobile radio to find the limitations at the low end. I kept adjusting the input signal level to maintain a constant meter indication on the GTX mobile's RSSI output pin.

Input Freq Signal Level
in MHz in dBm
------------ --------------
927.5 -117.0
926.5 -117.0
925.5 -117.0
924.5 -117.0
923.5 -117.0
922.5 -117.0
921.5 -117.0
920.5 -117.0
919.5 -117.0
918.5 -116.5
917.5 -116.0
916.5 -114.5
915.5 -113.0
914.5 -111.5
913.5 -110.0
912.5 Did Not Receive Anything
912.0 VCO did not lock - no audible beeps heard on front panel
902.0 Unit would not program any lower due to RSS limits

As you can see, the unit seems to work fairly well down to 919 MHz or so, then does begin to drop off probably due to the front end filters that are only supposed to pass 935-941 MHz or so. I don't know where the unit actually stopped receiving, but it was some place below 913.5 MHz.

I used an Agilent E4430B signal generator for this test, feeding the GTX 900 MHz mobile radio through a single 24 inch piece of RG-58 with a male N connector at one end and a male mini-UHF connector at the other.

Bob M.