Pro 107 reception issue on GMRS 1-8

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JimAckley

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I have a Pro 107 and I decided to program it with all of the frequencies from my Motorola MR350R. 1-14 (FRS) play back through my scanner just fine, but when I get to 15-22 (GMRS), my scanner freaks out a little.

I have the backlight for each object set to "on": so that it remains off in general (unless I hit a key), but comes on during reception so I can see the screen in the evenings and night.

When I xmit on channels 1-14, it comes through loud and clear, no breakup at all. When I key up 15-22, it will lock onto the channel most of the time, but the second I start talking, the scanner's backlight flashes off for a brief moment, and then the scanner scans forward to either of the two FRS frequencies that are above/below the GMRS frequency and locks on to one of those, and the backlight comes back on when it acquires the new channel. E.g., when I key up on Ch 21, it will lock on until I speak, and then it flips forward to either 6 or 7. Here's a channel table to help those who aren't familiar with the frequencies.

Even when I have the scanner on pause, the backlight will turn on when I key up, but when I start talking on the GMRS channel it shuts off, but obviously won't scan forward since it's paused. I've tried putting ATT on for all of those channels, thinking maybe it's just coming in too hot, but that didn't change a single thing.

For the record, I program it via the GRECOM EZ Scan-SD software since I can't find my RS software and it's not supplied online.

Anyone have an idea as to what's going on?
 

hertzian

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It might be that if you set your 350 to low power on gmrs channel 21, it is actually a bit higher than on FRS. I'm not familiar with the 350's actual power levels, so something to check there as you may still be overloading it even with the attenuator on the 107.

It sounds like it can handle an unmodulated carrier, but as soon as you speak, there's enough modulated power now that the 107 still goes into desense on 21.

Then when scanning, it can only handle the surrounding off-frequency interstitials on 6 and 7. But 21 is totally desensed when you speak so it will never land on 21 and stay there.

I think it is just a matter of being too close for testing, even with low power and attenuation. If you enlist the help of another, does it happen when you have some decent separation between the two?
 
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Mike_G_D

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If it were AM hertzian's advice would be good but since we are dealing with FM not so much. I am guessing that the FRS/GMRS radios you are using are switching between two FM deviation modes - they may use +/-2.5KHz deviation on the FRS channels (FRS rules permit only that deviation) and switch to +/-5KHz deviation on the GMRS channels (GMRS is allowed that deviation). I would try making sure that the mode you use for scanning the channels in the scanner is set to "NFM" on the FRS channels (1-14) and just "FM" for the GMRS channels (15-22). I am not familiar with the 107 but I am guessing it has those settings as the older 500 series does. In the 500 series, the "NFM" mode switches in a narrower IF filter when engaged. Basically, when dealing with FM, deviations which deviate outside the passband the receiver is designed for (or set at) will cause the radio to "lose" the signal during those particular deviations.

For FM analog voice, louder sounds deviate the carrier from the center more - the louder you talk the more it deviates. FM two-way radios are designed for a certain set limit for maximum deviation depending on governing rules and system needs (number of channels needed, etc.). In the US, most two-way narrowband FM has been, for many years, set at +/-5KHz maximum deviation and is now switching over to +/-2.5KHz maximum deviation. However, the FRS rules permitted only +/-2.5KHz maximum deviation from the beginning. The MURS channels 1-3 are also permitted only 2.5KHz maximum deviation while channels 4 and 5 are permitted to use 5KHz maximum deviation.

Another way to check this is to speak very softly into the transceivers when on the higher deviation GMRS channels and see if that helps when the scanner is set on that channel. If so, check the scanner's mode and see if you are using "NFM" and switch that to "FM" and try the test again - should be better at handling the higher deviation now.

This is also why it's not a good idea to speak louder into an FM radio transmitter when trying to be heard at marginal signal levels. Contrary to human nature, for analog FM voice transmission and reception it's better to speak softer into the transmitter so as to make sure that your full information is contained within the receiver's IF passband and its discriminator design/setting.

-Mike
 
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hertzian

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Mike is right - I had been doing so much AM airband monitoring lately, that I mixed up my modulation modes. :)
 
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