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Author Topic: HT Overmodulation?  (Read 220 times)

N4SRN

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HT Overmodulation?
« on: January 30, 2022, 03:45:27 PM »

I’m using an RFinder B1+ dual band 2m/70cm HT as well as GMRS and FRS radios to test RX on my ICOM IC-R30 HT scanning receiver. TX from my GMRS and FRS radios are picked up nicely from a few feet away (so I can see the R30 stop scanning on the proper channel and hear my TX).

When I TX on 146.7 from my B1+ while scanning 2m FM, the R30 stops on a range of frequencies 1-1.5MHz lower than the TX frequency, and voice is garbled. Again, I’m transmitting from just a few feet away so I can see the R30 HT scanner.

Is the discrepancy between TX frequency and the frequencies my R30 stops scanning on due to overmodulation? As GMRS and FRS are channelized, maybe that’s why they stop on the right channel despite overmodulation?

There’s a BlueTooth remote app for the IC-R30 I’ll try to then test from 5-10m away from my TX. Or I can engage the 10 dB attenuator on the R30.

Thanks for your thoughts!

Bret/N4SRN
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Bret/N4SRN
Bedford, NH  USA

WB8VLC

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Re: HT Overmodulation?
« Reply #1 on: February 02, 2022, 10:24:30 AM »

You really need a good spectrum analyzer to look at the output of that radio from around DC to 1 GHz for VHF TX but if you are looking at UHF, then a sweep to 2 GHz is probably better and this is the only real way to see what's going on spurious wise with that radio.

If you use one of the smaller Nano style of cheaper SDR spectrum analyzers note that because of the method that they are designed to cover such a wide band that if you don't use a number of good band pass filters throughout the sweep range that during a wide sweep you will experience aliasing issues that result in chasing ghost signals and as a result I only use either a real Tex or HP spectrum analyzer when things like this pop up.
 
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WA3SKN

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Re: HT Overmodulation?
« Reply #2 on: February 02, 2022, 02:00:02 PM »

Have you tried testing when transmitting into a known good dummy load?

-Mike.
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KM3F

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Re: HT Overmodulation?
« Reply #3 on: February 05, 2022, 10:09:57 PM »

I think your method of testing is faulty.
Do you really know what over modulation is on analogue FM?
FM is frequency modulation not amplitude modulation.
The dependency is how wide the frequency swing goes.
That in turn depends on the bandwidth of the receiver and the how far the Tx can be driven.
Any or all this can be said to be over modulation but a different form for FM when just listened to.
Over loading can sound the same.
For example, you can't slam 4 watts to a receiver 12 inched away. it's pure overload.
Too much mic gain over drives the early stage of the Tx
Deviation become too great etc.
Get your understanding and test procedures under control first.
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WB6BYU

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Re: HT Overmodulation?
« Reply #4 on: February 06, 2022, 05:38:07 PM »

I'd start by setting the radio to the expected receive frequency and
seeing if it can hear your other radio.

It is possible that, due to spurious outputs, or front-end overload, the
scanner picks up the lower frequency signal before it gets to the
expected frequency.  So the first step is to make sure that the signal
sounds clear on the transmit frequency.

Another problem with transmitting on 146.7 is that, if the HT has
Automatic Repeater Offset, it will actually transmit on 146.1, expecting
that it is working through a repeater with a 600 kHz offset.  In that
case, try running the test on 146.5 instead, which should be in the
simplex segment.


I'd run the test on the lowest possible power from the HT, and with
a dummy load or attenuator on the output rather than an antenna.
(And possibly a dummy load on the receiver as well.)
It is easy to overload some receivers and generate a lot of extraneous
signals.
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