Mains hum on FM Tx with radio in the charger

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VK3KYY
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Mains hum on FM Tx with radio in the charger

Post by VK3KYY » Tue Jan 21, 2020 7:01 am

Guys

I've noticed when doing some testing, that on FM I get quite a lot of mains hum on FM Tx.

The primary cause of this seems to be the mains charger unit, but also things are worse if I use a speaker mic.

Strangely, things also seem worse on 2m than 70cm, but that doesn't make any sense.

I tried various power levels, but I get the problem even at 250mW.

This is not going to be an OpenGD77 firmware problem, but I think that anyone running their radio in a charger all the time, like I do, may have degraded Tx performance both on FM and DMR because of the hum (though in theory it should not matter to the 4FSK FM signals that DMR uses)

DF9HJ
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Re: Mains hum on FM Tx with radio in the charger

Post by DF9HJ » Tue Jan 21, 2020 7:15 am

I had the same with mainly cheap chinese mics.
If I use Kennwood mics I newer had that problem.
At some points also DMR was affected.
73 Jens

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YT5HOK
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Re: Mains hum on FM Tx with radio in the charger

Post by YT5HOK » Tue Jan 21, 2020 9:14 am

VK3KYY wrote:
Tue Jan 21, 2020 7:01 am
...
I've noticed when doing some testing, that on FM I get quite a lot of mains hum on FM Tx.
...
It is a known problem, at least for me. Handheld transceivers usually have cheap chargers and hum is expected, if used while charging. I was always told by "older" amateur radio operators not to use it that way, for mentioned reason. I wouldn't worry about it. ;)

BTW, there's a mod to reduce RF noise, but you probably know that.
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kt4lh
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Re: Mains hum on FM Tx with radio in the charger

Post by kt4lh » Tue Jan 21, 2020 1:01 pm

The TP5100 charger drop-in also is said to improve the RFI on the charger, although it's still pretty significant. Weaker repeaters on one HT disappear as soon as I drop the GD77 or MD380 into the charger, so for me it's not so much "through" the charger as RFI from the charger.

I believe the original charger was worse, but if you're curious I can dig it out, I only modified one of the two.

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