RFI(?) kills audio from microphone

[Dieter Pelz (Faustdownunder)]

Hi everyone

When I use my linear amplifier at about 100W, I notice that the "Frm Mic" signal does not appear in the FreeDV window.

A friend of mine observed the same effect and assumes the cause is RFI. 

There is no such problem when I run my 7300 "barefoot" at 100W. 

Any hints or comments, suggestions would be most welcome.

73 & tnx

Dieter VK3FFB

[Mooneer Salem]

Hi Dieter,

What kind of microphone are you using? A USB headset or something plugged into the 3.5mm jack on your sound card?

Thanks,

-Mooneer K6AQ

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[Dieter Pelz]

Hi Mooneer

Thanks for your message. I am using a USB microphone plugged into the front USB connector of an older DELL PC.

73

Dieter VK3FFB

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[Mooneer Salem]

Hi Dieter,

For RFI related issues, I've actually had good luck with adding a USB isolator between the computer and USB device. Example: https://www.amazon.com/Cablemax-USB-2-0-Isolator-Protection/dp/B00QU232UM. Might be worth a shot.

Thanks,

-Mooneer K6AQ

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[Dieter Pelz]

Understood Mooneer. I was going to take such measures we typically use against RFI. My hesitation was because all other digital HF modes (Vara, WSJT) work fine when I use my linear amp. I can't see why FreeDV would be more sensitive to some residual RF but this may indeed be so.73

Dieter

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[Dieter Pelz (Faustdownunder)]

Hi Mooneer,
Only now I understood your point about the microphone correctly. I conclude that because WSJT and Vara does not involve a microphone, everything is fine with these modes. FreeDV does however use one (USB digital in my case) and therefore the RFI is probably upsetting the Mic. It has a ferrite clip-on device but it is near the USB PC plug. I'll try to RF-choke the cable so it cannot act as an "antenna" for local RF pick-up. If that does not work, I'll buy the isolator.
Thanks again and sorry for my late realisation of the likely nature of the problem.

73

Dieter VK3FFB

[jdow]

First run your transmitter power down really low or off and see what happens. Also clean up your cabling so you do not have any ground loops. (OK OK - impossible - agreed, Do the best you can.) If the microphone starts working then Bob's your uncle. Otherwise other debugging will be required. Ferrite chokes help but are not necessarily a solution.

{^_^}    Joanne/W6MKU

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[Dieter Pelz (Faustdownunder)]

OK, I fixed the problem. I simply plugged the Mic USB connector into an external USB multiport switch. That seems to work fine at 200W output power. The brand of the switch (multiport hub) is Anker. 4 ports.

Had to make the nessary changes for the Mic in the audio setup of course. 

73

Dieter VK3FFB

[Dieter Pelz (Faustdownunder)]

Thanks Joanne. I tried that before but to no avail. My working solution is now by using an extern 4-port USB hub (switch). Say hi to uncle Bob for me.

73 es tni tnx

Dieter VK3FFb

[Barry Jackson]

On 14/12/2025 03:40, Dieter Pelz wrote:
> Understood Mooneer. I was going to take such measures we typically use
> against RFI. My hesitation was because all other digital HF modes (Vara,
> WSJT) work fine when I use my linear amp. I can't see why FreeDV would
> be more sensitive to some residual RF but this may indeed be so.73
> Dieter
>
>
> On Sun, 14 Dec 2025 at 12:40, Mooneer Salem <moo...@gmail.com
> <mailto:moo...@gmail.com>> wrote:
>
> Hi Dieter,
>
> For RFI related issues, I've actually had good luck with adding a
> USB isolator between the computer and USB device. Example: https://
> www.amazon.com/Cablemax-USB-2-0-Isolator-Protection/dp/B00QU232UM

> <https://www.amazon.com/Cablemax-USB-2-0-Isolator-Protection/dp/
> B00QU232UM>. Might be worth a shot.
>
> Thanks,
>
> -Mooneer K6AQ
One point to bear in mind with those isolators is the capacitance
between input and output ground connections.
One that I tested was 30pF.
A local station was having issues on 40m only, so I put an appropriate
choke across the device in/out grounds to resonate it as a 40m trap
which did the trick.

Cheers,
Barry
G4MKT

[Dieter Pelz]

Good that apparently I don't need one as everything is fine if I go through a 4-port USB hub.

73

Dieter VK3FFB 

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