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K3: transmitted RF feedback with condenser mics?

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K1LI

K3: transmitted RF feedback with condenser mics?

I am getting transmitted RF feedback into the K3 when running high power
with a headset that uses a condenser element. With an otherwise identical
setup, I don't have any problems with either of my two headsets that have
dynamic mic elements, using either the front- or the rear-panel mic jacks.

I hear the distortion in the K3's own monitor, but I have confirmed the
distortion using my K2 as a second receiver (1" wire in the ANT jack, RF
gain near zero, ATT on to prevent front-end overload) and with on-the-air
tests with stations hundreds to thousands of miles away.

The problem headset is designed for aviation use and, as far as I can tell,
uses a shielded mic cable.

Has anyone else had and resolved this problem?

--
73 -- Brian -- K1LI
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Jim Brown-10

Re: [Elecraft] K3: transmitted RF feedback with condenser mics?

On Sun, 24 Jan 2010 14:53:28 -0500, Brian Machesney wrote:

>The problem headset is designed for aviation use and, as far
as
>I can tell, uses a shielded mic cable.

Since I'm an RFI/EMC specialist, I'll offer some ideas for you
to try.

1) Reduce the input gain as much as practical while still
giving you good modulation. There's a high a low gain setting.
Try to make the low gain setting work.

2) Pay careful attention to where you're connecting the shield.
It should go to the chassis, NOT "mic ground." The latter is a
classic "pin 1 problem" and can be a major cause of RFI (RF
feedback). I've been told by someohe who has studied the
circuit board that there IS a pin 1 problem at the front panel
jack. There is also a potential pin 1 problem at the rear panel
jack (in the form of an RF choke in series with the mic
shield).

3) Try a small multi-turn choke on the mic cable. A single turn
through a clamp-on is not likely to make a dent, but MULTIPLE
turns can help (by reducing the RF current on the cable).

All of the above is addressing RFI TO the K3. There's a small
chance the the RFI might be to the mic itself. The choke might
help that too.

You didn't mention what bands or how much power. A good test
when diagnosing things like this is to back off on the power
until the problem goes away. That reduction in power is the
degree of suppression (in dB) that you need.

>Has anyone else had and resolved this problem?

I had it with a dynamic mic on my FT1000MPs, and solved it with
chokes, as described. In that case, it was a pin 1 problem at
the front panel mic jack.

73,

Jim Brown K9YC

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W6ODJ

Re: [Elecraft] K3: transmitted RF feedback with condenser mics?

In reply to this post by K1LI

From: Oliver Johns <[hidden email]>
Date: 25 January 2010 09:12:00 PST
To: Brian Machesney <[hidden email]>
Cc: Oliver Johns <[hidden email]>
Subject: Re: [Elecraft] K3: transmitted RF feedback with condenser mics?

Try putting a snap-on ferrite bead ON the actual boom of the electret headset microphone. Put it right at the end, just before the microphone bulb. Maybe the RF is getting into the electret or its FET and is being rectified there.

Oliver Johns
W6ODJ

On 24 Jan 2010, at 11:53, Brian Machesney wrote:

> I am getting transmitted RF feedback into the K3 when running high power
> with a headset that uses a condenser element. With an otherwise identical
> setup, I don't have any problems with either of my two headsets that have
> dynamic mic elements, using either the front- or the rear-panel mic jacks.
>
> I hear the distortion in the K3's own monitor, but I have confirmed the
> distortion using my K2 as a second receiver (1" wire in the ANT jack, RF
> gain near zero, ATT on to prevent front-end overload) and with on-the-air
> tests with stations hundreds to thousands of miles away.
>
> The problem headset is designed for aviation use and, as far as I can tell,
> uses a shielded mic cable.
>
> Has anyone else had and resolved this problem?
>
> --
> 73 -- Brian -- K1LI
> ______________________________________________________________
> Elecraft mailing list
> Home: http://mailman.qth.net/mailman/listinfo/elecraft
> Help: http://mailman.qth.net/mmfaq.htm
> Post: mailto:[hidden email]
>
> This list hosted by: http://www.qsl.net
> Please help support this email list: http://www.qsl.net/donate.html
>

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Jim Brown-10

Re: [Elecraft] K3: transmitted RF feedback with condenser mics?

On Mon, 25 Jan 2010 09:14:21 -0800, Oliver Johns wrote:

>Try putting a snap-on ferrite bead ON the actual boom of the
>electret headset microphone. Put it right at the end, just before
>the microphone bulb. Maybe the RF is getting into the electret or
>its FET and is being rectified there.

Ferrite beads are tuned to about 150 MHz. Clamping on onto a cable
simply adds that 150 MHz resonance to the common mode circuit.
That's great if the RFI is on 2M, but next to useless if it's on
20M. To make a choke work on 20M, we must wind multiple turns
through the ferrite to more the resonance down to 20M. For a typical
ferrite clamp-on, that's about 5 turns. You can't do that on the
boom, but you can do it on the cable.

The original poster of this question emailed me this morning telling
me he had solved his problem by re-cabling the coax between his rig,
amp, and antenna switching, so the source of his RF was probably a
badly terminated coax connector.

73,

Jim K9YC

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W6ODJ

Re: [Elecraft] K3: transmitted RF feedback with condenser mics?

Hi Jim,

Well, the ferrite bead may not be self-resonant on 20 meters, but it still provides SOME impedance to the common-mode current. I found experimentally that it did help. And lots of people use these snap-on beads to good effect even in situations where multiple turns (which of course would be better) are not practical.

Oliver
W6ODJ

On 25 Jan 2010, at 9:31 AM, Jim Brown wrote:

> On Mon, 25 Jan 2010 09:14:21 -0800, Oliver Johns wrote:
>
>> Try putting a snap-on ferrite bead ON the actual boom of the
>> electret headset microphone. Put it right at the end, just before
>> the microphone bulb. Maybe the RF is getting into the electret or
>> its FET and is being rectified there.
>
> Ferrite beads are tuned to about 150 MHz. Clamping on onto a cable
> simply adds that 150 MHz resonance to the common mode circuit.
> That's great if the RFI is on 2M, but next to useless if it's on
> 20M. To make a choke work on 20M, we must wind multiple turns
> through the ferrite to more the resonance down to 20M. For a typical
> ferrite clamp-on, that's about 5 turns. You can't do that on the
> boom, but you can do it on the cable.
>
> The original poster of this question emailed me this morning telling
> me he had solved his problem by re-cabling the coax between his rig,
> amp, and antenna switching, so the source of his RF was probably a
> badly terminated coax connector.
>
> 73,
>
> Jim K9YC
>
>
> ______________________________________________________________
> Elecraft mailing list
> Home: http://mailman.qth.net/mailman/listinfo/elecraft
> Help: http://mailman.qth.net/mmfaq.htm
> Post: mailto:[hidden email]
>
> This list hosted by: http://www.qsl.net
> Please help support this email list: http://www.qsl.net/donate.html
>

W6ODJ

Re: [Elecraft] K3: transmitted RF feedback with condenser mics?

In reply to this post by K1LI

Brian,

I know a guy in Sonoma, CA, whose porch lights come on (dimly) when he transmits on 160!! Fortunately, my main problem (on 20 meters) seems not to touch the DSL, WiFi, etc. system. Fingers crossed. I think one of the hidden problems we all have is resonance with phone lines, household wiring, or metal weatherstripping, etc. The RF may be coming directly from the antenna, and no amount of RF proofing in the shack is going to help much then. Maybe move the antenna?

Maybe try putting 0.01 RF-bypass capacitors ACROSS the phone line at the jack where the DSL modem plugs in. Probably wouldn't hurt the DSL or POTS signal much, and might bypass the RF. Maybe it isn't even common mode.

Oliver
W6ODJ

On 25 Jan 2010, at 12:59 PM, Brian Machesney wrote:

> You are so right, Oliver.
>
> Now, if I could only figure out why 50W of RF on 160 kills my DSL line. Tried wrapping all of the DSL and wireless router cords through a high-mu ferrite toroid, but no joy. This has been a thorny problem; sensitivity depends on where the modem and router are placed. May have to go back to "wired" and see if that helps.
>
> No end of "fun" things to do! The XYL will not be happy when I kill the DSL during the CQ WW 160 contest this weekend!
>
> --
> 73 -- Brian -- K1LI
>
> On Mon, Jan 25, 2010 at 3:24 PM, O. Johns <[hidden email]> wrote:
> Hi Brian,
>
> Every solution is different, isn't it? Very glad yours worked for you.
>
> Oliver
> W6ODJ
>
>
> On 25 Jan 2010, at 9:25 AM, Brian Machesney wrote:
>
>> Thanks for the suggestion, Olvier. In the process of trying to figure things out, they went from bad to worse, so that I even thought I had terrible distortion with just 100W.
>>
>> Long story short, I think the problem was a bad shield connection in one of the current baluns that uses the small-diameter teflon coax; it's not mechanically robust because the coax doesn't fit snugly inside the PL259 reducer meant for RG58. I re-cabled the RF path from rig to antenna and, voila!, all is well.
>>
>> Brian
>>
>> On Mon, Jan 25, 2010 at 12:12 PM, Oliver Johns <[hidden email]> wrote:
>> Try putting a snap-on ferrite bead ON the actual boom of the electret headset microphone. Put it right at the end, just before the microphone bulb. Maybe the RF is getting into the electret or its FET and is being rectified there.
>>
>> Oliver Johns
>> W6ODJ
>>
>>
>> On 24 Jan 2010, at 11:53, Brian Machesney wrote:
>>
>> > I am getting transmitted RF feedback into the K3 when running high power
>> > with a headset that uses a condenser element. With an otherwise identical
>> > setup, I don't have any problems with either of my two headsets that have
>> > dynamic mic elements, using either the front- or the rear-panel mic jacks.
>> >
>> > I hear the distortion in the K3's own monitor, but I have confirmed the
>> > distortion using my K2 as a second receiver (1" wire in the ANT jack, RF
>> > gain near zero, ATT on to prevent front-end overload) and with on-the-air
>> > tests with stations hundreds to thousands of miles away.
>> >
>> > The problem headset is designed for aviation use and, as far as I can tell,
>> > uses a shielded mic cable.
>> >
>> > Has anyone else had and resolved this problem?
>> >
>> > --
>> > 73 -- Brian -- K1LI
>> > ______________________________________________________________
>
>
>