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Interesting K3 QRM problem

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Vic Rosenthal

Interesting K3 QRM problem

I live about 3.3 miles from a 50 kW BC station on 1080 kHz. It is on 24
hours a day, transmitting in Arabic, not that that matters.

I have two antennas: an R8 vertical and a Pixel Loop receiving loop.

With either antenna, I hear a wideband noise on my K3 that is
synchronized with the program material of the BC station on the 14, 10
and 7 MHz bands.

I first determined that it was not affected at all by the attenuator in
the K3. So I assume that it is not caused by front-end overload in the K3.

I noticed that if I listen on the Pixel Loop via the K3's RX ANT input
the noise completely goes away when I unplug the R8 from the K3's ANT 1
connector!

I also noticed that the noise is reduced when I bypass my amplifier or
my SWR/wattmeter, which are connected between the ANT 1 connector and
the R8. Reduced, but not eliminated.

I conclude that what is happening is that the BC RF is being picked up
by the R8, and the diodes in the wattmeter and the amplifier and the K3
finals are creating IMD products which I am hearing. The K3 seems to be
the major contributor.

I am thinking that maybe the IMD products are being re-radiated by the
R8 and picked up by the Pixel Loop.

I tried one more experiment: I have a BC band filter which is used with
the MFJ antenna analyzer when there are nearby BC signals. Sure enough,
putting this in series with the K3 ANT 1 connector kills the noise. Of
course I can't transmit through it.

Has anyone had a similar problem? I am thinking that the solution is a
highpass filter or trap at this spot. I don't operate on 160, so it
shouldn't be too hard to make.

--
73,
Vic, 4X6GP/K2VCO
Rehovot, Israel
http://www.qsl.net/k2vco/
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Arie Kleingeld PA3A-2

Re: Interesting K3 QRM problem

Vic,

Cannot really tell what's happening, but I have a suggestion for your BC
bandfilter.
Connect the filter to the RX ANT in and out connectors, and switch on
the RX ANT.

Now you can transmit the normal way and use the filter for receive only.

73
Arie PA3A

Vic Rosenthal 4X6GP/K2VCO schreef op 23-10-2014 12:58:

<snip>
> I tried one more experiment: I have a BC band filter which is used
> with the MFJ antenna analyzer when there are nearby BC signals. Sure
> enough, putting this in series with the K3 ANT 1 connector kills the
> noise. Of course I can't transmit through it.
>
> Has anyone had a similar problem? I am thinking that the solution is a
> highpass filter or trap at this spot. I don't operate on 160, so it
> shouldn't be too hard to make.
>
<snip>
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Charlie T, K3ICH

Re: Interesting K3 QRM problem

In reply to this post by Vic Rosenthal

I'd go for a series tuned trap to ground. That would seem to the least
disruptive, if that's the right word, to your system.

I remember working at Vectrol in Rockville MD fairly close to an AM transmit
antenna site. We designed & built big three phase SCR controllers and of
necessity, had to isolate any oscilloscope's chassis ground. You could
always see the low level AM signal on the scope. A similar trap worked, but
it would have required dozens of them to cover all the scopes etc..
Luckily, when you're looking a 60Hz waveforms, we just ignored the BCI.

73, Charlie k3ICH

----- Original Message -----
From: "Vic Rosenthal 4X6GP/K2VCO" <[hidden email]>
To: "Elecraft Reflector" <[hidden email]>
Sent: Thursday, October 23, 2014 6:58 AM
Subject: [Elecraft] Interesting K3 QRM problem

>I live about 3.3 miles from a 50 kW BC station on 1080 kHz. It is on 24
>hours a day, transmitting in Arabic, not that that matters.
>
> I have two antennas: an R8 vertical and a Pixel Loop receiving loop.
>
> With either antenna, I hear a wideband noise on my K3 that is synchronized
> with the program material of the BC station on the 14, 10 and 7 MHz bands.
>
> I first determined that it was not affected at all by the attenuator in
> the K3. So I assume that it is not caused by front-end overload in the K3.
>
> I noticed that if I listen on the Pixel Loop via the K3's RX ANT input the
> noise completely goes away when I unplug the R8 from the K3's ANT 1
> connector!
>
> I also noticed that the noise is reduced when I bypass my amplifier or my
> SWR/wattmeter, which are connected between the ANT 1 connector and the R8.
> Reduced, but not eliminated.
>
> I conclude that what is happening is that the BC RF is being picked up by
> the R8, and the diodes in the wattmeter and the amplifier and the K3
> finals are creating IMD products which I am hearing. The K3 seems to be
> the major contributor.
>
> I am thinking that maybe the IMD products are being re-radiated by the R8
> and picked up by the Pixel Loop.
>
> I tried one more experiment: I have a BC band filter which is used with
> the MFJ antenna analyzer when there are nearby BC signals. Sure enough,
> putting this in series with the K3 ANT 1 connector kills the noise. Of
> course I can't transmit through it.
>
> Has anyone had a similar problem? I am thinking that the solution is a
> highpass filter or trap at this spot. I don't operate on 160, so it
> shouldn't be too hard to make.
>
> --
> 73,
> Vic, 4X6GP/K2VCO
> Rehovot, Israel
> http://www.qsl.net/k2vco/
> ______________________________________________________________
> Elecraft mailing list
> Home: http://mailman.qth.net/mailman/listinfo/elecraft
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> Please help support this email list: http://www.qsl.net/donate.html
> Message delivered to [hidden email]

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Vic Rosenthal

Re: Interesting K3 QRM problem

Oh, I didn't think of a trap. Simpler than a filter. But I have already
designed a 4-pole HP filter that will knock out all of the BC band,
which in this location is probably a good idea. There are some very
powerful AM BC stations in the Middle East. If my filter doesn't work
I'll fall back to a trap.

On 23 Oct 2014 15:10, Charlie T, K3ICH wrote:

> I'd go for a series tuned trap to ground. That would seem to the least
> disruptive, if that's the right word, to your system.
>
> I remember working at Vectrol in Rockville MD fairly close to an AM
> transmit antenna site. We designed & built big three phase SCR
> controllers and of necessity, had to isolate any oscilloscope's chassis
> ground. You could always see the low level AM signal on the scope. A
> similar trap worked, but it would have required dozens of them to cover
> all the scopes etc.. Luckily, when you're looking a 60Hz waveforms, we
> just ignored the BCI.
>
> 73, Charlie k3ICH
>
>
> ----- Original Message ----- From: "Vic Rosenthal 4X6GP/K2VCO"
> <[hidden email]>
> To: "Elecraft Reflector" <[hidden email]>
> Sent: Thursday, October 23, 2014 6:58 AM
> Subject: [Elecraft] Interesting K3 QRM problem
>
>
>> I live about 3.3 miles from a 50 kW BC station on 1080 kHz. It is on
>> 24 hours a day, transmitting in Arabic, not that that matters.
>>
>> I have two antennas: an R8 vertical and a Pixel Loop receiving loop.
>>
>> With either antenna, I hear a wideband noise on my K3 that is
>> synchronized with the program material of the BC station on the 14, 10
>> and 7 MHz bands.
>>
>> I first determined that it was not affected at all by the attenuator
>> in the K3. So I assume that it is not caused by front-end overload in
>> the K3.
>>
>> I noticed that if I listen on the Pixel Loop via the K3's RX ANT input
>> the noise completely goes away when I unplug the R8 from the K3's ANT
>> 1 connector!
>>
>> I also noticed that the noise is reduced when I bypass my amplifier or
>> my SWR/wattmeter, which are connected between the ANT 1 connector and
>> the R8. Reduced, but not eliminated.
>>
>> I conclude that what is happening is that the BC RF is being picked up
>> by the R8, and the diodes in the wattmeter and the amplifier and the
>> K3 finals are creating IMD products which I am hearing. The K3 seems
>> to be the major contributor.
>>
>> I am thinking that maybe the IMD products are being re-radiated by the
>> R8 and picked up by the Pixel Loop.
>>
>> I tried one more experiment: I have a BC band filter which is used
>> with the MFJ antenna analyzer when there are nearby BC signals. Sure
>> enough, putting this in series with the K3 ANT 1 connector kills the
>> noise. Of course I can't transmit through it.
>>
>> Has anyone had a similar problem? I am thinking that the solution is a
>> highpass filter or trap at this spot. I don't operate on 160, so it
>> shouldn't be too hard to make.

--
73,
Vic, 4X6GP/K2VCO
Rehovot, Israel
http://www.qsl.net/k2vco/
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wayne burdick

Re: Interesting K3 QRM problem

Administrator

In reply to this post by Vic Rosenthal

Hi Vic,

We designed a high-power, high-pass filter that should solve this problem. You can transmit through it.

Your situation would be a good test of the filter, which we were considering offering as a product. Can we send you one of the prototypes?

73,
Wayne
N6KR

On Oct 23, 2014, at 3:58 AM, Vic Rosenthal 4X6GP/K2VCO <[hidden email]> wrote:

> I live about 3.3 miles from a 50 kW BC station on 1080 kHz. It is on 24 hours a day, transmitting in Arabic, not that that matters…..

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Al Gulseth-2

Re: Interesting K3 QRM problem

In reply to this post by Vic Rosenthal

There's an old adage about "if life hands you lemons, make lemonade." With
that kind of field strength (a rough estimate from a chart I found indicates
about 0.2v/m or so at your QTH) I'd think you could hook a wire to a tuned
circuit on the BC station's frequency and then rectify the RF from it to
charge a small battery and let them foot the bill for some of your energy
needs. You might even be able to run a K1 etc. on it. (My comment is based on
remembering plans for a simple "free power" radio in a mag some years ago.)

73, Al

> > ----- Original Message ----- From: "Vic Rosenthal 4X6GP/K2VCO"
> > <[hidden email]>
> > To: "Elecraft Reflector" <[hidden email]>
> > Sent: Thursday, October 23, 2014 6:58 AM
> > Subject: [Elecraft] Interesting K3 QRM problem
> >
> >> I live about 3.3 miles from a 50 kW BC station on 1080 kHz. It is on
> >> 24 hours a day, transmitting in Arabic, not that that matters.
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Jim Brown-10

Re: Interesting K3 QRM problem

In reply to this post by Vic Rosenthal

On 10/23/2014 3:58 AM, Vic Rosenthal 4X6GP/K2VCO wrote:
> I have a BC band filter which is used with the MFJ antenna analyzer
> when there are nearby BC signals. Sure enough, putting this in series
> with the K3 ANT 1 connector kills the noise. Of course I can't
> transmit through it.

Hi Vic,

A few suggestions. First, try the BC Band filter on the K3 RX loop. If
that works, work out a switching arrangement so that you can apply it to
either antenna. Second, build or buy a BCB filter that can handle the K3
output. Based on the frequency of the BCB transmitter, one of the lower
cost filters should work. I've measured the vintage ICE BCB filter, and
it's good for at least 40 dB on 1080.

Another thought is that this might be common mode on the coax, in which
case a serious ferrite choke tuned to 1080 might help. I'd start with at
least 20 turns of RG58 on a #31 2.4-in toroid, and I'd put it by the rig.

73, Jim K9YC
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Vic Rosenthal

Re: Interesting K3 QRM problem

In reply to this post by Al Gulseth-2

Oh, absolutely. I noticed that I can see the towers of the 50 kW station
from my roof!

On 23 Oct 2014 18:19, Al Gulseth wrote:

> There's an old adage about "if life hands you lemons, make lemonade." With
> that kind of field strength (a rough estimate from a chart I found indicates
> about 0.2v/m or so at your QTH) I'd think you could hook a wire to a tuned
> circuit on the BC station's frequency and then rectify the RF from it to
> charge a small battery and let them foot the bill for some of your energy
> needs. You might even be able to run a K1 etc. on it. (My comment is based on
> remembering plans for a simple "free power" radio in a mag some years ago.)
>
> 73, Al
>
>
>>> ----- Original Message ----- From: "Vic Rosenthal 4X6GP/K2VCO"
>>> <[hidden email]>
>>> To: "Elecraft Reflector" <[hidden email]>
>>> Sent: Thursday, October 23, 2014 6:58 AM
>>> Subject: [Elecraft] Interesting K3 QRM problem
>>>
>>>> I live about 3.3 miles from a 50 kW BC station on 1080 kHz. It is on
>>>> 24 hours a day, transmitting in Arabic, not that that matters.

Vic Rosenthal

Re: Interesting K3 QRM problem

In reply to this post by Jim Brown-10

It isn't necessary to filter the loop. What is happening is that BC RF
is coming into the ANT1 connector of the K3 from the vertical, where it
is mixing up with every other signal around, probably in the t/r switch
section. Then the IMD products (which are on numerous frequencies) are
getting re-radiated by the vertical, and they are picked up by the loop.

You can see this by hooking the loop to RX IN and listening. The noise
is only there when you plug the vertical into ANT1.

I made a 4-pole Butterworth filter that cuts off at 2.5 MHz using the
filter design program that came with the ARRL handbook, husky enough to
transmit through, and put it on the ANT1 connector. That did it -- no
more noise on either antenna.

If Wayne would like me to test the Elecraft filter I will be happy to do
so. It will be interesting to compare to my homemade one.

On 23 Oct 2014 20:09, Jim Brown wrote:

> On 10/23/2014 3:58 AM, Vic Rosenthal 4X6GP/K2VCO wrote:
>> I have a BC band filter which is used with the MFJ antenna analyzer
>> when there are nearby BC signals. Sure enough, putting this in series
>> with the K3 ANT 1 connector kills the noise. Of course I can't
>> transmit through it.
>
> Hi Vic,
>
> A few suggestions. First, try the BC Band filter on the K3 RX loop. If
> that works, work out a switching arrangement so that you can apply it to
> either antenna. Second, build or buy a BCB filter that can handle the K3
> output. Based on the frequency of the BCB transmitter, one of the lower
> cost filters should work. I've measured the vintage ICE BCB filter, and
> it's good for at least 40 dB on 1080.
>
> Another thought is that this might be common mode on the coax, in which
> case a serious ferrite choke tuned to 1080 might help. I'd start with at
> least 20 turns of RG58 on a #31 2.4-in toroid, and I'd put it by the rig.
>
> 73, Jim K9YC

alorona

Re: Interesting K3 QRM problem

In reply to this post by Vic Rosenthal

This doesn't apply in Vic's case here, but his situation reminds me of yet another benefit of using a link-coupled tuner with balanced line feeding a horizontal antenna in the center.

A balanced link-coupled tuner turns out to be a high-pass filter, so while you're tuning it up on a ham band, it's rejecting stuff in the AM broadcast band.

When tuned on 80 meters, I've measured the 1400 kHz rejection of my tuner at about 40 dB. Forty dB makes a 50 kW signal sound like a 5 W signal. I might be in bad shape if I didn't have this tuner.

Another good thing to remember when planning an antenna installation.

Al W6LX
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Al Gulseth-2

Re: Interesting K3 QRM problem

Pardon my ignorance here, but I thought a link coupled tuner would be more of
a bandpass than a high-pass filter. CLC T-network tuners (MFJ etc.) are the
ones usually associated with becoming high-pass networks if misadjusted.

73, Al

On Thu October 23 2014 12:55:15 pm Al Lorona wrote:

> This doesn't apply in Vic's case here, but his situation reminds me of yet
> another benefit of using a link-coupled tuner with balanced line feeding a
> horizontal antenna in the center.
>
> A balanced link-coupled tuner turns out to be a high-pass filter, so while
> you're tuning it up on a ham band, it's rejecting stuff in the AM broadcast
> band.
>
> When tuned on 80 meters, I've measured the 1400 kHz rejection of my tuner
> at about 40 dB. Forty dB makes a 50 kW signal sound like a 5 W signal. I
> might be in bad shape if I didn't have this tuner.
>
> Another good thing to remember when planning an antenna installation.
>
> Al W6LX
> ______________________________________________________________
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Jim Brown-10

Re: Interesting K3 QRM problem

In reply to this post by Vic Rosenthal

On 10/23/2014 10:36 AM, Vic Rosenthal 4X6GP/K2VCO wrote:
> It isn't necessary to filter the loop. What is happening is that BC RF
> is coming into the ANT1 connector of the K3 from the vertical, where
> it is mixing up with every other signal around, probably in the t/r
> switch section. Then the IMD products (which are on numerous
> frequencies) are getting re-radiated by the vertical, and they are
> picked up by the loop.

Maybe, and if you're right, my suggestion may not help. But you
misunderstood my suggestion. There is an insert point between the T/R
switch and the RX input. That's the RX loop I was talking about -- you
insert the filter there.

73, Jim K9YC
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Joe Subich, W4TV-4

Re: Interesting K3 QRM problem

> Maybe, and if you're right, my suggestion may not help. But you
> misunderstood my suggestion. There is an insert point between the
> T/R switch and the RX input. That's the RX loop I was talking about
> -- you insert the filter there.

The insert point is *after* the PIN diode T/R switches for both the HPA
(KPA3) and 10W LPA. Given Vic's description, it is likely the noise is
being generated in one or more of the PIN diodes in the T/R switching
and placing the highpass filter in the insert point will not resolve
the noise. Vic, you can bypass (disable) the KPA3 to see if the noise
generation is in the KPA3 T/R switch.

73,

... Joe, W4TV

On 2014-10-23 4:36 PM, Jim Brown wrote:

> On 10/23/2014 10:36 AM, Vic Rosenthal 4X6GP/K2VCO wrote:
>> It isn't necessary to filter the loop. What is happening is that BC RF
>> is coming into the ANT1 connector of the K3 from the vertical, where
>> it is mixing up with every other signal around, probably in the t/r
>> switch section. Then the IMD products (which are on numerous
>> frequencies) are getting re-radiated by the vertical, and they are
>> picked up by the loop.
>
> Maybe, and if you're right, my suggestion may not help. But you
> misunderstood my suggestion. There is an insert point between the T/R
> switch and the RX input. That's the RX loop I was talking about -- you
> insert the filter there.
>
> 73, Jim K9YC
> ______________________________________________________________
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>

Jim Brown-10

Re: Interesting K3 QRM problem

On 10/23/2014 1:52 PM, Joe Subich, W4TV wrote:
> Given Vic's description, it is likely the noise is
> being generated in one or more of the PIN diodes in the T/R switching
> and placing the highpass filter in the insert point will not resolve
> the noise.

I agree.

73, Jim
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Vic Rosenthal

Re: Interesting K3 QRM problem

Yes, the solution to the problem was a highpass filter between the
vertical antenna and the K3 ANT1 connector.

On 24 Oct 2014 00:34, Jim Brown wrote:
> On 10/23/2014 1:52 PM, Joe Subich, W4TV wrote:
>> Given Vic's description, it is likely the noise is
>> being generated in one or more of the PIN diodes in the T/R switching
>> and placing the highpass filter in the insert point will not resolve
>> the noise.
>
> I agree.
>
> 73, Jim

--
73,
Vic, 4X6GP/K2VCO
Rehovot, Israel
http://www.qsl.net/k2vco/
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alorona

Re: Interesting K3 QRM problem

In reply to this post by Al Gulseth-2

You're right, it will roll off on the high end, too, but the rolloff rate is usually much less steep especially if series-tuned. An analysis of my tuner shows that it would be only about 20 dB down at 30 MHz (when tuned on 40 meters) when parallel-tuned, and only about 10 dB down when series-tuned.

(A really versatile tuner allows you to switch the output capacitors between series and parallel. This allows it to match a very wide range of impedances, and I think you'd have to homebrew a tuner to be able to do this, unless one of the commercially made tuners can switch.)

Al W6LX

________________________________
From: Al Gulseth <[hidden email]>
To: [hidden email]; Al Lorona <[hidden email]>
Sent: Thursday, October 23, 2014 1:36 PM
Subject: Re: [Elecraft] Interesting K3 QRM problem

Pardon my ignorance here, but I thought a link coupled tuner would be more of
a bandpass than a high-pass filter. CLC T-network tuners (MFJ etc.) are the
ones usually associated with becoming high-pass networks if misadjusted.

73, Al

On Thu October 23 2014 12:55:15 pm Al Lorona wrote:
> This doesn't apply in Vic's case here, but his situation reminds me of yet
> another benefit of using a link-coupled tuner with balanced line feeding a
>
horizontal antenna in the center.

>
> A balanced link-coupled tuner turns out to be a high-pass filter, so while
> you're tuning it up on a ham band, it's rejecting stuff in the AM broadcast
> band.
>
> When tuned on 80 meters, I've measured the 1400 kHz rejection of my tuner
> at about 40 dB. Forty dB makes a 50 kW signal sound like a 5 W signal. I
> might be in bad shape if I didn't have this tuner.
>
> Another good thing to remember when planning an antenna installation.
>
> Al W6LX
> ______________________________________________________________
> Elecraft mailing list
> Home: http://mailman.qth.net/mailman/listinfo/elecraft
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Russ Tobolic

Fw: Interesting K3 QRM problem

In reply to this post by Joe Subich, W4TV-4

In this recent discussion here on K3 QRM from strong local broadcast stations, W4TV brought up the noise generated in the T/R switching diodes. In doing some experimentation with a bandpass flter in the loop between the RX antenna "out" and "in" on the KXV3A I have discovered something which has baffled me. I have a 2KW station on 1480 kc 1.5 miles north of me and a 20KW station on 1300 kc about 5 miles south of me. The 1300 station is directional north and I am right in the boresight. I am experiencing +30 to +40 dB over S9 intermod from these two stations on 1840 kc. An intermod calculator shows a 5th order intermod at 1840 from the two stations. I normally run QRP at 5W cw and what I have discovered is that when I crank up the power above 15W (activating the KPA3), the intermod drops to about +5 to +10 dB over S9. This behavior is very consistent when switching between low and high power.

Can anyone explain this or is something broke in my K3?

Thanks,
Russ, N3CO

On Thursday, October 23, 2014 4:52 PM, "Joe Subich, W4TV" <[hidden email]> wrote:

> Maybe, and if you're right, my suggestion may not help. But you
> misunderstood my suggestion. There is an insert point between the
> T/R switch and the RX input. That's the RX loop I was talking about
> -- you insert the filter there.

The insert point is *after* the PIN diode T/R switches for both the HPA
(KPA3) and 10W LPA. Given Vic's description, it is likely the noise is
being generated in one or more of the PIN diodes in the T/R switching
and placing the highpass filter in the insert point will not resolve
the noise. Vic, you can bypass (disable) the KPA3 to see if the noise
generation is in the KPA3 T/R switch.

73,

... Joe, W4TV
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Vic Rosenthal

Re: Fw: Interesting K3 QRM problem

The intermod is generated in the T/R switch, which is before the RX
IN/OUT circuitry, so putting a filter in the loop won't help. It must be
between the T/R switch and the antenna, which means it has to be able to
handle transmitter power.

There is a T/R switch in the KPA100 and one in the driver stage (which
is the final in the QRP K3). Possibly the two T/R switches behave
differently.

On 10 Nov 2014 18:17, Russ Tobolic wrote:

> In this recent discussion here on K3 QRM from strong local broadcast
> stations, W4TV brought up the noise generated in the T/R switching
> diodes. In doing some experimentation with a bandpass flter in the
> loop between the RX antenna "out" and "in" on the KXV3A I have
> discovered something which has baffled me. I have a 2KW station on
> 1480 kc 1.5 miles north of me and a 20KW station on 1300 kc about 5
> miles south of me. The 1300 station is directional north and I am
> right in the boresight. I am experiencing +30 to +40 dB over S9
> intermod from these two stations on 1840 kc. An intermod calculator
> shows a 5th order intermod at 1840 from the two stations. I
> normally run QRP at 5W cw and what I have discovered is that when I
> crank up the power above 15W (activating the KPA3), the intermod
> drops to about +5 to +10 dB over S9. This behavior is very
> consistent when switching between low and high power.
>
> Can anyone explain this or is something broke in my K3?
>
> Thanks, Russ, N3CO
>
>
>
>
>
> On Thursday, October 23, 2014 4:52 PM, "Joe Subich, W4TV"
> <[hidden email]> wrote:
>
>
>
>
>> Maybe, and if you're right, my suggestion may not help. But you
>> misunderstood my suggestion. There is an insert point between the
>> T/R switch and the RX input. That's the RX loop I was talking
>> about -- you insert the filter there.
>
> The insert point is *after* the PIN diode T/R switches for both the
> HPA (KPA3) and 10W LPA. Given Vic's description, it is likely the
> noise is being generated in one or more of the PIN diodes in the T/R
> switching and placing the highpass filter in the insert point will
> not resolve the noise. Vic, you can bypass (disable) the KPA3 to see
> if the noise generation is in the KPA3 T/R switch.
>
> 73,
>
> ... Joe, W4TV

alexandr.kobranov

Re: Fw: Interesting K3 QRM problem

In reply to this post by Russ Tobolic

Hi Russ,
the same problem was here with BC station abt 5km away. Only with KPA in the
chain.
What helps - increasing current in diode switch on KPA board - change of R25
to cca 65 Ohm and there are no IMD products as before.
Hope this can help in any similar case in very special configuration of
local BC station and non resonant ant used for higher bands.
(I had IMD problems from 14MHz up, with with LW ant, BC station on 1062kHz)

GL & 73!
Lexa, ok1dst

---------- Původní zpráva ----------
Od: Russ Tobolic <[hidden email]>
Komu: [hidden email] <[hidden email]>
Datum: 10. 11. 2014 17:17:51
Předmět: [Elecraft] Fw: Interesting K3 QRM problem

"In this recent discussion here on K3 QRM from strong local broadcast
stations, W4TV brought up the noise generated in the T/R switching diodes.
In doing some experimentation with a bandpass flter in the loop between the
RX antenna "out" and "in" on the KXV3A I have discovered something which has
baffled me. I have a 2KW station on 1480 kc 1.5 miles north of me and a 20KW
station on 1300 kc about 5 miles south of me. The 1300 station is
directional north and I am right in the boresight. I am experiencing +30 to
+40 dB over S9 intermod from these two stations on 1840 kc. An intermod
calculator shows a 5th order intermod at 1840 from the two stations. I
normally run QRP at 5W cw and what I have discovered is that when I crank up
the power above 15W (activating the KPA3), the intermod drops to about +5 to
+10 dB over S9. This behavior is very consistent when switching between low
and high power.

Can anyone explain this or is something broke in my K3?

Thanks,
Russ, N3CO

On Thursday, October 23, 2014 4:52 PM, "Joe Subich, W4TV" <[hidden email]>
wrote:

> Maybe, and if you're right, my suggestion may not help. But you
> misunderstood my suggestion. There is an insert point between the
> T/R switch and the RX input. That's the RX loop I was talking about
> -- you insert the filter there.

The insert point is *after* the PIN diode T/R switches for both the HPA
(KPA3) and 10W LPA. Given Vic's description, it is likely the noise is
being generated in one or more of the PIN diodes in the T/R switching
and placing the highpass filter in the insert point will not resolve
the noise. Vic, you can bypass (disable) the KPA3 to see if the noise
generation is in the KPA3 T/R switch.

73,

... Joe, W4TV
______________________________________________________________
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
Message delivered to [hidden email]"
______________________________________________________________
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Home: http://mailman.qth.net/mailman/listinfo/elecraft
Help: http://mailman.qth.net/mmfaq.htm
Post: mailto:[hidden email]

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Please help support this email list: http://www.qsl.net/donate.html
Message delivered to [hidden email]

Joe Subich, W4TV-4

Re: Fw: Interesting K3 QRM problem

> What helps - increasing current in diode switch on KPA board -
> change of R25 to cca 65 Ohm and there are no IMD products as before.

This would tend to explain why Russ sees a more of a problem at low
power than at high power. Current through the receive diodes in the
KPA3 is about three times as much as those on the RF board (13/200 vs.
7/320).

One would need to look carefully at the specifications of the PIN
diodes and the FETs being used for switching. However, the resistors
to "tweak" would be R58 on the mainboard and R25 (as Lexa mentions)
on the KPA3. The idea would be to keep the diodes turned on hard
enough to prevent them from turning off on RF peaks.

73,

... Joe, W4TV

On 2014-11-11 5:03 AM, [hidden email] wrote:

> Hi Russ,
> the same problem was here with BC station abt 5km away. Only with KPA in the
> chain.
> What helps - increasing current in diode switch on KPA board - change of R25
> to cca 65 Ohm and there are no IMD products as before.
> Hope this can help in any similar case in very special configuration of
> local BC station and non resonant ant used for higher bands.
> (I had IMD problems from 14MHz up, with with LW ant, BC station on 1062kHz)
>
> GL & 73!
> Lexa, ok1dst
>
>
> ---------- Původní zpráva ----------
> Od: Russ Tobolic <[hidden email]>
> Komu: [hidden email] <[hidden email]>
> Datum: 10. 11. 2014 17:17:51
> Předmět: [Elecraft] Fw: Interesting K3 QRM problem
>
> "In this recent discussion here on K3 QRM from strong local broadcast
> stations, W4TV brought up the noise generated in the T/R switching diodes.
> In doing some experimentation with a bandpass flter in the loop between the
> RX antenna "out" and "in" on the KXV3A I have discovered something which has
> baffled me. I have a 2KW station on 1480 kc 1.5 miles north of me and a 20KW
> station on 1300 kc about 5 miles south of me. The 1300 station is
> directional north and I am right in the boresight. I am experiencing +30 to
> +40 dB over S9 intermod from these two stations on 1840 kc. An intermod
> calculator shows a 5th order intermod at 1840 from the two stations. I
> normally run QRP at 5W cw and what I have discovered is that when I crank up
> the power above 15W (activating the KPA3), the intermod drops to about +5 to
> +10 dB over S9. This behavior is very consistent when switching between low
> and high power.
>
> Can anyone explain this or is something broke in my K3?
>
> Thanks,
> Russ, N3CO
>
>
>
>
>
> On Thursday, October 23, 2014 4:52 PM, "Joe Subich, W4TV" <[hidden email]>
> wrote:
>
>
>
>
>> Maybe, and if you're right, my suggestion may not help. But you
>> misunderstood my suggestion. There is an insert point between the
>> T/R switch and the RX input. That's the RX loop I was talking about
>> -- you insert the filter there.
>
> The insert point is *after* the PIN diode T/R switches for both the HPA
> (KPA3) and 10W LPA. Given Vic's description, it is likely the noise is
> being generated in one or more of the PIN diodes in the T/R switching
> and placing the highpass filter in the insert point will not resolve
> the noise. Vic, you can bypass (disable) the KPA3 to see if the noise
> generation is in the KPA3 T/R switch.
>
> 73,
>
> ... Joe, W4TV
> ______________________________________________________________
> 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
> Message delivered to [hidden email]"
> ______________________________________________________________
> 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
> Message delivered to [hidden email]
>