K3/100 RF Feedback/Spurious Oscillation Verified Via RX Input

[Scott Prather-2]

When I read about this report from Stewart last night (as well as the
responses indicating that other had experienced a similar effect) I decided
to set up the bench test to see if I could emulate the problem. I've since
discovered that these reports are valid, the K3 PA will generate a severe
spectral "comb" if there's insufficient isolation between the RX and TX
antennas when the KXV3 RX input is used.

To execute this test, I ran the K3 into a dummy load through an RF coupler
that displays 26 dB of isolation. The output of this coupler was split (3 dB
splitting loss) between a Tektronix 496 spectrum analyzer and a Tektronix
2701 50-ohm step attenuator. The output of the 2701 was fed into the K3's RX
input. Initially, the attenuator was set to 0 dB.

The K3 was keyed at full output (100W or +50 dBm), which results in a +21
dBm signal back into the K3's RX input. Under this condition, if the RX port
is enabled, the K3 will generate a spectral comb with the most severe spurs
only 10 to 20 dB down from the carrier. Turning the RX input off clears the
problem. I was able to create this comb spectrum on 160, 80, and 40 meters,
with 40 displaying the highest-power spurs.

Lowering the input power to the K3 via the 2701 attenuator indicates that
the "sweet spot" for generating this spurious oscillation occurs when
there's an input signal of about +20 dB. This would correspond to a minimum
of 30 dB of TX antenna to RX antenna isolation when operating at 100W output
power or 40 dB of isolation at 1 kW. I have spectrum analyzer photos of the
spurious signals generated by the K3 under these conditions if anyone is
interested.

I ran the same test under the same conditions with my K2/100, and this
problem does not occur.

The bottom line here is that a +20 dBm signal (100 mw) will create severe
spurs on the K3 but not the K2 when operating in the 40M band and below.
Above 40M I was unable to duplicate the problem, perhaps a higher power to
the RX input is required.

It's important to note that anyone who has +20 dBm or more coming back on
the receive antenna is asking for trouble in the first place, and additional
isolation and/or a shorting relay during TX operation is probably a good
idea.

>From the reports, it sounds like there have been a few folks who had rather
high power coming back in through the RX input for a while and didn't notice
it until this secondary symptom was noted in the K3. I haven't looked at the
RX input circuit in the K2 to see how it differs from the K3 so that I can
find the root-cause of this problem.

Scott
N7NB

K2 #1285
K3 #219


------------------------------

Message: 34
Date: Thu, 31 Jan 2008 16:14:13 GMT
From: Stewart Baker <[hidden email]>
Subject: [Elecraft] K3/100 RF Feedback/Spurious oscillations ?
To: Elecraft Reflector <[hidden email]>
Message-ID: <2008131161413.171620@shack>
Content-Type: text/plain; charset="iso-8859-1"

Very strange effect noticed today when tuning up on 20m.

At power levels above 20 w on 14.175 my external Palstar SWR meter
would suddenly leap from 1:1 to above 5:1. Also the K3 SWR display
showed similar. High current was also reported. No problems with
either a TS850S or K2, both OK up to 100W.

Switching the RX Ant switch to the main antenna or removing the RX
Antenna  BNC lead cured the problem.

I use the RX antenna input for 2 x Pennant loops for the LF bands,
but find that they sometimes give a better SNR than the main
antenna on higher bands. The main antenna system is a doublet fed
with open wire feeders into a fully balanced (no toroid on output)
ATU.

My initial thoughts is that there might be some spurious output
generated by an obscure feedback path from the RX antenna input.
If this output were on a frequency well separated from the desired
then it could show up as as high SWR.

What think Ye ?

73
Stewart G3RXQ




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[Stewart Baker]

Good investigation Scott, ties in with my findings.
Protem I have installed a relay to short the loop pre-amp output
on TX.

73
Stewart G3RXQ
On Sat, 2 Feb 2008 18:22:57 -0800, Scott Prather wrote:
>
> When I read about this report from Stewart last night (as well
as the
> responses indicating that other had experienced a similar
effect) I decided
> to set up the bench test to see if I could emulate the problem.
I've since
> discovered that these reports are valid, the K3 PA will generate
a severe
> spectral "comb" if there's insufficient isolation between the RX
and TX
> antennas when the KXV3 RX input is used.
>
> To execute this test, I ran the K3 into a dummy load through an
RF coupler
> that displays 26 dB of isolation. The output of this coupler was
split (3 dB
> splitting loss) between a Tektronix 496 spectrum analyzer and a
Tektronix
> 2701 50-ohm step attenuator. The output of the 2701 was fed into
the K3's RX
> input. Initially, the attenuator was set to 0 dB.
>
> The K3 was keyed at full output (100W or +50 dBm), which results
in a +21
> dBm signal back into the K3's RX input. Under this condition, if
the RX port
> is enabled, the K3 will generate a spectral comb with the most
severe spurs
> only 10 to 20 dB down from the carrier. Turning the RX input off
clears the
> problem. I was able to create this comb spectrum on 160, 80, and
40 meters,
> with 40 displaying the highest-power spurs.
>
> Lowering the input power to the K3 via the 2701 attenuator
indicates that
> the "sweet spot" for generating this spurious oscillation occurs
when
> there's an input signal of about +20 dB. This would correspond
to a minimum
> of 30 dB of TX antenna to RX antenna isolation when operating at
100W output
> power or 40 dB of isolation at 1 kW. I have spectrum analyzer
photos of the
> spurious signals generated by the K3 under these conditions if
anyone is
> interested.
>
> I ran the same test under the same conditions with my K2/100,
and this
> problem does not occur.
>
> The bottom line here is that a +20 dBm signal (100 mw) will
create severe
> spurs on the K3 but not the K2 when operating in the 40M band
and below.
> Above 40M I was unable to duplicate the problem, perhaps a
higher power to
> the RX input is required.
>
> It's important to note that anyone who has +20 dBm or more
coming back on
> the receive antenna is asking for trouble in the first place,
and additional
> isolation and/or a shorting relay during TX operation is
probably a good
> idea.
>
>> From the reports, it sounds like there have been a few folks
who had rather
> high power coming back in through the RX input for a while and
didn't notice
> it until this secondary symptom was noted in the K3. I haven't
looked at the
> RX input circuit in the K2 to see how it differs from the K3 so
that I can

> find the root-cause of this problem.
>
> Scott
> N7NB
>
> K2 #1285
> K3 #219
>
>
> ------------------------------
>
> Message: 34
> Date: Thu, 31 Jan 2008 16:14:13 GMT
> From: Stewart Baker <[hidden email]>
> Subject: [Elecraft] K3/100 RF Feedback/Spurious oscillations ?
> To: Elecraft Reflector <[hidden email]>
> Message-ID: <2008131161413.171620@shack>
> Content-Type: text/plain; charset="iso-8859-1"
>
> Very strange effect noticed today when tuning up on 20m.
>
> At power levels above 20 w on 14.175 my external Palstar SWR

meter
> would suddenly leap from 1:1 to above 5:1. Also the K3 SWR
display
> showed similar. High current was also reported. No problems with
> either a TS850S or K2, both OK up to 100W.
>
> Switching the RX Ant switch to the main antenna or removing the
RX
> Antenna  BNC lead cured the problem.
>
> I use the RX antenna input for 2 x Pennant loops for the LF
bands,
> but find that they sometimes give a better SNR than the main
> antenna on higher bands. The main antenna system is a doublet
fed
> with open wire feeders into a fully balanced (no toroid on
output)
> ATU.
>
> My initial thoughts is that there might be some spurious output
> generated by an obscure feedback path from the RX antenna input.
> If this output were on a frequency well separated from the
desired

> then it could show up as as high SWR.
>
> What think Ye ?
>
> 73
> Stewart G3RXQ
>
>
> _______________________________________________
> Elecraft mailing list
> Post to: [hidden email]
> You must be a subscriber to post to the list.
> Subscriber Info (Addr. Change, sub, unsub etc.):
> http://mailman.qth.net/mailman/listinfo/elecraft
>
> Help: http://mailman.qth.net/subscribers.htm
> Elecraft web page: http://www.elecraft.com

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[Barry N1EU]

I believe this phenomenon was responsible for my Acom 2000A generating arc fault errors if I ran >500W on 160M with rx connected to a Beverage that's about 100ft away in its general direction.

It's still early to say, but the problem seems to have been cured by adding an ICE 196 RF Limiter in the Beverage feedline.

73,
Barry N1EU