Posted by
Stewart Baker on
Feb 03, 2008; 9:48am
URL: http://elecraft.85.s1.nabble.com/K3-100-RF-Feedback-Spurious-Oscillation-Verified-Via-RX-Input-tp459688p459690.html
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
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