K2/100 AM (MW) Broadcast Intermods on 80m

[Steve Kavanagh]

I just had my first opportunity to use the K2/100 on a
fairly large antenna for 80m (inverted L with total
wire length around 150 ft., through a low-pass type of
antenna tuner). I noticed a few fairly strong
intermodulation products from AM (medium wave)
broadcast stations on and around 80m.  There are
several 50 kW stations within 10 or 20 km, but none of
them are really close.  For example, I found the
mixing product of 2 times 1310 kHz and 1150 kHz when
tuned to 3770 kHz (meter reads S9 with attenuator and
preamp off) and the mixing product of 2 times 1310 kHz
and 580 kHz at 3200 kHz.  

These signals were not present on a Drake TR-7
connected to the same antenna/tuner.  They also were
virtually unaffected by switching the KPA100 in or out
(either with power setting above or below 10 W or
using the PA ON/OFF menu settings).  They appeared to
be affected by the attenuator/preamp settings in the
same way as "real" 80m signals, which seems to
indicate that they were generated in the T-R switch.

Has anyone else experienced this (i.e. is it normal ?)
or should I be looking for problems in or near the K2
T-R switching circuitry  ?

73,
Steve VE3SMA


       

       
               
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[Don Wilhelm-3]

Steve,

Since these are likely products of the K2 mixing scheme rather than 'real
signals', comparison with another receiver that does not use the same mixing
scheme as the K2 will not necessarily produce the same results - but your
comparison with the Drake should confirm that they are indeed mixing
products.

I would consider having several 50kW stations within 10 to 20 kilometers
within the range of quite close!!!  I wonder just how many volts of
broadcast band energy you have on that antenna - an inverted L would likely
pick up more of that energy than a horizontal antenna.  In the area South of
my current location we have an AM broadcast station on 680 kHz that is
capable of putting a volt of 2 of RF energy onto telephone lines as far as 7
miles away from their antenna field (yes, I used to live there), so having a
lot of RF on an antenna that is somewhere around a 3/4 wavelength long at
the AM broadcast band center does not seem unusual to me at all.

You may want to try adding a high pass filter on the front end of the K2
with a cutoff in the 1700 kHz range.  The bandpass filter of the K2 is
intentionally broad and just may not be filteing out enough of the energy,
or as you have surmised, if the broadcast band energy is high enough, it
could even be demodulated in the T-R switch and mixed with another signal.
To my mind, adding a high pass filter on the antenna input would be the
easiest solution since this may not be the only mixing product that you
might encounter.  A properly designed high pass filter could be left in-line
all the time and have no adverse effects (conversely, a poorly designed one
may cause significant attenuation) - I recall that either QEX or QST has had
published some BC band filters designed by Zack Lau that I would consider
good designs, but then you could design your own if you are so inclined -
size the components for your transmit RF level.

73,
Don W3FPR

> -----Original Message-----
>
> I just had my first opportunity to use the K2/100 on a
> fairly large antenna for 80m (inverted L with total
> wire length around 150 ft., through a low-pass type of
> antenna tuner). I noticed a few fairly strong
> intermodulation products from AM (medium wave)
> broadcast stations on and around 80m.  There are
> several 50 kW stations within 10 or 20 km, but none of
> them are really close.  For example, I found the
> mixing product of 2 times 1310 kHz and 1150 kHz when
> tuned to 3770 kHz (meter reads S9 with attenuator and
> preamp off) and the mixing product of 2 times 1310 kHz
> and 580 kHz at 3200 kHz.
>
> These signals were not present on a Drake TR-7
> connected to the same antenna/tuner.  They also were
> virtually unaffected by switching the KPA100 in or out
> (either with power setting above or below 10 W or
> using the PA ON/OFF menu settings).  They appeared to
> be affected by the attenuator/preamp settings in the
> same way as "real" 80m signals, which seems to
> indicate that they were generated in the T-R switch.
>
> Has anyone else experienced this (i.e. is it normal ?)
> or should I be looking for problems in or near the K2
> T-R switching circuitry  ?
>
> 73,
> Steve VE3SMA
>
> No virus found in this incoming message.
> Checked by AVG Free Edition.
> Version: 7.1.371 / Virus Database: 267.14.7/214 - Release Date: 12/23/2005
>
>

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[Vic K2VCO]

Steve Kavanagh wrote:

> Has anyone else experienced this (i.e. is it normal ?)
> or should I be looking for problems in or near the K2
> T-R switching circuitry  ?

If you really can't hear them on a different receiver, check the bias
voltage on the t/r switch.  It should be between 90 and 150v.  Low
voltage here can cause this problem.  The t/r switch is used whether or
not the KPA100 is activated.
--
73,
Vic, K2VCO
Fresno CA
http://www.qsl.net/k2vco
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[Sverre Holm-2]

 

> -----Original Message-----
> If you really can't hear them on a different receiver, check
> the bias voltage on the t/r switch.  It should be between 90
> and 150v.  Low voltage here can cause this problem.  The t/r
> switch is used whether or not the KPA100 is activated.
> --

Another test you may consider to do is to remove the KPA100 and test for
intermod with the barefoot K2. That will also tell you if the t/r switch in
the KPA100 is the source of the problem.


73

Sverre
LA3ZA
http://www.qslnet.de/la3za/
 

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[Steve Kavanagh]

As far as I can see from the manual, the KPA100 T-R
switch is bypassed in QRP mode.  As the intermods
occur in both QRP and high power modes the KPA100 T-R
switch can not be the only culprit. With more careful
observations I see that there are fairly small changes
in the S-meter readings on the intermod products (on
the order of one S-unit) when I switch between QRP and
high-power modes, so the high power T-R circuitry may
not be completely off the hook either.

As a matter of interest, I measured the composite
input voltage from the antenna with an oscilloscope
today; it is around 1.5 volts peak-to-peak.  Not an
insignificant signal environment to deal with !

73,
Steve VE3SMA


--- Vic K2VCO <[hidden email]> wrote:


> The t/r switch is used whether or
> not the KPA100 is activated.



       

       
               
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[Vic K2VCO]

Steve Kavanagh wrote:
> As far as I can see from the manual, the KPA100 T-R
> switch is bypassed in QRP mode.

No, that is incorrect.  If the KPA100 is installed, the t/r switch is
active.  Note that the bias oscillator (what I fondly refer to as the
'birdie generator') is fed from the 12CTL line in the ribbon cable.

If you really want to test this, replace the KPA100 with the low-power
lid and see what happens.
--
73,
Vic, K2VCO
Fresno CA
http://www.qsl.net/k2vco
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