[Elecraft] two k3's on the same band

[Fred Jensen]

On 3/21/2012 12:24 PM, Matt Murphy wrote:
> Just wondering if anyone has any experience using two K3's on the same band
> (for example, one for running, one for mults in a multi-op single
> transmitter scenario).

Yes, accidentally at N6A in the 2011 Cal QSO Party.  There was no 
lockout mechanism, it was late at night, it was the first time we'd ever 
done an M/2 operation, and we probably should have known better, we had 
over 200 years of combined ham experience aboard.

Is there any danger of damaging the receiver(s) by
> transmitting with high power so close by (different antenna, same
> property).

Yes, we fried the PIN diodes in the K3 front end.  The other rig was an 
ICOM Pro and the K3 was never keyed so it survived.

   Is there a difference if the transmission is a few KHz away vs
> (accidentally) on the same frequency?

Depends.  The critical issue is whether or not the antennas are in each 
others near field.  If they are, they become coupled elements just as 
parasitic elements in a beam are coupled to the driven element.  Coupled 
power in antenna 2 from same-band antenna 1 can then be large enough to 
fry the input circuits.

The size of an antenna's near field is roughly, kind of proportional to 
the gross size of the antenna.  Thus, lower frequency antennas, being 
larger, have a larger near field than higher frequency antennas.  The 
two 80/75 meter antennas [Inverted V's] at N6A were definitely in each 
other's near field, something that won't happen this year. :-)

It doesn't matter where the two radios are tuned in the band.  The 
damage happens at the very front end, often in the diodes of the TR 
switch or the first RF amp device.

If the antennas are definitely NOT in each other's near fields, you 
probably won't sustain any physical damage.  Whether or not the radios 
can be used will then depend on their dynamic blocking and IMD 
characteristics.  Jack, KF6T, is about 2 km from my station and runs 
legal limit.  With a Kenwood TS-850, I could not operate on the same 
band with him, I heard his keying all over the band at 30 over, 
generated in my receiver.  With my K2, I can get within 3 or 4 KHz 
before I begin to hear products.  With my K3, I can get within a KHz or 
less.
>
> Just curious what sorts of precautions (if any) ought to be taken and what
> solutions exist (if necessary).

1.  Make certain the two antennas are definitely NOT in each others near 
fields.  When BPL was a real possibility, I modeled the power lines near 
us and my antenna with EZNEC4.  It indicated a coupling factor [the loss 
between my power and what would show up in the power line] of about -35 
to-38 dB on all bands except 160.  There, it was -15 dB, because at 160, 
the antenna and the distribution lines were in each others near fields.

2.  Run K3's.

73,

Fred K6DGW
- Northern California Contest Club
- CU in the 2012 Cal QSO Party 6-7 Oct 2012
- www.cqp.org