[Elecraft] FW: K2 Frequency Shift On 80 Metres

Tue May 3 14:37:27 EDT 2011

Ray, when you raised it you made the coax run longer. Where did the extra
coax come from? Did you have some extra coiled up, perhaps? 

It sounds like you have changed the length of the coax enough to cause a
voltage 'loop' (peak voltage point) along the standing waves on the outside
of the coax shield to occur at the rig. The balun at the feed point will not
interrupt the currents on the outside of the coax. That RF is induced on the
outside of the shield by the RF field around the radiator. (The only
exception is when you feed a balanced - i.e. center fed - radiator and run
the coax away from that radiator at exactly right angles. In that case there
are still currents induced onto the outside of the shield, but - in theory -
they are equal and opposite from opposite sides of the radiator and so they
cancel each other. In practice it's seldom that neat ;-)

Even if you simply uncoiled or rerouted some coax without adding any for the
extra height you shifted the standing waves on the outside of the coax
shield. 

If you have more coax, try adding a few feet to the feed line, coiled up,
and see if that fixes the issue. 

Another approach is to put ferrite beads around the coax to interrupt the
currents - shortening the electrical length of the coax on the outside while
leaving the inside where the RF you want is flowing unchanged. 

Ron AC7AC

-----Original Message-----
From: Ray Spreadbury [mailto:ray2.s at btinternet.com] 
Sent: 03 May 2011 10:25
To: 'don at w3fpr.com'
Cc: 'ho13dave at gmail.com'
Subject: RE: [Elecraft] K2 Frequency Shift On 80 Metres

Hi Don & Dave

Thanks for the advice re RF in the shack.
I tried it with a dummy load on 80 metres & the problem disappeared (!!!!),
so good call both of you -I wasn't looking forward to trying to find a
problem in the rig & ordering replacement parts from Elecraft in USA!

The antenna is currently fed with RG213 cable and there is a 4:1 balun at
the feed point so as the impedance at the dipole is supposed to be about 300
ohms I thought it would be a reasonable match for the 50ohms cable?

Anyway the antenna was OK until yesterday, when I raised it a bit, but
obviously changed something to now cause the rf in the shack.

I might put up a balanced feeder to the OCFD or have a dedicated 80 metre
dipole instead, what do you think or do you have other suggestions ?

73 & many thanks
Ray, G3XLG

-----Original Message-----
From: Don Wilhelm [mailto:w3fpr at embarqmail.com] 
Sent: 02 May 2011 23:08
To: Ray Spreadbury
Cc: elecraft at mailman.qth.net
Subject: Re: [Elecraft] K2 Frequency Shift On 80 Metres

  Ray,

Before you do anything desperate, try the same thing into a dummy load.  
If it does not happen with the dummy load, you can conclude that it is 
RF getting into the K2 somehow.

Off-center fed antennas are notorious for creating RF in the shack 
unless you take drastic steps to suppress it.  The proper place to do 
that is within the antenna system - maybe adding a balun at a point 
outside the house where the feedline changes from balanced to unbalanced.

73,
Don W3FPR

On 5/2/2011 5:21 PM, Ray Spreadbury wrote:
> Hi Elecrafters
>
> I have recently bought a second-hand K2 with the KSB2 SSB module, the
KPA100
> 100 watt amplifier&  the KAT100-1 antenna tuner. Everything appears fine
> running either QRP or 90+ watts on all bands except 80 metres. I am using
an
> off centre fed dipole (133') and the KAT100-1 antenna tuner to match.
>
> Running in excess of 10 watts or so on 80 metres however when I switch the
> microphone, the tuning apparently shifts to another frequency on the band.
> The shift may be a few Hz or even a few 100Hz. The receiver is then on
that
> new frequency. Reducing power back to a QRP level, the transmitter&
> receiver behave normally.
>
> I guess there's something wrong with the 80 metre sections of the KPA100
> and/or the KAT100-1.
>
> Any clues of what I should check please?
>
> 73
>
> Ray, G3XLG
>
>
>
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