[Elecraft] KX3: nearby noise post-mortem

[Ralf Wilhelm]

You have written earlier that you could pull the coax plug "half out" (disconnecting the shield) and that this also helped. So maybe you have common mode current on the coax  which would also increase the coupling to the wiring (because not only the antenna itself but also the coax would emit the local oscillator). Since this could also  increase the risk of r.f.i. when you transmit I would recommend that you add a common mode current choke (a few windings of the coax?) close to the rig - and a second one near the antenna perhaps... 

Greetings

Ralf, DL6OAP

Am 10.02.2014 um 18:49 schrieb Nicklas Johnson <nick at n6ol.us>:

> Makes sense.  And that's why the RX SHFT also works, because then the reflected signal is shifted away from the RX frequency by the same amount as the shift.  That was a great explanation, and I thank you for it!
> 
> Another solution in my case, then, might be to just move the antenna further away from the house so the local oscillator's signal isn't mixed with and reflected by the house wiring (and/or whatever may be plugged into it).
> 
> I noticed that I didn't have the same problem with the inverted-V dipole up on the roof, I imagine in part because it's so much further away from the house wiring (and the house).
> 
>    Nick
> 
> 
> On 10 February 2014 09:40, Ralf Wilhelm <ralf at super-deutschland.net> wrote:
> The problem with all receivers using a local oscillator (direct conversion or superhet) is that they emit a small part of the local oscillator power via the mixer to the antenna port (maybe a typical number of L.O. to R.F. Port Isolation of a mixer is 40 dB or something like that). In a superhet, the local oscillator's frequency is far away from the rx frequency and the front end filtering keeps the oscillator signal from leaving the RX. 
> In a direct conversion RX, the oscillator is on the same frequency as the RX. So basically, you have a very little transmitter that is always exactly on your rx frequency. In principle, this signal can be reflected by some structure and get back into the receiver (which would cause a d.c. offset) or in your (and my) case, it can be "absorbed" by the wiring, modulated by the 60/120 Hz in one of the power supplies and than be re-emitted by the wiring before being received via the antenna. In this case, you receive a carrier plus 60 Hz sidebands, but the carrier is at zero beat so you only hear the sidebands. 
> 
> The additional isolation (the oscillator signal "tries" to pass the amp in the "wrong direction") of the preamp makes the emitted fraction of the local oscillator power maybe 20 dB weaker so that is buried in the atmospheric noise.
> 
> -- 
> N6OL
> Saying something doesn't make it true.  Belief in something doesn't make it real. And if you have to lie to support a position, that position is not worth supporting.