[Elecraft] [K2/KX3] Tuning a magloop

[Fred Jensen]

On 12/4/2015 1:09 PM, Ron D'Eau Claire wrote:

> IMX I tune the loop for maximum noise in the receiver. Properly
> designed small loops have very high Q, so the peak is very "sharp".

Yes.  While the RX noise peak *is* quite narrow, especially on 40 and 
30, it is still quite a bit broader than the resonance point.  I've 
occasionally gotten the look to resonance with the RX noise peak, but 
it's pure luck.
>
> If I then transmit and find that the SWR is high, I'm sure the
> coupling between the loop and the feed line is not correct. That is,
> the loop at resonance is not presenting a 50 ohm impedance to the
> feed line.

Ummm ... not my experience with either the Alex or MFJ, nor does that 
match theory.  The Alex is designed to present 50 ohms *exactly at 
resonance*, nothing else is required. Also true for MFJ.  Once I've 
peaked the noise, I put the K2 into TUNE and adjust for an SWR of 1.0:1. 
  It is much narrower an adjustment than the RX noise peak.  No ATU, ever.

> If you are not worried about feed line losses, I would expect the
> loop to work just fine using the ATU at the rig to compensate for the
> odd impedance presented to it by the feed line, but be sure to check
> the loop tuning after the ATU does its job since it will interact
> with the loop and shift its resonant frequency somewhat.

Uhhh ... no.  No ATU, ever.  Everything is done with the little knob on 
the loop while watching the SWR.  Conveniently, my K2 displays the SWR. 
  No ATU, ever.

This is an amazingly simple antenna to adjust, I'm a little surprised at 
the length of the thread.  Many years ago [OK, decades ago :-)], I built 
a little analog phase detector from a QST, CQ, or 73 article.  It 
powered a small DC motor connected to a variable cap that drove the 
reactance on the 75 m antenna on Dad's car to zero.  Worked real good. 
Not sure why it wouldn't work with the loop since "resonance" = zero 
reactance.

73,

Fred K6DGW
- Northern California Contest Club
- CU in the Cal QSO Party 1-2 Oct 2016
- www.cqp.org