[Elecraft] [K2/KX3] Tuning a magloop

[Ron D'Eau Claire]

I was speaking of small transmitting loops in general, not a particular
design. 

A small loop, like any other antenna, can be designed for any impedance feed
line. If a given loop happens to present a 100 ohm impedance at the feed
point, 50 ohm coax will see a 2:1 SWR. That will result in some oddball
impedance at the transmitter, depending upon the electrical length of the
feed line. If the transmitter output network (e.g. ATU) can match that
impedance, all is good providing you are happy with whatever losses occur in
the feed line.

As I said, it's a balancing act. Messing with the termination at the
transmitter end of the coax to match the impedance presented will shift the
resonant frequency of the loop; the adjustments are interactive. 

If your loop is designed so that it always presents a 50 ohm termination to
a 50 ohm feed line at all frequencies you need not adjust the rig's output
network. 

None of the homebrew loops I've built maintained that match across the
various bands. 

73, Ron AC7AC



-----Original Message-----
From: Elecraft [mailto:elecraft-bounces at mailman.qth.net] On Behalf Of Fred
Jensen
Sent: Friday, December 4, 2015 2:00 PM
To: elecraft at mailman.qth.net
Subject: Re: [Elecraft] [K2/KX3] Tuning a magloop

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