[Lowfer] RE: Worthwhile going to Shielded loop?

[Stewart Nelson]

Hi Alberto and all,

Several lowfers have the tuning capacitor in the shack,
and it works quite well.

However, for some applications, there are problems with high-Q loops.

There are, of course, two big advantages in tuning a loop:

1. More signal power is delivered to the preamp, so amplifier noise
   is not an issue.

2. Strong interference sources, e.g. AM broadcast signals, are
   greatly attenuated, minimizing intermod problems.

But there are some disadvantages:

1. The narrow bandwidth reduces the effectiveness of most noise
   blanking techniques.  This includes the hardware blankers typically
   found in receivers, as well as software clipping and gating schemes.

2. It is more difficult to simultaneously capture multiple signals
   with e.g. ARGO, when they are spread across a kilohertz or more.

3. If a transmitted signal is synchronized to GPS or another standard,
   or sourced from an atomic oscillator, you can measure phase changes
   to see propagation delay variations.  But if you have, say, a Q of
   300, then a loop inductance change of only one part in 10000, caused
   by temperature variation, will shift received phase several degrees.

4. If you have two loops at right angles, either for direction finding
   purposes, or to allow remote nulling of an unwanted signal, then
   phase shifts can also be problematic.

IMO, if your loop is big enough, advantage 1 may be unimportant.
You can use a passive LPF between loop and preamp to eliminate
intermod from MF, etc.

A possible compromise is to add some resistance to the tuned circuit,
so the resulting Q is modest, perhaps 10 or 20.

73,

Stewart KK7KA

----- Original Message ----- 
From: "Alberto di Bene" <dibene@usa.net>
To: <lowfer@mailman.qth.net>
Sent: Tuesday, October 28, 2003 12:25 PM
Subject: [Lowfer] RE: Worthwhile going to Shielded loop?


> Talking of tuned loops, are there any drawbacks in tuning the secondary 
> side
> of the coupling toroid transformer, as opposed to tune directly the loop
> winding itself ?
> 
> This would have the advantage that, placing the, let's say,  1:6 (turn 
> ratio)
> transformer directly at the loop location, then bringing its secondary 
> into the
> shack via a length of coax, you could place the tuning variable 
> capacitor in the
> shack, without the need of remote control.  The capacitance of the cable 
> would
> of course be in parallel with the tuning capacitor, reducing the tuning 
> range, but
> this could be accounted for.
> 
> Another advantage is that the loop would be balanced, if the only connection
> it does have is that to the primary of the transfomer.
> 
> I have to build a new receiving loop, so would like to hear opinions on the
> above before starting.  Thanks.
> 
> 73  Alberto  I2PHD