[Lowfer] WA test coil.... also loop size radiation resistance

[Paul Cianciolo]

 Hello Bill,

OK... I promise not to moan and groan....

The reason I used the Litz wire was to simulate the anticipated Rac of my
loop when finally get that room temperature super conductor wire up in
place.

Actually according to theory, at that point I should be able to make a loop
5 feet on a side work as well as a full size one, but that's another story.


I guess I just want to play around a little with the final and set up a test
jig for later on if we get the 136Kz band.

Yes I would be very interested in borrowing a calibrated test coil.  Since
my current transformer is indeed home brewed it would be good to know if it
is accurate or not.

On another note Jay and I were having a conversation about size of a loop
versus radiation resistance.  It seemed to me that the relationship between
area and radiation resistance would be proportional.  IE 1/4 the area  -6 db
gain.  Jay disagreed.  He thought it would be much more than that. he came
up with a figure that was hard for me to believe. So using the 10 meters per
side and 20 meters per side I ran some numbers using the following formula
quoted from this page

http://you.genie.co.uk/carl.littlejohns/magloop/magmath.htm

The radiation resistance of the loop is
31171 ( PI to the fourth power x 320 ) multiplied by the Area squared (A*A),
divided by the Wavelength to the fourth power (W**4)


Not including copper losses, or ground effects I came up -11.54 db for the
10 meter loop when compared against the 20 meter loop.

Do you have any thoughts on this?   6 db seems appropriate at first... 1/4
the area  -6db but the numbers prove otherwise.

Comments anyone?


PauLC
W1VLF
"USA"


----- Original Message -----
From: "Bill Ashlock" <ashlockw@hotmail.com>
To: <lowfer@mailman.qth.net>
Sent: Friday, January 17, 2003 12:50 PM
Subject: Re: [Lowfer] WA test coil


> Paul,
>
> >Can you tell us a little more about your test coil?
>
> If you all promise not to groan and mutter: "Not another plug for those
dam
> loops"
>
> >I believe you made it to try to simulate a 50 x 50 loop.
>
> Right. I have a number of test coils with Rac ranging from 0.07 to 1.2
ohms
> and inductances close to 100uh to match the loop I happen to be using at
the
> time. The physical size of these coils is typically 4" of winding length
on
> a 3" dia form. They have proven VERY useful in checking out my old standby
> dual final on the bench when there is any question about the performance,
> since they load the final exactly the same as the actual loop. I use a
test
> coil to test any new final that I build since a resistive load doesn't
tell
> the whole story. They can also be connected in place of the loop out in
the
> woods as a test. Litz is only needed when you are simulating a loop with
an
> Rac of <0.2 ohms but does allow a smaller coil for the higher Rac types.
>
> >I built a coil from Litz wire with about 100 uH and got it to resonate
> > >with about 7000 pF. Now to match it and try putting some power into it.
>
> You have to calibrate the coil with the actual Rac. This requires an
> accurate current probe. I like to use the sine wave from my Wavetek signal
> generator (50 ohm output Z) as the signal source. Use a transformer ratio
> that is sufficient to eliminate any distortion of the generators's output
> and do a Rac = voltage/current calculation of the voltage out of the
> transformer and the current through the coil at resonance. You can also do
a
> Q measurement and calculate the Rac from this. The Q is the voltage, at
> resonance, across either the capacitor or the coil, divided by the voltage
> out of the transformer.
>
> Let me know if I've missed anything. Also, I can very easily send you a
> calibrated test coil (if the the RAC is >.5 ohms) as I have done for a
> couple of other guys. This way you can calibrate you current probe, which
I
> think is a home brew?
>
> Bill
>
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