On Wed, 02 Aug 2006 22:09:43 GMT, "Lee"
wrote:

Hi!....

I`ve just built a magloop 5`square and i find the variable cap only tunes
down to 4.45mhz..
can i pad the cap out with a coaxial ur213 stub to reach 3.5mhz ???....

Lee.......G6ZSG.....

(The 1metre loop works fine - 15 to 7mhz ).. :-)


This is a riddle, right?

If you are after a few pF, then the losses in the stub will be very
low, but at lower reactances, the resistance or loss element becomes
more significant.

So you have hinted a bit at this loop.... lets do some guessing:

An inductor of 1.5m square and 20mm conductor diameter has an
inductance around 5uH, thats about 140 ohms at 4.45MHz... so it looks
like your tuning capacitor is 250pF.

You probably need around 400pF at 3.5MHz, so lets add another 200pF in
shunt from an RG213 o/c stub. You need an admittance of ????+j0.0044
S.

Using the line loss calculator at http://www.vk1od.net/tl/tllce.php ,
it looks like 1.95m of RG213 terminated with 1e8+j0 will have an input
admittance of 0.000004+j0.004401. So this stub is dropping around
4e-6S of loss conductance across the loop, or 250kOhms in shunt if you
like. This shunt resistance is somewhat lower than you would expect
with a good quality capacitor.

You can estimate the effect by plugging the 250KOhms resistance into
your design model and examining the change in efficiency.

Owen
--