[Lowfer] USA loop details

Sun, 16 Nov 2003 02:01:37 -0600

It sure is nice to have a guy like Bill around with the abilities to do the
math in a snap of the fingers. I sure wish I could remember all those
formulas I used to use years ago.

Mike>WE0H

-----Original Message-----
From: [email protected] [mailto:[email protected]]
On Behalf Of Bill Ashlock
Sent: Sunday, November 16, 2003 12:39 AM
To: [email protected]
Subject: Re: [Lowfer] USA loop details

Hi Paul,

>2) Voltage across 50 Ohm resistor at feeding loop.  6.5 volts

Are you saying that you have a transmitter in the shack and are feeding the
loop stepdown transformer (located at the loop) with a coax feed line?
....And the voltage at the end of the coax is 6.5v p-p when loaded with a 50
ohm resistor? This calculates to 100mw so I figure this what you are doing.

Since you have a 10 to 1 transformer ratio (or a 100 to 1 impedance ratio)
you are figuring the 5/8" loop has an Rac of 0.5 ohms. That's about 2x
higher than I would have figured but not a big deal at the moment.

When you changed to the 7/8" loop conductor and found the transformer ratio
increased to 14 to 1 (196 to 1 impedance ration) that amounts to a loop Rac
of 50/196 = approx .25 ohms. Since the ERP is proportional to the Rac that
would be a 3 db improvement at a distance receiving sight.

So what happens to the loop current? Since Isq = .1/Rac the current should
have gone up by 1.4X if the Rac dropped by 1/2.  I figure you have a problem
in you current transformer. What are you using for a core? Also, are you are
loading it with a resistor that reflects back a small portion of .25 ohms?
For example: If you have 50 turns and the load resistor is 100 ohms the
reflected resistance is 100/2500 = .04 ohms and would be marginally OK.

BTW, with a coax feed one can expect all kinds of weird effects from line
mismatch at all but the exact res freq of the loop. Not nice to try and fool
mother nature. <G>

Bill A

PS: USA is looking good! Minimal FM tonight.

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