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From: "Ashlock,William" <William.Ashlock@edwards.boc.com>
To: rsgb_lf_group@blacksheep.org
Subject: LF: RE: Loop Amperage ~~~~~~
Date: Wed, 29 May 2002 23:44:52 -0400
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Couple of addendum's to my earlier post:

>The ERP for 1w of input is now 81mw. To reach a 1w ERP the current
multiplier will need to be
>only 1/.081 = 12.3 X 1.5 = 18.5A.

It should be sq rt(1/.081) yielding only 3.51 X 1.5 = 5.3A for the 450ft
wide loop to produce an ERP of 1w. The power input would be only I^2 x R =
28 x 3.42 = 99w. (Think there is a small error in this calculation and
should be closer to 110w) So the increased size does make a lot of
difference!  

Rik Strobbe, however, pointed out the importance of taking into account the
radiation resistance difference between 185k and 136k, which is a forth
power effect for loop antennas. (185/136)^4 = 3.42. So the radiation
resistance, hence the radiated power, must be lowered by this factor. My big
loop at 100w would only produce .9/3.42 = .26w ERP at 136k ........ (Darn!).
But to be fair, the vertical has a (185/136)^2 = 1.85 radiation resistance
factor and that does give the vertical a 2.67db advantage over the loop
between the two bands.

So back to the original question about the input power needed to reach 1w
ERP (at 136k):
That would be 3.42 x 110w = 376w. The current in the loop based on a Rac of
3.5 ohms would be 10.4A. 

Here's the numbers for a low cost Europeon #12 AWG loop at 136k (where the
input limit is 1KW):
Rac total for a 450'x50' loop = 10.5 ohms. At a 1w input the loop current is
sq rt(1/10.5) = .31A. The ERP is 12.3 x 10^-3 x .31^2 = 1.18mw. In order to
reach an ERP of 1w the input power will need to be 1/.00118 = 848w. The loop
current to develop 848w is sq rt (848/10.5) = 9.0A. So put away the fire
hose! 

Bill A 


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