[Lowfer] Medfer TX Loop Question(s)??

Sat, 16 Feb 2002 11:54:14 -0500

Brian

There is a world of difference between 185 and 510 kHz when it comes to loop
performance. I can only speculate what happens at 510k but here's what I
suggest if YOU want to be the '510k loop guru':

1. Rac is everything! Try different conductors and measure the Rac at 510k
for each. The shield you mention may or may not be a good conductor. Very
hard to predict (even at 185k). Make your measurements with the loop high
enough above ground so that soil losses are not part of the reading. You
will have to determine how high this has to be.

2. You will need a good volt meter and a good current meter to allow you to
make theses measurement. By "good" I mean reasonably accurate, reasonably
linear, and repeatable. The voltmeter can be a scope or a voltmeter with a
simple RF probe, and the current meter can be simple home-brewed meter using
a ferrite core.

3. Understand the math behind loop antennas (ARRL Antenna Handbook, loop
section) as well as the math in the measurements. 

4. Be careful of the rjeloop program's loss values. Reg indicated to me that
his program was based on the 160 meter band and I find it gives losses way
too high at 185k. I predict you will have soil losses about three times
higher than 185k losses (which are less than 20% will this area's sandy
soil, if you keep the lower leg >5ft above gnd).

5. You will need to be able to make field strength reading in the far field
so you can determine the net effect of all changes. The loop current and the
size of the loop will tell most of the story, but not all. This distance at
510k will be >1/2mi. The math will tell you what signal level to expect.

6. And most importantly: Have fun at this. The intrinsic rewards are
incredible!

Bill

-----Original Message-----
From: Brian Helms [mailto:[email protected]]
Sent: Friday, February 15, 2002 9:03 PM
To: [email protected]
Subject: [Lowfer] Medfer TX Loop Question(s)??

Loop gurus/ Smart people:),

I am working on a loop this weekend to put on the air in the low end of the
medfer band.  After using the rjeloop program to model different loops I
found that I needed big diameter conductor for loop as I had already read
about Bill's experience.  I thought about using multiple #14 wires for my TX
loop but then I found a large piece of shielded multiconductor cable that
was about .75 inches in diameter.  My thoughts are that multiple #14
conductors would not be near as effective or as easy as using the shield on
the multiconductor cable.  If you use multiple small conductors, can you
figure the diameter of each conductor, add them together and figure them as
a single conductor w/t the sum of the smaller coductors diameter?  Would
spacing the conductors out make the antenna radiate better?  I am going to
use the shielded cable for now for it has more surface area and it is a
single cable which makes it easier to put into operation.  The loop that I
modeled for 510khz was approx 6db down from a optimized loop at 185khz.  Is
this correct or did I make a mistake on the calculations?

I hope these are reasonable questions to ask.

Thanks
Brian Helms
KD4RLD
www.qsl.net/kd4rld

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