[FoxHunt] 28 MHz RDF Loop.

[Jay Hennigan]

KD7JYK DM09 wrote:
> I am looking at the 28 MHz RDF loop in the ARRL Handbook, 1994, page 38-12.

I haven't seen the article so these answers are somewhat generic to 
antenna design in general.

> I have a few questions:
> 
> The design shows RG-11 for the loop.  Must it be RG-11 or can a rigid 75 Ohm
> co-ax with greater structural integrity be used?

I don't have the article but would think that any 75-ohm co-ax should 
work.  Getting good connections to aluminum outer conductors can be 
tricky or require the exact connectors for that type of co-ax, and 
vibration in a mobile environment can cause fatigue failure.  I believe 
you can get Andrew Heliax in 75 ohm which would have a copper outer 
shield.  Sourcing small quantities could be a problem.  Sometimes you 
can get an engineering sample by asking nicely.  Usually a loop requires 
a gap in the shield, follow the design.  You might be able to adapt it 
to 50-ohm co-ax depending on how the matching is done.

> The lead-in description mentions 67 inches of RG-59/U (82 inches if the
> cable has foamed dielectric).  What exactly determines this length?  A
> difference of 15" seems to indicate more than just compensation for velocity
> factor.

It looks like the goal is an electrical 1/4 wave.  They're pretty close.

67 inches = 1.7 meters / .66 (poly velocity factor) = 2.57 meters
82 inches = 2.08 meters / .8 (foam velocity factor) = 2.6 meters

> The description mentions mounting the 28 MHz RDF loop to the roof of a
> vehicle with a suction cup.  Would there be problems using a magentic mount?

Not that I can think of.  Static magnetic fields shouldn't be an issue.


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Jay Hennigan - CCIE #7880 - Network Engineering - jay at impulse.net
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