[FoxHunt] 28 MHz RDF Loop.

[Mark Harrison]

Hi Kurt,

I think the differences in dimensions for cable types are likely to be
due to different inner to outer conductor capacitance and also the
inner conductor loop inductance.  The whole thing should resonate
together with the tuning capacitors in the base.

We used to make a similar loop for 11m, although it didn't use tuning
capacitors as in the ARRL design.

Instead, from memory, we took a length of coax and soldered the inner
conductor to the braid at the end.  The jacket was removed to expose
the braid at a specific point from the end of the coax (like about 28"
or something). The coax was formed into a loop 9" diameter (or
whatever) so that the coax end could be soldered to the exposed point
on the braid.  Then a strip of jacket and underlying braid was removed
at the top of the loop.  

This formed a single turn coil inside an 'electrostatic' shield.  The
shield had a break in it at the top of the loop so that it didn't
effectively short out the inner coil, but also acts as a capacitor to
tune the inner coil.  From memory the loop could be tuned by cutting
away more shield to make a bigger gap.  All the dimensions were fairly
critical (my examples will be wrong!), but different coax or hardline
could be used with appropriate adjustments.

73,
Mark VK3BYY






> -----Original Message-----
> From: foxhunt-bounces at mailman.qth.net 
> [mailto:foxhunt-bounces at mailman.qth.net] On Behalf Of KD7JYK DM09
> Sent: Saturday, 1 December 2007 6:23 AM
> To: Radio Direction Finding
> Subject: [FoxHunt] 28 MHz RDF Loop.
> 
> I am looking at the 28 MHz RDF loop in the ARRL Handbook, 
> 1994, page 38-12.
> 
> 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?
> 
> 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.
> 
> 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?
> 
> Kurt
> 
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