[ARDF] 2M Turnstiles

[Jay Hennigan]

On Tue, 24 Feb 2004, Kenneth E. Harker wrote:

> So, when we first build it and get 1.5:1 or 1.6:1 in the target frequency
> range (144-146 MHz) what should we be doing (if anything) to tune element
> length?  How do you know that the element length is right?

Optimize a single dipole for resonance.  Measure it.  Cut another to the
same dimensions.

> Right - that I follow just fine, but once I soldered the solder lugs on,
> the overall length is no longer 16" if you include the solder lugs in the
> length.  So, what I am wondering is for each end (if you'll excuse ASCII
> artwork):
>
>       A   B    C              ___
>        _____                     \\    braid/jacket
>       /     \_____        ________\\\_____________
>       |  /\    ___|______|        ||||
>       |  \/  _____|      |________||||____________
>       \_____/
>                            dielectric
>        Solder lug  center
>
> Should the 16" length be measure from A (end of the solder lug) to A,
> from B (center of the solder lug, where the machine screw goes) to B,
> or some point C (on the solder lug) to C?  The way I actually soldered
> this first antenna, was more like a 16" C-C distance, but should I really
> be soldering so that the overall B-B length is 16"?  Or does it matter
> that much if it's off by 1/16" or 1/8" total length?

The short answer is that the phasing section should be an electrical
quarter-wave.  The long answer is that at VHF with the distance inside
the PVC pipe between the feedpoints it's hard to say where the phasing
section ends and the dipole begins.  My calculations for 1/4 wave were
closer to the letter "t in "Dielectric" than the solder lug itself, for
the matching section.  With 1.25 inch ends this would be 16 inches to
point "C" overall.
If you consider that the coaxial section ends when the shield splits
from the center conductor, this is going to be closer to reality.
A portion of the braid and a portion of the center conductor will be
acting as element instead of phasing section.  Of course there is
propagation delay within elements just like phasing sections, so it
all goes into the mix.  It's really sort of cut-and-try.  Keep in mind
that the 16 inch dimension is for the raw length of the coax before
preparing the ends.  Subtract 1.25 inch for the prep on each end and
you get 13.5 inches of intact co-ax which is 1/4 wave at 146 MHz. with
a 0.69 velocity factor.  This gave me good results.

If this were a 10-meter turnstile with a center hub of 1 1/4 inches
it wouldn't be as critical a factor, natch.

Others have pointed out that a bead balun at the feedpoint as well as
a quarter wave away would be a good idea.  It certainly won't hurt
anything.

-- 
Jay Hennigan - CCIE #7880 - Network Administration - jay at west.net
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