FW: [FoxHunt] Turnstile antennas for 2M

[WolfBob]

Kuon & Dale Hunt wrote:

> 
>     Actually a single dipole provides a pattern similar to an
>     Adcock pair - and a shortened dipole even more so.  A pair
>     of crossed half-sized dipoles might be a good choice for
>     a portable system.
> 


You are absolutely right and this would make for a neat design of what
I call the Adcock system. simply use the turnstyle feed for the sum channel
  and modualte the two difference channels right at the antenna onto sub 
carriers,
  ala AOR (and many others). It is no problem to carry a small LCD 
screen around
to display the sine/cos signals.

To get around the multipath errors in the TDOA systems simply take the 
measurements while in motion and average the spread. This is the 
advantage of these systems in that you don't have to stop to get a 
bearing. You can easily get them to divulge amplitude information as 
well as bearing.


> 
>     As always, it is a tradeoff.  The beam allows you to differentiate
>     between a direct signal and a somewhat weaker reflection in a
>     different direction.  The TDOA and switched-pattern methods
>     will simply give you an inaccurate bearing.  (The Watson-Watt
>     or AOR will show both signals, but it isn't particularly convenient
>     to carry a 'scope in the bush.)
> 
>     Among serious ARDF competitors, antenna size (boom length) is
>     a common debate.  Longer booms mean more gain/sensitivity and
>     a sharper pattern, and require more clear space to take a
>     bearing.  I was on one hunt where the winner used a 6-element
>     2m yagi with aluminum elements.  I'd get tired just lugging
>     the antenna around!  But that hunt didn't require any
>     bushwacking.  (For those who want to try it, I believe the tips
>     of the elements were connected by string to prevent any single
>     element from catching on a branch.  Anyone who saw the
>     WM5R/W5JEN yagis after the last National ARDF Championships
>     will be familiar with the pitfalls of nominally-rigid
>     elements.)  On the other hand, 2-element antennas such as
>     the HB9CV are popular because they are compact.  (The Moxon
>     should be even better, though the closed design may catch
>     on branches.)  The loss in directivity can be made up by
>     using a threashold limiter in the receiver.  3 elements
>     seems a reasonable compromise, and will still give some
>     directivity if one of the parasitic elements gets broken.
> 
>     The TDOA method may work if you can reduce the problem with
>     reflections and provide a meaningful signal strength indication:
>     strength is an important piece of information, both for
>     deciding which transmitter to hunt next, and for knowing
>     whether you might overrun the transmitter during the next
>     OFF cycle.  I encourage you to try it and let us know how
>     it works!
> 
> 
>>I wonder about Halos. They are a wavelength in circumferance...
> 
> 
>    Actually they are a HALF wavelength in circumference,
>    basically a dipole bent into a circle.  For true omni
>    coverage it isn't really a circle - W4RNL has a good
>    6m design on his web site (www.cebik.com) that can
>    be scaled for 2m.
> 
>    Hopefully this will inspire some creative experiments
>    among the list members.  We always need to question
>    whether the current common convention for doing things
>    is really optimum, and come up with creative new methods.
>    For example, there no reason why one could not build a
>    tunable converter and antenna system to use a basic
>    L'Per box with LEFT/RIGTH reading on 80m signals...
> 
> 
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