FW: [FoxHunt] Turnstile antennas for 2M

[Kuon & Dale Hunt]

>When I said Adcok, I was referring to the 4 element "stationary" one
>with four dipoles that uses lobing to generate sin/cos...

    Many of these are simply two two-element Adcocks at right
    angles to each other, so one responds to sine and the other
    to cosine of direction.

    Actually, if I were going to build a stationary array, I'd
    consider using a slotted cylinder antenna:  on 2m this
    would be similar to an 8' length of copper pipe (perhaps
    4" diameter?) with a slot cut down most of one edge.
    Not as portable, but much better gain than some of the other
    possible designs.  I've thought about using this for a
    transmitter antenna - as long as your forest is tall enough
    to hide it and you don't have to carry too many of them at
    once!

    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.


>...A horizontal Yagi doesnt do well in brush in
>that you have to get it clear to make measurements where other systems
>might be much better. Depending upon an amplitude change due to moving
>an antenna pattern around is just not as neat as TDOA methods that are
>friendly to FM radios.

    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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