[FoxHunt] IARU Transmitter question

[Kuon & Dale Hunt]

>In Portland I was able to get one at about 200 meters out. While others did
>not go that far.


    This was a common problem in Portland - several people punched
    the transmitter that didn't count for their category just
    because they were running close to it and the oscillator
    was strong enough to hear.

    I used the "OXO" transmitter kits from Kanga Products for
    the 1999 event.  The oscillator runs full time, and the
    final is keyed.  For this application they work better
    if the oscillator is keyed along with the final.
    (I ordered the kits about a week before the event, so
    obviously I didn't get much chance to try them out ahead
    of time!)   The ones I use now are the "Pippin" circuit
    from the G3PTO web site, a directly-coupled circuit with
    both stages keyed.

    With the MFC controllers the 2m PTT output can be used to
    key the oscillator on during the transmit cycle.  The
    ATX-80 design should have an extra inverter stage before
    the final so keying it when the oscillator is off doesn't
    smoke the final.

    The new Region 1 ARDF rules state that the there should be
    no detectable radiation from the transmitter except during
    the minute when it is supposed to be transmitting.  The
    leakage through the final on the ATX-80 may be much less
    than on the OXO transmitters, but you will have to see
    how far you can hear it in a shielded box with an antenna
    attached to judge if it is weak enough.



>
>The antennas in Albuquerque used a 22 foot vertical wire with two 22 foot
>radials.  A loading coil matched the antenna.

>>While we are on the question of questions :), what kind of an antenna
>>was used on the 80M transmitters in both Georgia and Albuquerque? We
>>have typically been throwing some 20 feet or so of wire up over a tree
>>limb with a radial of about the same length laying on the ground. Has
>>anyone done any work on a quicker-to-set-up type of antenna? I was
>>thinking about an 80M mobile whip, but haven't done any research into
>>finding out if that would work for championship events.


    The Portland antennas were 8m (26') wires with 3 radials each
    5m (16') long.  These were connected across a loading coil wound
    on a T-130-2 toroid core and link coupled to the transmitter.
    My transmitters ran about 1 watt output and that appeared to be
    quite adequate with a well-tuned antenna.  I've tried the same
    technique with shorter wires (down to about 8') and it still
    works, though the efficiency is less and it takes a larger
    loading coil (meaning smaller wire on the same size toroid
    core.)

    Sam used long fishing rods with wire running up them and the loading
    coil wound around the base.  (At least that is what they looked
    like to me!)

    I have sometimes used a 75m helical mobile whip for practice,
    though I never bothered to resonate it properly on the TX
    frequency.  You could try a HamStick, though it probably will
    need an extended stinger to tune low enough in the band.

    Personally I never found much problem setting out wire antennas
    quickly.  One key part is to wind the wire in a figure-8 across
    your palm between your thumb and little finger.  This allows
    the wire to pull out without twisting or kinking:  rolling it
    up in the common manner is just inviting tangles.  Usually I
    just tie a weight on the end of the wire, though for tall
    branches I would use a piece of mason's twine as an extension.
    Of course, other parts of the country may not have the problem
    that the lowest branch of the convenient tree is 30 to 50
    feet off the ground!  For use in the southern California chapperal
    you need something much shorter.

    I have a couple test antennas wound on PVC pipe out in the barn
    that I've never gotten around to testing out.  One design that
    I want to try is a variant of the "EH" antennas:  basically
    a length of PVC or cardboard mailing tube perhaps 4" in diameter
    with aluminum flashing or copper foil covering the outer
    third at each end.  These form the "wires" of a center loaded
    dipole, with the loading coil wound on the form between them.
    (It may be important that the flashing on the ends not extend
    quite all the way around the form so it doesn't make a shorted
    turn coupled to the loading coil.)  The feedline would be link
    coupled to the loading coil, probably with the coax exiting
    out the bottom of the tube.  I suspect this would give a
    shortened vertical that could give about the same signal
    strength as a typical mobile whip, but without the radials.
    Worth a try, anyway, though it means you have to lug these
    long pieces of PVC pipe through the woods.

    Since I did fairly well with one watt output, you can make
    the antenna less efficient and make up for it by increasing
    transmitter power (presuming your batteries are large enough.)

    A center-loaded vertical wire would be more efficient.  This
    was my first design:  adding enough inductance in the center
    of the radiator to resonate it on 4 MHz, which would leave
    enough base load inductance that the feedline could be coupled
    to it.  This required that I throw a rope all the way over
    the branch and back down to the ground to pull the antenna
    up due to the added weight of the loading coil.  However, this
    is probably a good approach to improve the efficiency of
    a shortened wire.


    Hope this gives you some ideas

         - Dale WB6BYU