[ARDF] Collapsible pole verticals

[Kuon & Dale Hunt]

...
>Since the only description of the antenna construction I have uses a 26'
>vertical wire, and I'm sure the transformer winding is dependent on that
>length...

   Ken -

      I developed the original matching system, and there is nothing
   critical about the antenna dimensions.  Except that longer wires
   generally mean stronger signals.

      We're blessed with plenty of trees here in Oregon, so I just
   used plain wire and throw it up over a tree branch.  Since I've
   been stringing portable HF antenna wires for years, it is no
   problem getting it over a 20 - 30 foot branch.  (And I take
   some string along in case the lowest branch is higher than that.)
   The 26' length is rather an artifact of chance.  (I had actually
   cut a pair of 13' wires to try a center-loaded vertical wire, and
   when I decided to try base loading I just tied the two wires
   together.)  Here is how to build an antenna that fits your
   available poles:

   First, decide how long you want the antenna wire, and the number
   and length of the radials.  Obviously if you are using a 20' pole
   then 20' might be a good choice.  If you aren't throwing the wire
   over a branch, then adding some center loading might make the
   antenna more efficient, but probably not worth it.  The radials
   also improve the signal, while also increasing the opportunities
   for tangling in the foot of a runner.  I didn't have any problems
   with the set of 3 radials, each 16' long (again, just numbers I
   grabbed from thin air) as I could usually run them through
   bushes or other places where the runners were not likely to
   pass.  They can also be staked down to the ground with  big staples
   made from stiff wire.  Efficiency seems to be better with more
   short radials than one or two longer wires, but are more effort
   to set out.

   The next step is best done with a dip meter, though an SWR analyzer
   will also work.  I took a big chunk of coil stock and connected it
   between the antenna and radials, then moved the coil tap up and down
   until the dip meter showed the coil+antenna+radials were resonant
   at the desired frequency.  I then measured the inductance of the
   coil and decided how I wanted to build a reproducible coil with
   the same inductance.  In my case I had a number of T-130-2 toroid
   cores handy, so I used them.  But you can also wind the coil on
   PVC pipe (I think there is a description on the Ron Graham Electronics
   website.)  Allow for some room for adjusting the actual number of
   turns.  I used #22 wire because it would fit on the core and I had
   a big spool of it handy, though larger wire would be better.

   Once the antenna is resonant, the next step is to match it to
   the transmitter.  Wind about 5 turns of wire around the loading
   coil and connect them to a short length of coax with a PL-259
   on the other end.  Plug this into an SWR analyzer (or SWR meter)
   and adjust the number of turns for lowest SWR.  In my antennas
   there is no direct connection between the coax and the antenna
   or radials - just inductive coupling.

   Once you get a design that works with one wire you can build
   a whole set of them.  You may find that they need some individual
   adjustment, but I think mine weren't off by more than a turn or
   two.  The number of turns in the main coil sets the resonant
   frequency, and the number of turns in the link sets the input
   impedance, though there is a bit of interaction.

   The whole process is very pragatic - you just do what you need
   to to make it work for you.  Nothing is magic, or critical.
   If you want to allow for field adjustment you can add a large
   mica trimmer capacitor in series with the coil (check Dan's
   Small Parts - they have some that have a shaft that will take
   a knob.)  In that case I'd suggest a field strength meter with
   an audio tone output:  put it about 30 - 50' from the transmitter
   and adjust the capacitor for highest pitch tone.  This will
   also let you hear what effect your body has in the vicinity
   of the wire antenna and radials.  But, so far, I haven't
   bothered about field adjustment.

   I've used the same approach with a 12' wire and a pair of 8'
   radials for a test transmitter, though it was a chore to get
   enough turns of wire on a T-68-2 core.

   As convenient as it may appear to put the loading coil inside
   the transmitter box and just bring out connections for the
   wire, do make sure you have a coax link somewhere in the
   system:  this allows for measuring power out of the transmitter
   and SWR on the antenna.  I built one with the link/loading
   coil integrated into the transmitter output stage, and
   have no idea if either is working properly!

   With the original 26' wire I've heard my 1 watt transmitters
   over 7 miles away - clearly this long coverage is not
   needed on a normal ARDF course, but it is helpful with
   weaker receivers (such as the PJ-809's, which couldn't hear
   TX #1.)  Using the ON7YD transmitters at 3 - 5 watts output
   will allow some reduction in antenna efficiency while still
   maintaining ample signal levels.


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