[ARDF] 80 meter antenna question

[Dale Hunt WB6BYU]

It isn't hard to build a suitable antenna for 80m.  There has been one
in the ARRL Antenna Book for 40 years using a ferrite rod:  about 20
turns around the rod resonated with a capacitor, two turns link to the
coax, and a 15" whip with adjustable loading coil and resistor to
adjust the sense performance.

Ferrite rods have the advantage of small size, but unless you order
it direct from the source you can't be sure of the actual  
characteristics,
and some AM BC ferrite rods are actually pretty lossy on 80m.  I like
the air core loops because they pick up more signal and don't depend
on finding the right type of ferrite.  There are many different  
construction
techniques, but putting the wires inside a piece of plastic tubing like
I did for my QST receiver is simple and withstands the rigors of an
ARDF course.


The problem, however, as Jay pointed out, is that the TH-F6A is NOT
a good ARDF receiver because it doesn't have a wide range gain
control.  To hear the null best you want to turn off the AGC, which
makes the receiver prone to overload on strong signals.  You can
also get pickup through the case when the signal is strong and this
will tend to shift and/or blur the nulls.  One solution (much as we do
on 2m) is to make a converter up to, say, 5 MHz, basically an offset
attenuator with a 80m loop attached.  You can use an SA602 mixer
IC fed directly from the loop (if you use about 4 turns  on the tuned
loop and a 1-turn coupling loop feeding the 1500 ohm input.)  If
you use a separate transistor oscillator rather than the built-in one
you can sink current from the input circuit and get 40dB or so of
gain control from the IC.  That plus a switched input attenuator
with a couple of 20dB steps might be enough range.  (You do have
to be careful because most such receivers are designed for a
short antenna, and strong signals will overload them very easily.)


This means you now have an active box rather than a passive
antenna connected to the radio, but I think it will give you the
best performance.

In the end, however, I'd have to echo Jay and say that if the
Russian receiver is designed for ARDF and is repairable, it
might be the best way to go.  (Yes, the old Altais are rather large
compared to some of the newer sets.)

One other factor to keep in mind is, if you ever want to attend an
international event, the TH-F6A will not be allowed on the course
because it is a transmitter.  We haven't enforced that rule in our
National and Region 2 championships because some folks may
still be using a HT and attenuator on 2m.

But give it a try and see how it works.

Good luck!

	- Dale WB6BYU


On 08 Nov 2009, at 18:15, Mike (KA5CVH) Urich wrote:

> I have an old Russian receiver that I bought off Bob Frey that works
> OK but I'm looking to change things and get serious about ARDF.  I
> recently bought a new Kenwood TH-F6A which has hf receive
> capabilities.  So I'm wondering about making a DF antenna for 80
> meters and use the F6A.  If I remember correctly someone at Bastrop in
> 2008 was using a similar set up.  I looked at Joe, K0OV's, web-site
> and it seems that this is an appropriate method.
>
> So after a overload of information googling antenna designs for a 80
> meter DF antennas does anyone have any good suggestions they would
> recommend? I'm considering building a few different ones over the
> winter and see what I like best.   Also I remember seeing a web-site
> that had a plug and play micro 80 meter transmitter.   I would like to
> find something simple that I can use around the neighborhood, I.E. get
> my daughter to go hide it somewhere and see if I can find it using
> different antennas.
>
> -- 
> Mike Urich
> http://ka5cvh.com
> http://twitter.com/KA5CVH
>
> You know you're over the target when you start taking flack.
>
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>
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