richard,
How you adapt the directional system depends entirely upon what kind of
signal is available from it.

Let's assume the unit to be modified has a regular old fashioned meter
 movement  which indicates how well the antenna is aimed at the fox.  If the
meter simply reads "higher" when more closely pointed, it's pretty easy.

  In that case you need a VCO which can look at the voltage across
the meter,  usually 50 to 100 Milivolts

A slightly trickier system uses a so-called "zero center" meter which shows
whether you're aimed left or right of the fox. These meters actually measure
current in either direction  This would need a bit of fiddling before being
applied to a VCO.


There are gobs of such vco circuits available from transistorized ones
designed in the 1950s to specialized chips and microcontrollers. 

If all you need is a "relative" indication, not the ability to get an exact
measurement, I'd suggest the transistorized circuit for its simplicity.

You and your friend can find many such circuits in plain text format in:
The Smith-Kettlewell Technical File
https://legacy.ski.org/rehab/sktf/index.html

Search the magazine for "audio gimmick" circuits or "meter readers" and
you'll find more than you'll ever want.

the only issue for your presumably sighted friend is that the schematics are
text discriptions not schematic drawings. Sometimes it's fun to have the
sightlings be at a small disadvantage <GRIN>

If I can help with more specific choice of circuits or related stuff, please
email me either on list or directly.
Tom Fowle WA6IVG

On Wed, Nov 25, 2015 at 09:36:41AM -0800, Richard B. McDonald wrote:
> Hi!
> 
>  
> 
> In a few months, my club is going to do some fox hunting activities.  My
> question here relates not so much to making the "antenna" itself per say,
> but rather to making an audible tone direction indicator.  I think this
> thing is called a "V C O?"  Basically, the V C O causes the pitch of a tone
> to change as the direction finding antenna is pointed towards the fox.
> 
>  
> 
> A very techy friend of mine in the club is going to build this contraption
> for me, including the antenna itself.  He has some ideas.  However, I wonder
> what others here on our list have done?
> 
>  
> 
> 73,
> 
> Richard KK6MRH