2 Meter ARDF Attenuator Project- project build notes

Prologue:  When I was a 12 YO kid, just getting into ham radio- the old guy hams who smoked cigars would take me along in their sedans to turn the broom handle with the semi-directional loop antenna atop it- outside the car window.  The loop antenna was connected to a Heath Twoer, a  2 meter AM transmitter/receiver on the front seat of the car.  

The Hidden Transmitter or Bunny or Fox- another Twoer or 2 Meter transmitter was located in another sedan parked in an alley or driveway within a designated area or neighborhood.  

The  Fox or Bunny ham would transmit for a while to taunt the hunters to find him- then the hunters would rotate their broomstick loops to determine a direction to drive towards.  I have fond memories of tagging along with the old guys to find the elusive Bunny in the '49 Chevy in the alley behind the Jones' house.  Now I am that old ham- but no cigars.  

Heathkit Twoer- AKA "Benton Harbor Lunchbox" 2M AM Transmitter-Receiver.  Circa 1960.  

Now-a-days, it's 2 Meter FM Hidden transmitter hunting, AKA Hidden T Hunting or Fox Hunting or Bunny Hunting.  

For effective hunting- an  attenuator is needed between the Directional Antenna and the Hunters receiver.  Beginners as well as advanced hunters use a Tape Measure Yagi Antenna, and an Attenuator that is connected to a Handy Talkie receiver.  

Simplified schematic of ARDF Attenuator.

Factoid:  S meters on most HTs  operate at full scale while receiving a very low level.  When looking for 2M hidden transmitters- the HT can not see any difference in receive signal level when rotating the Yagi- unless the transmitter is really far away.  In rare cases the direction can be discerned by listening to the receive audio.  The direction of the most noise free audio is probably the direction to head.  

S-meter bar graph shows full scale at -101 DBm- a very low level.

Two popular attenuators are Resistive with switches to switch in various levels of loss and the Active Attenuator which contains electronics.  This blog addresses the  Active Attenuator.

Resistive Step Attenuator.  Switch in dB increments of loss between the antenna and receiver.

 A Bunny Hunt Active Attenuator 

The AA  reduces the received level of the hidden transmitters signal so the S-Meter on the hunter's HT will show a High as well as a Low reading when rotating the Yagi.  It does this (see schematic below) by controlling the amount of level that passes from the ANT to the RCVR via the diode mixer by varying the level of the MPF102 XTAL Oscillator signal into the mixer via the 1K pot.    

A huge benefit of the AA is that the hunter can quickly and easily reduce the signal level being receiving with a simple level control- sort of like a volume control- before the Hidden Transmitter times out and goes QUIET for its programmed interval.  

Caveats:  The Active Attenuator offsets the receive frequency by plus or minus the Local Oscillator frequency.  The AAs that I build and use... have a 4.0 MHz Oscillator which sometimes can be a problem if the hunter does not remember to  offset their input frequency + or - 4 MHz of the Bunny / Fox's output frequency. 

Example:  If the hunter is receiving 147.570, then the HT must be set at 143.570 or 151.570.

Another caveat is-  the AA unit does not have Zero loss when set to minimum attenuation.  It will always have an inherent or minimum loss of about 30 dB.  Normally the added loss is not a problem, unless the Bunny / Fox signal level is quite low.

AA schematic

The AA I build comes from this website:
http://theleggios.net/wb2hol/projects/rdf/rdf.htm

DIY BUILD NOTES:   The enclosure fabricated from Copper Clad PCB Material cut with a Dremmel tool and soldered together.  This view shows the AA upside down.  The cover- to the left- is the units bottom.   The interior depth dimension is 3/4" which makes the finished product 1" thick.  A commercially made project box could also be used.

I will not go into a lot of detail of how I DIY'd this- but I do use dead-bug blob solder electronic construction.   Measure and cut out the printed circuit material enclosure to fit a the battery holder.  It turns out that the oscillator circuit wouldn't oscillate using 3 VDC battery- but will oscillate at 4.5V.  A 9V battery fits well in the battery chamber.  The current draw is very small.  The 1K ohm Local Oscillator level control is equipped with an ON/OFF switch wired to the red battery wire.

Note that the Mixer has its own shielded compartment.  Past experimenting revealed with no shielding- the JFET oscillator running full blast would bleed into the diode mixer area and wipe out the Output Level Controls sensitivity- especially with the control set to provide close to maximum attenuation- when the hunter is very close to the transmitter. 

Finished product.  The enclosure corners are rounded over with sand paper to remove sharp areas.  The antique instrument 'big knob' gives a little bling to an otherwise boring electronics device.  Stick on Velcro is applied to the bottom of the OA- so it will adhere to the  pad on my TM Yagi (blue plastic w/ black strips in photo below).  

 Build Instructions

73 Rich KR7W