Amateur Radio Orienteering
www.myardfsport.com

Radio Sport
Penang

Hunter's Tool Box
A collection of useful devices homebrewed and tested, to assist fellow hunters to have a quick start into this exciting hobby. Some of the designs were formerly provided by other hams, and may have been modified to suit local conditions.

Last update
Feb 16th, 2012

 

Antennas

Attenuators

Receivers
 
 

Antenna Designs

1.

3 Element Measuring Tape Antenna

 

  • Original design by Joe WB2HOL whose website can be found by this link:
       http://home.att.net/~jleggio/projects/rdf/rdf.htm

    Note: The "U" match at the Driven Element is made of magnet wire of
    approx. 1 ~ 1.2 mm diameter and of total length of 5 inches.
    Prefer not to use the hose clamps and using tapes to reduce the overall weight.
 

 

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Attenuators

1.

4 Mhz Off-set Active Attenuator with -39 dB passive attenuator

 

  • This is modified version using other FET's instead of MPF102
     This device will give you a great advantage when used with the 3 element ARDF antenna,
    especially when  approaching a strong signal source. The 4 Mhz off-set greatly helps to
    prevent the source signal from being picked up through the radio's body.

Schematic diagram
  Veroboard Layout And Component List
 
 

R1
R2
R3
R4
R5

C1
C2, C3
C4

Q1

Q2

D1
Led1

L1

J1-3

S2

  

1K6
1K
4K7
1K
150

0.047
150
220

BF410

4

1N4148
 

10 Turns

 

SPST

 

ohms
ohms
ohms
ohms variable
ohms

uF
pF
pF  optional

  FET

Mhz Quartz

  Diode
  LED

  RF Choke

  Wire jumpers

  Switch

 

 
Passive Attenuator Components
S1
R6
R7, R8		 DPDT
2K4
51		   Switch
ohms
ohms		      Veroboard hole - not connected
   
  Connection for -40dB Passive Attenuator

Black shielded wire is from the active attenuator output and the grey shielded wire goes to the output BNC connector.
   
 

Side View                                                              Top View
 

 

Inside View
 
 

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2.

4 Mhz Off-set Active Attenuator

 

  • This version uses a normal NPN transistor as the oscillator.
     This device has similar functions to the earlier design BUT using a cheap NPN transistor instead of the FET.
    You might want to incorporate the passive attenuator section from the above circuit for better advantage.
    Disclaimer:
    I have just completed this circuit BUT have not field tested     for its performance.

 

 Component listings

 R1 4K7 ohms C1 330 pF
 R2   10K ohms - potentiometer C2 330 pF
 R3 100 ohms C3 10 nF
 R4 4K7 ohms C4 330 pF
 R5 1K ohms Inductor 15 Turns (approx. - See picture)
 VR1 1K ohms  variable resistor LED Red Light Emitting Diode
 Q1 BC549 NPN Transistor D1 1N4148 Signal Diode
 X1  * 4 Mhz Crystal
Input & Output BNC Connectors
SW1 Power Supply Switch Battery 3 Volts ( Watch type 3V lithium button cell )
 
             
  *  May use ceramic resonators instead of crystals like the 500 Khz (Murata) resonators,  the off-set will be ±500 Khz.
 
 

 

   PCB Side
 
 

   Component Layout
 
 

Component Side
 

Current consumption of this circuit is very low, about 10 mA of which majority is consumed by the LED power indicator, as such I selected R5 to be 1K ohm to reduce the current flow to the LED and preserve the battery life. For the inductor coil, I wound about 15 turns of enameled copper wire (taken from old transformers or motor) over a 1.4mm rod, to form an air wound coil.  A watch type Lithium button battery should easily last for many fox hunts. For the passive attenuator section, just "dead bug" the components to the switch selector ... see earlier schematic design above.
 

 
 

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ARDF Receivers

1.

PJ-80 The little low cost 80M ARDF receiver from China.

   

In early January 2012, I received in kit form, the China made 80 meters ARDF receivers.
These are low cost 80M receivers meant to introduce 80 meters ARDF to enthuses.
The good part of being in kit form, is that all required parts including the earphone unit is packed together.
(Batteries 4x AA size Not included)
 
The printed circuit board is made out of strong fiberglass material (FR4), something I seldom want to work on
if I were to make my own PCB from scratch! It's a hard material and difficult to drill.
The receiving antenna consists of a ferrite bar housed inside the plastic casing with a telescopic (sense) antenna.
   
 
Based on the supplied schematics and component layout, it is easy to assemble the kit.

(Caveat: Definitely expected that the assembler must have some electronics knowledge
to read the resistor colour codes, capacitor values and able to handle a soldering iron.)

The manual which is included in the kit is in Chinese!
However, English versions are available on the internet.

This is a direct conversion receiver with a twist.
The oscillator is running on sub harmonic (half the received frequency) Variable Frequency Oscillator (VFO).
It is using the Russian technique called "Polyakov" style.
{Reference material: www.ke3ij.com/DC-80.htm}

Advantage of this setup is better stability and less microphonic effects.
The earphone jack also functions as the ON/OFF switch. Just pull out the earphone plug and the set is disabled.
 
  Alignment.
  Alignment is quite simple. You will need to have a low RF source like a signal generator. In my case, I had constructed my own single transistor 80M beacon which I used as my RF reference, since I will be mainly hunting on only a single spot frequency.
 
 
  • Put the RF signal source near the receiver.
  • Set the variable resistors,RP1 to maximum and RP2 to the mid-point.
  • Adjust T2 (white core) until you pick up the signal source.
  • Adjust T1 (black core) for higher audio output. May need to move or reduce the RF source and reduce RP1.
  • Finally, adjust capacitor trimmer C1 for best output.
  Field Test.
  The same single transistor 80M beacon was used as the RF source except that a long wire vertical antenna (6 meters) and a ground counterpoise (6 meters) was attached to it.
   
Simple wire antenna attached to the fishing pole.
 

 

 

 

 

 

 

Enlarged picture of the QRP 80 meter beacon
   
  Moved about 100 meters away and was able to comfortably read the beacon signals.
Extended the sense antenna and pressed the side button S1, and was able to have the
cardio-pattern response, thus was able to get a front-to-back ratio directivity.

Decided to move as far away from the beacon to find out the distance the PJ-80 can still pick up the beacon signals. Found that it could still receive the weak beacon RF source for more than 500 meters.

That is almost equivalent to my homebrewed Superhet 80M ARDF receiver - MYRX80!
Well, this may be just a fluke-shot as most recommended usable distance was 300 meters.

So far, I have assembled 5 units and will soon look for an opportunity to carry out a fox hunt on 80 meters.
 
  Pros and Cons
  Very affordable kit set allowing beginners to be introduced to 80 meters ARDF.
Best used in small area hunts.

Did not like the center click position of variable resistor - RP2.
   
 

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