By Bryan Ackerly, VK3YNG.
Note: A PCB for this project is provided with every 80m sniffer kit or board set, but components are not supplied and will need to be additionally sourced.
The 80 metre sniffer shown last month used a simple variable potentiometer to provide all the attenuation required. While this is the simplest and cheapest way to provide gain control, there are more intuitive ways of doing so.
Most of the sniffing equipment now in use by active foxhunters indicates the amount of attenuation required in discrete steps of around 15 to 20dB per step. It is easier to refer to range numbers as a relative measure of distance than a setting on a potentiometer. A typical sniffer normally uses between 7 and 10 ranges and most modern sniffers on the two metre band insert the required attenuation automatically.
On two metres the receiving antenna is normally large compared to the wavelength involved. A large number of ranges are required to attenuate the signal as you get very close to the transmitter and couple more and more energy into the receiving antenna. Up to 130dB of attenuation can be required on this band. The need for extra attenuation occurs more and more rapidly as you get closer to the signal source, hence why many of the more competitive foxhunters now use automatic ranging on this band.
On 80 metres however, the receiving antenna is quite small compared to the wavelength and a lot less ranges are normally needed as you get close to the transmitting antenna. Typically the attenuation required is about 30dB less than that required for two metres. Therefore since the need for range changes occurs less often on 80 metres a manual range control is usually quite adequate for use on this band.
This circuit basically replaces the attenuation control (RV5) on the 80m sniffer. The schematic is shown in figure 1 . R2 (or R1) provides full 5V supply to the gain control line when the range control (SW1) is set to the lowest two ranges. This sets the sniffer to maximum sensitivity.
For ranges 2 through 8, RV1 through RV7 load down the supply from R2 providing a number of preset voltage divisions. Ten turn trimpots are used as the levels can be rather difficult to adjust with single turn pots, particularly on the higher ranges. Lower values of resistors for the last two ranges assist with setting these levels. Discrete resistors were not used due to the spread of gain versus voltage characteristics between sniffers. The multiturn pots are more expensive but result in a simpler solution.
When SW1 is set to the lowest range (“A”), current is drawn through R4 (or R3) which turns on Q1. The collector of Q1 provides 5 volts to the “Audio Switch” line on the sniffer. This effectively overrides the sniffer’s tone switch and forces it into audio mode. For ranges 1 through 8, R6 (or R5) ensures that Q1 remains off and the sniffer returns to the mode selected by the mode (Tone/Audio) switch on the main board.
Construction is straightforward. There are no coils to wind or otherwise special components. There is one surface mount component. Mount Q1 as shown in figure 3 . A good pair of tweezers and a small soldering iron is all that is needed to mount this device. The orientation is fairly obvious.
Next mount the fixed and variable resistors as shown in figure 2 . If preferred, 0805 case surface mount resistors can be used instead of through hole parts. Note the orientation of the multi-turn trimpots.
Mount the rotary switch next. You may need to cut the shaft to a more suitable length. The top of the switch also contains a ring which determines the amount of positions the switch has. Set the switch fully counter-clockwise and remove the ring. Set the tab of the ring into the “9” position.
Finally mount the wiring to connect the range control to the main board. This board replaces sniffer gain control RV5. A fourth wire connects to TP7 which is located immediately below U4 on the sniffer main board. A right angle header can optionally be used to allow the board to be removed if this is desired. If this is done, make sure the header, plug and cabling clear all components on the main board when the box is assembled.
The metalwork drawings for the sniffer assume the stepped range control will be used so there should be no need to drill any additional holes to fit this
The easiest way to align the range control is with the assistance of an RF signal generator with a calibrated attenuator. Otherwise a 3.5MHz signal source with a low resistance pot (about 1K to 5K ohms) configured as a voltage divider may do.
Connect the signal generator to the antenna port of the sniffer and set it to 3.58MHz with a signal level of about –115dBm. Switch the sniffer to the “Tone” position. Select “A” on the range control and tune the sniffer to ensure the signal is tuned “mid band”. Select range 1 and ensure the sniffer reverts to Tone mode. Take note of the pitch of the tone. Set the sniffer to range 2 and increase the level of the signal generator by 15dB. Adjust RV1 until the same tone pitch is heard as on range 1 with –115dBm. Switch the sniffer to range 3 and adjust RV2 for the same tone pitch again. Continue this for RV3 through RV7 until all ranges have been aligned.
The exact signal level is not that important as the main idea is to make sure that there is some overlap between the ranges. The job is just made a bit easier with a proper signal generator, but it is by no means compulsory.
Use the signal generator to step by 1dB steps and verify check that the ranges provide a reasonable amount of overlap. The sniffer should be able to handle levels up to about 0dBm. This should be adequate for normal ARDF foxhunting and should allow you to take bearings right up to the transmitter antenna.
The range control is set up to effectively allow one-handed operation on an ARDF foxhunt without the need to fiddle with any extra switches. The tone mode does not normally function when signals are very weak. In this case, setting the sniffer to the “A” position will switch the receiver to the Audio (LSB) mode without the need for changing the mode switch. This allows very weak signals to be tuned and rough bearings can be determined. For this to work the mode switch should normally be left in the “Tone” position.
As the signal gets stronger the range control is stepped up through the ranges from 1 through 8. Tone mode is automatically selected when any of the numbered ranges are used. If Audio mode is ever required with attenuation, setting the mode switch to “Audio” will force the sniffer range control to function in audio mode only.
With a bit of practice you will be able to get a good idea of the distance to the transmitter for each range. You will find this much easier and more intuitive than trying to use a continuously variable range control, especially if you are used to hunting on 2 metres with an auto-ranging sniffer.
R2 - 10K, 1/8W or 1/4W axial resistor
R4, R6 - 100K, 1/8W or 1/4W axial resistor
RV1, RV2, RV3, RV4, RV5 - 50K Bourns 3296W PCB mount multiturn vertical trimpot or equivalent ( Jaycar RT-4654)
RV6, RV7- 5K Bourns 3296W PCB mount multiturn vertical trimpot or equivalent (Jaycar RT-4648)
Q1 - BC857, SOT-23 SMD PNP transistor ( Farnell 300-0709)
SW1 – Lorlin 12-way PCB mount rotary switch ( Jaycar SR-1210)
X1 - 5 pin 2.54mm right angle header (optional)
R1, R3, R5 – Not fitted 0805 SMD resistor (optional, replaces R2, R4 and R6)
The Victorian ARDF group web page: http://www.ardf.org.au
Joe Moell’s (USA) foxhunting web page: http://www.homingin.com
The Author’s web page: http://www.foxhunt.com.au
The Author’s email address: email@example.com