Construction of Attenuator

1 Suitable for fox hunts at 144mhz

2 Good screening

3 Attenuation selectable to suit S meter on radio

4 Easy to make with no special tools

5 Low cost approx 5.00 if parts have to be bought

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One of the most essential tools required to perform well on 2 meter DF hunts is a good-switched attenuator to reduce the RF level received within the range of the radio S meter to enable peaking of the incoming signal.

I have now built three attenuators and have found a two-way PI-network unit to be the most practical with 10db and 20db switched attenuation. This gives three settings up to a maximum of 30db. Previous experiments suggest that greater attenuation ahead of a handheld radio typically used on DF hunts the incoming signal would bypass the attenuator anyway and get straight into the rig through the plastic case. Pi-network configuration was chosen rather than T-network as Pi gave a smaller neater unit with less lead lengths. The chosen attenuation values where selected to give optimum performance with the dynamic range of the LCD S meter in my Standard C500 handheld. I tried various values by experimentation and found a value that when the S meter was just hitting maximum, switching in the lowest value reduced the S reading just above minimum so further peaking of the incoming signal could be done. This is what I refer to as the dynamic range of the S meter. This arrangement gives four ranges of hitting full scale on the S meter with just two switches!

Parts required are a chassis BNC plug, chassis BNC socket, 2 DPDT (double pole double throw) toggle switches and 6 resistors values selectable to suit the dynamic range of the S meter on your radio. The box was made as small as possible out of double-sided PCB and an old Duckhams oil can cut up and soldered to the PCB to complete the screening. The overall size of the finished housing is unit is 37mm 27mm 17mm excluding sockets and switches. This enables the input and output connectors to be directly soldered to the switch poles and keeps all leads as short as possible. If a chassis plug is used this means that you don't need a patch lead to connect it to the rig. Connectors can be chosen to match the rig in use.

Cut the top, bottom and ends out of double sided PCB and drill the holes for the switches and sockets.  Solder your chosen R3 resistors and the shorting links on the back of the two switches and fix to top PCB. The ends can now be soldered in place and the input output connectors fitted and soldered. The bottom piece of PCB can now be soldered into place and the R1 R2 resistors fitted as shown in the picture. The open three-way attenuator shown in the photographs containing three, six and twelve db steps was built for the purpose of varying the Tx RX drive to a transverter but other wise is the same construction as my two-way DF unit.  I used tin snips to cut up an old oil can to make the side screens. After degreasing and when you are happy with your chosen attenuation values solder the sides on to the PCB frame to complete the screening.

Table of resistor values for Pi network 50ohm attenuator.

            R1      R2      R3

3db     292     292      18

6db     150     150      37

10db     96      96      71

12db     84      84      93

15db     72      72     136

18db     64      64     195

20db     61      61     248

24db     57      57     395

Circuit of Attenuator

Use the nearest preferred values available e.g. in the case of 10db use 100 ohm instead of 96 and 68 instead of 71. It does not matter that it ends up as 9.6db and 51.9 ohms impedance.

Now this construction page has been added to the clubs web-site it is hoped that other members may upload articles about their home construction that may be of interest to others locally or further a field.

In a future article a "sniffer" will be described for use when within 100 meters of the fox.

My thanks to Colin G3RLA who kindly took the photos

Denis G3UVR

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