145MHz Slide Attenuator:
One of the problems when transmitter hunting, can be the overwhelming strength of the hidden transmitter. As mentioned in the Beginners only section of this site, when the signal gets too strong it will be difficult to obtain directional information from your aerial.
Probably the simplest attenuator you can build is this ‘slide’ type, and whilst not a calibrated piece of test equipment, it will allow you to easily insert large amounts of attenuation in the aerial lead and so control those ‘overload’ situations.
How it works
The attenuator is basically two coils inside a screened tube.
Inserted between the hunting aerial and your radio, it acts like a variable transformer, coupling the radio signal from one coil to the other. The further the two coils are apart, the weaker the coupling and so the greater the attenuation. (Fig.1)
The two coils are contained inside a copper (rather than plastic) pipe, to electrically ‘screen’ the coils from everything but each other. You must remember that any external ‘stray’ pickup will defeat the effect of the device.
Components
You only need a handful of components to build this unit...
1 x BNC panel mount socket
1 x BNC cable mount socket
1 x 6 to 8 inch length 15mm dia copper tube
3 x paper clips
1 x piece UR76 coax, length to suit
2 x 20 swg. Short lengths of enamelled copper wire (diameter not that critical).
PVC tape, solder etc.
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Farnell 324-1440
Farnell 309-515
DIY store/junk box
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Construction
This really is a ‘junk box’ project, if you can use a saw, file, heavy duty soldering iron, cutters and pliers then you can build one of these devices.
The attenuator is basically a length of 15mm copper water pipe with a small coil fixed inside it at one end, a similar coil fitted to a ‘slug’ that can slide back and forth inside the pipe is inserted in the other end.
Using the 20 s.w.g. copper wire, make two coils like the one shown here. The overall diameter of the coil should be about the same diameter as the threaded portion of the BNC.
Clean the enamel from the ends of the wire and tin with a soldering iron, now solder one of the coils to the BNC socket as shown. Solder the centre connection first, now make sure the other end is pressing tight against body of the connector without external assistance. (If you fail to do this, then later when you solder the connector in the tube, this connection may ‘spring’ undone as the solder softens and cause an open circuit!)
Make sure the nut is nice and tight on the BNC as this will eventually be soldered to the end of the copper pipe.
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The second coil will be used for the other half of the transformer, and will be attached to the sliding section of the attenuator.
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Take the second BNC connector and file the shoulder until the connector is an easy slide fit inside the copper tube. With a 1mm diameter drill, bore 3 holes at 120 degree spacing in the front face of this shoulder to take the three spring contacts. Cut the three contacts from the paper clips and solder into place as shown in Fig 4.
Remove excess solder from the edges of the connector with the file.
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Take the UR67 coaxial cable and fit this modified connector to it. Once you have decided how long to make the lead you may then fit a connector to the other end to match the aerial input on your radio.
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Now fit the coil in place, clean the ends of the copper wire and tin with a clean iron. Insert the centre wire of the coil into the ‘pin’ of the BNC connector and test for length. Trim the outer wire length to just overlap the outside of the connector. Carefully heat the inner wire and run some solder around the joint, now solder the outer wire to the BNC body as shown in Fig. 5.
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Try the sliding coil/slug for fit in the copper pipe and adjust the paper clip spring contacts to give a smooth tight contact. Wrap some PVC tape around the back of the connector to take up some of the free movement at the rear of the ‘slug’ (see Fig 5). The tape keeps the slider running true in the bore of the pipe and also relieves any sideways ‘twisting’ pressure on the spring contacts.
Clean one end of the copper pipe using wire wool or fine emery cloth. With your heavy duty soldering iron, lightly tin the edges, and about 3mm down the outside of the pipe.
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Carefully align the ‘fixed’ BNC connector and coil assembly in the end of the pipe, keeping them concentric, solder the two together.
Take care not to get solder on the connection part of the BNC, yet apply sufficient solder to the joint to make a smooth looking assembly.
If you fitted the coil to the BNC correctly, then it shouldn’t be affected by all the heat!
Once cool clean off the residue with a little methylated spirit or an alcohol based computer ‘screen wipe’.
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Operation
‘Simplicity itself’, connect your aerial cable to the fixed connector on the end of the copper pipe, connect the sliding lead to your radio aerial socket and away you go!
If you want to try to add some calibration marks, then one idea is to wrap a white self adhesive label around the coax where it enters the tube. Then by borrowing a suitable rf attenuator, it will be possible to make comparative measurements to aid your calibration process.
Attenuation steps
This attenuator is of course continuously variable and fairly linear over a 90 dB range.
The minimum attenuation is around 20dB (10x) which is a good starting point for the addition of attenuation when hunting.
To increment in 10dB steps, you will find that the marks are quite close together (about 3mm) but still perfectly useable.
Fig. 7 shows the label with some marks for each 10dB of attenuation
Experiment
The principle of this device is so simple that you should be able to experiment with whatever materials you have to hand. Try PL259’s and larger diameter pipe, spring wire or strip instead of paperclips. The idea behind this project was to see just how simply we could make a wide range attenuator.
Have fun!
copyright © silent design associates. 2001. all rights reserved.
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