Offset Attenuator

An Offset Attenuator

It may be a bit late for this year's DF fox hunt, but it could be something you may care to have a look at for the future. Peter G8YNC found an article on the net by Joe Moell K0OV which could help you to win next year's hunt.

The full article runs to several pages and goes in some detail into the problems of DF hunting, and if you want to look at the item in full the webpage appears at the end.

The schematic diagram is shown here, and is almost simple enough for even me to follow!
It consists of a local oscillator (LO) X1 and a diode mixer with a potentiometer to control the attenuation. (U1 is a voltage regulator which can be changed if required.)

When hunting the fox with no attenuator the signal will usually cause the S-meter to hit the end stop long before you get anywhere near the transmitter. Using a passive resistive type attenuator is fine except that it is prone to RF leakage through the case or cables etc..
This reduces the effectiveness of your highly directional beam you are using for the hunt !
A better way to get bearings is to convert the strong on-frequency signal to a weaker off-frequency signal. You then tune your receiver to the offset frequency and measure its strength versus direction with your RDF beam or the “body shield” manoeuvre.

This system causes a shift in the received signal of ± 4.0 MHz. If the DF signal was on say, 145.45 you would tune to 141.45 or 149.45. The higher the LO level, the higher the amplitude of the offset signal applied to the receiver. To increase attenuation, decrease the LO signal into the mixer with the control. (You could use a different local oscillator module and tune accordingly.)

The circuit can be built on veroboard or similar and can be mounted in a small diecast box that can accommodate the board and a PP3 battery. As the circuit uses about 27mA a bigger battery might be a good idea, or use a CMOS oscillator that uses less current.
In the original article details of construction are given if you feel the need to use them.

In use, get the fox's bearing with S1 turned on and the dial fully clockwise, which is minimum attenuation (about 4 dB). Listen on the fox's frequency. Increase attenuation as necessary by turning the knob counterclockwise. At the stop attenuation is about 20 dB.

When this is not enough, tune up or down to the first offset frequency, knob fully clockwise again (about -30 dB). Continue the hunt increasing the attenuation as you go. When you get to -100 dB you will be within a few feet of the source.

This unit can be used with any VHF transceiver or scanner that has a removable antenna. The receiver must have sufficient frequency coverage to permit tuning it 4 MHz away from the hunt frequency.

The author states that the offset attenuator operation may be degraded on frequencies that are exact multiples of the oscillator, such as 144.0 or 148.0 MHz. Also if there is a strong signal from another source that offsets onto the frequency you are monitoring, you may experience cross-modulation interference.

It is not a good idea to transmit through this attenuator ! Your antenna will emit strong spurious signals and you may burn out D1, so disable the transmit section or switch to the lowest possible power output. Also remember that an offset attenuator does not greatly reduce the level of on-frequency signal into your radio, so there is no protection for the receiver's front end. If you touch your receiver antenna to that of a transmitting fox you may damage both the receiver and the attenuator.

Have a look at the webpages that this article was extracted from. There is useful information there about components and a kit which is available.

Who's going to get the cup next year for winning the SARC Fox Hunt then ?

See :- http://members.aol.com/joek0ov/offatten.html

[With grateful acknowledgements to the author Joe Moell K0OV] - Mike Roedel M0ASA

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