Using this 3 element Yagi along with a modern 2m receiver, signals from a FOX station can be detected up to 20 miles away in open country. As far as the receiver is concerned, an S-meter is not essential, with the aid of an adjustable attenuator, you can use the signal to noise ratio of the monitored transmission to establish the relative strength of the signal.
Building an effective adjustable attenuator at 144MHz is not easy and requires a good knowledge of screening and coupling techniques. This is where the active attenuator scores, as virtually no screening is needed; mine only uses a tin enclosure because it was available and I could solder the 50 ohm sockets directly to the chassis, providing good continuity between both.
The active attenuator presented, is really only a simple mixer, combining a relatively low frequency local oscillator, in this case generated by a clock oscillator module running at 4MHz. The level of the 4MHz local oscillator signal is easily controlled using a simple potentiometer, rather like a volume control in an audio circuit.
You may be thinking at this point, why not just use a potentiometer to attenuate the FOX signal directly? Well it doesn't really work at frequencies above HF, it's down to screening again, there is too much coupling and bypass at VHF of the variable resistor. In addition the feed impedance to the antenna will be very poor causing its directional properties to be compromised.
Mixer
The mixing in this design is performed by a single silicon diode, it takes the incoming FOX signal, for instance 144.625MHz, via the beam antenna and combines it with the 4MHz local oscillator drive. This produces mixer products and there will be two close-in images of the FOX transmission, one at 148.625MHz and the other at 140.625MHz – these are the FOX frequency plus and minus 4MHz!
If we tune the receiver to either of these mixer products they are far enough away from 144.625MHz not to swamp the receiver when close to the FOX transmitter. In addition if we reduce the level of the low frequency local oscillator, we also proportionally reduce the mixer product we are monitoring. It's a simple idea and was first aired by the ARRL back in the late 1970s, The design works remarkably well and produces a maximum attenuation range of zero to in excess of -60dB. Which with a small rubber ducky antenna will enable you to get within touching distance of the FOX transmitter, while the Yagi is used for initial cross country bearings further out.
The design shown in Figures 1 & 2 is not mine and was borrowed from Joe Moell - KØOV, there are many other derivations of the design some using discrete low frequency crystal oscillators some are available as mail order kits.
If we tune the receiver to either of these mixer products they are far enough away from 144.625MHz not to swamp the receiver when close to the FOX transmitter. In addition if we reduce the level of the low frequency local oscillator, we also proportionally reduce the mixer product we are monitoring. It's a simple idea and was first aired by the ARRL back in the late 1970s, The design works remarkably well and produces a maximum attenuation range of zero to in excess of -60dB. Which with a small rubber ducky antenna will enable you to get within touching distance of the FOX transmitter, while the Yagi is used for initial cross country bearings further out.
The design shown in Figures 1 & 2 is not mine and was borrowed from Joe Moell - KØOV, there are many other derivations of the design some using discrete low frequency crystal oscillators some are available as mail order kits.
Figure 1. Complete mixer circuit, the grey line around the circuit is the enclosure.
Figure 2. Internal layout of my attenuator in a small tobacco tin.
Antenna
The 3 element Yagi is constructed from a budget 5m steel measuring tape, plastic electrical conduit and fittings from Screwfix. It cost less than £15 to build, excluding some scrap coaxial cable, self-tapping screws, cable ties and Velcro patches. Again I claim no originality for the design of the antenna shown in Figure 3 & 4, being sourced from Jeffrey Bail - NT1K web pages and mechanically modified to work with the cheapest fittings I could get.
Figure 3. Completed antenna with elements folded back on themselves, it can be used in this configuration when thrashing through hedges causing less element tangle.
A hair-pin match is used to balance the antenna feed to the coaxial cable, which is then coiled around the boom to form an RF choke, helping preserve the antennas directivity with reference to feeder position.
Figure 4. Hair-pin match and built in RF choke.
The antenna has a good front to back ratio and can be used back to front in order to provide more attenuation when approaching the FOX station. Please follow the homingin.com link below, as it's a mine of useful information on fox hunting techniques. Radio fox hunting is great fun and can be done on foot, in a local park or wood, or by car, although fuel being at a premium this is a less attractive proposition. It’s always nice to adjourn to a local hostelry when the FOX has been found, for refreshment and a wash-up meeting to compare notes.
73 Rick G6AKG
References:
Text and artwork © 1998-2019 by Joseph D. Moell - KØOV. All Rights Reserved. http://www.homingin.com/index.html#toc
3 ELE. TAPE MEASURE YAGI, Prints By: Jeffrey Bail - NT1K. All Rights Reserved.
https://nt1k.com/wp-content/uploads/2012/11/3L-Tape-Measure-Yagi-Assy.pdf
