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VHF/UHF Mixer box from 1995 ARRL handbook
During a VHF transmitter hunt, the strength of the received signal can vary from roughly a microvolt at the starting point to nearly a volt when you are within inch of the transmitter, a 120-dB range. If you use a beam or other directional array, your receiver must provide accurate signal-strength readings throughout the hunt.
Zero to full scale range of S-meters on most hand-held transceivers is only 20 to 30 dB, which is fine for normal operating. but totally inadequate for transmitter hunting. Inserting a passive attenuator between the antenna and the receiver reduces the receiver input signal. However, the usefulness of an external attenuator is limited by how well the receiver can be shielded. Anjo Eenhoorn, PA0ZR, has designed a simple add-on unit that achieves continuously variable attenuation by mixing the received signal with a signal from a 500 kHz oscillator. This process creates mixing products above and below the input frequency. The spacing of the closest products from the input frequency is equal to the local oscillator (LO) frequency. example, if the input signal is at 146.52 MHz, the closest mixing products will appear at 147.02 and 146.02 MHz.
The strength of the mixing products varies with increasing or decreasing LO signal level. By DFing on the mixing product frequencies, you can obtain accurate headings even in the presence of a very strong received signal. As a result, any hand-held transceiver, regardless of how poor its shielding may be, is usable for transmitter hunting, up to the point where complete blocking of the receiver front end occurs. At the mixing product frequencies the attenuator’s range is greater than 100 dB.
Varying the level of the oscillator signal provides the extra advantage of controlling the strength of the input signal as it passes through the mixer. So as you close in on the target, you have the choice of monitoring and controlling the level of input signal or the product signals. which ever provides the best results.
The LO circuit, Fig 23.97 [see below], uses the easy-to-find 2N2222A transistor. Trimer capacitor Cl adjusts the oscillators frequency. Frequency stability is only a minor concern; a few kilohertz of drift is tolerable. Q1’s output feeds an emitter follower buffer using a 2N3904 transistor, Q2. A linear-taper potentiometer (R6) controls the oscillator signal level present at the cathode of the mixing diode, D1. The diode and coupling capacitor C7 are in series with the signal path from antenna input to attenuator output.
This frequency converter design is un-orthodox; it does not use the conventional configuration of a doubly balanced mixer, matching pads, filters and so on. Such sophistication is unnecessary here. This approach gives an easy to build circuit that consumes very little-power. PA0IZR used a tiny 1.4-V hearing-aid battery with a homemade battery clip. If your enclosure permits, you can substitute a standard AAA-size battery and holder. [Photo below uses AA]
Construction and Tuning
ARRL Headquarters will supply the PCB board etching template and parts overlay for this project. Request the Eenhoorn Active Attenuator template package from the ARRL Technical Secretary. A circuit board is available from FAR Circuits. The prototype uses a plated enclosure with female BNC connectors for RF input and output. C7, Dl, L2 and R5 are installed with point-to-point wiring between the BNC connectors and the potentiometer. S1 mounts on the rear wall of the enclosure.
Most hams will find the 500-kHz frequency offset convenient, but the oscillator can be tuned to other frequencies.[Notes from VK3VT - with the components shown my unit came out around 1MHz which works well.] If VHF/UHF activity is high in your area, choose an oscillator frequency that creates mixing products in clear portions of the band. The attenuator was designed for 144-MHz RDF, but will work elsewhere in the VHF/UHF range. [Has been used successfully on 50MHz, 144MHz, 440MHz and 1296MHz]
You can tune the oscillator with a frequency counter or with a strong signal of known frequency. It helps to enlist the aid of a friend with a hand-held transceiver a short distance away for initial tests. Connect a short piece of wire to Jl, and cable your hand-held transceiver to J2. Select a simplex receive frequency and have your assistant key the test transmitter at its lowest power setting. (Better yet, attach the transmitter to a dummy antenna.)
With attenuator power on, adjust R6 for mid-scale S-meter reading. Now retune the hand-held to receive one of the mixing products, Carefully tune C1 and R6 until you hear the mixing product. Watch the S-meter and tune C1 for maximum reading.
If your receiver features memory channels, enter the hidden transmitter frequency along with both mixing product frequencies before the hunt starts. This allows you to jump from one to another at the press of a button. When the hunt begins listen to the fox's frequency with the attenuator switched on. Adjust R6 until you get a peak reading. If the signal is too weak, connect your quad or other RDF antenna directly to your transceiver and hunt without the attenuator until the signal becomes stronger.
As you get closer to the fox, the attenuator will not be able to reduce the on-frequency signal enough to get good bearings. At this point, switch to one of the mixing product frequencies, set R6 for an on scale reading and continue. As you make your final approach frequently adjust R6 and take new bearings. At very close range, remove the RDF antenna altogether and replace it with a short piece of wire. It’s a good idea to make up a short length of wire attached to a BNC fitting in advance, so you do not damage J1 by sticking random pieces of wire into the center contact. While it is most convenient to use this system with receivers having S-meters, the meter is not indispensable. The active attenuator will reduce signal level to a point where receiver noise becomes audible. You can then obtain accurate fixes with null-seeking antennas or the "body fade" technique by simply listening for maximum noise at the null.
Fig 23.97 — Schematic of the active attenuator.
Resistors are 1/4W, 5%-tolerance carbon composition or film.