by Clint Turner, KA7OEI
What is an "offset
Suppose you have narrowed down the source of the signal to a city block. At this point it is impractical to drive much closer - all you can do is circle. Your mobile DF unit isn't particularly portable, so you can't use it on foot and the signal is so strong that no matter where you point your portable Yagi (which, on 2 meters, isn't particularly portable) you get a full-scale reading. You get out of your car, or off your motorcycle, or off your horse and start looking for the source of the signal on foot: You are now doing "close-in" DFing.
One of the tools you can use to do this sort of direction-finding is the "Homing Circuit" - a device that will tell you whether or not the signal is to your left or right or even behind you. In close quarters, however, these devices are prone to being "fooled" by multiple reflections. In these cases it is nice to have a number of tools to use and a good 'ole signal level meter might be useful: The higher the reading, the closer you are to the transmitter's antenna.
It would be handy to be able to use the S-meter in your radio to help find the signal, but these signal meters (usually a bar graph on an LCD display) have quite limited range - typically between 20 and 30 db - depending on the radio. Furthermore, these meters will often read "full scale" on signals that are not really very strong at all: The Yaesu FT-530's meter is quite typical - on 2 meters and 70cm it will typically read full scale with a signal level of just 5 microvolts!
There is another, rather simple, method that can be used to attenuate the signal for which you are looking: Move it somewhere else!
Using a simple mixer circuit one can make the signal for which you are looking appear on another frequency. Why is this helpful? To provide this conversion, you would mix the signal being sought with a bit of signal from an offset oscillator and create a new signal: The more signal you supply from your offset oscillator, the stronger the new signal is and vice-versa. Conversely, if there were no signal from your offset oscillator, then there would (theoretically) be no offset signal.
Typically, you would pick your offset oscillator to be a nice, even frequency- like 1 MHz or 4 MHz (that is, one that you can add/subtract in your head.) If, for example you were tracking a signal on 146.52 MHz and you used a 1 MHz offset oscillator, you would also be able to hear the 146.52 MHz signal 1 MHz away - on 145.52 and 147.52 (as well as progressively-weaker conversions on 144.52, 148.52, etc.) and you would pick whichever frequency was handiest and the most free of interference.
"Gotchas" when using the offset attenuator:
The offset attenuator is not without its quirks and problems, however:
Using the Offset attenuator:
How does this help track signals then? By varying the amount
signal from the offset oscillator you can also vary the efficiency of
frequency conversion. Because the receiver is no longer tuned to
the frequency of the original signal, the strength of that signal is
solely on the efficiency of that conversion. Simply by
the signal level from the offset oscillator, you can adjust the
strength of the converted, up and down, to keep it within the range of
your radio's S-meter. In reality, you can vary the strength of
"new" signal from "full" (between 30 and 50 db down) to, for all
purposes, infinity. That is, you can be right next to the
transmitter, and still adjust for an on-scale S-meter
Another tactic for using the offset oscillator is to adjust the
attenuation such that the received signal becomes very noisy: As
you approach the signal (or aim the beam toward its source - if you are
using one) the signal will become less noisy. This scheme has the
advantage that it can be done by ear without having to stare constantly
at the S-meter - which is particularly helpful if you are wandering
around in the dark while looking for the signal.
What sort of antenna should one use with the offset
attenuator? A Yagi is rather obvious, as it is directional and
one can simply points the antenna for the best signal, but one can also
get good results with a simple rubber duck antenna and using the "Body
Shielding" method. In this method, one holds the rubber duck
against the chest and then adjusts the attenuator for a noisy
signal. One then turns the body (while holding the antenna still)
until the signal is the weakest (adjusting the attenuator as necessary)
and at the point of the weakest signal, one knows that the signal (or a
reflection) is coming from directly behind: One then walks in
that direction for a while, and does the steps again to verify the
direction. In this way, even the simplest equipment can allow you
to find a signal. (Comment: Don't forget to hold the
antenna horizontally or at an angle for some of the readings, too -
just in case the signal you are looking for happens to be horizontally
A Practical Offset attenuator:
How it works:
Power is supplied using a 9 volt battery, passed through the LED to
a voltage regulator. The +5 volts output from the regulator
a stable source of power for U2, the 1 MHz oscillator. Please
note: Use only "normal" Red, Green, or Yellow LEDs as
have a voltage drop of between 1.9-2.1 volts. The "Ultra High
types (All blue, white and many green or yellow) have drops of 4 volts
or so of drop and are unsuitable for this application.
The output of U2, a 1 MHz "crystal can" oscillator, is filtered with L1 and C4 to provide 40-50 db of harmonic attenuation at 2 meters. The filtered signal is applied to one end of R1, a 5k linear taper potentiometer (on which the power switch is mounted) and the wiper of this pot goes to a 1k resistor, R2, which "flattens" out its "rotation versus attenuation" characteristics, preventing as many db of change from being "scrunched" into as small an angle of rotation. An audio taper pot may also be used - in which case you may not even need R2.
Also connected to R1's wiper is D1, our mixer diode. Practically any diode will work to some degree, but a good, cheap device to use is the venerable 1N914 or 1N4148. Connected to the other side of this diode is L2: This coil provides a low-resistance DC ground path for the diode, but a high-impedance RF path Its value is not critical and could be practically anything from 4.7 uH to 220 uH. This end of the diode is coupled to the antenna terminals via R3, a 150 ohm resistor. The purpose of this resistor is largely to prevent the diode/potentiometer from being destroyed if one accidentally transmits "through" this unit. In the prototype, no damage was done by brief (2-3 second) transmissions of 5 watts.
S2 switches a simple attenuator in and out. When in the "out"
position R4, the 47 ohm resistor is in parallel with the 1.5k resistor
(R5) and that total resistance is placed between J1 (the "Receiver"
and J2 (the "Antenna" port.) The attenuation caused by this
is typically in the range of 4 and 6 db, depending on a number of
such as cable lengths and antenna/receiver characteristics. When
S2 is in the "-30db" position, the "bottom" end of R4 is grounded and
received signal is attenuated by approximately 30db.
Comment: R4, the 47 ohm resistor, is stated as being
a 1/2 watt unit only so that it can withstand a few seconds of being
accidentally transmitted-into at several watts of transmitter
power. A 1/4 watt unit can be used if you are quite sure that you
won't be transmitting into it - but even that should survive very brief
"accidental" bumps of the push-to-talk button on your radio.
This attenuator is extremely simple - and is, in fact, not a "true" 50 ohm attenuator. Also, when the switch is in the "out" position, there is still some attenuation in line. Do we care? Not really: If you are so close to the transmitter than you need to be using the offset attenuator box anyway, the extra 4-6 db of loss is irrelevant. The 30db attenuation isn't precise, but its sole purpose is to knock the signal way down and prevent receiver overload (see the sidebar below.) Note: If you were to use a DPDT switch, you could completely avoid the intrinsic 4db loss of this circuit and use a "true" 50 ohm attenuator. A simple SPDT switch was used because it was available, and this one had a very short handle.
There is at least one situation that could cause you chase your tail when using an offset attenuator. This can happen when you are very close in proximity to the source of the signal. Front end overload!
What happens is this: You have tuned in the signal for which you are looking, say, 1 MHz away. As you get closer to the source, this offset signal also gets stronger - but so does the original. In this case, you dutifully turn "up" the attenuation to compensate. Doing so has no effect on the original signal, however.
As you get even closer, it is possible that the RF stages (and mixer) in your receiver are starting to overload. As this happens, the receiver will become less sensitive to other signals - including the offset signal being generated by the offset attenuator - which seems to start to get weaker. This effect is often called blocking.
Different radios manifest effects of blocking in different ways and at different signal levels. The FT-530, for example, starts to exhibit signs of signal overload at just -30dBm. (Note that this radio is more susceptible than many, but most Handie-Talkies are susceptible to this problem.)
The result? The closer you get to the source of the signal, the lower the reading you may get from the offset signal. This can, of course, greatly confuse the attempt at locating the signal.
What can you do about this?
Performance of the prototype:For the bands 6 meters through 222 MHz, the minimum attenuation of the offset attenuator is about 30 db - going up to about 50 db on 70cm. This frequency response characteristic is most likely a result of the properties of the diode itself: A microwave Schottky diode (like an HP 5082-2835) will likely provide more consistent performance across the VHF and UHF range, but is likely to be much more fragile than the (cheaper) 1N914/1N4148. The attenuation/mixer characteristics will be very strongly dependent on the diode you choose!
The maximum (and minimum) attenuation will also depend on which "offset" you pick. If you picked something 1 MHz away, attenuation of 90 db or more should be easily obtainable. This may not be enough if the transmitter you seek is high powered and/or very very close to you. In this case, moving 1 MHz farther away will reduce the signal even more. In the unlikely event that that was still inadequate attenuation, you could move still farther away (in 1 MHz steps) from the original signal.
A few comments:
The addition of L1 and C4 reduced the amount of 1 MHz harmonic energy by between 40 and 50db on 2 meters by removing many of the harmonics of the 1 MHz oscillator. These harmonics may actually be weak enough that they will not cause a full-scale S-meter reading, allowing use of this device to locate a transmitter on an even 1 MHz harmonic. If it is desired, one may use a higher frequency oscillator (such as 2 or 4 MHz) to reduce the number of frequencies within the band that will be covered by the harmonics. A change of oscillator frequency will, of course, require that the values of L1 and C4 be modified - or, if the presence of strong harmonics doesn't bother you, removed altogether. If a 4 MHz oscillator is being used, suggested values for L1 and C4 are noted on the schematic.
A warning about using an offset attenuator with some inexpensive Chinese radios:
With the advent of very inexpensive (less than $100) Chinese-made radios there has arisen an issue when they might be used with an offset attenuator. Many of these radios (e.g. Baofeng) use a single "transceiver on a chip" for all receive and transmit functions including IF filtering, frequency synthesis and demodulation, leaving only the RF preamplifier and filtering and transmit power amplifier functions to external components.
While these "radio on a chip" devices work quite well under most circumstances, they will sometimes fail in the presence of a strong, off-frequency signal. It is precisely this condition that can occur when when is using an "offset converter" and is getting close to the transmitter.
Here is what can happen:
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Note: Neither the author or UARC officially endorse any vendors mentioned above. The level and satisfaction of performance of any of the above circuits is largely based on the skill and experience of the operator. Your mileage may vary.
This page updated on 20170116