An Offset Attenuator is often used by people participating in a transmitter hunt; those who are hunting the transmitter. One significant problem for transmitter hunters is that when they are near the hidden transmitter the signal can become so strong it pegs the receiver’s S-meter (the S-meter goes to full scale). No matter where the directional antenna is pointed, the S-meter becomes useless as a tuning indicator, because the received signal is so strong.
Because of this problem the hunter might add an attenuator between the directional antenna and the radio receiver. The attenuator might be as simple as a potentiometer, or as sophisticated as a laboratory grade step attenuator. These will help, to a degree. But eventually the hunter will again come close enough to the hidden transmitter that some portion of the transmitter’s signal will leak around the attenuator and again the receiver’s S-meter will be pegged. You might think that you would need to be within tens of feet of the transmitter for this to occur, but leakage or sneak paths can be fairly problematic at VHF and UHF frequencies. For example, my home is 1.6 km from my local 2 meter and 440 repeaters, with a hill in between. Yet when I connect my two watt HT to a commercial quality dummy load using a two foot section of good quality coax, I can still bring up both repeaters. In that case the leakage signal is strong enough to be heard one mile distant. So consider the hidden transmitter at two watts on a proper ground plane antenna. How well will it radiate around your attenuator when you get within a few hundred meters of the hidden transmitter?
An offset attenuator can provide an effective solution to the “leak around” problem. An offset attenuator consists of a local oscillator and an RF mixer, to produce an intermediate frequency output. This is very similar in operation to the first mixer and local oscillator of a superheterodyne radio receiver. One input to the mixer is the hidden transmitter signal from your directional antenna. The other input to the mixer is a fixed low frequency local oscillator *and* the amplitude of that fixed low frequency local oscillator provided to the mixer passes through a logarithmic taper (audio taper) potentiometer allowing you to control the amplitude of the fixed low frequency local oscillator applied to the mixer. Recall that the amplitude of the signal output from a mixer is proportional to the amplitude of each of the two signals going into the mixer. What that means is that as the signal from your antenna varies in amplitude (as you change the azimuth of your directional antenna), the output of the mixer will also vary and do so proportionally. Additionally, when the amplitude of the local oscillator signal is varied, that also will cause the mixer output signal amplitude to also vary and do so proportionally.
Let’s talk about a concrete example of an offset oscillator. Suppose that the hidden transmitter was transmitting on 146.52 MHz (maybe not the best choice of frequency, but we’ll still use that frequency for this example). So that will be one input to the offset attenuator’s mixer. And let’s have the local oscillator within the offset attenuator be at 4.000 MHz. With these to signals applied to their respective mixer inputs within the offset attenuator, that means that there will be at least two signals (sum and difference) at the mixer output: 150.52 MHz and 142.52 MHz. So now tune your receiver to either one of those frequencies (either one will provide you with the same result; and yes they are both out of band, but you are only listening on that frequency, not transmitting). Now imagine that there is a potentiometer (a basic audio volume control potentiometer) between the output of the 4 MHz oscillator and the mixer input. As the potentiometer is adjusted from maximum to minimum, the 4 MHz signal applied to the mixer goes from maximum to minimum, which causes the output of the mixer at 150.52 and 142.52 to also go from maximum to minimum. Thus, by varying the position of the potentiometer we can vary the amplitude of the offset signal out of the mixer that is passed on to the receiver. How much variability can we get by using this offset attenuator? Reports of up more than 70 dB of variability are common.
So why doesn’t the hidden transmitter signal on 146.52 MHz sneak around (or through) the offset attenuator to the receiver? The answer to that is “it does”; some portion of that 146.52 MHz signal is hitting the antenna jack of your receiver. But it doesn’t matter because your receiver isn’t listening to 146.52 MHz. Instead your receiver is listening to either 150.52 or 142.52, and there’s no signal at those frequencies coming from your antenna to sneak around the offset attenuator. So why doesn’t the 4 MHz oscillator sneak around (or through) the potentiometer? Well, because 4 MHz signals, being relatively low frequency, are not very effective at being sneaky, whereas a signal at 146 MHz, being a very high frequency, is almost always very sneaky.
This is how the offset attenuator works around the issues of leakage and sneak paths, while also providing a wide range of attenuation.
Are there any downsides to using an offset attenuator? Yes there are. The output of the mixer will produce signals at many different frequencies, not just the primary sum and difference frequencies. This is rarely a problem, but still something to keep in mind. An additional downside is that if you accidentally transmit into your offset attenuator you will probably blow up a few components within the offset attenuator. How much will that cost to fix? Probably not more than a dollar.
There’s some good info on the internet about building and using an offset attenuator for transmitter hunting. My favorite link is:
http://www.homingin.com/joek0ov/offatten.html
I liked his design so much that I modified that design slightly to use components that are more commonly available in 2023 and created my own printed circuit board. You can see a photos of my version of the offset attenuator in the photo below.
Figure 1 – Photo of Offset Attenuator
I should have the schematic, parts list and a Gerber ZIP uploaded here in a few weeks.