The advert is from Practical Wireless February 1987.
In my last blog I tried to explain how a QRM Eliminator works. Here’s some more information.
Suppose the signal you are trying to hear is a sine wave (shown in blue). The signal as received (shown in red) will have some noise added as it travels to you.
The QRM eliminator allows you to pick up the noise with your noise aerial and phase shift by 180º. As shown in this chart.
If you add the blue and red signal together you’d get a zero signal. So if you mix the inverted noise with the noisy signal as received you’ll recover the original signal.
Of course reality is different and the recovered signal won’t be as clean as that. Also if you don’t match the noise and main signal amplitudes properly, you’d then get something like this.