Recently I saw an idea for a very useful marker for 47.088GHz. Some things put out a reasonable amount of harmonics, others very few. Over recent years (in the UK) there has been quite a lot of surplus 22GHz link equipment become available (and we'd like more!) which was DRO based. This was on 22-23.5GHz nominally. Unfortunately the harmonic output was almost non existent and so something was needed to generate a 2nd (and more) harmonic. It is almost certainly putting out a 3rd harmonic - just right for 76GHz. G3PYB came up with the very simple idea shown below.
The mixer diode is taken from a 12GHz satellite LNB/downconverter - these are available, either for free or a few $, secondhand. These have two diodes in them and these are in series. This is mounted at the bottom of an SMA socket between the pin and ground. A small piece of thin wire acts as the DC return and a choke. That's it! The pin acts as the antenna. I can copy mine over several feet when fed (in my case) with a 20mW Xtal derived source (part of the narrowband project which was to hand) on 23.5GHz, using a WBFM system - it should be an enormous signal with an NB system. Since I was driving it quite hard I put a 50R chip resistor in the ground leg of the choke but I haven't ascertained whether that helps or hinders. I measure 10mV across it.
So, if you want a cheap, easy to build, signal source, start looking for an LNB.
UPDATE
At the Rutherford Appleton Laboratories there was an opportunity to look on a spectrum analyser to 47G. The results were very good. With a small horn on the Tektronix mixer the signal was above the noise floor up to about 4 ft away. Since there was no coupling into a guide the best estimate of output was -60dBm to -40dBm. These are respectable signal levels! It makes you wonder what it would be like on a bit of Duroid and coupled into a guide. Back to the home page