The Capacitive Bottom-fed Fat Monopole.

Alternative title:- Serendipity rules, O.K.

This post is not about an obese Polish person who has a huge appetite, lives alone and has to be fed anally. I know a hilarious suppository joke, but it's not really suitable for this blog.

Having stayed up all night reading nearly 800 comments on Jack Kruse: Neurosurgeon. Leptin Reset and Cold Thermogenesis. Controversy, I noticed Sean's comment about Antennas.

I designed antennas for 225MHz to 400MHz portable man-pack radios (also a 1GHz to 3GHz Ultra Wide Band monopole antenna). There were two existing man-pack antennas, affectionately known as "The Bird-cage" and "The Egg-whisk". Electrically they worked well, but they would both catch in branches when the radio was used in woods.

My mission (should I choose to accept it, which I did) was to design an antenna that had a good impedance match over 225MHz to 400MHz, a good gain and couldn't get caught in branches.

The reason why the original antennas were shaped like bird-cages and egg-whisks was because barrels and inverse cones give a better impedance match than a piece of wire. Don't ask me why. The answer is extremely complicated and even I don't understand it!

The antenna had to be a bottom-fed monopole (an antenna which is designed to work with a ground-plane) with a connector at the bottom which plugged into the radio's RF connector. The RF system impedance was the standard 50 ohms.

As fat cylinders give a better impedance match than thin cylinders (a wire being an extreme case of a thin cylinder), I went for the fattest cylinder that would be acceptable on a man-pack radio. I designed an impedance-matching transformer using one of THESE made out of THIS. I connected the transformer to the end of the fat cylinder and examined the impedance using a 8753C Network Analyzer (or even older model).

During some faffing-about, a wire snapped and I was amazed to see that the match to 50 ohms improved. This led to other improvements being made, resulting in the Capacitive Bottom-fed Fat Monopole. It was rugged, it couldn't get caught in branches and it had a high gain. You could even bash somebody over the head with it without breaking it. It worked well on field trials. There were two versions - a short one for covert use which had a lower gain and a longer one for normal use which had a higher gain.

I hope you found that interesting. It's nice (for me) to blog about stuff that I have qualifications in!