Saturday, 7 April 2012

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!


praguestepchild said...

No pics of the actual antenna?

Nigel Kinbrum said...

It's a military product, so no.

It's an Olive Green Drab metal tube of about an inch diameter and a foot and a half long, with black plastic caps at top and bottom, a chemically-blackened barrel spring at the bottom and an N-type RF plug beneath that.

LeonRover said...

I thought at the beginning you might discourse on the magnetic monopole, long seen as a missing member of the set of elementary particles.

I recall one of my (ancient) physics profs, who himself made his bones researching high energy gammas in the 1920s using balloons, giving a talk to 2nd years on this topic. (We were baffled until quantum in our 4th year).

I do not believe that physics teams near Geneva, currently (sic!) hunting the Higg's Snark will find it either.

The debacle with synchronizing satellite clock links, thus "finding" hyperluminary neutrinos was somewhat bizarre.

Slainte and hope you enjoyed Boat Race.

Nigel Kinbrum said...

I'm a "hands-on" kinda guy who likes solving techy problems. Theoretical Physics goes right over my head.

Writing about stuff that I designed yonks ago is helping my memory, which on occasion has resembled Emmenthal.

I shall do some more posts about stuff that I designed without going into too much detail, as I'm still constrained by the Official Secrets Act.

LeonRover said...

In a former life I worked as an outside consultant for DGW(N), Bath. It was tasked for procurement of weapons and other equipment for installation at first build and refits for RN.

I was negatively vetted to level Secret non-Nuclear ( can no longer recall the jargon). This was during Balcombe St. etc.

"But that was in another country,
And besides, the wench is dead"

PS It a VERY slow holiday Monday!!

Nigel Kinbrum said...

I was vetted to a level high enough to allow me to work on hand-held radios which contained plug-in encryption units to a high & secret encryption standard.

Funnily-enough, the above radios were for the security forces in NI. Radios for other customers contained Racal encryption units.