Thread: what does the choke do?

Are you referring to the little coil in the antenna whip itself ?

It serves a number of functions but mostly it is for impedance matching, i.e. so the "load" seen by the radio/coax is correct (50 ohms ). With RF, unlike DC, the load is not purely resistive, but has capacitive and inductive elements. You'll notice the shorter 1/4 wave whip doesn't have that coil, because it is inherently a 50 ohm load.

The effective ground plane in those antennas is provided by the shiny tube at the base. If you can imagine the normal sheet of metal you'd mount the base on twisted up into a tube. ( well, in simple terms anyway ).

I think that's right, but I am trying to recall my radio theory from thirty years ago.

indeed its the shiny tube thingy (great technical term isnt it!) that i'm curious about.... whats in it and how does it function?

also, am i to take it that the whip that sits on top of that nice shiny tube is for all intents and purposes a ground dependant antenna?

which choke are you refering to?

the RF choke
the noise suppression choke?
and I think theres one in the tuning bit as well..

umm....

basically.... wtf is the difference between a ground dependant antenna and a ground INdependant one?

some require a specific ground plane to operate on and others work, essentially, off of the planet itself.

very very generically

the ground plane of a vehicle mounted antennae is the panel to which it is mounted which is why some antennae work better on a guard than on a bull bar

non ground depenant antennae are like the ones that your hand held 2ways use or the antennae that dont mind where they are mounted on the car.

Originally Posted by Sprint

umm....

basically.... wtf is the difference between a ground dependant antenna and a ground INdependant one?

There's a few designs of ground independent antennas.
Most of the ones you see on 4WD bull bars with the "shiny tube thingy" are the elevated or raised feed design. ( which is the type I was referring to in my reply above ).
These have the advantage, when mounted on a bullbar, of getting the whip section a bit higher up, and also are fairly simple and robust.

Here's a fairly good definition of the different types

Untitled Document

interestingly, speaking with my local communications shop, they reckon the raised feed base i have will be fine with basically any 477mhz monopole antenna section screwed onto the base.....

Originally Posted by Sprint

interestingly, speaking with my local communications shop, they reckon the raised feed base i have will be fine with basically any 477mhz monopole antenna section screwed onto the base.....

Yes, although its probably best to stick to the whip that came with it.
If you lose it any whip will do as long as it's tuned for the correct band, and the threads match.