UHF antenna base

[jase]

A question
I just bought a couple of cheap UHF antenna bases from Jaycar
I have just notice that the Threaded centre "post" (that the antenna screws onto) is not insulated from the ground (body of the car?)
is this correct?
I doesn't sound right to me?
did I get duds?
thanks
jase

[VladTepes]

Depends on the aerial type.

I know with 27 MHz some antennae are ground dependent some are ground independent.

I think that all UHF are ground independednt (not sure about this though). Perhaps the bases you have are actually for a ground dependednt 27MHz antennae rather than a UHF one ?

[jase]

they are defiantly UHF (26TPI vs 25 TPI off the top of my head)
perhaps they are supposed to be grounded? I don't know it just looks wrong (and I thought I would have noticed on UHF bases before now?)
why run a shielded cable to an antenna then ground it? (the cable not the shield)
I could just connect the antenna cable straight out the back of the UHF to the chassis (not that I would just that's the same logic)
come on where the radio techs P38 Ron are you out there?
Jase

[one_iota]

There are two types of UHF aerials:

One that relies on attachment to a metal surface to radiate the signal and the other (Ground Plane Independent) that doesn't.

It sounds like the bases that you have belong to the former type.

Some useful info on UHF aerials here:

http://www.mobileone.com.au/antennas/default477.htm

[waynep]

If those are the bases I think they are, they have a threaded brass post that runs up the middle. This is what the antenna whip screws on to and the centre conductor of the coax connects to. There should be an insulator where that post goes through the metal of the car.

If you could post a photo of it, it might help us tell if something is missing from the kit.

[jase]

sorry can't post a pic (I tried last time and couldn't get it to work ??)
it's like you say but NO insulator :eek:
like you I imagined the "center post" (that the coax conductor connects to (not the shield) & the antenna screws onto) should be insulated from the body of the vehicle but in this case it is not??
It's connected to (short circuited to) the shield and therefore the body of the car??
J
"Help me" radio techs ;)

[langy]

Fear not - it being a UHF base, the insulator for the centre wire is inside the mount. To fit it you need to strip the sheath and some of the outer conductor of the coax off for about 3cm, and wind the cable into the base. Then you solder it at the top.

[jase]

thanks Langy, you're spot on :D , I knew it was a "solder the top type" but couldn't see how it was separated / insulted from the bottom half
but a check with a multimeter tells me no connection top to bottom (the copper to air ratio is correct :) )
looking inside you can barely see a line, and disassembled nothing on the outside? the join is obviously sleeved under the moulded plastic lower section somehow?
anyway Thanks heaps
I'll install it now, much happier that it's correct

Jase