Frank

Wouldn't you be concerned over presenting a slight Z bump of a 75 ohm
piece of material? I'll admit I cant see it making a huge difference,
but as one is building test gear it might pay to be pedantic.

I'd also be concerned that pushing wire into the air gap would lower the
Z in an unpredictable way. Pushing it just under the shield keeps it at
a reasonably fixed dielectric spacing from the inner conductor.

If it works however, then I won't argue!

Bob VK2YQA


Highland Ham wrote:
Instead of using RG213 coax I would use a short piece of 75 Ohms coax
(here in the UK used for sat dish feeder) with air gap dielectric .
The 10 mm (3/8 inch) OD variety is particularly suitable .
Without removing the outer sheathing a bare or teflon insulated wire can
be readily pushed through.

Bob Bob wrote:
Frank

Wouldn't you be concerned over presenting a slight Z bump of a 75 ohm
piece of material? I'll admit I cant see it making a huge difference,
but as one is building test gear it might pay to be pedantic.

I'd also be concerned that pushing wire into the air gap would lower the
Z in an unpredictable way. Pushing it just under the shield keeps it at
a reasonably fixed dielectric spacing from the inner conductor.

If it works however, then I won't argue!
==========================
I'll check the anticipated 'bump' with the MFJ259B analyser.
First connected to a high quality professional dummy load good for freqs
to 2 GHz (ex cellnet industry),thereafter with the homebrew SWR sensor
inserted,testing in 2 stages : First with just the short piece of 75
ohms coax (obviously having the braid connected to chassis at one end
only) ;Secondly with the wire pushed into the air gap.

This is a useful and interesting NG thread.

Frank GM0CSZ / KN6WH