In reply to MM0FMF:
See Wireman-J-Pole and search for his 2m measurements and diagram. I have the following lengths for my version.
A = 130cm
B = 42cm
C = 3cm
My gap is only 5mm not 10mm.
I used the transparent 300 ohm flat twin feeder from Maplin and feed with approx. 5m of RG174 (quite lossy). It’s built as per the website but with my lengths. Hanging free in the shack from a light fitting with the feeder partly coiled and partly draped over the junk in here, my MFJ259B says
145.5MHz SWR=1.3 R=63 X=5
I think I made it the length in the article originally and trimmed it a bit. I wanted a lightweight backup antenna for VHF as the 817 and rubber duck was not up to the job in Scotland, a better antenna was needed most of the time due to lower population density and less flat areas.
The SWR can be finicky! I normally attach the top of the J-Pole to the top of my fishing pole and attach the bottom of the twin feeder to the fishing pole with a double sided Velcro strap. The feeder is allowed to hang free and terminates in a BNC plug. I have a 1m(ish) length of extra flexy RG58 that has an SMA male on one end (W.H. Westlake) and a BNC cable end female. This gives me the ability to use the FT 817, VX 170 and IC-80AD with the antenna. The 817 seems to often get RF feedback problems and I think a ferrite choke on the microphone cable is needed on mine on 2m. I have issues with the rubber duck on the 817 at 5W, might just be my 817 though. No problems observed with either 2m capable handheld.
Slightly OT: the VX 170 is better in high external VHF RF fields than the IC-80AD. There was some QRM/desense on Cyrn-y-Brain with the IC-80AD that I’ve not seen with the VX 170. No problems observed from near the main mast cable anchor on Winter Hill using the IC-80AD, so 700MHz RF (and many dBW of it) doesn’t affect it!
Andy
MM0FMF