Kannad 406 ELT 'Low RF power' error

[SarahC]

So, I took a gamble on a 2nd hand Kannad 406 AF-Compact ELT complete with remote panel and ELT10-773-3 antenna.
Made up a RG400 cable, checked no shorts on cable. Connected everything on kitchen table and did an ELT self test according to manufacturers manual, which passed with single beep and our scanner gave a warble on 121.5.
So far so good.
Then 1st problem as all other RV8 builders have found: where to put a 23" antenna. Did not want in in cockpit for kids to play with (break) or people to loose an eye on. No room on rear turtle deck as canopy slides too far back.
At 23" the antenna is too long to mount on the rear bulkhead under tail fairing.
So we've tried installing a bracket a couple inches forward of the last bulkhead. The antenna is mounted to this bracket and has electrical connection to airframe. The antenna goes through rubber grommet to extend under tail fibreglass fairing.
I know this mounting is not ideal, but I'm a bit stumped on where else to put it.
Did another self test of the ELT. This time it failed with 3-2 beeps 'Low RF power'. Still got a warble on 121.5 on the scanner.
Has anyone else had a 'Low RF power' fault, or know what I could try to fix this?
Am I flogging a dead horse with this antenna installation?
Has anyone got a recommendation for an antenna short enough to fit under tail fairing that with works with Kannad 406 AF?

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[fl-mike]

I would bench check it again, just in case you have a VSWR issue in the installed cable or antenna. If you have an RF dummy load, that would be quickest.
Unfortunately, if the fault code remains, the CMM says "send it in".