How to measure/diagnose/cure weak VHF output?

[Gentlemanracer]

I suspect my VHF output is weak. For the last 2 weeks cruising in company, other boats owned by friends have only been able to hear me when very close. Even my tiny handhelds work better. Receiving seems better, though not great. I have not accidentally selected 1w transmit; the 25w is clearly showing in the display

Also my boat doesn't show on marine traffic despite ais transmit being activated, and using the same antenna as the voice VHF. This suggests my ais transmissions also have weak range and are not reaching the base stations, which points to a problem with the antenna or antenna wiring. That all looks ok on visual inspection

The position of the antenna is ok: my previous boat worked fine and shows reliably on Marine Traffic

I think this problem has existed since boat was new ( boat is 2013 squadron 78 with garmin 300vhf and garmin 600 ais. Boat antenna is in the standard position)

So, what do you think might cause this? Loose connections? Faulty antenna? Is there a meter I can buy that measures the output signal strength? Thanks for any ideas

With Garmin RADAR, plotter, AIS & ICOM VHF, same problem discovered this season when cruising in company. Probably a latent defect from 2008 build & installation. From my experience, do try VHF after powering down the RADAR; if VHF then works then you need no assistance in drawing sobering conclusions. They should, of course, all work to specification simultaneously and - lesson learned - only accepted into service thus! I'm very keen to follow this challenge being chased down; in my case, having returned cross channel into dense fog just south of the Nab anchorage and needing VHF and AIS and RADAR working simultaneously it's bye bye Garmin, hullo Raymarine. We survived via hand held VHF at max power to very nearby company (with Raymarine kit) & wake following. Yes, I could invest more in chasing it down further but modest research tells me that particular investment is well over-due from the manufacturer and no place for me. Good luck, listening...