Greetings,
I'm not a ham, but I am hoping a radio pro here can offer some advice. I am a private pilot and the VHF radio in my plane (118-137 MHz) is not behaving well. Reportedly my transmissions are weak and only partially intelligible. I sent the unit back to the factory a year ago, they replaced some bad capacitors and adjusted the TX frequencies, but now the problem is back.

I bought a handheld unit (Yaesu FTA-550) and am considering just using that in flight--it is legal, and performance is supposed to be adequate if used with an external antenna. But, my installed radio (a Val 760 model) is a little more powerful and it is a less "kludgy" fit in my aircraft's cramped interior.

I'd like to do some testing to find out if my existing radio really has a problem, or if I've got a problem with my cable or connectors to the external antenna.
Ideally I'd like to be able to see field strength and look at the spectrum of the TX. Is there an inexpensive meter for doing this? I've seen some very impressive add-ons for Android devices or USB devices for laptops that do analysis of various kinds, and was hoping something already existed for at least VHF field strength measurement. I won't use it often, so I don't want to spend a lot of money, but an avionics shop will probably charge me $100 just for a one-time diagnosis--I'd rather have the tools to do it myself.

Thanks in advance for any assistance, and accept my apologies if I've used incorrect terminology, etc.

Mark