Funny this topic pops up now. I just ran an involuntary test about just this type of thing. To wit:
Two days ago I was in a hurry to get on the road. Since I was going on a 600 mile trip, I wanted to bring a CB to help avoid traffic problems, but unfortunately the CB was not in the car I was going to use. So, I quickly grabbed my travel kit comprised of a plugged-up Uniden 520 radio, an Italy RL 203P amp and an SWR meter, brought it out to the car and plugged it to dedicated 12vdc ports in the car. I then put a magnetic Wilson 500 antenna on the roof of the car and promptly forgot to hook the thing to the SWR meter and thus to the transmitting equipment. Bad ju-ju.
As I always do, after starting the engine, I mashed the transmit button on the mike just to make sure my SWRs are OK. I do this because you never know what could go wrong. Well, for the first time, and to my surprise, the SWR meter reported really, really bad values. Of course, to check my sanity and make sure I was testing everything right, I repeated the process several times, with and without the amp, and consistently the meter pointed only to failure of both the radio and the amp achieving decent SWRs. Then, in a eureka moment, I realized that I hadn't plugged the antenna into the system. Well, I thought, that's an easy way to throw your money away.
I then hooked up the Wilson to the SWR meter and thus to the rest of the system and things immediately looked normal, with the SWR at about 1.1 from the 2-watt radio solo and 1.3 with the 60-watt amplifier. But there's more:
After I got back from the trip, on which my equipment seemed to be working well, I decided to verify that it actually was working as well as it should be. Since my car was parked next to a metal building, I inserted a 50-ohm dummy load in place of the antenna and an SWR/watt meter in placer of the SWR meter in order to test the transmitting wattage from my equipment. When I keyed the mike, the SWR meter and the watt meter both were acting funny, quivering and then moving in tandem back and forth and doing all sorts of things that made me fear that I had screwed up my equipment from the original sin of transmitting without an antenna. After fiddling around with this a while and finding only unsatisfactory and totally erratic results, I went inside for a cup of coffee feeling sad and mad and stupid and, well, you know what I mean.
After the coffee perked me up a bit, I returned to the car to begin troubleshooting. I figured the easiest first step was to check the dummy load device, which I had bought a year or so ago at a cheap price from ebay and seemed to work just fine on a couple of projects, but you got to start somewhere. So, I removed the dummy load and screwed on the Wilson antenna coax and, guess what, my SWRs suddenly looked fine and the watt values seemed healthy. I confirmed that my radio actually was producing its nominal two watts deadkeyed into the amp, and the amp was producing its nominal 60 watts deadkeyed into the antenna, and that ole Democrat refrain "Happy Days Are Here Again" began playing in my head, and I'm no Democrat.
So, I threw away the cheap dummy load like a saved cripple his crutches, and I began to hope that the 15 or 20 times the radio and amp were keyed to bad loads have not caused harm down the road. All I know is that at this juncture, things look OooooKkkkk.
So, if you, dear reader, are still with me, the moral of the story "seems" to be that yes, transmitting without a proper antenna load may be bad, but if you are watching your instruments and don't mistreat your equipment too excessively, then you can get away with it, at least, maybe, sometimes.
