Originally Posted by nakman
Dave I'm not sure we're saying different things.. as you still say it's technically possible, but not a good idea. agreed.
But if you were transmit on 70cm with a 2m antenna to a truck that's is 50' behind you, I'm not sure he wouldn't hear you. We can try with one of your radios..
You are very right that for RX the 2m antenna would be marginally different in the real world than a tuned antenna on 70cm. It's possible that a 5/8λ 2m might even receive slightly better than a proper 70cm.
I read that this was for TX on 70cm potentially using a FT-8800 with a 2m-only antenna since the FT-2900 does not even receive 70cm. That would not be a good idea at all.
The RF that comes out would be due to the incidental and reflected waves on the coax itself. The antenna wouldn't be doing much work. In reality it would almost the same radiated energy you get from a dummy load, which works BTW. Running 100W into a power resistor or even an incandescent light bulb radiates energy and you can actually hold a QSO using a light bulb as an antenna if band conditions are good. It takes quite a bit of power to do that and the field you generate is about equivalent to a super QRP radio into an tuned antenna (think 200mW).