Need some info for JR 642 radio
I don't care what the specs say on the literature, they have the same RF board and it is tuned the same way. The official specs must be bunk.
If your antenna feels loose there is something wrong. You may have a loose/broken antenna mount. Open the radio up and look it over. the antenna mount should be well soldered to large pads on the RF board. The nut should be tight against the lock washer on the antenna mount screw. It it has worked loose, tighten it. If the antenna mount has broken off the RF board, re-solder it. This damage is caused by dropping the radio on its antenna or the radio taking a hard hit against the antenna, such as bumping around inside a vehicle.
I don't know much about the JR/Graupner radios they are not sold over here in Missouri and I rarely see them.