I have some GMRS walkies that work very well and are 5 1/2” tall including the antenna. It seems like there are no other GMRS walkies available that are less than 6” tall. I’m assuming this is because 6” or so is 1/4 wave at these frequencies. Can anyone explain how much less powerful my 5 1/2” walkie should be than one with a 6.1” antenna at GMRS frequencies? I believe the radiated power of mine are about 1.75 W.
Actually, I don’t think anyone can tell you that. Gotta measure it to be sure. i really doubt if the difference in antenna length will make any -significant- difference. Holding the thing over your head instead of by your mouth will make about the same difference… maybe.
Great. It’s good information to know that the difference won’t be dramatic especially for short or medium distances. I thought there might be a little more “magic” having an antenna length an exact fraction of a full wave. Now I realize that this may not be the case.
It sounds to me that:
Two GMRS radios with the same output power that are both tuned properly for their antenna size should work about the same (short and medium range of 2-3 mi), even if one has an antenna that is 90% as long as the other.
I asked my original question because my old 5 1/2” walkies seem a lot more portable than 6.1” walkies and I didn’t understand why no one manufactures shorter than 6.1” any more. There appears to be no good reason (other than for long range situations) not to make them smaller.
UHF radios at the bottom end of the band have an antenna with a slightly inconvenient length, so for years now, many manufacturers have artificially shortened them by gentle or not so gentle helical winding. In practice the smaller antennas are just a bit less efficient, but better in the pocket. It’s never occurred to me to ever measure any of my radios, and I have quite a lot in my hire stock. With hand-held operation A speaker mic to enable the radio to be in the clear always works better than a so-called better antenna in a trouser pocket!
Interestingly, I took the (Cobra CXR825) 5 1/2" walkie apart and discovered the helical antenna (which looks just like a spring) does not run the length of the 5 1/2" case. It is actually only 2 1/2" long! This really surprises me because with what I’ve experienced (and read), the performance of these is better than average for GMRS walkies.
The good performance you’ve observed could be because helical antennas are very good at going through obstructions that would otherwise cause trouble. I have no experience with them in radios (unless they’re buried in some of my FRS/GMRS radios), but I’ve used them quite a bit for 2.4ghz wifi.
I assume the 2.5" you measured is the length of the coiled segment? Remember that the antenna length is the actual length of the wire if you were to stretch it out straight.