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.
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I’m afraid the prior post is wrong in a number of ways. That’ coiled part on the antenna contributes to the antennas resonance/efficiency. Change it and things go to ■■■■ very fast. A helically wound antenna is typically a less than efficient type antenna. It’s shorter than an unloaded antenna, but that’s it.
I still don’t fully understand the technical benefit that the helical design may add or even if the added length and coil improves performance. In my experience however I strongly agree with Paul that, “smaller antennas are just a bit less efficient”. I don’t believe there is any performance loss for normal use except over long line of sight distances.
My shorter (low profile) walkies are also rated for up to 30 miles line of sight. I know of course this is likely a made up number but it is similar to what is claimed for most taller units. This is why I am surprised that no one makes a shorter, more portable GMRS walkie any more.
An antennas length determines what frequency it will operate most efficiently. An antennas length may be physically shortened by adding inductance (a coil). The length of the wire making up that coil has no significance. How tight you wound that coil does make a difference in both the length of wire used and the resulting inductance. A long skinnny coil and a short fat one can have the same inductance regardless of the amount of wire used to make either of them.
…confused yet??
Sorry for the very late reply; apparently I wasn’t notified of the new messages.
@W5LZ is confusing loading coils with helical (circular-polarized) antennas. They are entirely different things. My statements on helical antennas are correct. To work well, they have to be wound to very precise diameter and coil spacing matched to the frequency and desired performance (directional or omni, and gain). There’s quite a bit of info available on the web, should you wish to look into them.
Helicals, especially VHF ones are not the ¼wave physical length wound around to make it shorter because the helix is of course a form that has inductance, so the wrap tightness and diameter have an impact on the actual length of wire. It’s pretty close but a helical cut for say 166MHz pulled out is not the 45cm the ¼ wave would be. The helical design also gets tweaked to cope with the poor ground plane a hand presents - so it’s a compromise antenna.
The helical is just more convenient. My experience of VHF helicals is that the usual physical size has sufficient capture area to do a pretty good job, and compared to the impracticality of a full ¼ wave it’s a winner.
Here in the UK, back in the 70s, the Police used UHF handhelds, with really low power - typically 2-3W and the helicals were only an inch long. Crazily small, tiny capture area yet the performance was actually pretty amazing. Base station antennas on a tower and a reliable coverage area of around 5 miles in flat country. We took some of these radios and converted them to work on our ham repeater and got better range with them than the Police did! These antennas really shouldn’t;t have worked that well, but they did - held in front of the face up in the clear. In a pocket, they were useless!
Helicals can be surprisingly good. All the manufacturers have been using them for so long now that they really are proven designs - the only design issues is the small gain increase with length tempered with the inconvenience of that extra length.
With ham and hobby operation, experienced users who understand how antennas work can swap to ¼ wave, hold the radios appropriately and squeeze every last metre/yard out of the radio’s performance. Commercial users have very different expectations. Radios are often annoying to have, and are expected to cope with people whispering at two feet away, or being inside a jacket, with the user too lazy to remove it from being pressed against a warm body. The radio call will work or not work. If it doesn’t it’s the radio’s fault. Hobby users will happily walk up a short rise, or hold the radio up high and use a remote mic to squeeze that little extra but commercial users don’t do this.
One thing - NO radio is rated for up to 30 miles. None of them. Range is topographical. My best two hand-held distance conversations were on a low power handheld that ran 1.5W. This got me to an astronaut in the early Skylab spacecraft. It also got me over 100 miles across the North Sea to Holland beach to beach - during a tropospheric duct. A Watt on flat ground might get you four or five miles to a similar radio, but that’s doubtful. Put a decent antenna the other end and that Watt might be happy doing ten - but further than that is something like a lottery. Bump it up to 10W and very little improves. The issue is path loss - and geography is more critical than power. In hilly country - being on top of the hill certainly means 30 miles to a distant hill is possible, and that’s how manufacturers justify their crazy claims. The important thing is to remember we rarely have both radios on the top of hills. If you had a laser, could you make the beam hit the other person? That’s the only question. If there is anything in between so the laser light was blocked, you can bet the radio signal will die too.
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(Brief delay in responding due to pandemic path losses). All makes sense about a very small, carefully designed helical antenna working incredibly well and geography being most important in determining whether communication will take place. This solves the mystery for me about my small helical antenna.
I’m still overwhelmed by the performance of my little radios that are no longer made. I haven’t found any made today as small, with such a small antenna, vibrate ring, and battery life of several days that perform as well. I decided it was worth it to perform surgery on the innards to barely fit 1100ma 14500 batteries as an option to the 850ma proprietary lithium battery.