FMpleas1981
Newbie
Does anybody know exactly how to tune this antenna? And at what length to cut it for the 460-470mHz range? The overall length is 31.5".
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Maxrad 5db 200 watt gmrs antenna
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sloop
Member
If the antenna you are referring to is the MUF4505....it covers the 450 to 470 freq. range. Any minor adjustments should be included with the instructions and usually are as simple as moving the antenna slightly in or out of the mount using set screws..
mmckenna
I ♥ Ø
The antenna will ship with a 'cut chart' that shows you what length to cut the bottom whip to. You should cut it a bit longer than suggested, then use an SWR meter or antenna analyzer to check the SWR and then make short incremental cuts to get the SWR as low as possible. Usually the cut charts will get you really close. There can be some variation depending on your exact installation, so following up with the SWR meter is a really good idea.
You can look at the cut chart here:
You can look at the cut chart here:
Without trying to rat-hole this thread, I'm very interested in Sloops comment. I've read, I think on the cutting instructions that came with a GMRS antenna, that one should cut it to length and fully insert it before tightening the screws. That was the only antenna I've ever cut for tuning before and I did as recommended (cut to specified length and fully seated). On a cheap 2m/70cm mag mount antenna, I once loosened the screws only to find it wasn't fully inserted when I'd received it. Without any cutting, I did fully insert it while wondering if I'd just screwed up the tuning. I still use it to this day, however, and the performance has been good so I leave well enough alone. But I've often wondered about tuning with the set screws - whether that was possible.
mmckenna
I ♥ Ø
Not sure I'd ever trust an antenna coming from the factory 'pre-tuned'. There are some low profile ones that are not tunable, but I'd still check them before using them.
Usually following up with an SWR meter or an antenna analyzer is standard operating practice. Using the set screws to adjust whip length would be required.
I've only adjusted antenna length by cutting and then fully seating to the bottom before tightening set screws. However, I've seen videos where folks have not seated it fully and used the set screws, making me wonder if that's even effective. The Laird antenna I cut recently mentioned in the instructions to fully seat it, so that's what I did, but none the less, I wonder whether anyone could - if desiring to retune to a different frequency - just pull it up and retighten the set screws. Seems, on it's face, that having it seated fully and making metal-to-metal contact at the bottom would be the wiser practice. I just wondered if, electronically speaking, not fully seating it would still work. Know some antennas are also coated to make them black, so not sure how well contact would be made with just the set screws in such a case.
There would be no difference in performance with the whip bottomed out or just barely held with the set screw just grabbing the bottom edge of the whip. Nearly all of the RF currents at VHF/UHF are riding on the surface of the metal and they don't penetrate very far, so RF will flow from the top of the base coil or metal stud to the outside of the spring then transition to the outside of the whip and it will never see the set screw contact. Any metal contact deep down inside the whip hole is out of the circuit. Except at DC or when measuring with an ohmmeter.
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mmckenna
I ♥ Ø
The set screws bite through the coating. Some whips don't have coating on the bottom bit of the whip.
It's really easy to file/sand off a bit of the coating if that's a concern, but after installing/testing antennas for 30 years now, I've never had an issue with it. The set screws need to be dogged down pretty tight.
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