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Amateur Radio Antennas - For discussion of all amateur band designed antennas and related accoutrements. This includes base, handheld, mobile and repeater usage. For commercial antennas on the amateur bands please use Commercial Radio Antennas below.

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  #21 (permalink)  
Old 07-02-2017, 8:21 AM
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Originally Posted by mmckenna View Post
And what's the UHF side?
Another matter all together.

Larsen says that on 440-450 MHz it's a "Colinear" with 3 dBd of gain.
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Old 07-02-2017, 8:51 AM
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And what's the UHF side?
If I remember right the top section was about 16" (5/8 wave) and the bottom section was about 18" (almost 3/4 wave).
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Old 07-02-2017, 9:07 AM
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A Motorola AVA on an ATV? You must be a glutton for punishment!


UTV. The ATVs are specifically waiting for the GME CM60 to get through CAP compliance and become available.


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If a repeater IDs and no one is on frequency to hear it, does it even make a sound? Yes, because KC5MVZ is monitoring you…
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Old 07-10-2017, 8:44 PM
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Hey everyone, thanks for contributing to my question. I ended up getting a Larsen 0270B and intend on mounting it on the roof of my flatbed. Confident it will most closely meet my needs and I just wanted thank you all again for the help.
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Old 07-11-2017, 12:43 AM
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Old 07-26-2017, 7:07 AM
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Default Trimming the NMO antenna length.

Hi guys, I've been looking through out the whole radio reference website for this kind of information and cannot find it so I thought I would ask an expert here. I'm just looking at where I should start my measurements, when trimming the physical length of the antenna element on my and NMO magnet mount roof antenna. Do I make my measurement from the button on the top of the NMO mount to the tip of the antenna? Do I make my measurement from the beginning of where the threads are on the stub to the tip of the antenna? Do I measure from the base of the magnet to the tip of the antenna? Do I measure from the roof of the car from the tip of the antenna? I'm just wondering where I do begin to make my measurements so I can measure it accurately (I'm just setting up my length for a scanner I do not have the ability to transmit). I am trying to cut the length to the middle of the railroad frequency range. I already have the length I need, just need to know where to start measuring!

Any help would be greatly appreciated!
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Old 07-26-2017, 8:32 AM
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Originally Posted by Mikejo View Post
I'm just looking at where I should start my measurements, when trimming the physical length of the antenna element on my and NMO magnet mount roof antenna.
The antenna starts where the coax sield and ground plane ends.
How you measure it depends on the particular model antenna and mount and how long the unshielded metal in the mount is.

If the antenna instructions has a cutting chart then go by that. If the antenna is for receive only then the length isn't critical.
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Old 07-26-2017, 9:08 AM
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Well, I guess it's difficult to determine where the shield for the cable is attached inside the magnet mount antenna. The cutting guide just gives you basic numbers "18 inches for 152-162 MHZ" they don't tell you where to start your measurement from, that is why I'm trying to determine i where and how much to cut the element element to it to get as close to an accurate SWR electrical length without having to purchase the fancy SWR meter and a ham radio to transmit with in order to achieve a close match.
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Old 07-26-2017, 10:11 AM
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Quote:
Originally Posted by Mikejo View Post
Well, I guess it's difficult to determine where the shield for the cable is attached inside the magnet mount antenna. The cutting guide just gives you basic numbers "18 inches for 152-162 MHZ" they don't tell you where to start your measurement from, that is why I'm trying to determine i where and how much to cut the element element to it to get as close to an accurate SWR electrical length without having to purchase the fancy SWR meter and a ham radio to transmit with in order to achieve a close match.
An easy way to measure it. If the antenna is adjustable (uses a set screw to hold the whip in place) then you simply measure the whip length of just the whip (blunt cut to static ball). If it's something like a traditional 1/4 wave NMO antenna you'd take the "chrome nut" off and measure from the brass button to the static ball however, those antennas typically don't get cut (Sti-Co flexiwhip is the only exception I can think of) as they are built for a specific band and will generally perform with a 1.5:1 SWR with a 1/4 wave radius ground plane for the advertised frequency range.
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Old 07-26-2017, 10:32 AM
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It's a shame, that when you buy the antenna, they don't give you all the information you need to make an accurate cut. I've already shortened another element that I have and I have a achieved way better performance than with the stock antenna cut according to their cut chart! I guess I'll just have to get another element and keep-snipping until I get it close then I can do a little math and figure out what I need to know for the future.

Thanks for your help much appreciated!
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Old 07-26-2017, 11:09 AM
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When you are playing in the commercial equipment world, it's expected that you have some experience in this stuff. At least that is the way every manufacturer I've dealt with does it.

Even with the manufacturer provided cutting charts, they are a "best guess" thing that will get you sort of close. Think of it as a "good enough" thing. It'll work. Following up with an SWR meter or an antenna analyzer is aways a good idea. In fact, the antenna analyzer approach will tell you a whole lot about where the antenna is resonate and give you a better idea of how far you need to move it.

Usually the cutting length is measured from the top of the very whip. If you put a tape measure over the end and cut to the suggested length, that is often close. Co-linear type antennas usually measure from the bottom of the loading coil to the bottom of the whip.

As always, follow up with an SWR test if you've got the tools.
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