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Wide-band VHF-H/UHF Mobile antenna

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I'm looking for some suggestions for a new dual band antenna. What I was using was a Comet CA2X4SR that recently began performing very poorly after a tree branch strike. Well, I pulled the antenna and tested it and it has an internal short somewhere. Lo and behold, also damn near pulled the mount out of the roof but I was able to fix that with a sheet metal persuasion device (aka DBH).

Anyway, looking for something that will perform well in the 144-148 MHz ham band, 440-470 MHz part of the UHF ham, GMRS, and commercial bands as well is a bit more flexible and able to withstand an impact such as say…a tree branch at 30 MPH roughly 2/3 the way up the antenna from the base.
 

mmckenna

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Something with a spring base sounds in order.

Not sure Larsen still makes them, but they used to have an NMO2-70 dual band antenna with a spring base. I have one at home somewhere. Designed for the ham bands, it's likely not going to be an outstanding performer for GMRS, being 20MHz out of the designed band, but still, might be worth a shot.

Another option would be to use just a NMO 1/4 wave VHF. They'll work as 3/4 wave on UHF and will tune up on the 70CM band no problem. I did that a few years ago with a dual band radio. The SWR on 70CM band was actually lower than the SWR on 2 meters. The radiation pattern is a bit funky with a 3/4 wave, but it worked just fine for what I needed.
 
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I'm liking the idea with the 1/4 wave VHF. I did that once and never really thought about it until you mentioned it. That would also be pretty LP with my 900 MHz antenna. Twas just a low hanging tree. I think I got the two problems with the Comet narrowed down so I think I might go with a 1/4 wave and then just keep the Comet up for road trips or save it as a spare incase I kill the one on my Cherokee.

Anything UHF is strictly on repeaters for the most part so I think I could probably take the 25W limit off my UHF radio and let my 75W VHF radio run a little hotter than usual and still be okay.
 

popnokick

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I had a Comet CA2X4SR and snapped it off driving into a garage. Replaced it wit a Comet SBB-1 16" dual band flexible whip. It's been great on 2m, 440, and GMRS. No more worries about garages or tree branches. Here's a link:
http://www.cometantenna.com/products.php?CatID=1&famID=4&childID=4
 

BirkenVogt

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I can't speak to the tuning on UHF, but for broad band performance you need a 1/4 wave and the broad band ones such as the Laird B132S or the Larsen equivalent are even better. They work because they have a large chunk of copper in the black housing where the coil would be and the mass of the spring at the base all help to make the antenna thicker than a simple quarter wave and therefore more broad.

I only use single band radios but I use a B132S to cover the ham band to the Federal band and it kind of works. I don't think I keep it below 2:1 SWR, more like 2.5:1 at the edges but it is tricky to get it even that good. But it works, good enough anyway. An SWR analyzer where you can view a plot is almost essential to this type of installation, otherwise, you spend a lot of time fine tuning the edges.

I will try to remember to sweep my B132S and see what it looks like on UHF.
 

mmckenna

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I agree, looking at the useable bandwidth of a 1/4 wave on a good antenna analyzer is an eye opener. While higher gain antennas have their place, it's pretty impressive what a good 1/4 wave can do.
 
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Yea, my current vhf/uhf setup consists of a yaesu ft-2900 and a Icom F221. About to swap the Icom for a Kenwood TK-880.
 

BirkenVogt

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I looked at my B132S today and found it tunes very well across the ham band all the way to 174 and beyond in VHF. In UHF, its center frequency is around 455-460 and stays good to the top of the commercial band, but it also does not go much above 1.5:1 to 1.7:1 a the bottom of the ham band.

However, my experience with VHF antennas on UHF has been that they are a little suspect. They may present a good SWR to the radio, but somehow the actual performance seems to not be as good as hoped.
 

mmckenna

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However, my experience with VHF antennas on UHF has been that they are a little suspect. They may present a good SWR to the radio, but somehow the actual performance seems to not be as good as hoped.
Yep, the radiation pattern gets pretty funky due to it being 3/4 wave on UHF. Works fine out where I am where there are lots of hills and mountains, but likely not best if you were out in the plains.
As an easy/low buck solution it works.
 

Boatanchor

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While quarter wave antennas do offer reasonable bandwidth, they are not miraculous.

The bandwidth of a quarter wave antenna is directly related to the radiators diameter (and quality of it's ground plane obviously). Larger/thicker diameter radiators provide greater bandwidth than do thinner elements. If you could make a VHF 1/4 wave antenna 2" in diameter, you would have about 20Mhz of bandwidth. A thin stainless steel whip may only offer 8Mhz of bandwidth on 146Mhz.

And a previous poster was correct in that while you will obtain VSWR nulls on odd multiples of the fundamental design frequency (on UHF), the radiation pattern would be awful.

You can also wind a choke into the VHF 1/4 wave, so that the bottom 6" or so forms a UHF 1/4 wave and the remainder of the choke plus the top portion of the antenna forms a VHF 1/4 wave. There are designs for this on the net, but be warned, this reduces the VHF bandwidth considerably.

Stick with the Comet CA2X4SR if at all possible :)
It is a pretty slick antenna for broadband coverage and gain.
 

mmckenna

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If you read the original post, you will see the OP has already suffered a failure of this antenna and is looking for a replacement. Replacing it with the same antenna will likely result in similar failures.

A properly designed 1/4 wave whip on VHF with a proper ground plane will have a lot more than 8MHz of useable bandwidth. Not sure where you are getting your numbers from, but I've confirmed this with the test equipment I use at work. We're not talking magnetic mounts or trunk lip mounts, but a real NMO in the center of a truck roof. Not using a proper mount will certainly result in the poor performance you see from your antennas.

When the amateur grade antennas break, it's time to look at something better. As I said, the 1/4 wave VHF whip used on UHF isn't the ideal solution, but it will work.
 

dksac2

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You really cannot beat the Comet 4x4 antenna, it's fantastic.
I use one and it does have a fold over feature when going somewhere that it might get hit.
I also carry 1/4 wave 2 meter and 70 CM antennas that I put on if I'm going off road or someplace the antenna may get hit.
Just be aware that it's up there and stop and fold it over or change it out to a 1/4 wave if necessary. You can go with another antenna, but none will work as well as the 4X4 for wideband use and the gain it has does help quite a bit in tough places signal wise.
It also does not bend much in the wind at speed, which is a good thing also.

73's John
 
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You really cannot beat the Comet 4x4 antenna, it's fantastic.

I use one and it does have a fold over feature when going somewhere that it might get hit.

I also carry 1/4 wave 2 meter and 70 CM antennas that I put on if I'm going off road or someplace the antenna may get hit.

Just be aware that it's up there and stop and fold it over or change it out to a 1/4 wave if necessary. You can go with another antenna, but none will work as well as the 4X4 for wideband use and the gain it has does help quite a bit in tough places signal wise.

It also does not bend much in the wind at speed, which is a good thing also.



73's John

The 2x4 is a good antenna. I originally had 2 of them. Keep one on my jeep on a hood channel mount. The fact that it is such a stiff antenna is it's downfall. The fold over I didn't discover until after I trashed the one on my trailblazer. Was driving through a neighborhood at night that was poorly lit. Didn't see a tree hanging over the street. Hit it and it took the antenna out. Had to pull the NMO mount apart and beat the roof back where tabs on the bottom of the NMO tried to pull out. Not gonna run one of those on the roof again.


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