1/4 wave VHF antenna on UHF

mmckenna

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I recently installed a VHF mobile in my wife's new truck. I used a Larsen NMOQ 1/4 wave VHF whip permanently installed in the center of the truck roof. I have an Agilent spectrum analyzer with tracking generator that I borrowed from work to test it before I called it done.

Often hams will ask about various dual band antennas. I've pointed out a few times that a 1/4 wave VHF whip will act as a 3/4 wave on UHF. It is important to point out that a 1/4 wave VHF whip will have a nice radiation pattern on 2 meters, and it'll also have a funky radiation pattern on 70cm, often with major lobes above the horizon. Not a great solution for dual band use, but it is an option if you don't frequent UHF often. Still, a dual band antenna is probably going to serve you better.

But, while I had the test gear at home, I decided to run a sweep not only on the VHF band, but also UHF, just for giggles.
Here's the VHF sweep on a Larsen NMOQ cut for about 148MHz. You can see that a standard 1/4 wave permanent mount on the center of a truck roof will give a nice low SWR across the VHF band:


Here's the UHF band on the same antenna, a Larsen NMOQ cut for 148MHz:


-Remember, on the UHF band, this antenna is going to toss radiation lobes up above the horizon. Not ideal if you are out on the plains, but probably going to work well enough in the mountains where a repeater may be well above the horizon. I ran a Kenwood TM-D710 with a similar antenna for years.

Point is, the SWR is more than acceptable across a nice chunk of useable UHF spectrum. Easy enough to cover the 70 centimeter band, but also GMRS.

So, not trying to sell you anything, just sharing some screen shots for discussion. I still think a dedicated dual band antenna is probably your best choice for 2 meter/70 centimeter use, but using a simple 1/4 wave VHF antenna isn't going to result in damage to your radio, at least not from high SWR.

Here's a sweep of everything from 136-500MHz:


Center of the roof, ideal ground plane, no regrets.
 

mmckenna

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Just about to do just this on Sat...but why cut for about 148? I was thinking 146.5.
Because I run a couple of VHF systems at work that are in the 154-159MHz range. I tune it a bit higher than 2 meters so I'm not sacrificing performance up there. But as you can see, it doesn't make a big difference.
 

K4EET

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On @mmckenna's discone thread found HERE, this thread could have parts carved out and added to the suggested Reference Wiki that I mentioned in @mmckenna's discone thread. These sweeps are great references not only for purchasing decisions but also for learning various applications.
 

popnokick

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For hams, 1/4 wave antennas rock for 2M and 70cm particularly in mountainous areas. Our club did a railroad mobile event a couple of years ago and the train route took us through many steep ravines and hills well above the train. We had a mag mount 5/8 wave on the caboose and had trouble calling any of our "base" stations along the route. Switched to a 1/4 wave mag mount (owner wouldn't let us drill the caboose ;-) and VOILA! Coverage in the ravines. So the high angle of radiation worked to our advantage. However, if you are in mostly flat terrain (e.g. Florida or the Midwest) you may want to consider a 5/8 wave for 2M mobile. It pulls the radiation pattern downward toward the horizon... and offers some gain as well (typically 3dB). You do lose the UHF coverage BUT you will be able to use the 2M 5/8 wave as a 6M 1/4 wave!
 

mrweather

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Very nice write up, OP! If you're into satellite work, having 3/4 wave on 70cm works is advantageous because of the high-angle radiation lobes.
 

jhooten

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If only you were closer. I have a fat quarter wave made out of 1” copper (it was a slow day and the plumber had thrown a bunch or ‘scraps” in the trash) pipe, a couple caps, and some 3/8X24 hardware I would like to see swept. The MFJ-266 showed under 1.8 from 136 to about 162. But that was just before it died.
 

mmckenna

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If only you were closer. I have a fat quarter wave made out of 1” copper (it was a slow day and the plumber had thrown a bunch or ‘scraps” in the trash) pipe, a couple caps, and some 3/8X24 hardware I would like to see swept. The MFJ-266 showed under 1.8 from 136 to about 162. But that was just before it died.
I may try out a Larsen NMOQ and a Larsen NMOWBQ later today to see how they compare.
 

mmckenna

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I may try out a Larsen NMOQ and a Larsen NMOWBQ later today to see how they compare.
 

tunnelmot

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Props to mmckenna...solid data...no bs
Duly noted.
I truly think that a band specific 1/4 wave is the winner for many installs. But they're not as "sexy"..that includes myself as I have a few bux invested in low profile/covert/can type antennas. Some are excellent, some are dummy loads. But after some years....1/4 waves are tried, true, reliable and simple. Can't fight rf physics.
 
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mmckenna

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Why this antenna over the QW144?
Laird makes good stuff. The QW144 is a good antenna. Larsen makes a similar antenna, NMOQSPEC.
They are inexpensive and work well.

Problem is, the whip extends all the way down to the button where the contact is made to the NMO mount. The rubber cone is what keeps the water out. Over time, flexing, wear, vibration and temperature changes results in moisture making its way down inside the mating contact between the antenna and the mount. That can result in corrosion. If you are careful and check it frequently, you can keep it clean.

The Larsen NMOQ's have a separate sealed base and the whip screws on top. Costs a few bucks more, but long term it is worth it.
 

OhSixTJ

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Laird makes good stuff. The QW144 is a good antenna. Larsen makes a similar antenna, NMOQSPEC.
They are inexpensive and work well.

Problem is, the whip extends all the way down to the button where the contact is made to the NMO mount. The rubber cone is what keeps the water out. Over time, flexing, wear, vibration and temperature changes results in moisture making its way down inside the mating contact between the antenna and the mount. That can result in corrosion. If you are careful and check it frequently, you can keep it clean.

The Larsen NMOQ's have a separate sealed base and the whip screws on top. Costs a few bucks more, but long term it is worth it.
Ah! That’s good enough reason. I experienced this 2 vehicles ago. That explains it.
 
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