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Old 03-09-2013, 11:29 AM
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Default Mobile antenna solution

This might beling in the antenna section, but lets see if it gets moved.

My Fire Dept has a Kawasaki Mule, and we have a motorola Maxtrac (42-50) installed in the utility pocket to the left of steering wheel. The antenna for this is of course a 31.3" whip with spring strain relief. It is mounted via NMO on angle bracket to the roll cage on top of the vehicle.

This vehicle is being used for off road-woods calls and used as a medical response vehicle for special events and the county fair.

Our issue is the antenna. Its too tall. I know that it is already a 1/4 wave, but hear me out.
I would like to use a Motorola helicoil antenna with the MX connection, or a similar antenna with similar properties. I know that the Motorola helicoil antenna is capable of handling 50 watts (per documentation). I would like to know if there is an adapter that will convert the NMO mount to the MX of the Motorola Helicoil. I would of course turn the output power of the radio down until I reach a tolerable SWR.

Perhaps there is a rubber duck that is capable of the same thing, with the ability to be permanently mounted to the roll cage of the Mule.

I know that the mobile radio is not intended to use portable antennas, but this is a unique situation. This vehicle would not be performing any long distance communications, it would mostly be used for fireground communications.

Any input would be great. Thanks!
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Old 03-09-2013, 12:28 PM
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I can't help you with the other antenna options but why dont you think about re-mounting the antenna as low as you can on the unit? Basically using the height of the vehicle.
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Old 03-09-2013, 2:45 PM
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Rescue Squad here, I know exactly what you're talking about. We have a Rhino and a Gator for up on the mountain. We are VHF so ours is only about 20". We use magnetic mounts on both. They both have plastic roofs over the roll cage, so I epoxied a 4x4 blank cover plate from an electrical box to the roof. The plates are available at any building supply, cheap, galvanized so they won't rust. If you knock a mag mount off, just put it back. And if you know it's gonna hit, just take the antenna off and stick it to the roll cage horizontally ..

My $.02, your mileage may vary...
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Old 03-09-2013, 3:02 PM
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Quote:
Originally Posted by m-gerty View Post
Rescue Squad here, I know exactly what you're talking about. We have a Rhino and a Gator for up on the mountain. We are VHF so ours is only about 20". We use magnetic mounts on both. They both have plastic roofs over the roll cage, so I epoxied a 4x4 blank cover plate from an electrical box to the roof. The plates are available at any building supply, cheap, galvanized so they won't rust. If you knock a mag mount off, just put it back. And if you know it's gonna hit, just take the antenna off and stick it to the roll cage horizontally ..

My $.02, your mileage may vary...
Good advice, however the 4" x 4" box cover isn't providing enough of a ground plane for your antenna to work efficiently and provide a suitable match to the radio. A better solution is to get a piece of sheet metal that is as big as will go on the roof. You ideally need 19 inches all the way around the antenna location to provide a proper counterpoise on VHF.
The other option is to replace the quarter wave 20" antenna with a half wave. They don't need a ground plane to provide a proper match.

Second issue for all these installations is that the vehicle operators are getting exposed to some pretty high RF doses in these sorts of installations. There are limits for occupational exposure.

We are running a 2 Polaris Rangers, Yamaha Rhino and two polaris rzrs. Roof top mounts seem to work the best, but the tree branch issue is always a problem. The thinner 1/4 wave whips provide some flexibility. For Low Band, mounting off the bed or front bumper might be a better choice. You will have ground plane issue, but that can be overcome. On the Polaris Rangers, there are mounting holes for accessories along the edges of the bed. I took some 2" by 2" angle stock and mounted it on there. That provides a flat vertical metal edge along the bed even with the outside of the bed. I've used that to mount ax and shovel clamps. A ball mount could be installed on there with some modifications quite easy. Getting the whip lower will help your clearance issues, and at those frequencies, likely not affect things too much.

We are running the two Polaris Rzrs's with mounts attached to the roll cage. Works OK. There are also companies that make roll bar clamps that have an NMO mount on them. Doesn't address the ground plane issue, but it does provide for a sturdy mount for a half wave antenna.
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Old 03-09-2013, 3:15 PM
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Wirelessly posted (Mozilla/5.0 (iPhone; CPU iPhone OS 6_1_2 like Mac OS X) AppleWebKit/536.26 (KHTML, like Gecko) Version/6.0 Mobile/10B146 Safari/8536.25)

Does the vehicle always respond alone, or is there usually/always another larger/regular over-the-road vehicle on the call and within a mile or two? I'm thinking UHF vehicular repeater in the "escort" vehicle linking the 4-wheeler with regular dispatch/comm channel. This would let you use very short UHF antenna on the off-road vehicle. And yes... It will cost much more to do this....
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Old 03-09-2013, 8:05 PM
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Thanks for the input folks. There are some good ideas there, however I am still curious if there are adapters to do what I had described before.

I do not want to mount antenna low do to signal reflection and RF exposure. I would like to keep the antenna as high as possible. I also need to keep the operation is seamless as possible, without having to remember to take an antenna down.

I honestly think my best bet is to place a rubber duck antenna on top of the roll cage.
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Old 03-09-2013, 9:39 PM
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A short rubber duck is going to be a very bad mismatch for VHF low band on transmit and a crummy receive antenna as well. It may even increase the unwanted RF output from the coax feedline and connectors. Expect the worst.
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Old 03-09-2013, 11:01 PM
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RF exposure isn't going to change much between an antenna on the roof and an antenna on the back of the bed. Actually, you can probably get the antenna farther away from the occupants on the rear of the machine than on the roof.

Not sure anyone makes an adapter for allowing an MX antenna to go on the roof. What you'd need to do is find a machinist that would make an adapter that would go on the Larsen NMOQ antenna mounts (I think it may be 5/16x18) on one end and allow the MX antenna to thread in the other end. One issue you will likely run into with this design is that the SWR is going to be so bad that the radio will fold back the power to protect the final stage. This is going to result in drastically reduced power output. At that point, you might as well just carry a hand held, the performance will likely be pretty close.

For low band, you really need a tuned antenna. The flexible antennas for a low band hand held are not going to be tuned. I'd really suggest either a base loaded mobile antenna with a spring, or a full 1/4 wave whip. Find a way to mount them so they don't get damaged.

Other option would be to use a VHF high band radio and use some of the interoperability frequencies. Set up a radio in one of the trucks to handle traffic and manually relay on the low band radio.
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Old 03-10-2013, 12:34 AM
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Something like this Vertex ATL-1C - $24.95 : The Antenna Farm or this Laird Tech EXL-42-MX
$36.95
may work if you can locate the proper adapter.
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Last edited by W8RMH; 03-10-2013 at 12:37 AM..
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Old 03-10-2013, 10:18 AM
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I agree with mmckenna's comments. Keep RF exposure and ground plane issues in mind.

What is smacking you is a law of physics (a law with no appeal process, I might add). The transmitter expects to see a half-wave antenna- 62" of whip and 62" of ground plane from the Mule to operate. Base loaded coils physically shorten the antenna, but still react like a full length whip.The more you shorten the whip, the less effecient it becomes. An full-length old-fashioned quarter wave whip and massive spring mount on the front or back of the machine may be your best option.

I fabricate a tip-over base attached to the back of the roll cage for base loaded antenna situations. When a low branch sweeps the roof it pushes the antenna over instead of knocking it off. Still tall, and requires manual righting, but it saves antennas.

Larsen makes their NMOTEST1 adapter- NMO on the bottom, UHF(F) on top. You may be able to field-modify your rubber antenna into a PL-259 connector (for RG-8). VHF-Lo is a bit more forgiving than UHF or 7/800, so you may be able to build a get-by antenna. No planned failure point, so a branch is gonna break something.

I would also be concerned that turning down the TX power of your Maxtrac may lead to internal PA overheating. I know that is a problem with many of the similar Radius products- been there, done that. Yes, low TX output overheads the radio.
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Last edited by jeatock; 03-10-2013 at 10:21 AM..
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Old 03-10-2013, 7:02 PM
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"Larsen makes their NMOTEST1 adapter- NMO on the bottom, UHF(F) on top. "

There may still be some portable antennas with the male PL connector on them in the Laird line of port antennas.

Lowband Maxtrac/Radius radios are really finicky with 'not' antennas!
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