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VHF antenna height?


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We run VHF in the 154.xxx frequency. I am adding a Motorola CM 300 to my Jeep. I had been thinking about a 4’ steel whip, clearance is modeled with a 4’ fiberglass fence post. I was planning on a steel whip, maybe I should go to 5’? 
the pickups have 18” magnetic mounts on the roof. 

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If you're planning on connecting your radio to the bottom of a 4 or 5' mast as it sounds, you will not have a good impedence match for transmitting.  You can buy longer antennas with matching coils at the bottom, but these are not cheap.  If I were doing it, I'd make a way to mount a 19" quarter wave antenna on top of your mast for some height or you may find using a mag mount on your hood will work almost as well with less work.

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I think our base loaded antennas to match that freq range is about 31", would need to measure for sure.

Center roof mounted of course.

On my cj5, I mounted to hood just as Howard suggested and it gave a good ground plane, worked well

 

 

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I guess i was envisioning the jeep it would be bolted to being a pretty big ground plane, I guess I understand even less than I thought🧐

your both saying it needs a large flat area at the base, not just lots of iron? 

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1 hour ago, vtfireman85 said:

Would something like this work? Or am I back in the same boat?

https://www.cabelas.com/shop/en/Cobra-5-Fiberglass-VHF-Antenna

 

Not sure what VHF freq that is tuned to.

I just know on our fire radios, we pick the freq we use the most and trim the length to that.

Go off a chart.

Being in the truck I don't have access to to my files and CRS can't pull it off the top of my head .

Would that antenna work well enough?

Couldn't say, I have always matched because out here distance is everything and hard to come by.

The big rough sandhills are great at absorbing radio waves.

 

 

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22 hours ago, vtfireman85 said:

I guess i was envisioning the jeep it would be bolted to being a pretty big ground plane, I guess I understand even less than I thought🧐

your both saying it needs a large flat area at the base, not just lots of iron? 

Yes, in theory your ground plane should be all around your antenna or you may experience some directionality toward where the metal is which is why many mount their antennas in the center of their roof and it should be at least 1/4 wave in length (about 19"). 

Practically, it may not be all that critical as I use some strange mounting points such as a mag mount mounted horizontally on the back of a pickup cab for clearance purposes for my 2 meter ham radio, but don't notice great improvement when moving it to the top if the cab and many folks mount them on a corner as you are looking at or on the edge of a roof (clamped to a rain gutter--remember them?)

That Cobra antenna would be a good choice.  Their specs say it is tunable up to 161 Mhz so no problem at 154.

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