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UHF Ground plane radial lengths

bobcrean

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I'm building an antenna for 460 Mhz, and started with a ground plane around an NMO mount that I'll attach something like an OPEK-Autotek UH=417 mobile whip on. I know that if I were just building a plain old ground plane antenna, the length of the radials would be quarter wavelength. But the question is, why is the length of the radials (I calculate 6.4" in my case) so important? When you use such a mobile antenna, isn't the whole roof of the car the ground plane?
 

ka3aaa

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the car roof is the ground plane when the antenna is mounted on an automobile however when the antenna is mounted in free space like on a piece of pipe above a roof you need a artificial ground plane of some type such as radials. The length of the radials is dependent on the frequency you are trying to receive.
 

prcguy

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An infinite ground plane like a car roof at UHF is different than tuned radials. When two or more radials are tuned to 1/4 wavelength the RF currents in them are making a 1/2 wave round trip from the feedpoint to the end of the radial and back to the feed point where they present a low impedance at the feedpoint. Any time RF takes a 1/2 wavelength trip it will try and recreate the same impedance as where it started, which is low in this case.

Make the radials 1/8 wave or 3/8 wave or something that ends up with a multiple of 1/4 wave round trip and you end up with them providing a high impedance back to the feedpoint and the overall antenna will not match well to 50 ohm coax.

Since you would have more than two radials the RF currents will be dividing and traveling on the radials equally but in opposite directions, canceling out radiation leaving the vertical part to radiate. Unlike a single counterpoise radial, which radiates because you have made a dipole in that case.
 

bobcrean

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An infinite ground plane like a car roof at UHF is different than tuned radials. When two or more radials are tuned to 1/4 wavelength the RF currents in them are making a 1/2 wave round trip from the feedpoint to the end of the radial and back to the feed point where they present a low impedance at the feedpoint. Any time RF takes a 1/2 wavelength trip it will try and recreate the same impedance as where it started, which is low in this case.

Make the radials 1/8 wave or 3/8 wave or something that ends up with a multiple of 1/4 wave round trip and you end up with them providing a high impedance back to the feedpoint and the overall antenna will not match well to 50 ohm coax.

Since you would have more than two radials the RF currents will be dividing and traveling on the radials equally but in opposite directions, canceling out radiation leaving the vertical part to radiate. Unlike a single counterpoise radial, which radiates because you have made a dipole in that case.
Wow. Thank you. So are more than 4 better? I've seen some antennas with bunches...like at 30 degree increments.
 

prcguy

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A huge infinite solid ground plane is best. If you can't do that I would go with at least four. Every time you double the number of radials you will see a little improvement but after about 32 it starts getting into the area of diminishing returns. Even going from four radials to a solid sheet of copper is not a huge difference unless you want to squeeze the most from the antenna.

There are studies pointing out the distance between the ends of the radials comes into play and there is a goal, something like .02 wavelengths between radial tips that gets you nearly equivalent to a solid sheet of metal, but it takes a lot of radials to do that. Going further, some antenna designs have very short radials like 1/8 wavelength or less but there are a lot of them so the distance between the tips is very small simulating a solid sheet. I've seen some research where a dozen or more very short radials can be better than four that are a full 1/4 wavelength.

I've gone too far now. Just make four 1/4 wavelength radials and be happy.

Wow. Thank you. So are more than 4 better? I've seen some antennas with bunches...like at 30 degree increments.
 
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robertmac

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If this is for 460 not sure what it has to do with amateur radio. But I have never really worried about ground plane as I let the vehicle do it. I know nothing about the OPEK-Autotek UH=417.
 

bobcrean

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That would be 6.5 inches @460. and 6.58 inches @ 449 Ham band.
Actually, my plan is to cut the radials (made out of solid .125 ss) short, and then slide thin wall brass tubing over them so that I can use the antenna for both 449 and up to 460...sort of like a trombone slide.
 

nd5y

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Actually, my plan is to cut the radials (made out of solid .125 ss) short, and then slide thin wall brass tubing over them so that I can use the antenna for both 449 and up to 460...sort of like a trombone slide.
That would be a waste of time. Ground plane radials only need to be at least an electrical 1/4 wave at the lowest frequency.
6.5 inch radials would work fine from 440-470 MHz as long as the antenna is designed to cover that range.

Using dissimilar metals like that could cause intermod problems.
 

freddaniel

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The radials are the minimum length at 1/4 wavelength, at the lowest operating frequency.
To get more bandwidth on any antenna, increase the diameter of the radiating element ONLY.
A commercial ground plane that covers 380-480 MHz is a Sirio GP430 LB/N, and is real vivid example.
Also, look at their broadband Sirio GP 430 LBN 380 - 480 MHz $114.JPGUHF beams.
 

prcguy

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This antenna is not a good example of why you should or should not bend radials down. Its got a very fat vertical element with much more capacitance to the ground radials than a normal ground plane, plus there is some passive matching stuff inside the hollow element. I have several of these and took one apart and was surprised to see a wire and inductor running the length of the hollow vertical element.

The radials are the minimum length at 1/4 wavelength, at the lowest operating frequency.
To get more bandwidth on any antenna, increase the diameter of the radiating element ONLY.
A commercial ground plane that covers 380-480 MHz is a Sirio GP430 LB/N, and is real vivid example.
Also, look at their broadband View attachment 83095UHF beams.
 

bobcrean

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Feb 3, 2020
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So, my plan changed a bit. I got an OPEK VU-1509, with an NMO mount. It calls out unity gain at 144-148 Mhz, and 2.5db gain at 430-450 Mhz.
If I do want to operate at the lower frequencies, I'd cut the radials to 20.5". Does that sound correct?
 
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