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I'm just trying to make sure I understand the theory correctly.

I have some triband VHF/UHF/800MHz antennas, I need to mount on a tower so I plan on using a "mobile to base" conversion kit. The kits i'm mostly looking at are specified for VHF/UHF, which in my mind should be sufficient for 800MHz.

The way I understand it is that the radials should be at least 1/4 of the wavelength of the antenna, so if the antenna is operating in the 800MHz range the radials that are much longer than 1/4 wave of the 800MHz frequency will not be detrimental in any way at that frequency.

Does anyone see any problem with my reasoning?

[Update] ---

I just edited my post to provide an example photo/diagram. I'm looking at a system like this, but the anennas I'm using are multiband VHF/UHF/700/800. So my resoning is that the ground plane radials should be sized for the VHF band, and that the extra length for the higher frequencies will not negatively affect the antenna. My understanding is that the radials provide a reflective plane which just must be at least a quarter wave length. I hope this provides some clearification.

enter image description here

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    $\begingroup$ Are you talking about radials on the ground, or elevated radials (a "counterpoise")? $\endgroup$ – rclocher3 Jul 13 at 15:22
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    $\begingroup$ This question might be of interest: When and why does the size of a ground plane or radials matter? $\endgroup$ – Kevin Reid AG6YO Jul 13 at 15:25
  • $\begingroup$ The was an article in QST ages ago (1970's) where a guy did empirical studies and determined that 600 radials each 5/8 wave was as close to ideal as you could get, anything beyond that was diminishing returns. His experiments were for HF, but that may apply. $\endgroup$ – Duston Jul 14 at 14:05
  • $\begingroup$ @rclocher3 I edited the post so you can see what I'm talking about. An elecvated plane on the mount. $\endgroup$ – Frank Jul 20 at 18:09
  • $\begingroup$ @Duston Thanks, but what I'm really trying to determine is if there is any negative affect by adding extra length to the radials? I was thinking I need to accomidate the lowest frequency, and the higher frequencies will not be negatively impacted by ground radials that are "too long". $\endgroup$ – Frank Jul 20 at 18:11
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According to this guidance from Pulse Electronics, makers of Larsen amateur antennas, the only limitation on the "ground plane" is that it exceed a minimum size:

Ground plane availability is [a] critical factor in mobile antenna performance ... Ground plane requirements vary given the type of mobile antenna and the frequency of operation. A typical 5/8 wave antenna at 150 MHz requires a ground plane at least 42” in diameter. At 450 MHz a 15” diameter ground plane is required, At 800 MHz a minimum of 8” is considered sufficient.

Note that 42" is a bit more than a half-wavelength at 150-MHz, so the radius is about a quarter-wavelength, as one would expect for a ground plane antenna using a traditional monopole radiator.

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  • $\begingroup$ That's for a mag mount. But what about radials on a tower-mounted vertical ground plane, which is what the question is about? $\endgroup$ – Mike Waters Aug 21 at 2:47
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I think Kevin's link in the comments to "when and why does the size of a ground plane matter" is spot on for this question. You should definitely hit the link for a high quality and well explained answer, but the tl;dr of it is:

Perhaps consider that the objective of the ground plane is to present a low impedance. At the feedpoint, the hope is to have all the current go into the antenna, and none of it on the coax common-mode. The lower the impedance of the ground plane, the less current will be on the coax common-mode due to its relatively high impedance.

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If the idea is to make the ground plane as small as possible, an electrical quarter wavelength radius is a good length.

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This quarter-wavelength size is especially critical in elevated monopoles, that is those without radials buried in soil.

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