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I have an appliation where i need to mount a multiband (VHF/UHF/700/800/900 MHZ) mobile antenna (picture 2) on a tower. The spec sheet for the antenna says it used a 4 foot diameter ground plane for testing. I'm considering using a mobile to base converson kit like the one in the picture with a radial ground plane or counterpoise. My question is on the size I need.

From the way I thought I understand the theory, the radials act as a reflective ground, and should be at least 1/4 wavelenth of the antenna frequency. So is there a situation where the radials would be too long?

I'm thinking I should use radials that are 1/4 wave length of the VHF frequency, and that the extra length when operating at the UHF and higher frequencies would not negitivly impact operation. Is my reasoning correct?

Also, will I need some sort of balun?

radio ground plane Antenna

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  • \$\begingroup\$ unless you want to illuminate the sky, you'd want to mount that upside down on your tower, by the way. (the choice of antenna might be a problematic one, overall) \$\endgroup\$
    – Marcus Müller
    Commented Jul 20, 2020 at 20:12
  • \$\begingroup\$ What does this mean: 7000/800/9000 MHZ? \$\endgroup\$
    – Andy aka
    Commented Jul 20, 2020 at 21:43
  • \$\begingroup\$ @Andyaka sorry that was a typo. Supposed to mean it operates in VHF, UHF, and 700 - 900 Mhz frequencies. \$\endgroup\$
    – Frank
    Commented Jul 20, 2020 at 22:53
  • \$\begingroup\$ @MarcusMüller That's a good point. Actually these are for multiple purposes. For direct transmit to units in the field, but also to mountain top repeaters. But, this gives me something else to consider. \$\endgroup\$
    – Frank
    Commented Jul 20, 2020 at 22:54
  • \$\begingroup\$ VHF top limit is 300 MHz - have you fully specified the full usage spectrum? \$\endgroup\$
    – Andy aka
    Commented Jul 21, 2020 at 7:20

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If you are using radials, they should have a length of \$\lambda /4\$ (or an odd multiple of that). Consider the end of the radials as a high impedance (open) point that gets transformed by a \$\lambda /4\$-transformer to a low impedance (ground) point.

A balun is not necessary, since both the antenna and the cable are asymmetrical. In practice, however, the use of a common mode filter (coaxial core) may still be necessary.

You can try to improve the impedance matching by pointing the radials slightly downwards. In theory, radials that are perpendicular to the radiating element will result in an impedance of about 36\$\Omega \$. If the radials point straight down, the theoretical value is about 73\$\Omega \$.

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  • \$\begingroup\$ ok, but considering that the antenna is used for a radio that is operating in VHF/UHF/700/800/900 MHz do I just go with radial that are the 1/4 wave of VHF frequencies? Will the extra length be a problem at the higher frequencies? If so is there anything I should consider if I need to operate in all thos bands? \$\endgroup\$
    – Frank
    Commented Jul 20, 2020 at 22:51
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    \$\begingroup\$ As a compromise, you could use several radials with different lengths. Since the radial that corresponds to a given frequency produces a low impedance point, it will short circuit the other radials. Simply put, the antenna automatically selects the radial with the correct length. \$\endgroup\$
    – Hufi
    Commented Jul 20, 2020 at 23:01

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