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When using whip style monopole antennas (similar to those found on a Baofeng push to talk radio) as feeds for software defined radios, what “quality” of ground plane is required for best performance? Is it sufficient to connect this style antenna to the SDR via coaxial cable? Or would it be better to, say, connect the antenna to an SMA bulkhead mounted to a large metal plate (and connect the other side of the bulkhead to the SDR)?

Either way the antenna is connected to the radio via coaxial SMA cable, but in the latter case the cable shield side is also electrically connected to large metal plane via the bulkhead.

Like many applications, my definition of “better” would be to maximize gain particularly in the azimuth direction.

Thank you!

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  • \$\begingroup\$ To maximize gain in a certain direction you should be fiddling with your antenna design, not asking for the best way to connect coax to a monopole. Any of your ideas sound like reasonable ways to connect coax to a monpole. \$\endgroup\$
    – Criticizing Israel not allowed
    Commented Mar 1, 2023 at 13:27
  • \$\begingroup\$ Thanks for your quick reply. I guess I should say I’m content with the gain I’ll get from the antennas I purchased. I just don’t want to hinder myself. The overall antenna design is simpler without the metal ground plane, so if the metal plane is not necessary I’d like to avoid it. \$\endgroup\$
    – user1858431
    Commented Mar 1, 2023 at 13:29
  • \$\begingroup\$ does it help at all, if you think of a monopole as a dipole with one pole not specified? With a ground plane, the second pole is the reflection of the first pole in the ground plane, which kind of requires a relatively strong ground plane to act as a mirror. But you can also shrink the ground plane down, and instead of having a mirror, make the reflection a real thing, and then you have a dipole. \$\endgroup\$
    – Criticizing Israel not allowed
    Commented Mar 1, 2023 at 13:32

1 Answer 1

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Not all "whip style" (also sometimes called "rubber duckies") antennas are actually monopoles, and the answer depends on what's inside.

If the antenna is really a simple, quarter-wavelength monopole, then some kind of ground plane is helpful -- without it, RF return current will flow on the outside of the coaxial cable. This can cause an unpredictable antenna pattern, impedance, and VSWR. (If you're transmitting, it can even radiate signal into nearby devices causing interference.) In handheld UHF radios with monopole antennas, you're actually part of the ground plane, whether you like it or not.

A solid metal plate isn't necessary. For pole-mounted antennas, it's common to use a set of radials which are a quarter-wavelength long, angled downward at a 45 degree angle. This gives a good 50 ohm impedance match and a predictable radiation pattern. For pole-mounted antennas, a wider frequency range is provided by a discone antenna.

If the length of the antenna isn't an issue, consider a coaxial dipole instead. These are just a half-wave dipole where the feedline comes up inside the lower element. Here's one example that's been disassembled (from allaboutcircuits.com) enter image description here

Longer variations are available with a few more dB of gain in the horizontal plane. Antenna datasheets will give you the information you need.

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  • \$\begingroup\$ Thank you mark. This is very helpful. You started out by saying “if the antenna is really a simple quarter wavelength monopole…”. What other types of construction is there for rubber duck type antennas? Would some of them be perfectly happy without any type of ground plane? \$\endgroup\$
    – user1858431
    Commented Mar 3, 2023 at 12:08
  • \$\begingroup\$ @user1858431 I've added a picture of a coaxial dipole, disassembled from it's insulating casing. These are considered "ground-independent" -- they don't require a ground plane -- but as with any antenna, metal objects nearby can still affect their pattern and performance. \$\endgroup\$
    – Mark Leavitt
    Commented Mar 3, 2023 at 21:13
  • \$\begingroup\$ I decided to open up one of my spare antennas to examine its construction. There is a helical coil of wire connected to the SMA center conductor, and very little metal (just some structural material) connected to the SMA shielding. I will experiment further with receive signal strength and ground plane vs non ground plane antenna mounting. \$\endgroup\$
    – user1858431
    Commented Mar 5, 2023 at 5:40

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