2
\$\begingroup\$

I've read a lot online, and have found seemingly contradictory ideas about the purpose. Supposedly the purpose of "ground plane [Totally separate concept from electrical ground?]" is to reflect the signals so the antenna appears twice its own height. Will metal mesh work for this? Could multiple sheets of metal be used, to make a larger ground plane area? I read here that you must [electrically I assume?] ground an artificial ground plane like that to the vehicle chassis / vehicle ground. Does this mean the ground plane is actually also electrically grounded? Could it be grounded to earth ground instead? Or a home outlet ground? I've also seen it mentioned that the "antenna must be grounded". I would assume this is through the [electrically grounded?] ground plane, however my antenna base has a rubber pad on the bottom, so I am guessing it doesn't ground through the ground plane. Do I have the right ideas here?

\$\endgroup\$
2
  • \$\begingroup\$ "however my antenna base has a rubber pad on the bottom" , but the coax shield on CB is connected to the case, which is connected to the chassis. Not so good, but it's the price to have fast mount antenna on your car. You can make a hole in the centre of the roof of your car and then you'll get connection with the plane (roof). Not sure is a good idea if you will sell the car one day. \$\endgroup\$ Commented Jun 16, 2017 at 7:50
  • \$\begingroup\$ A common way of installing a ground is to run radials under (or above) ground - like the spokes of a wheel. Four is a good number. Depth doesn't matter; they are easily installed by cutting shallow slits with a spade if the 'ground' is soft. \$\endgroup\$
    – Chu
    Commented Jun 16, 2017 at 8:36

5 Answers 5

4
\$\begingroup\$

Will metal mesh work for this?

Yes if the holes in the mesh are significantly smaller than the wavelength of the signal (for CB at 27 MHz that's 11 meter) the mesh will appear like a solid metal plate to the signal.

Does this mean the ground plane is actually also electrically grounded?

Yes and the ground is your vehicle's chassis.

In a stationary situation you would indeed use earth ground. You could use a conductive pipe stuck in the earth.

my antenna base has a rubber pad on the bottom

Indeed it has no electrical connection there. The ground/earth connection will come from the receiver or via the shield of the cable to the antenna.

I think you have right ideas about this.

\$\endgroup\$
3
  • \$\begingroup\$ Thanks for the quick and effective response. What is the limitation of a artificial ground plane then; could it be a single wire circle with a diameter less than 11 metres? \$\endgroup\$ Commented Jun 16, 2017 at 6:29
  • \$\begingroup\$ I don't think so, a single wire in a loop will behave differently I guess. However, if you'd make the loop and add a few wires in the circle from one side to the other, like a star. Then I believe that would work. But 11 meters is a lot, you would be using that with a non-moving antenna. Then you can also just use the earth as the ground plane. \$\endgroup\$ Commented Jun 16, 2017 at 6:54
  • \$\begingroup\$ The mesh size should be /significantly/ smaller than the wavelength, and the plane size should be larger than the antenna. It doesn't /matter/ if it's not -- you just get a different antenna-- but a different antenna normally has more loss, the wrong input impedance, and not the radiation pattern you wanted. A single wire loop will have very little effect in most directions, and also energy radiated into the ground will simply be lost. \$\endgroup\$
    – david
    Commented Jun 16, 2017 at 7:35
2
\$\begingroup\$

The 1/4 antenna is only one half of the antenna, the other half is the ground plane. Ideally you would connect the antenna on top of the earth ground with the connection of buried grounding rods, etc..

In absence of the possibility to directly ground the antenna, then a good praxis is to use a large conductive object. This can be a car chassis or conductive plates. The antenna will work even without direct connection with the earth, except it have a little more SWR ratio.

\$\endgroup\$
1
\$\begingroup\$

The 1/4 wavelength of the whip antenna becomes effectively double the length with the reflected ground signal to become 1/2λ antenna. TO become a good reflector , the base must be close to the ground plane and at least as large as the antenna. A partial size works with slightly less performance.

No ground plane leaves it as a 1/4 λ which reflects 100% ideally meaning a short becomes an open circuit and visa versa and 50~300 Ohm still becomes high imepdance and far above free space 377 ohms causing a major loss in transmitted signal being reflected back to the source.

\$\endgroup\$
0
\$\begingroup\$

Recalling vehicle radiation patterns for CBs, a bumper-mounted whip antenna has the strongest radiation over the roof (metal) and weakest out into free-space off the bumper.

Thus roof-mounted loaded-whip (some inductance in middle of the antenna) produces symmetric radiation.

\$\endgroup\$
0
\$\begingroup\$

For a vehicle mounted cb antenna, if you want omni directional radiation and reception, you need to mount the antenna right in the center of the roof as close to the center of the car as possible. Even if it's a magnet mount it will be grounded to the chassis via the coax shield and provide the needed ground plane. An alternative that has not been mentioned is the co-phase harness to use two 1/4 wave antennas such that they work in phase. By mounting them on either side of the vehicle you can get some directional gain both forward and backward compared to the center of the roof single antenna. You can get the same effect by mounting your antenna at the front of the trunk lid so that the best transmission/reception is forward with good side and fair reverse direction gain. This seems a logical path to me because you mostly want to know what's ahead of you. I used a co-phase harness and twin 1/4 wave whips on an old dodge I used to have. They were mounted behind the rear windows on the quarter panel above the fenders and gave really good range ahead and behind. Regardless of which idea you use the car chassis is the ground plane. Having a good ground plane/counterpois is very important. I have a 2 meter handheld and can nearly double the range by attaching a wire the to the outside of the antenna connector and letting it dangle. Even a large pizza pan or just a big sheet of metal will help if you are using a mobile style antenna in a home radio system.

\$\endgroup\$

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.