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I'm new to CB radio's and finally setup a home one. The kit I bought was cheap and was made for a car. I got all the wiring done correctly and able to get some fuzz back when I turn the unit on and scroll through the channels.

I'm just realizing now that I need a "ground plane" I large metallic surface to give and receive signals. Normally this would be your car. I have the antenna on my fire escape now, do you know a DIY object I can build / place under it to act as a good ground plane?

Thanks in advance.

The antenna in question is here: http://www.amazon.com/Cobra-Base-Load-Medium-Magnet-Antenna/dp/B00005N5X2/ref=cm_cr_pr_product_top

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  • \$\begingroup\$ When I built my first crystal set years ago, I used a radiator pipe for a ground - ahh, them were the days :) \$\endgroup\$ Nov 1, 2011 at 9:37
  • \$\begingroup\$ for receiving, the fire escape (assuming it's made of metal) should be more than adequate. for transmitting, key the mic briefly and check the swr. you might not do too badly with a fire escape for a ground plane, if the stars are lined up right. \$\endgroup\$
    – JustJeff
    Nov 1, 2011 at 11:05
  • \$\begingroup\$ My fire escape is rust and full of layers of paint. I couldn't send or receive a signal. :( I even tried a metal pan and my metal kitchen garbage can to no avail. I'm assuming the CB radio (which was only $30) or the antenna are broken. Also I rented a zipcar and put the antenna on the roof - still no signal. So yes, most likely broken. \$\endgroup\$ Nov 2, 2011 at 22:30

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Not all antennas require a ground plane. A basic antenna like a dipole is self-contained and requires no ground plane. The concept of a ground plane is roughly to build a antenna half as long as a normal self-contained one, then add a mirror so that it looks like a complete antenna from the antenna side of the mirror. We call this mirror a ground plane. such antennas are more compact, but don't work right without the ground plane (mirror to make it look like a real full size antenna).

So your first order of business is to find out what kind of antenna you have. If it was meant to be mounted on the top of a metal car, then it very likely requires a ground plane. A good ground plane extends about a wavelength underneath the antenna, although less can be used as long as this is taken into account when the antenna is designed, else the impedance won't be as expected. CB frequencies are in the 27 MHz range, which means the wavelength is 11 meters. No car has a roof 22 meters accross, so a car top antenna will have been designed with a small ground plane in mind. A metal sheet about the size of a car roof, or a bit bigger if you can manage it, should do fine.

If you have or end up getting a self contained antenna, then it is best to keep it away from anything conductive. It is meant to work on its own, so conductors in the near field will mess up its resonance and change its impedance. Again a whole wavelength clearance would be great, but in reality you can live with less.

If you really want to get into this, get a SWR (standing wave ratio) meter and experiment. Some transmitters may have SWR meters built in already. If you want to get into this some more, you can measure what the antenna impedance is, know what load the transmitter wants to see, and put matching components in between so each side sees a nicely matched load. There is something called a "Smith chart" that helps with all this, but that's serious decent into the black magic of RF.

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You presumably mean an antenna with a ground-plane, such as a 1/4 wave vertical. They are often made with four grounded radials, at 45 degrees, as this gives a good match to 50 ohms.

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  • \$\begingroup\$ Why the downvote? \$\endgroup\$ Nov 1, 2011 at 13:58
  • \$\begingroup\$ Not me. Your answer is short, but that doesn't make it wrong or deserving of a downvote. I upvoted it to set it right. \$\endgroup\$ Nov 1, 2011 at 14:06
  • \$\begingroup\$ Thanks Leon, as an fyi, I did not downvote either. \$\endgroup\$ Nov 1, 2011 at 17:42
  • \$\begingroup\$ I actually built one of these based around a SO-239 connector when I was a kid and needed a cheap antenna for my 2m handheld. The OP should be able to do the same for CB I'd imagine just do the calcs for the appropriate frequency. example: w7tck.org/_misc/so-239_ant.html \$\endgroup\$
    – Craig
    Feb 18, 2012 at 15:11

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