CB Radio Antenna Tuning

Several years ago I was asked on the Ford Ranger Forum to explain antenna tuning. This is what I came up with and thought it might be useful to someone on here. This seemed like the most appropriate forum to use.


“There is no voodoo or black magic involved. First, yes, you must have an SWR meter and jumper cable to do this. (CB’s with built in meters are an exception). There is no other way. I have worked on satellite, air to ground, ground to air, ground to ground and Troposcatter (bouncing the radio wave off of the upper atmosphere) communications systems for many years. The frequencies vary but all radios have either an external or built in SWR meter.

To keep the antenna as small as possible, Radios require you to have an antenna the length of 1/4 of the physical length of the radio frequency you are TRANSMITTING on.

The length for receiving is no where near as important.

The frequency difference between channel 1 and channel 40 is relatively small in the grand scheme of things. There is no adjustment that will be perfect for all channels. That's ok as long as it is close.

SWR means Standing Wave Ratio. That is a ratio between how much power your radio transmits and how much is reflected back into the radio. What makes it bounce back? The antenna not being exactly 1/4 of the wavelength of the channel you are on.

You don't adjust the antenna every time you change channels! You set the antenna for the best compromise for ALL of the channels. Again, it's a relatively small difference.

Logically, we will try to get the SWR reading as close to 1:1 as possible on channel 20. The antenna will be slightly short for channel 1 and slightly long for channel 40.

One quarter wavelength of the CB frequencies is about 108". (27 KHz). Using ball park numbers, that would mean a transmitter at 100 KHz would have a 1/4 wavelength of around 27". Lets say your cell phone is transmitting on 1 GHz, about 100 KHz X 1000. Now we can take 27" / 1000. That's a small antenna! And that's why modern cell phones have built in antennas.

An SWR meter has two coax connectors on the back (or sides). The antenna will connect to the one labeled ANT. The jumper will connect to the connector labeled RADIO. The other end of the jumper will connect to the CB.

READ EVERY WORD THAT FOLLOWS BEFORE YOU ATTEMPT TO ADJUST YOUR ANTENNA. MAKE SURE YOU UNDERSTAND EXACTLY WHAT I AM TELLING YOU TO DO!

DO NOT KEEP THE RADIO KEYED UP ANY LONGER THAN NECESSARY! A FEW SECONDS SHOULD BE PLENTY OF TIME. IF YOU ARE HAVING PROBLEMS, DO A CHECK AND GIVE THE RADIO 5 MINUTES TO COOL OFF BEFORE YOU DO YOUR NEXT CHECK!

FAILURE TO DO SO COULD DAMAGE YOUR CB! THE WARRANTY DOES NOT COVER THE TRANSMITTER TRANSISTORS BECAUSE YOU DON'T KNOW WHAT YOU ARE DOING!

Turn on the CB and set it to channel 1. Key the microphone (press it in) and hold it. Set the switch to FWD. Adjust the knob until the meter reads EXACTLY full scale. (The last mark on the meter scale). While still keying the mike, flip the switch to REV. Hopefully the meter will will read 2.0 or less. Unkey the mike and write that number down.

Repeat the last paragraph except set the CB to channel 40. Unkey the radio and write that number down as well. Hopefully it will be 2.0 or less as well.

If the reading on channel 40 is higher than the reading on channel 1, the antenna is too long. Loosen the set screw and lower the top rod a little and tighten the set screw. (Of course, if the reading is higher on channel 1, you must lengthen the antenna slightly).

Check the SWR on channel 1 and 40 again. Repeat this until the readings are the same.

If the readings are much above 2:1, you are risking CB damage as too much power is being reflected back into the radio. That won't hurt your reception, but you won't be able to transmit very far. Again, you are also running the risk of damaging your radio.

Lets say you lower the adjustment rod and the SWR on channel 40 is still higher than channel 1 and the rod is all the way down. What do you do? You still have to make the rod shorter, so, with lineman pliers, hack saw, or whatever you have, remove 1/4" from the rod and slide it all the way into the sleeve. Tighten the set screw and check the SWRs.

Repeat as necessary.

If the SWRs you encounter are, say 3 or more, you more than likely have an antenna grounding problem. The best ground is the frame of the truck. Next is the body panels (if some paint is scraped off).

A tool box is not a great place. It's metal but there are only screws or clamps holding it to the bed rails. Stick some foam or a bed liner between the bed and the box and the ground will be even worse.

Under any and all circumstances, you will not and can not damage your radio by listening to it. Period! Transmitting on a misadjusted radio is a whole different matter!!!!!!

Again, overall, the antenna will have an SWR of say 1.5:1 on channels 1 and 40. Not perfect, but close. Channel 20 may have an SWR of 1.2:1. If you graph the reading of all of the channels on a piece of graph paper, it should be a shallow "U" or “V” shape.”

I hope this explains how radios work with virtually no math. I would be more than happy to go into this deeper if there are any questions. I would greatly appreciate your feedback.



Mike

 

I am looking to drill the roof and install a 6db antenna for GMRS...