How To: Tune CB Radio Antenna

[Mike In Bama]

Im sorry this is so long. I tried to make it as close to covering all the bases as possible.

First, yes, you must have an SWR meter (and jumper cable) to do this. 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 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 MHz). Using ball park numbers, that would mean a transmitter at 100 MHz would have a 1/4 wavelength of around 27". Lets say your cell phone is transmitting on 1 GHz, about 100 MHz 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!

I also don't want you to come back and tell me I'm a dumba**, 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.

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" shape.

All installations are different, so sometimes what doesn't look right on paper works anyhow. Testing is the only way to know. 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.

2005 2.3L XL Electronics Tech since '70's

[Greenroomsurfer]

This needs to be a sticky
Awesome write up man you made it easy enough for everyone to do themselves and also gave some knowledge in laments terms to help people understand cbs

[Mike In Bama]

Thanks for the kind words. I've been doing CB radios for about 40 years. I was the only geek in my group of friends. I had to learn how to break things down for my buddys to understand it.

Not knowing how to do a sticky, I thought this would be the best place to put this.

2005 2.3L XL Electronics Tech since '70's

[red_nek]

I wish this would have been posted long ago. I know how to set antennas, but explianing something like this is not my strong suit.

[Shredxcam]

Use a wattmeter?

Is what I use at work

[red_nek]

Originally Posted by Shredxcam

Use a wattmeter?

Is what I use at work

A wattmeter only tells u how much power is being made, not whats being reflected back to the radios finals(transistors). Thats why a swr meter is being used.
Most swr meters have a small scale for checking power output, but that is a different subject altogether.

[Shredxcam]

U use a wattmeter to determine output power
Then measure reflected power for your vswr

[Mike In Bama]

You're right. A watt meter will help you do the same thing. I first learned by using the ratio gauge. On some of the 1 and 2kW stuff I worked on in the Air Force, we used the watt meter exactly as you described.

2005 2.3L XL Electronics Tech since '70's

[Shredxcam]

Yea. That's how we do it today in the USAF. Did this week on a satcom radio

[Mike In Bama]

You might be in my old job. I was Wideband Comm, which was Radio Relay before my time. My last assignment (during Desert Storm) was Barksdale AFB. All of our satcom gear was overseas and I never saw it before I got out.

2005 2.3L XL Electronics Tech since '70's

[Shredxcam]

I'm 2a2x1 Hh-60g avionics com/Nav mission systems

[Mike In Bama]

I'm with you. All the jobs have different designators now. I was 304X0. Main job was being AT&T of the battlefield. Bounce the signal long distances off of the upper atmosphere.

Thank you for carrying on the tradition!

2005 2.3L XL Electronics Tech since '70's