Techniques - CB radio installation and aerial tuning
CB radio - Types and prices
CB radio - Installation tips
CB radio - Aerial SWR adjustment instructions
You will need an SWR meter, a patch lead, and tools to adjust the length of the aerial. (see note * below re “Dial a Match” type aerials )
IMPORTANT NOTE = Don’t touch the aerial when the microphone key is pressed, you can get a bit of an unpleasant electric shock from it !!
1, Connect an SWR meter between the CB and the aerial lead, using a patch lead, make sure the SWR meter is connected the right way round, the 2 sockets will be labelled “ANT” (i.e. the Aerial !) and “TX” (i.e. the CB set, TX is short for “Transmitter”) - It should be easy to figure out which is which !!
2, Switch the CB on and set on channel 20, use the low power setting switch if you have one.
3, Set the switch on the SWR meter to the position marked “FWD” or “CAL” and turn the adjusting knob ½ way up.
4, Briefly press the microphones transmit button / key, and note how far the needle on the SWR meter moves, now adjust the knob until the needle just moves neatly to the right hand side of the scale when the transmit button is pressed.( to the position usually marked “CAL” or “SET” ) Don't press the mike for more than 2 or 3 seconds at a time. If you can't get the needle to go far enough over, switch the CB to high power.
5, Now set the switch on the SWR meter to the position marked "REF”, (or sometimes "SWR") don't alter the setting of the knob.
6, Briefly press the mike key again and quickly note how far the needle moves, if it goes more than 1/2 way be careful as pressing the mike key for more than a second or two could damage the CB set.
7, The idea is to adjust the aerial up or down in length until the needle on the SWR meter moves as little as possible. ( see note * below re “Dial a Match” type aerials )
8, Most aerials will have some means of adjusting the length that is fairly obvious, the amount of adjustment is largely dependant on the size of the aerial, a small car aerial should be moved about 1/4 of an inch at a time, a large base station aerial may be moved 6 or more inches at a time until fine adjustment. On some car aerials the shortest position may still not be short enough to get the reading down and so you might need to cut some off the tip. Obviously care is needed here as it is difficult to add it back if you cut too much off! Cutting just a ¼” at a time is recommended, whilst each time checking the effect on the reading of the SWR meter.
9, When this has been done the SWR reading should be less than 1.5 ideally, but a bit over this will not do any harm and will hardly affect the signal.
10, Finally check the readings on channel 1 and channel 40 using the same procedure, but without adjusting the aerial, Note = The readings on channel 1 and 40 may be slightly higher than the reading in the middle, (i.e. channel 20), this is normal on car CB aerials.
P.S. while the efficiency of the aerial is at its best when the SWR reading is lowest, anything below 2 is considered ok, above 2 will start to make a difference, above 3 is bad for the CB set. If you need further assistance call an expert ( Me !! )
- Note re “Dial a Match” type aerials = On these you don’t alter the length, you screw the little chrome rings up and down the threads to get the minimum reading on the meter (when set to “REF”), as in step 7 above. The rings must be moved together and gently locked together when the correct position is found. A small movement up and down can make a big difference to the setting on these so fine adjustment may be required.