Properly installing a CB radio Part 2

In part one of this arti­cle, we cov­ered the gear and tech­niques nec­es­sary for a prop­er instal­la­tion of CB radio in a vehicle…so now that the sys­tem is set up, lets learn how to opti­mize its trans­mis­sion capa­bil­i­ty. In part two, we will dis­cuss a few com­mon prob­lems that can cre­ate unwant­ed noise. This isn’t com­pre­hen­sive but it will cov­er most prob­lems that will arise and address how to fix them.

SWR
The first thing we need to address is SWR. SWR is short for Stand­ing Wave Ratio… essen­tial­ly, it mea­sures how well your equip­ment will trans­mit and receive on the spe­cif­ic fre­quen­cy you intend to use. If you have every­thing ground­ed prop­er­ly, your equip­ment is imped­ance matched, and you have a decent anten­na mount­ed in the cor­rect loca­tion, the SWR should be ok. An SWR read­ing of 1:1 is opti­mal but a read­ing of 1.5:1 is excel­lent, a read­ing of 2:1 is con­sid­ered good (actu­al­ly great for most appli­ca­tions), but any­thing high­er than 3:1, well, you pret­ty much wast­ed your time with the instal­la­tion. Get­ting the best SWR on your spe­cif­ic rig is a mat­ter of tri­al and error… in my expe­ri­ence, you can’t go wrong if you ground every­thing well and place the anten­na on top of a met­al roof or mount it where at least a por­tion of the anten­na clears the roofline.
An SWR meter mea­sures the SWR (duh!!) and can be picked up at any radio shack or elec­tron­ic sup­ply house. Most come com­plete with direc­tions and are pret­ty easy to use, even for a novice. If you’re unhap­py with the SWR you get from your set­up ini­tial­ly, don’t wor­ry, you can improve it by tun­ing your anten­na. All anten­nas are tun­able, but some are tuned eas­i­er than oth­ers. Some need to be cut and some need to be bent to retard the oscil­la­tion on the part past the bend. The K40, for exam­ple, is one of the eas­i­er ones; it has a small whip that sticks out the top on the anten­na and is moved up and down using a sup­plied Allen key; by adjust­ing the length of the whip, you can receive a bet­ter SWR read­ing.

Noise Elim­i­na­tion
Even if you are hap­py with the SWR you get on your sys­tem, you may still have prob­lems with noise (one doesn’t nec­es­sar­i­ly effect the oth­er) so lets learn how to elim­i­nate that noise.
Most radios come with an inter­nal noise fil­ter… a but­ton or tog­gle switch on the face of the radio that elim­i­nates much of the squelch noise from the radio out­put. The prob­lem with this fea­ture is that it also makes dis­tant trans­mis­sions dif­fi­cult to hear. If you want to (or need to) address the prob­lem fur­ther, know that noise on a CB unit (while the engine is run­ning) is nor­mal­ly caused by two things… 1) noise com­ing through the hot­line of the radio or 2) noise being picked up by the anten­na.
(Note: You need to remem­ber, a CB picks up 27MHz radio waves and an engine or oth­er vibra­tions can cause inter­fer­ence and dis­tor­tion of those radio waves. Pow­er win­dows or seats can cause feed­back… that’s nor­mal­ly caused by the elec­tri­cal motor. An old­er engine can cre­ate oscil­la­tion heard on a CB… chances are it’s the points spin­ning in the dis­trib­u­tor. So noise isn’t nec­es­sar­i­ly just an elec­tri­cal hot­line prob­lem… you need to elim­i­nate both pos­si­bil­i­ties men­tioned above.)
A few sim­ple tests can iso­late where the noise is com­ing from.
Hook the radio up to the bat­tery direct­ly or bet­ter yet, a sep­a­rate bat­tery not hooked into the truck’s elec­tri­cal sys­tem. This will bypass and elim­i­nate any noise caused by the alter­na­tor or fir­ing of the cylin­ders. If you still get noise, it’s com­ing in through the anten­na.
Dis­con­nect the feed­line from the anten­na or the feed­line from the radio. This will elim­i­nate any noise being received on the anten­na. If you still get the noise, it’s com­ing from the power/ground lines.
I’ve had both types of prob­lems (both on the same rig once)… so here are the fix­es I used to elim­i­nate most (not all) of the noise.

Anten­na noise
To elim­i­nate noise caused by the anten­na receiv­ing unwant­ed sig­nals, put in an “anten­na choke.” Dis­claimer time: I have an idea why this fix works but Im not quite sure if I under­stand the radio the­o­ry is cor­rect as I under­stand it….and I haven’t got­ten a straight answer from any­one on the  mat­ter so do me a favor and don’t ask because all I can tell you is that it does work. Take about 6 feet of the feed­line and wrap it into 6 or 8 loops (kind of like wrap­ping up an elec­tri­cal cord or piece of rope but about the diam­e­ter of a cof­fee can) then tape the loops togeth­er. If an anten­na choke doesn’t suc­cess­ful­ly elim­i­nate all noise, there are oth­er meth­ods to try. Try chang­ing the loca­tion of your anten­na to a spot on the vehi­cle where it is shield­ed from the engine. Radio waves are line of sight recep­tion and some­times sim­ply hid­ing the anten­na from the con­stant oscil­la­tion of the engine can do the trick. Anoth­er method is to try using short­er or longer lengths of coax… but unless you know how to sol­der a PL259 con­nec­tor that is an expen­sive exer­cise in tri­al and error and… try the oth­er meth­ods first.

Hotwire noise
First off, try attach­ing the hot wire(s) from the radio direct­ly to the bat­tery for the vehi­cle. Much of the noise picked up through the hot wire comes from the alter­na­tor feed­ing cur­rent into the sys­tem.
This method should work, but if not, try installing an exter­nal noise fil­ter onto the hot wire and ground wire of your CB. They are small cylin­ders (about the size of a bicy­cle han­dle) and can be picked up at Radio Shack or oth­er elec­tron­ics stores. Sim­ply attach the hot wire(s) from the radio to the red wire of the fil­ter, then the red wire on the oth­er side of the fil­ter to a pow­er source. Attach the ground wire from the radio to the black wire on the fil­ter, then the black wire on the oth­er side of the fil­ter to a chas­sis ground.

These tech­niques should help you set up a radio prop­er­ly, even if you run into dif­fi­cul­ties. It may take some time and trou­ble shoot­ing on your part but you’ll be left with min­i­mal noise and decent reception/transmission capa­bil­i­ties. The cost shouldn’t be too bad either… all can be done for under $100 with new equip­ment but if your bud­get allows, feel free to spend more.