I just posted this in another thread helping someone, but thought I would create a how to thread for future reference to help everyone out.
For Jeeps, etc... I am going to suggest getting a good SWR meter from Radio Shack. (There are much better, much more expensive meters out there, but the Radio Shack meters ARE reliable & won't break the bank.)
First Off, for those who are new to this, having a good Standing Wave Ratio is a vital key in your rig, if it is not set right you will end up killing some key components in your radio. ( final transistors, resistors, AM regulator, etc. usually the finals will be the ones to go, that is, if your radio is turned up, which I do not recommend) Then you will have to send your radio off to be repaired do reflected power back to it's components causing a strain on them. "Why?" You may ask, well, SWR indicates the quality of the match between your coax & the antenna's feed point ( Where you connect the coax coming from your radio to the antenna )
SWR (Standing Wave Ratio), is also a measurement of how efficiently your antenna system will radiate the power available from your radio. In simple terms, your radio would like to radiate all of its power, but can only do so if the other components cooperate. Bad coax and mounts, or inefficient antennas and ground plane can cause kinks in the radiation of RF power. The easiest way to understand the concept is to think of it in the flow of fluid through plumbing. That is, if you put a one inch reducer/bushing on a two or three inch main feed pipe, your potential output will be restricted by the one inch outlet which will cause backpressure & a considerable strain on the pump that is pumping the water ( With a radio, think of your final transmitting components as the water pump ) & the overrall result is less flow out of the other end of the pipe due to the restriction. So goes with antenna systems. Setting your SWR of your antenna will reduce the reflected power that puts a strain on your radio's / amp's components( think of reflected power as the back pressure I mentioned above ) & of course, the restriction of radiated power. Another thing is that the length of your antenna has ALOT to do with SWR, considering the antenna length resembles a portion of the wavelength of the frequency you are talking on & if it is resonate to that frequency, but we can get into antenna length vs. wavelength in another thread.
Having an SWR of 2.0:1 or less is a MUST for decent transmitting (Also, some will say it can burn up your radio, but if your radio is legal & hasn't been "turned up" your radio should live
). Having an SWR of 1.5:1 or less is considered optimal for best transmit & recieve.
What You Will Need
GOOD SWR Meter
GOOD Coax Jumper ( a 3' jumper is usually sufficient )
Tools to adjust your antenna ( crecent wrench, allen wrench set, wire cutters, even some screwdrivers maybe, depends on the antenna )
To start off with, you really can't put a whole lot of trust in your radio's internal SWR meter. ( if it has one ) They work & are fairly accurate for the most part. The best thing to do is get an external SWR meter, & don't get one of those that you see in the truck stops, they aren't worth a dime in my opinion, I have personally tested them against a Bird & Palstar meter.
You can usually find a decent meter for around $50 nowadays and yes, the Radio Shack meter is decent for checking SWR but that's about it. The watt meter is questionable in those. As you get into the hobby more, you will want a better meter that gives accurate RMS (Root Mean Square, or average power) and PEP (Peak Envelope Power) power reading along with reflected power. Bird directional watt meters are the best and most accurate. The Palstar & Daiwa meters are also darn good meters as well. The ultimate in antenna tuning is an antenna analyzer that can check the antenna's resonate frequency or impedance. For now, we just need an SWR meter.
Step #1 ) Find a place that is clear of trees, buildings, other obstruction and other antennas that could give you false readings. The areas should be clear of objects by 50 feet or more in all directions.
Step #2 ) Place the meter inline between the radio and antenna using your coax jumper.
Step #3 ) Hook the jumper from radio to the TX or transmitter side of the meter and antenna to the ANT or antenna side of the meter.
Step #4 ) Turn the radio on and set it to channel 1.
[ Special note: If you will only be broadcasting on the CB bands & if your radio has SSB ( Single Side Band ) &/or is an export, make sure the selector knob is ine the "AM" position & the frequency display ( if your radio has one ) on Channel 1 shows 26.965 Mhz / Channel 20 Shows 27.205 Mhz / Channel 40 Shows 27.405 Mhz ]
Step #5 ) Turn the adjustment knob to the minimum or full counter clockwise position. Set the meter to the CAL or calibrate position.
Step #6 ) Key the mic and turn the calibration knob so that the needle moves to the CAL mark on the scale.
Step #7 )Set the meter switch to the SWR setting without touching the calibration knob.
Step #8 ) Key the mic and record the SWR meter reading.
Now repeat steps 5-8 on channel 40.
Also, just for reference, repeat steps 5-8 on Channel 20 ( which is the center of the 11-meter CB Band )
If the SWR is lower on channel 1 than channel 40, your antenna/whip is too long. If the SWR is higher on channel 1 than channel 40, your antenna/whip is too short.
Adjust your antenna in ╝ inch increments. Some antennas have a set screw that allows you to adjust the height of the whip and others must have some of the whip cut or ground off to adjust the SWR. Many of the whips are stainless steel and very hard to cut. A hacksaw, cutoff wheel, or bolt cutters will be necessary to cut these antennas.
After you make an adjustment, repeat steps 5 through 8 until your SWR is the same at both channel 1 and channel 40.
The goal is to get the lowest SWR possible. If your SWR is below 2.1:1 you are ok, 1.5:1 or less is what I would recommend . If you have or plan on getting an amp, you need an SWR of 1.3:1 or less. If you have tuned the antenna but still have a high SWR reading, you may try moving the antenna to another location on the vehicle or you may need to add ground straps to the vehicle for better bonding. Grounding & bonding is dicussed in another thread.
Here's some info on adjusting SWR on a few different antennas.
ADJUSTING LONG ANTENNAS
If the SWR on channel 40 is greater than that on channel 1, your antenna is considered to be a little "TOO LONG" and reduction of physical height and/or conductor length will correct this situation. Depending upon antenna model, this entails screwing down the tunable tip on a Firestik II & similar antennas, or, removing the tip, making short slits in the plastic covering and unwinding and clipping off wire on a Firestik 1 & similar antennas. Most coil & whip style antennas require loosening the allen screws and lowering or raising the metal whip.
ADJUSTING SHORT ANTENNAS
If SWR on channel 1 is greater than that on channel 40, your antenna is considered to be "SHORT" and increasing the physical and/or electrical length of the antenna is required to correct this situation. Because we make our antennas extra long, readings which indicate "Short" normally stem from ground plane deficiency (lack of vehicle metal surface for the antenna to reflect its signal rom). This condition is often corrected by adding a spring and/or quick disconnect to increase the physical height. Ground plane deficiencies can also be compensated for by using dual (co-phased) antennas or special no-ground-plane antenna kits.
NOTE: The shorter the antenna, the more sensitive it is to adjustments. For example, removing two wire turns on a 4 foot antenna might move the SWR by 0.3; the same amount removed from a 2 foot antenna may move the SWR by 1.0. Make smaller adjustments on shorter antennas.
Measurements and determination of proper antenna length are the same as the single antenna procedure. However, when tuning co-phased antennas, if you adjust one antenna, it is advisable to adjust the other in equal amounts to keep them in perfect balance.
Firestick I (And similar) Antennas
If you take the cap off of the top, there will be some "wire" coiled in the top. This is what you trim to tune these antennas. Make sure you do not trim too much at at time.