I sort of remember my electronics theroy on SWR and the importance of having an antenna length in specific divisions of the wavelength, from eons ago.

What I can't remember at all is why the antenna isn't already setup for use on a specific type of transmitter. Say if I have a CB radio and buy a CB antenna, why isn't it already pre adjusted for the CB wavelength?

What is it about my setup that requires a final fine adjustment? Is it how it or where it is mounted? Is it the length of the coax?

 

Yes to all, and more. The antenna is usually not at resonant length, so it needs tweaked to be electrically resonant. Usually this is done by adding inductance. The ground plane is also a major variable.

 

Although there can be exceptions, minimum SWR corresponds with maximum power output. Both are considered desirable.

 

Even the best antennas have tuning stubs that help tweak the SWR for best performance over the frequency range to be used.

If SWR is high, the entire transmission line, antenna and transmitter will be radiating and will have an unpredictable radiation pattern.

If really bad, it might even make your lips tingle when they touch the mic...

During manufacturing, the antenna has various mechanical tolerances for one, and even the tower it is mounted on can detune it as well as adjacent trees--they cannot test and tune every antenna that is manufactured because there are so many variables that they have no control over.

Regarding the transmission line, if properly matched, it may be any length--if mismatched, the length does all kind of strange things.

I recently did a job for a guy that hired someone assemble and mount a 3 element cubical quad CB antenna--the SWR was so bad that he could not use it--eventually he found me via advertising on craigs list. I took one look at it and had to climb the 40' mast, take it down, reassemble it according to instructions, do a little improvisation because some of the tuning parts were a little different, get it back up the tower, and tune it for best SWR (it was already pretty good when assembled properly). He was really tickled at how well it then worked as it had really high gain.

 

An improperly tuned antenna length can reflect power back into the radio and damage the final output section. It is kind of like leaving a diode out and getting a spike from a stepper. The SWR is set to accomodate length, resonance and loss in the coax. A high SWR will not transmit very far and will sound muffled or tinny depending on the cause. The old CB craze taught a number of enthusiasts the value of proper adjustment when they used linear amps in the 1000 watt range and fried coax and transmitters and damaged the amps coils. Thats what I remember from the old CB days gone by. Hope this helps with a plain english explanation.
Bob