08-08-2010, 07:54 PM
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Join Date: Dec 2008
Location: Silicon Valley CA, Storm Lake IA
Originally Posted by spider87
I know, I just meant the 50 ohms part. I don't know what that part refers to. Sorry.
When an antenna is being transmitted thru; it needs to have an impedance
(that is resistance from an AC circuit. Radio freq's are 'AC") that matches the output of the final transmit transistors. 50 ohms is what your radio would like to see and to work without heating up. The SWR meter is used to check what kind of impedance your antenna is presenting to the radio's output transistors. Adjusting the height of the antenna whip (either up or down) will change this impedance value; the meter is used to dial it in the zone. You will need to tune the antenna manually - usually with a small Allen wrench - to get this adjustment correct. The antenna subsystem is the most important part of your CB system. It's tuning can make you or break you. When checking the SWR or the "Standing Wave Ratio", you will be hooking up a simple form of a directional watt meter between your antenna and the CB radio.
The meter measures Forward power to the antenna system and then compares it to the reverse or reflected power from the antenna system. Instead of showing you power, the meter is calibrated to show the RATIO of coupling from radio to antenna system. If there is all forward power and no reflected power the antenna system has a one to one (1:1) ratio with the transmitter.
By checking the SWR at channels 1 and 40 on your CB, you will have a good idea if the antenna is too "long" or too "short" for the center of the desired band. You will also be able to evaluate your system performance and troubleshoot problems. For instance a SWR too high (over 3:1) all over the band can be an indication of a bad part or junction, poor ground or poor location. An SWR too low over the whole band (1:1 for all 40 channels) can indicate a low efficiency antenna or lossy component.
Safe operation of your CB should be with an SWR of 2:1 or less. Most people would prefer to be 1.5:1 or less. A good quality antenna and proper installation (location very important) should easily result in an SWR of less than 1.5:1 and for most long antennas it will be that low across all 40 channels (plus a few). Shorter Antennas tend to be more narrow banded and may have as high as a 2:1 at the top and bottom of the band. Here's one of the places you can evaluate your antenna. We EXPECT a high quality "short" antenna to be narrow banded. If you have an antenna that's say, three feet or less, and the SWR tunes great (less than 1.5) across all the channels than maybe your antennas feedline coax or the antenna coil is "lossy".
In "Lossy" I mean that maybe the design or construction material is inferior and is wasting some of your energy as ground losses or increased heat dissipation. This causes both the forward and reflected power to be partially absorbed, thus showing a lower SWR on the meter.
What the meter shows on an SWR meter reading - is the amount of power being reflected back into the radio. Too much 'reflected' power is what can damage a radio; that is why we told you that it is important! We want to adjust that antenna so that it reflects very little power back into the radio - and on to the antenna will it will transmit better. A SWR reading of 3 to 1 or above is considered unacceptable. A SWR reading of 1.5 to 1 or less is considered desirable - and that is the target to reach. Sometimes you will not be able to get it to 1.1 to 1; so you must get it as close as you can.
Read this completely; than ask some more qustions about it: SWR
BASE: Kenwood TS-2000 w/Alpha Delta DX-CC, Diamond X50A, Sirio SY27-4, and IMAX 2000.
MOBILE: Galaxy DX99V w/RF Limited CR-577 mic, Sirio Z-180. Yaesu FT-8800R & Diamond NR-770.