Trying to tune a Firestik antenna connected to Magnum 257 on my '98 Jeep Wrangler. Having trouble with SWR reading as it is reading too high (~4) after I set needle on FWD and then switch back to REF. I'm only using it as a CB radio for now and am getting high SWR REF readings on both Ch 40 & 1. Any ideas? Should I just cut the transmit power knob down as far as it will go on the Magnum 257?
Start with a basic inspection of the coax, connectors, and antenna. Unless it is a shunt fed antenna or uses a tapped loading coil, you should find no continuity between center pin and shield on the coaxial connector when checked with an ohmmeter. If there is a short in the connector or coax your SWR will be abnormally high. I have found this several times after pulling my hair out over SWR. Without any other equipment, the best way to tell how to tune it will be to sweep thru the channels and see if you are able to tell which direction is resonant point is (higher or lower than the CB band apparently). Plot on a piece of paper the SWR on all the channels and see which end of the band the SWR is trending lower. The resonant point should be that direction also. If the SWR is lower towards channel 1, the antenna is electrically long, shorten it. If the SWR is lower towards channel 40, your antenna is electrically short, lengthen it.
It is also very advisable to place the meter inline at the antenna to tune it properly first. Then check it with the meter at the radio to determine if standing waves are present due to problems in the transmission line (coax).
A good guide can be found at www.firestik.com/Tech_Docs/Setting_SWR.him
(Running higher than rated power can also skew results unless the meter you are using is designed for more than 4W, just saying)