Antennas and Coax

Back to Education
Double-click here to edit the text.
Antennas and Coax
 There seems to be a great amount of confusion as to SWR, coax length, and antenna tuning. I can't go into great detail here, but would like to help propagate some factual information that may be helpful to those seeking good results.
 There is nothing that an antenna tuner can do to change the tuning of an antenna. There is no use in trying to use your tank circuit in your amp to try to make the load look like it should. The entire antenna system is the feed line and the antenna if the antenna is not 50 Ohms +/- j0.
 If the feedpoint of the antenna is other than 50 Ohms +/-j0, and if you are using 50 Ohm coax. Coax length may alter reading taken at the transmitter. This is indicative of the feed point at the antenna being other than 50 Ohms +/-j0.
 The coax has a particular amount of capacitance per foot, and that reactance may tune out opposite inductive reactance in the antenna. Though the meters may say you have a good SWR, the meter is now looking at everything connected to it. The Coax, and the antenna included. No ifs ands or buts.
 An antenna tuner may save your radio from running into a bad SWR. An antenna tuner will deliver the maximum power into the load. The load however, is the entire coax, and antenna. You may be asking how to get it right. First, Tune your station into a dummy load. This is a 50 Ohn +/-j0 load.
 It is what the antenna is supposed to look like.
 The Coax is 50 Ohms, and it is only that if the source and load is 50 Ohms +/-j0.
Once the antenna is tuned correctly, then it does NOT matter what the coax length is as long as you are NOT taking measurements on that coax. If you are taking measurements, the coax must be exact multiples of electrical 1/2 waves.
This is accomplished by multiplying 492 times the Velocity factor of the coax that you are useing, and deviding that by the frequency that you are operating in MHz.
Always treat the input of an amp the same as an antenna. the previous stage is also trying to look at 50 Ohms +/- j0