The table below is from the "ARRL Antenna Book," page 26-21, 19th Edition (ARRL). I've added a couple columns to make it a little clearer to a beginner.
For anyone that does not know, it is very important to use either a coax choke, or balun, on your feedline connecting your transceiver to your antenna, unless it happens to be designed for a single frequency and the entire system is well matched.
In the event the feed line is NOT perfectly matched to the antenna (it normally is not) there will be some degree of "reflection" resulting in part of the RF energy bouncing off the antenna and heading back to the transceiver. This reflected wave compared to the forward wave (your transmission) results in the SWR (Standing Wave Ratio - VSWR is Voltage SWR, and both are the same thing, only the term used differs).
A meter measuring this SWR is designed to measure two energy "flows" - the forward and the reflected wave. If there were a third wave of energy present, for which the meter is not designed to measure, the SWR reading would not be accurate.
Due to "skin effect" electrical (AC, alternating current) energy produced in our antenna systems flows on the outside of the metal conductor, not through the entire wire (as would DC). The higher the frequency the closer to the surface the energy flows. Your transmitted signal flows along the outside of the center conductor inside your coax to your antenna. The refelcted wave flows down the outer layer of the coax shielding, but on the inside of the braid (or foil). These are the two energy flows your SWR meter is designed to measure.
There does exist the possiblity of a third wave of energy, one that flows down the outside of one's coax shielding. It is this undesirable flow of energy which the coax choke (or a balun) is designed to eliminate.
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COAX CHOKE - A number of feet of coax wrapped into a coil, without any turn overlapping another. Can be wrapped on a non-conductive form or simply taped together. BALUN - Transfer point in one's feedline from a BALanced feedline to an UNbalanced feedline. Twinlead (also called Ladder Line) is balanced, whereas coax is unbalanced. |
Low SWR, Wrong Reason |
For more specific information on this topic, read "Reflections II".
| Freq. MHz | Band | RG-213, RG-8 | RG-58 |
|---|---|---|---|
| 3.5 | 80 Meters | 22 ft, 8 turns | 20 ft, 6-8 turns |
| 7 | 40 M | 22 ft, 10 turns | 15 ft, 6 turns |
| 10 | 30 M | 12 ft, 10 turns | 10 ft, 7 turns |
| 14 | 20 M | 10 ft, 4 turns | 8 ft, 8 turns |
| 21 | 15 M | 8 ft, 6-8 turns | 6 ft, 8 turns |
| 28 | 10 M | 6 ft, 6-8 turns | 4 ft, 6-8 turns |
| Freq. MHz | Band | RG-213, 8, 8X, 58, 59 |
|---|---|---|
| 3.5 - 30 | 10 - 160 M | 10 ft, 7 turns |
| 3.5 - 10 | 30 - 80 M | 18 ft, 9-10 turns |
| 7 - 28 | 10 - 40 M | 12 ft, 7-8 turns * |
| 14 - 30 | 10 - 20 M | 8 ft, 6-7 turns |
"Wind the indicated length of coaxial feed line into a coil (like a coil of rope) and secure with electrical tape. The balun is most effective when the coil is near the antenna. Lengths are not highly critical."
- Emphasis mine, from the "ARRL Antenna Book"