Antenna Diameter Vs VSWR
An interesting discussion on the effect of the diameter on the bandwidth of an antenna taking place on 40 meters a few days ago caused me to think about the subject of what effect the size of the antenna conductor had on the VSWR and bandwidth of the antennas used. So two antennas were modeled in this article and the results were graphed.
The two antennas modeled were on two 40 meter verticals, one made of 2 inch (5.08 Cm.), the other made of #12 wire. These are common antennas used by hams, the first being a typical 40 meter ¼ wave vertical that is a common DX antenna. The other one is what a QRP backpacker would use if he could find a suitable support for it. Both antennas are the same height, 33.ft (10.03 meters), and are matched on 7.1 MHz. A ground radial system is also assumed to be in place and the antennas are simulated in a moderately clear area.
Graph 1 shows the results of a frequency sweep from 6.7 to 7.5 MHz. It is readily apparent the large diameter antenna has a greater bandwidth, as the VSWR curve is a lot flatter than the VSWR curve of the wire antenna. This is also one of the reasons the antennas used in the days of spark contributed to the extremely wide band signals emitting from the spark transmitters. The pipe antenna VSWR at 6.7 MHz is below 2.5:1 and the wire antenna is at 3.8:1 on the same frequency. This is due to the Length-to-Diameter ratio. All of this contributes to the bandwidth of the antenna. Of course, the higher in frequency you go, the more effect the diameter has on the bandwidth. When you move up to the UHF range, such as the 420-450 MHz band, the diameter of the elements has a definite effect in overall bandwidth.
Originally posted on the AntennaX Online Magazine by Richard Morrow, K5CNF
Last Updated : 5th March 2024