Air Band Antenna Analysis

I created this page to get a better idea about the effect of dipole element diameter on the SWR and Gain. I chose the Air Band because it has a fairly large band width (29 MHz). The results are pretty interesting.

According to Wikipedia.org:

The VHF Airband uses the frequencies between 108 and 137 MHz. The lowest 10 MHz of the band, from 108–117.95 MHz, is split into 200 narrow-band channels of 50 kHz. These are reserved for navigational aids such as VOR beacons, and precision approach systems such as ILS localizers.

To me, this means that the area between 108 and 118 MHz doesn't provide the casual scanner person with any meaningful information. So the upper 19 MHz (118–137 MHz) would be the main area of interest.

Element Diameter	Dipole Length
#14 AWG (0.00163mm)	45-7/8" (1.16562mm)
#12 AWG (0.00205mm)	45-13/16" (1.16327mm)
#10 AWG (0.00259mm)	45-11/16" (1.16072mm)
1/4" (0.00635mm)	45-3/16" (1.14840mm)
1/2" (0.01270mm)	44-11/16" (1.13496mm)
3/4" (0.01905mm)	44-1/4" (1.12455mm)
1" (0.02540mm)	43-15/16" (1.11549mm)

So I picked a center frequency of 122.5 MHz and ran few dipole design simulations, using different diameter elements. I designed seven dipole antennas using different diameter elements. The first three are solid conductor wire of three different gauges. The other four are intended to be copper tubing. The table at the right shows the seven dipoles. Initially note that, even though the design frequency was not changed, the length of the dipoles does. As the element diameter gets larger, the antenna length gets smaller.

Then I used the antenna simulator to extract the SWR and Gain figures from each antenna, as I scanned from 100 to 150 MHz. Initially I used data every 100 KHz, but that was too much data and caused the web page to be too slow. So I trimmed the data down to every 500 KHz. The SWR assumes that the antenna will feed a amplifer with 75 Ohms input impedance or drive 75 Ohm cable.

The switches below interact with the graph below them. The switches allow you to turn on the SWR and Gain traces for the various antennas. SWR is indicated by the left vertical axis and Gain by the right vertical axis. What the graph shows is that, a Aircraft Band antenna can be made from a variety of different diameter conductors. But while small gauge wires may be convenient, larger diameters will provide you with a wider operational bandwidth.

100

110

120

130

140

150

1:1

2:1

3:1

4:1

5:1

6:1

7:1

8:1

9:1

10:1

11:1

SWR

1.4

1.5

1.6

1.7

1.8

1.9

2.0

2.1

2.2

2.3

2.4

Gain (dBi)

122.5 MHz

108 - 137 MHz Aircraft Band

AFC

1290k