Electrically Shortened 2 Band Dipole (ES2B)

The drawing on the right illustrates the derivation of the Electrically Shortened 2 Band Dipole (ES2B). At the top is Fig. 1A which is a Trap Dipole. A Trap Dipole consists of two dipoles, A (lower frequency) and B (high frequency), that are electrically seperated by the L/C traps (L1A/C1A & L1B/C1B). These traps are tuned to resonate at the higher frequency. This isolates the trap, and the wire past the trap, when the antenna is excited with the high frequency (FHigh). When the antenna is excited with the lower frequency (FLow), the "effective length" of the trap inductor shortens the end wire needed to resonate the antenna.

Fig. 1B shows a Electrically Shortened Dipole. This type of antenna is usually intended for one band and uses a loading inductor (L2A & L2B) in each leg to electrically and physically shorten the antenna.

Fig. 1C then shows a combination of the Trap Dipole and the Electrically Shortened Dipole. For the higher frequency, (FHigh) the trap (L1A/C1A & L1B/C1B) effectively isolates L2A, L2B, and the rest of the antenna from the inner section. For the low frequency (FLow), L1A/L1B and L2A/L2B combine to form L3A/L3B, and function as the loading inductor .

ES2B Calculator

A calculator for the ES2B is below. Input data is entered on the left and output data is displayed on the right. Initially, select two bands for the antenna and enter the center frequency for each band under Low Band (FLow) Center Frequency and High Band (FHigh) Center Frequency. If you enter them in reverse, the calculator will swap them. Then enter the High Band (FBot) Bottom Frequency which is the low end of the FHigh band. FBot is used for designing the trap. You want the trap to resonate a bit below FHigh.

Then enter the Available Space (A) for the antenna. Below that entry area are the maximum and minimum lengths for A. These limits are placed on the antenna because, the calculator initially creates a full size antenna for FHigh and then a reduced size antenna for FLow. If you entered a value for A that is near the minimum length, the antenna for FHigh will work fine, but may not work very well for FLow. And, the inductor, L2, will be very large.

The calculator will allow you to enter almost anything. But that doesn't mean it will generate any useful data. For example, if you enter a Available Space that is more than required, you will get some negative inductances. That isn't very useful, is it? But, it's possible that you didn't notice that you entered bad data. So, for the example listed, an error in the Available Space, is indicated by changing the color of Max Length (A) or Min Length (A) to Red.

The last two entry areas are Wire Size and Trap Reactance. The Wire Size is intended for the antenna elements and is used in the calculation of the Loading Inductor (L2). Larger wire sizes will increase the antenna bandwidth and create a smaller inductor. But, of course, larger wire means heavier antenna element and inductors.

The default Trap Reactance is 375 Ω and should be kept in the 300-450Ω range. Higher reactance values will increase the Q and isolate the sections better. But as Q increases, bandwidth decreases. The default value of 375 Ω is reasonable for most uses.

Low Band (FLow)
Center Frequency
MHz
High Band (FHigh)
Center Frequency
MHz
High Band (FBot)
Bottom Frequency
MHz
Available Space (A)
Max Length (A)
xx
Min Length (A)
xx
Wire Size
Trap Reactance
Ω

The value for C1 is a Standard Value Capacitor or SVC. While there isn't much you can do with that value, you have some leeway with the inductors.

Inductor Details
Inductor Inductance Form Dia # Turns Wire Form Length TPI L/D
L1
L2

The inductor values (L1 and L2) are brought down from the calculator and added to the Inductor Details table. The details for each inductor are then filled in. The Wire Size specified in the calculator is assumed for both inductors.

The Form Diameter can be different for each inductor. If you are using PVC pipe as a form, remember that the printed pipe size is not the actual outside diameter. For example, 1" PVC pipe has a Outside Diameter of 1.315" (33.401 mm). The table on the right provides the actual outside diameter for several standard PVC Pipe types.

Pipe Size 3/8" 1/2" 3/4" 1" 1-1/4" 1-1/2"
Outside
Diameter		 0.675"
17.15 mm		 0.840"
21.34 mm		 1.050"
26.67 mm		 1.315"
33.40 mm		 1.660"
42.16 mm		 1.900"
48.26 mm
Pipe Size 2" 2-1/2" 3" 3-1/2" 4"
Outside
Diameter		 2.375"
60.33 mm)		 2.875"
73.03 mm		 3.50"
88.90 mm		 4.0"
101.60 mm		 4.50"
114.30 mm

The calculation for the Inductor Details assumes that the space between turns is the same as the wire diameter. This directly drives the TPI (Turns per Inch). The number of turns is intentionally made a even number so that it will be easier to construct.

An important parameter is the L/D Ratio (Length/Diameter Ratio). As you decrease the form diameter, the L/D Ratio increases. The form diameter should be chosen so that the L/D Ratio is around 2:1.