| Introduction |
This page looks at vertical antennas that are shorter than 1/4 λ, for any particular frequency. Almost any vertical element can be use on any band, if a loading coil is included. The loading coil helps to make up for the lack of element length. This is very common with mobile installations where a 102" vertical radiator is resonated with a loading coil to operate on the lower HF bands.
But a loading coil can also be used with stationary installation. For example, the 43' vertical. This vertical is often advertised as usable from 160 Meters to 10 Meters, with a tuner. It does tune nicely for 40 Meters to 10 Meters, but has trouble being tuned on 80 Meters and 160 Meters. It's certainly still usable on those bands, but with the help of a loading coil, it would be easier to tune.
The Single Element loaded vertical (left) has a loading coil inserted at the base of the antenna element. This is often the simplest and sturdiest situation, but it is also the least efficient. For better efficiency, a Double Element loaded vertical (right) could be used. You could move the loading coil all the way to the top, for maximum efficiency. But as you increase H1 and reduce H2, the required loading coil gets larger. Usually the loading coil gets too big to support easily.
Notes
- End effect calculations are purposely omitted to ensure that an antenna is electrically overlength. This is so that resonance at the design frequency can be obtained easily by removing a turn or two from the loading coil.
- Optimum loading coil Q is attained when the coil Length/Diameter ratio is at or near 0.5. Diameter of loading coil conductor (wire or tubing) should be as large as practicable. Loading coil turns should be removed, NOT shorted, for tuning purposes.
- Ref. THE ARRL ANTENNA BOOK, 17th Edition, pages 16-5 TO 16-11
| Single Element Vertical Antenna Calculator |
This page contains the Single Element calculator on the left and support information on the right. Initially, enter your Frequency of operation, the diameter of the vertical element (H1), and the length of the vertical element (H1). The diameter can be entered in inches or mm. The length can be entered in inches, feet, mm, or cm.
Under the entered data will be the calculated values for the Single Element specified and it's associated Loading Coil. This calculation is automatic, triggered simply by changing the data in the entry areas.
As a guide to your choices, the chart on the right (Single Element Typical Values) will show you typical values for H1 lengths from 10° to 90°, in 5° increments. You might note, in that chart, that the feed impedance (Radiation Resistance) is small, even with H1 elements that approach 90°. You will need to match this impedance to your transmitter.
Then, under Loading Coil Specifications, enter your Wire Size, Wire Type, and select the Length/Diameter Ratio. The Turns-Per-Inch (TPI) area is only needed if you are using insulated wire. See the notes below.
The calculator will determine a form diameter based on the selected Length/Diameter Ratio. If you want to use a form with a different diameter, the PCV Pipe table can be used. You can select any of the pipe sizes listed, or manually specify a diameter.
The default values are an example of designing a antenna for 14.1 MHz using a standard stainless steel whip antenna that is 102" long and 1/4" diameter. To make the whip resonate at 14.1 MHz, a loading coil of 4.511 uH would be required. The loading coil is 10.7 turns of #14 AWG Enameled wire on a 1.416" diameter form. As an alternate form, a section of standard 1" diameter PVC pipe is used. This pipe has an actual diameter of 1.32". This slightly changes the number of turns and L/D Ratio, but should work effectively.
| Initial Antenna Dimensions | ||
|---|---|---|
| Frequency MHz |
H1 Diameter |
H1 Length: |
| Single Element Vertical Antenna Parameters | |
|---|---|
| Length H1 (Electrical Degrees), H1 = | x |
| Degree-Ampere Area, A = | x |
| K Constant, K = | x |
| Radiation Resistance, RR = | x |
| H1 Characteristic Impedance, Km = | x |
| Inductive Reactance, XL = | x |
| Loading Coil Inductance = | x |
| Loading Coil Specifications | |||
|---|---|---|---|
| Wire Size |
Wire Type |
Turns per Inch |
L/D Ratio |
| Initial Coil Output Data | |
| Number of Turns = | x |
| Wire Type = | x |
| Wire Size = | x |
| Turns Spacing = | x |
| Coil Form OD = | x |
| Coil Length = | x |
| L/D Ratio = | x |
| Turns_per_Inch = | x |
| Single Element Typical Values | ||||||
|---|---|---|---|---|---|---|
| Length H1 | Radiation Resistance (Ohms) | Antenna Impedance (Ohms) | Antenna Reactance (Ohms) | Loading Coil (uH) | ||
| deg | cm. | in. | ||||
- The Turns per Inch area, above, is used as a Output when Bare or Enameled wire is selected, however, it is used as a Input when Insulated wire is selected.
- For Bare Wire, the spacing between turns is calculated as twice the wire diameter. When winding, a length of the same size wire is used as a spacer, and then removed.
- For Enamled Wire, the spacing between turns is calculated as the wire diameter + 0.005" (.127 mm), which is the approximate thickness of the enamel coating.
- For Insulated Wire, you need to determine the number of Turns-Per-Inch (TPI) when the wire is close wound. To determine this, wrap some of the wire you will be using around a ruler and count the number of turns in 1 inch (25.4mm). Then enter that data in the Turns-per-Inch input area.
Custom Loading Coil Output Data
It's nice to have some accurate calculations. But material like PVC pipe only comes in some standard sizes. And, the listed Pipe Size doesn't exactly match the Actual Pipe Size. So the loading inductor specified above needs to be adjusted to the Actual Pipe Size you select. The selected pipe size should be as close to the calculated value as possible.
| PVC Pipe | |||
|---|---|---|---|
| Pipe Size | Outside Diameter | Pipe Size | Outside Diameter |
| 3/8" | 0-11/16" (0.66" | 17.1 mm) | 1/2" | 0-13/16" (0.84" | 21.3 mm) |
| 3/4" | 1-1/16" (1.05" | 26.7 mm) | 1" | 1-5/16" (1.32" | 33.4 mm) |
| 1-1/4" | 1-11/16" (1.67" | 42.2 mm) | 1-1/2" | 1-7/8" (1.90" | 48.3 mm) |
| 2" | 2-3/8" (2.38" | 60.3 mm) | 2-1/2" | 2-7/8" (2.88" | 73.0 mm) |
| 3" | 3-1/2" (3.50" | 88.9 mm) | 3-1/2" | 4" (4.0" | 101.6 mm) |
| 4" | 4-1/2" (4.50" | 114.3 mm) | ||
| Custom Coil Form Diameter | |||
| Custom Loading Coil Based On Selected PVC Pipe | |
| Number of Turns = | x |
| Wire Type = | x |
| Wire Size = | x |
| Turns Spacing = | x |
| Coil Form OD = | x |
| Coil Length = | x |
| L/D Ratio = | x |
| Turns_per_Inch = | x |
| Double Element Vertical Antenna |
The other situation is the Double Element short vertical. In this situation, the loading coil is at some height above the base of the antenna. The loading coil can be anywhere along the length of the antenna. The higher the loading coil, the greater the efficiency, however, that also means the loading coil will necessarily be larger.
- End effect calculations are purposely omitted to ensure that an antenna is electrically overlength. This is so that resonance at the design frequency can be obtained easily by removing a turn or two from the loading coil.
- Optimum loading coil Q is attained when the coil Length/Diameter ratio is at or near 0.5. Diameter of loading coil conductor (wire or tubing) should be as large as practicable. Loading coil turns should be removed, NOT shorted, for tuning purposes.
- Ref. THE ARRL ANTENNA BOOK, 17th Edition, pages 16-5 TO 16-11
| Initial Antenna Dimensions | ||
|---|---|---|
| Frequency MHz |
H1 Diameter |
H1 Length: |
| H2 Diameter |
H2 Length: |
|
| Double Element Vertical Antenna Parameters | |
|---|---|
| Length H1 (Electrical Degrees), H1 = | x |
| Length H2 (Electrical Degrees), H2 = | x |
| Degree-Ampere Area, A = | x |
| K Constant, K = | x |
| Radiation Resistance, RR = | x |
| H1 Characteristic Impedance, Km = | x |
| H2 Characteristic Impedance, Km = | x |
| Inductive Reactance, XL = | x |
| Loading Coil Inductance = | x |
| Loading Coil Specifications | |||
|---|---|---|---|
| Wire Size |
Wire Type |
Turns per Inch |
L/D Ratio |
| Initial Coil Output Data | |
| Number of Turns = | x |
| Wire Type = | x |
| Wire Size = | x |
| Turns Spacing = | x |
| Coil Form OD = | x |
| Coil Length = | x |
| L/D Ratio = | x |
| Turns_per_Inch = | x |
| Double Element Typical Values | |||||||
|---|---|---|---|---|---|---|---|
| Length H1 | Length H2 | Radiation Resistance (Ohms) | Loading Coil (uH) | ||||
| deg | cm. | in. | deg | cm. | in. | ||
- The Turns per Inch area is used as a Output when Bare or Enameled wire is selected, however, it is used as a Input only when Insulated wire is selected.
- For Bare Wire, the spacing between turns is calculated as twice the wire diameter. When winding, a length of the same size wire is used as a spacer, and then removed.
- For Enamled Wire, the spacing between turns is calculated as the wire diameter + 0.005" (.127 mm), which is the approximate thickness of the enamel coating.
- For Insulated Wire, you need to determine the number of Turns-Per-Inch (TPI) when the wire is close wound. To determine this, wrap some of the wire you will be using around a ruler and count the number of turns in 1 inch (25.4mm). Then enter that data in the Turns-per-Inch input area.