Horizontal Indoor Antenna Calculator... VE3KL

Input Area 1

Used to Calculate the Allowable Maximum and Minimum Length of Wire #1

Frequency [MHz]
( 3 to 30 MHz )

Select Units

Maximum Length of Wire 1
Minimum Length of Wire 1

Input Area 2 Input the length of Wire1 based on your room size Keep length between Max and Min limits given above
Length of Wire 1

Length of Wires 2,3
Gain [dBi]
Vswr

Antenna Description

The antenna shown in the above diagram is basically a horizontal half wave dipole that is bent on both ends to fit into an attic or other confining space.
The antenna is modelled as three wires. Wire number 1 is connected to a Balun at its centre while wires 2,3 are connected to the ends of wire 1. Wires 2 and 3 are equal in length.
This antenna is balanced (symmetrical) which means that a current balun will inject equal currents into each half with no current injected onto the outside of the coaxial cable. Hence it is a very good antenna for use indoors as RF current will not travel down into the radio shack.

What the Calculator Does

The calculator computes the length of Wire 1, Gain and VSWR of the Indoor Antenna.

The input parameters are the Frequency [MHz] and the length of Wire 1.

Since the dipole is "bent" around corners, its VSWR, Gain and overall length depend on the frequency and the chosen length of wire 1. The critical input is the length of wire 1. First, a plan should be made to find out how much space is available for the antenna. This will determine the feasibility of using this antenna for a given band.

How to Use the Calculator

First, input the desired centre frequency and select the desired units (Metres, Feet or Wavelengths) for calculation in the box named Input Area 1. Then press Calculate.

The calculator then computes the Maximum and Minimum length that is allowed for Wire 1 and outputs the results in the boxes below Input Area 1.

Then insert the length of Wire 1 into Input Area 2 and press Get Output. Remember to keep the value of Wire 1 within the limits defined by the calculator. The length of your room will determine the Maximum length of Wire 1 that you fit into the space. A ratio of the length of Wire 1 to the Length of Wires 2,3 of 3:1 gives the best overall results but this is not critical.