Object is to make a simple loop to track down interference using AM demodulation on the 2 meter radio in the car. After that, it will be interesting to try the antenna to reach the Anoka County Radio Club repeater. The same size antenna works well for both.
$ ./SimpleLoopFactory.pl > SimpleLoop.nec $ xnec2c SimpleLoop.nec &
From http://www.66pacific.com/calculators/full-wave-loop-antenna-calculator.aspx for 146.670 MHz Length of full-wave loop:
- 6 feet, 10.2 inches
- 2.09 meters
The calculated length is approximate. In practice it's best to make the antenna a little longer than the calculated value and then trim it to get the best SWR value.
Feedpoint impedance and coaxial matching section
The feedpoint impedance at the design frequency is about 100 ohms, so full wave loops are often fed with an impedance-matching section made from a piece of 75-ohm coxial cable one quarter wavelength long (shortened to compensate for the velocity factor of the cable).
Length of an impedance-matching section with 75-ohm cable with a velocity factor of 0.66 (polyethylene dielectric):
- 1 feet 1.3 inches
- 0.34 meters
Length of an impedance matching section made with 75-ohm cable with a cable velocity factor of 0.80 (foam dielectric):
- 1 feet 4.1 inches
- 0.41 meters
The formulas for calculating the approximate length of a full-wave loop antenna are:
- Full-wave loop in feet = 1005 / frequency in MHz
- Full-wave in meters = 306 / frequency in MHz
The formulas for calculating the length of an impedance matching section are:
- Quarter-wave matching section length in feet = 246 * cable's velocity factor / frequency in MHz
- Quarter-wave matching section length in meters = 75 * cable's velocity factor / frequency in MHz
The velocity factor for polyethylene coax is usually 0.66. For foam-dielectric cable, it's usually 0.8.
146.670 MHz -> 0.333 meters radius for full wave loop at Anoka County Radio Club repeater frequency.