13.56 MHz Antennas

FCC Part 15 rules for the industrial, scientific, and medical band at 13.56 MHz include radiation limits. Maximum field strength from 13.553 to 13.567 MHz is 15,848 V/m (84 dBV/m) at a distance of 30 meters. Experimenters exploit this limit to see how far tiny signals can propagate. I used NEC-2 to calculate maximum input power for legal field strength for several antennas over three types of ground.

Radiation patterns are for average ground quality. Conductors are #14 bare copper wire. The models do not include insulator end effects. I calculated the electric field 30 meters from the origin and 1 meter above ground.

Dipole

The wire is 425″ long and 30 feet above ground. The red dot marks the feedpoint.

Ground      Legal Power   Impedance
Very good      112 mW     83.7−j2.8 Ω
Average         89        81.8+j0.7
Very poor       52        79.2+j4.3

Inverted V

The apex is at 30 feet. Each wire slopes 45 and is 216″ long.

Ground      Legal Power   Impedance
Very good       49 mW     55.2−j1.5 Ω
Average        115        52.2−j0.7
Very poor       72        48.8−j0.2

Inverted V Turnstile

The apex is at 30 feet and each wire slopes 45. The inverted V currents are in phase quadrature.

This overhead view of the offset antenna shows one V with 209″ wires and the other with 224″.

A 2″ jumper (perhaps just coax leads) fed at the center joins wire pairs of unequal length.

Ground      Legal Power   Impedance
Very good       26 mW     52.0+j3.4 Ω
Average         45        51.9+j3.5
Very poor       65        52.0+j3.5

Ground-plane

Base height is 10 feet. The radials slope 10 with ends 82″ high. Each wire is 216″ long.

Ground      Legal Power   Impedance
Very good      2.8 mW     36.3−j1.2 Ω
Average        4.4        34.7−j0.3
Very poor      6.6        32.9+j0.9

Low Dipole

Ground      Legal Power   Impedance
Very good      473 mW     53.6+j9.0 Ω
Average        412        58.7−j0.2
Very poor      228        65.2−j8.0

Lowering a horizontal antenna reduces low-angle radiation, but it also decreases the field strength near ground, which allows higher input power. The patterns show relative radiation for dipoles at 10 and 30 feet with each fed legal power. Radiation is similar at low angles while total radiated power is much greater for the low antenna. The disadvantage of a low antenna is that ground more strongly affects its impedance. But if you can measure it, you can turn this to your advantage by using the value to calculate ground characteristics and legal power.

The dipole is 417″ from end to end and 120″ above ground. Support the feedpoint to keep the wire straight. The antenna should be in the clear with no structures, large objects, or other conductors nearby. The antenna model does not account for wire loops at insulators. Try to devise a way to eliminate them. You might tie-wrap the entire wire to dacron line. Place a female SMA connector at the center.

Measure the antenna impedance at 13.56 MHz with a NanoVNA. Attach it to the antenna feedpoint with a double-male adapter. Calibrate the VNA at the adapter. Use a ladder, binoculars, or Bluetooth to read the impedance. To use the VNA at ground level, attach feedline with a common-mode choke. Calibrate at the far end.

Enter measured R and X values in this program to calculate ground permittivity, conductivity, and legal antenna input power. The program interpolates results for 100 antenna models that cover a wide range of permittivity and conductivity values.

Ground characteristics vary with moisture content and temperature. You may want to measure the antenna impedance at different times of the year.

Low Turnstile

The wires are 201″ and 217″ long with a 2″ jumper.

Ground      Legal Power   Impedance
Very good      855 mW     57.5+j0.5 Ω
Average        412        58.1+j1.4
Very poor      197        58.0+j0.7

A turnstile launches a circularly polarized wave toward ground. The circularity sense reverses upon reflection and the antenna is insensitive to the cross-polarized reflected wave. This makes the impedance insensitive to ground characteristics and precludes using it to calculate legal power.

A turnstile offers omnidirectionality. For average ground at three elevation angles, the legal radiated signal level of a low turnstile is higher than that of a low dipole at each antenna's worst azimuth by these amounts:

 10     20     30
3.3     4.4     4.1 dB

Antenna Comparison

                      10     20     30
Dipole at 10′        0.0     0.0     0.0 dB
Dipole at 30′       +0.6    −0.3    −1.5   
Inverted V          −0.5    −1.2    −2.0
Turnstile at 10′    −1.7    −2.0    −1.9
Turnstile at 30′    −6.5    −7.3    −8.0
Ground-plane       −14.3   −17.9   −21.3

This table compares legal radiated signal levels for average ground at peak azimuth and three elevation angles.

Ground Quality

          Permittivity Conductivity
Very good      20           30 mS/m    Pastoral, low hills, rich soil
Average        13            5         Pastoral, medium hills and forestation, heavy clay soil
Very poor       5            1         Cities, industrial areas

Local permittivity and conductivity can differ greatly from these nominal values. They also vary with frequency.

For antennas other than a low dipole, use a ground probe, VNA, and this program to measure permittivity and conductivity at 13.56 MHz. Then interpolate legal power results to estimate the value for your local ground.

Transmitter RF Voltage

If you can, measure the feedline loss and the impedance at the transmitter end of the feedline. Otherwise, calculate these values as described below. Increase legal power by the feedline loss and use a calibrated, wideband oscilloscope with low-capacitance probe to set the resulting RF voltage at the transmitter.

Let's say you install a dipole 10 feet over average ground and feed it with 100 feet of Belden RG-58C/U. In this calculator enter 58.7 for load R, −0.2 for load X, 13.56 for frequency, 100 feet for length, and Belden 8262 for cable type. Click calculate. Total loss is 1.6 dB (substitute measured value), which is a power ratio of 100.16 = 1.45. Legal transmitter power is 1.45 × 412 = 597 mW. Impedance through the feedline is 51.2−j5.8Ω (substitute measured value) so the legal transmitter RF voltage is √(0.597(51.2+5.8) ∕ 51.2) = 5.56 V RMS (15.7 V P-P).

Common-Mode Choke

Use a common-mode choke to inhibit feedline radiation. You can make a very effective choke by coiling about 12 feet of coax into a 1-foot diameter solenoid with roughly four turns. Tape the coil so that adjacent turns never overlap and everywhere touch. Connect the shields of the coil leads to the two ports of a VNA. Vary the fractional number of turns until self-resonance occurs at 13.56 MHz (maximum through attenuation). Use appropriate lead dress. Add connectors and insert the choke at the antenna feedpoint, or duplicate it by coiling the feedline in exactly the same way. Mount the coil perpendicular to the antenna wires and away from anything metallic.

FCC Part 15

15.23Home-built devices.

(a) Equipment authorization is not required for devices that are not marketed, are not constructed from a kit, and are built in quantities of five or less for personal use.

(b) It is recognized that the individual builder of home-built equipment may not possess the means to perform the measurements for determining compliance with the regulations. In this case, the builder is expected to employ good engineering practices to meet the specified technical standards to the greatest extent practicable. The provisions of 15.5 apply to this equipment.


January 21, 202288108 MHz