Alternative Techniques in Small Antennas

Updated 20180620

Tests for 2018

1. Install and test top-band version. A prototype was successful with 10-12dB better signals than my 40m long windom. This doesn’t “prove” anything scientifically but gives a useful 160m antenna at just 7m high.

2. An attempt to prove combined electric TE10 and magnetic TM10 mode radiation is possible.

2017 - Conventional loaded antenna comparison

For a frequency near 28MHz, a Boomerang 27W was mounted in the same position, just slightly higher to accommodate its long bottom dipole element. The boomerang is 1.75m x2 = 3.5m long, which is 0.67 of a half wave dipole. It was compared with horizontal, tilted and vertical polarisation to a rebuilt capacitive antenna of 0.7m height. The MMI-dipole used for horizontal and vertical, the inverted-V for tilted polarisation.

Boomerang signal readings off KiwiSDR:

Horizontal = -86dBm

Tilted = -74dBm

Vertical = -75dBm

Capacitive signal readings off KiwiSDR:

Horizontal = -96dBm

Tilted = -75dBm

Vertical = -72dBm

EH antenna signal readings off KiwiSDR:

Horizontal = -96dBm

Tilted = -80dBm

Vertical = -80dBm

It’s difficult to “average” these results because they’re absolute values in dBm, but it’s clear the Capacitive is very close to the Boomerang. Taking half the Boomerang length as a fair comparison, it’s 1.75/0.7 = 2.5x longer than the Capacitive antenna. As said on the previous page, you should assume some uncertainty in these measurements. Boomerang photo…

Bandwidth of Boomerang 27W, 2:1 = 1.2MHz.

Bandwidth of capacitive antenna, 2:1 = 600kHz, albeit centre frequency 500kHz higher.

Exactly half the bandwidth.

Chu-Wheeler Limit Comparison

Any suggestion a short radiator can match a far longer one is controversial. Does it contravene the Chu-Wheeler criterion? It measures as strongly vertically polarised, so the standard Chu-Wheeler formula applies.

Radian-sphere 0.75 high, 0.4 wide = 0.84m (by Pythagoras)

Frequency = 28MHz

Efficiency (eta η) estimated by measurement at 0.5 (-3.0dB down on half-wave vertical)

There are several documents transposing the basic Chu formula to give VSWR bandwidth, here is a link to one. Prof. S.R. Best gives equation (6) with a handy calculation including SWR (s). Working out on paper gives 0.28MHz. Measured 2:1 SWR bandwidth = 0.6MHz, so about twice the conventional theory. The document also gives bandwidth comparisons for many proven small antenna designs.

Of course this doesn't include the size of the ground-grid (or ground plane). The grid may not be acting purely as a reflector. If anyone likes to correct my arithmetic above, go ahead. With all uncertainties involved, I cannot confirm the antenna breaks conventional theory. But it’s definitely a good antenna for its size.

H-Field Probe Test

H-field probe test… where is the H component generated? To get some idea, I made a simple H-field probe. Consisting of a shielded loop, diode detector, and moving coil meter. The coax braid stops the electric field, but the magnetic field penetrates the braid. Shielding is important to minimise pickup from the strong high voltage electric field present in antenna testing.

Showing the results on paper is difficult. The largest magnetic field seems to be the side of the grid over the edge of the ground-grid. The side near the middle of the ground-grid has hardly any magnetic reading. This test needs to be repeated under more controlled conditions.

This experiment disproves what some say about displacement current between two plates of a capacitor, caused by an electric field. A magnetic field can only be made by current flow in wires. As you can see I measure no magnetic field when the meter is moved away from the plates. This is shown mathematically by Jefimenko’s equation.

The antenna obviously generates a high electric field (E). But, a corresponding magnetic field (H) is needed for generation of far field radiation, with the E x H vector product. An H-field is generated across areas of the metal high voltage grid, and from the transformer coil. Improving the H-field characteristics is probably the key to progress.

Updated 20180620

Tests for 2018

1. Install and test top-band version. A prototype was successful with 10-12dB better signals than my 40m long windom. This doesn’t “prove” anything scientifically but gives a useful 160m antenna at just 7m high.

2. An attempt to prove combined electric TE10 and magnetic TM10 mode radiation is possible.

2017 - Conventional loaded antenna comparison

For a frequency near 28MHz, a Boomerang 27W was mounted in the same position, just slightly higher to accommodate its long bottom dipole element. The boomerang is 1.75m x2 = 3.5m long, which is 0.67 of a half wave dipole. It was compared with horizontal, tilted and vertical polarisation to a rebuilt capacitive antenna of 0.7m height. The MMI-dipole used for horizontal and vertical, the inverted-V for tilted polarisation.

Boomerang signal readings off KiwiSDR:

Horizontal = -86dBm

Tilted = -74dBm

Vertical = -75dBm

Capacitive signal readings off KiwiSDR:

Horizontal = -96dBm

Tilted = -75dBm

Vertical = -72dBm

EH antenna signal readings off KiwiSDR:

Horizontal = -96dBm

Tilted = -80dBm

Vertical = -80dBm

It’s difficult to “average” these results because they’re absolute values in dBm, but it’s clear the Capacitive is very close to the Boomerang. Taking half the Boomerang length as a fair comparison, it’s 1.75/0.7 = 2.5x longer than the Capacitive antenna. As said on the previous page, you should assume some uncertainty in these measurements. Boomerang photo…

Bandwidth of Boomerang 27W, 2:1 = 1.2MHz.

Bandwidth of capacitive antenna, 2:1 = 600kHz, albeit centre frequency 500kHz higher.

Exactly half the bandwidth.

Chu-Wheeler Limit Comparison

Any suggestion a short radiator can match a far longer one is controversial. Does it contravene the Chu-Wheeler criterion? It measures as strongly vertically polarised, so the standard Chu-Wheeler formula applies.

Radian-sphere 0.75 high, 0.4 wide = 0.84m (by Pythagoras)

Frequency = 28MHz

*ka*= 2xPI/λ x 0.84 = 0.50Efficiency (eta η) estimated by measurement at 0.5 (-3.0dB down on half-wave vertical)

There are several documents transposing the basic Chu formula to give VSWR bandwidth, here is a link to one. Prof. S.R. Best gives equation (6) with a handy calculation including SWR (s). Working out on paper gives 0.28MHz. Measured 2:1 SWR bandwidth = 0.6MHz, so about twice the conventional theory. The document also gives bandwidth comparisons for many proven small antenna designs.

Of course this doesn't include the size of the ground-grid (or ground plane). The grid may not be acting purely as a reflector. If anyone likes to correct my arithmetic above, go ahead. With all uncertainties involved, I cannot confirm the antenna breaks conventional theory. But it’s definitely a good antenna for its size.

H-Field Probe Test

H-field probe test… where is the H component generated? To get some idea, I made a simple H-field probe. Consisting of a shielded loop, diode detector, and moving coil meter. The coax braid stops the electric field, but the magnetic field penetrates the braid. Shielding is important to minimise pickup from the strong high voltage electric field present in antenna testing.

Showing the results on paper is difficult. The largest magnetic field seems to be the side of the grid over the edge of the ground-grid. The side near the middle of the ground-grid has hardly any magnetic reading. This test needs to be repeated under more controlled conditions.

This experiment disproves what some say about displacement current between two plates of a capacitor, caused by an electric field. A magnetic field can only be made by current flow in wires. As you can see I measure no magnetic field when the meter is moved away from the plates. This is shown mathematically by Jefimenko’s equation.

The antenna obviously generates a high electric field (E). But, a corresponding magnetic field (H) is needed for generation of far field radiation, with the E x H vector product. An H-field is generated across areas of the metal high voltage grid, and from the transformer coil. Improving the H-field characteristics is probably the key to progress.