Alternative Techniques in Small Antennas 3
Updated 20180520
This page has results from 2017 and looks forward to the 2018 season.

Conventional loaded antenna comparison
For a frequency near 28MHz, a
Boomerang 27W was mounted in the same position, just slightly higher to accommodate it’s 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…
27W

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

Bandwidth of capacitive antenna, 2:1 = 600kHz, albeit centre frequency 500kHz higher.
0.7m high VSWR

Exactly half the bandwidth. A photo of the antenna that gave the above VSWR plot and relative signal readings.
Mini mesh

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 = 0.50
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.
H-field probe
Showing the results on paper is difficult. I made a YouTube video. 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.


Advances from 2017

1.
As confirmed by NASA, mesh sheets give nearly the same capacitance when spaced apart as solid metal. A 10% density mesh gives 90% of the capacitance and therefore E-field. Vastly reduced weight, visibility and windage. Taking the concept further the ground grid becomes a perimeter-connected radial system.

2. Balancing by moving the tapping point doesn’t greatly reduce common mode current with the ground-grid design. One or two turns does help some with the common mode and has the advantage of greater flux linkage. So, I now connect the feed line 2 turns from the ground-grid end.

3. Power dissipation of the choke was estimated at 5% of the total power with a crude temperature rise test. So, blocking the common mode current doesn’t waste significant energy in the choke. A completely conventional ferrite choke is therefore selected, as used in windom or other standard designs of antenna.

4. A larger coil to reduce losses. This idea is the combination of loop and capacitor, or more descriptively an open capacitor loop antenna. Its possible to directly couple across 1-turn of a 2.5 turn loop and get an optimal match. This design seems to combine TE10 and TM10 radiation as described by McLean’s theorem.

These 4 advances can be built on during 2018…

Tests for 2018
1. Optimisation of the ground grid size and the top grid height. Previous tests indicate spacing of the top grid must be 4-6% λ for best efficiency. I need to put some solid far-field numbers behind that.

2. Construction of top-band prototype. Using a large grid raised 7-8m above a perimeter connected radial system. If this outperforms a 40m long windom on 160m it will be installed permanently.

3. Open capacitor loop. Construction of 10m and 20m prototypes with thick copper tube and low resistance connections throughout. This should bring the loop aspect into full effect. It may introduce some directivity and contrary to what normally happens with loops, maintain several percent of 2:1 SWR bandwidth. Suitable tube is sold for industrial refrigeration.
s-l1600
4. The shape of the top-grid was changed from a flat rectangle by adding mesh pieces to make triangular and X-shape grids. The triangular grid produced a 10% reduction in frequency, the X-shape only 5%. This counter-intuitive result needs a repeat of both the frequency reduction and far-field measurement.

There are several lower priority tests which may have to wait for another year, such as proving where the H-field comes from.