Small Antennas 2015
Updated 20160605
This page describes aerial experiments in 2015, to setup a link over 4 miles between my house and my father's house. The objective to make something comparable to a large half-wave vertical operating on 27-29MHz.

The
Is
RF_Hazard
otron, SRA (super rad antenna) and my own EEF (enhanced E-field) are small antennas under test on this page. The Isotron was developed into the MicroVert, and I developed another new version. The central feature is radiation from plates. Not to be a Health & Safety grumpy, but the antennas shown can generate high voltages and emit RF radiation in confined areas.

Test Results 2015
For received signal measurements with good precision I used the Softrock Mk.3 rig with PowerSDR. Receiving antenna was the end-connected windom (OCFD). Distance between stations about 4 miles with a chalk hill between. For comparison some readings were also taken with a vertical at the receiving side to get an idea of polarisation effects. Results on a vertical at the Rx station showed similar relative differences between the Isotron and EEF, indicating there is mixed polarisation from them.

These results are NOT accurately calibrated in dBm but the relative levels are believed to be valid. The numbers are taken as an average on several transmissions.
It was noted wet conditions reduced signals by 1-2dB, so results were taken in dry weather.

Bilal Isotron, coax connected to alloy pole: -67.3dBm
Bilal Isotron, modified with floating coax: -68.5dBm
EEF (cylinder) 1 with 2.0m coax: -61.5dBm (repeated at later date)
EEF (cylinder) 2 with 2.0m coax: -62.5dBm
SRA disc type, coax 2.0m coax: -63.6dBm
Sloping reference half wave dipole: -67.0dBm
"Venom" half wavelength CB antenna: -53.8dBm
EEF (cylinder) 1 with 2.5m tail, air core choke: -61.4dBm
EEF (cylinder) 1 with 2.0m tail, 4x FT140-61 choke: -61.5dBm (repeat of prior test)
EEF (cylinder) 1 with 1.5m tail, 4x FT140-61, top plate added: -61.1dBm
EEF (cylinder) 1 with 1.5m tail, 4x FT140-61, top plate added, coax raised: -61.4dBm
EEF (cylinder) 1 with 1.5m tail, 4x FT140-61, T106-6 toroid: -61.1dBm
Grid EEF with no coax tail: -63.2dBm
Sirio Boomerang 27W, helically loaded dipole: -61.0dBm
Balanced Grid EEF, no common mode: -62.0dBm

The rest of this page has pictures and construction details of the home-made antennas, then comments on these test results.


Cylinder EEF Antenna
This antenna was inspired by an experiment to modify an EH antenna, which produced a slightly better result. Here is a photo of the roof test site, with EEF cylinder 1 installed. It's smaller than the photo suggests, at 1.4m high.
TLC antenna

This shows the top section, and coax coming down winding round the fibreglass pole. The coax is seen coming off near the base of the pole, and it is mostly vertical. The green string is for moving the frequency adjuster. It would be possible to have a small motor inside to adjust the frequency remotely.

The coax outer goes to the bottom of the coil; this is behind the pipe in the picture. The inner goes to a coil tap which give lowest VSWR. To find the lowest VSWR a network analyser is used, and the tapping point moved to be 1:1. There is a sliding section side the main cylinder which moves the resonant point, but maintains a low VSWR as show here

TLC10m

The antennas were mounted on a roof tile bracket as shown in the photo near the top of the page, allowing for easy swapping and adjustment of prototypes.

A high impedance coaxial choke (balun) is placed a distance down from the top section to stop common mode current at a defined position. The choke stabilises the tuning to allow reliable adjustment and also marks the edge of the radian-sphere as defined in the Chu-Wheeler ESA calculations. One choke tested was made of four FT140-61 ferrite rings. Ferrite 61 offers a good combination of inductive and resistive impedance above about 20MHz. The RG58 cable was wound 4 times through a stack of 4 rings. Effectiveness of the choke was confirmed by the VSWR reading on the analyser staying unchanged when the cable below the choke was touched or connected to anything.

TLC balun1

Several tests were made on the length of cable between the top section and choke. It was found that for coax lengths approaching 1/4λ between the top section and choke, an air cored choke could be used. But signal results for that arrangement were no better than for much shorter cable lengths. For shorter lengths than 1/4λ high impedance ferrite chokes had to be used because air core didn't have enough impedance to stabilise the tuning.

Super-Rad Antenna (SRA)
The Japanese SRA design has a link coil rather than a tap. Every one I made produced VSWR of <1.5:1 immediately, and with simple adjustment <1.1:1 at the centre frequency. Search for "SRA super rad antenna" on Google. Most of the websites are Japanese.

I made several SRA test prototypes. One with a horizontal plate is used in this test, the other prototypes were for other frequency bands. The horizontal plate version was made with a former designed for winding coils onto. The plate was made from a piece of fibreglass PCB material cut into an octagonal shape. Some 18SWG copper wire was then wound around, and the number of turns reduced to minimise the VSWR at 27.5MHz. The link coil was made out of PVC covered flex weave to make it easy to move around. Moving the link coil up and down minimises the VSWR.

SRA1
This odd looking device was placed onto the same length of coax as the EEF antennas. Performance was slightly down on the tapped coil design. Adjusting the frequency is more difficult because the plate on the top can't be "lengthened".

The Japanese think highly of this design. They say the top plate acts as a shorted turn and exchanges energy with the coil. Based on experiments with the cylindrical designs this is not true. A gap in the "shorted turn" does not alter the effectiveness of the antenna, it just reduces the resonant frequency slightly. If the energy was being exchanged a break would stop it, but it does not.

The common mode current is equally as much a problem with the link coil as the tapped coil. Given the 1-2dB less signal produced by the SRA design, I went back to a tapped coil instead. Having a flat plate instead of a cylinder would be good if the antenna had restricted height, such as mobile operation.

Grid/Mesh EEF
It was suggested by this paper that a metal plate spaced about 0.1λ away from a "ground" plate can produce far field radiation. The mechanical construction is very different to the cylinder type, as shown in the next photo.
EEF up2
This is 0.7m high measured from the bottom of the grey coaxial choke box to the top. In terms of Chu-Wheeler radian sphere, the radius is <0.5m. Electrically it's more similar to the other types than it may appear at first sight.

Radiation from spaced plates
There were great arguments over radiation from "EH antennas" and the like by common mode currents on coaxial cable. A way to greatly reduce this effect is to make the antenna balanced. A further experiment was to feed two plates (actually aluminium grids) from a high voltage step up transformer. Two turns of coax through a cheap small ferrite makes the tuning stable.

The first prototype with 2 spaced grids was briefly tested at a field day site. Made a few European contacts under difficult radio and environmental conditions. It was 5% λ in size with minimal common mode current. Moving the grids closer increases dead capacitance and the structure fails to radiate.

The second prototype was 4% λ with the grids in an 'X' pattern. The grid antenna is not a dipole, so feeding at any point on the grids makes no difference to the VSWR. The coil and feed line connection can be placed in mechanically optimal positions. It performed poorly, but experiments with plate spacing seem to confirm the assertions of the Landstorfer and Meinke academic paper.

CM current is the enemy of this type of antenna and allowing it does not improve effectiveness. It appears charge displacement radiation is acting against the small antenna. The main problem encountered in getting a low VSWR is the "single turn" effect. An autotransformer can only be tapped at 1.0, 2.0, 3.0 turns etc, there are no partial turns.

Further experiments and comments
This exercise was not done to challenge the accepted theory that electromagnetic waves are generated only by charge acceleration in a conductor over a large percentage of a wavelength. It was to make something good for communicating between 2 points, with restricted space at one end.

In
Maxwell's equations, and Ampere's Law, the alternating EM field is generated by both magnetic and electric components. This antenna is called enhanced E-field because the E-field is stepped up by a transformer and phase shifted. The classic paper by Landstorfer and Meinke theorises that electromagnetic radiation is possible with short electric field antennas. Their assertion that efficient radiation is possible from 0.1λ above a ground plane appears to be valid, but their paper failed to make a practical suggestion on implementation. The antenna I made seems to be an implementation of their theory.

Initial prototypes used type 61 ferrite toroids to block common mode current. There was always some loss associated with the chokes, as they got hot under high power. Despite the choke being lossy the first EEF prototype was 5.8dB higher signal than the Isotron which has no common mode choke. An Isotron is a simple device with the pole as ground rather than a "floating" coax. I removed the pole ground and replaced with the same coax used for other tests, the Isotron produced even less signal.

Various types of coaxial choke were tested. It was found an air core choke can reduce common mode current enough to make tuning possible but difficult, when placed approximately 1/4λ down. This placement at a current minima is the reason why previous designs such as SRA or MicroVert place their chokes at this position.

As an estimate based on temperature measurements a ferrite choke is losing 25% of the transmitted power, ~1dB. There's an approximate formula for toroid temperature rise vs. dissipation
in this document. An air-cored (series) choke at 0.25λ from the top section produced equal signal to the ferrite 1.5m (0.13λ) from the top. Previous experimenters with E-field antennas incorrectly state the coax has to be 1/4λ long, because they have were deceived by common mode current.

Another test was to put a T106-6 iron dust ring in the coil. The ring allows frequency to be maintained as the coax is reduced. The signal with 1.5m coax measured marginally higher than 2.5m coax. A 40% reduction in length (of the whole assembly) should result in reduction of efficiency, but produced a tiny increase. Raising the whole coax to the top of the roof made no measurable change.

Performance of a dipole was poor at the test site. It was sloping with its highest point where the EEF top section was, the lower end just below the apex of the roof. It would be hard to set up the dipole at a higher position. The lower average height of the dipole is the possibly the reason for the poor performance. Dipole bandwidth was twice as wide as the small antennas but adjusting the resonant point harder because of getting a ladder and cutting wire.

A full length dipole, even at lower average height should perform better than something 0.06λ long. The dipole was sloping at ~20 degrees off horizontal and 1-2m above the roof tiles. The Rx antenna is mostly horizontal and they are not end-on to each other. Therefore the path is not significantly cross-polarised, but the EEF antenna gives higher signal than the dipole by several dB. Theory states the dipole in
free space will be >90% efficient. This anomaly is unresolved.

The dipole and other antennas could not be used at the same time. A near equivalent of a dipole is a half wave vertical, and
this was tested. There was 7.2dB improvement with the half wave compared to the best EEF result and the large antenna has much wider bandwidth. It is somewhat unfair to compare a 0.5m high device with a 5.5m high pole. Scaling up to 160m, the small antenna would be 8m high and the half wave 80m high. The lower effectiveness of the small antenna should be seen in context of the difference in mechanical engineering.

A Sirio Boomerang 27W is a helically loaded dipole with 1.5m elements. I regard it as a fair comparison to the small antennas. The Sirio performed just 0.2dB better than the small antennas, though its bandwidth was wider. This indicates the small antennas are comparable in performance to conventional methods of antenna size reduction. The Sirio is approximately twice as long, and therefore its Chu sphere radius is twice as large. Practically however, the wind resistance of the designs is similar.

None of the small antennas appeared to be directional, though only limited testing could be carried out.


Small antennas which radiate in undefined ways generate
controversy among antenna experts. The EEF is related to the EH antenna which has been debunked many times with a NEC2 simulator like EZNEC. The results here indicate simulations in NEC2 are incorrect. This is probably because NEC only simulates radiation by current, whereas my antenna works primarily by voltage. Further experiments in 2015 were stopped by the arrival of winter.

Conclusions
1. Radiation from the coax tail is small compared to the antenna itself
2. Effectiveness is ~7dB down compared to a conventional full size half wave
3. Bandwidth is narrower than a conventional antenna
4. It is easy to outperform the Isotron design by ~6dB
5. A comparably sized helically loaded antenna performs roughly equally

A screenshot of the PowerSDR software with my homemade radio measuring signal levels.
TLC2 2015-07-07 at 22.01.05

Comparing the size of BEEF antenna (left) and half-wave vertical. Is the big one worth the extra signal?

EEF comparison
2010-2014 Experiments
In former years I tested or studied these types of antennas:
1. Poynting Vector Antenna (PVA)
2. Stub matched “metamaterial” dipole
3. Non-Foster matching

Other techniques and designs I looked at briefly were:
  • Inductive/Capacitive Loading
  • Line loading
  • Dielectric resonators
  • Small magnetic loops
  • Isotron/MicroVert
  • Slow-wave antennas
  • Faraday Rotation vs. Chu Limit
  • Direct Antenna Modulation
Poynting Vector Antenna (PVA)
The most controversial design ever made; synthesis of the Poynting Vector directly at the antenna. It's claimed the E + H phase shift causes the wavefront to form directly within the small space of the two cylinders. I made several EH antennas for 10m, and one for 20m. 10m types use 40mm diameter plastic pipe, the 20m one uses 50mm. Copper foil is used for the elements. Two ways to improve on the cylindrical dipole construction are shown by this 20m band version:
Large EH

Firstly the pipe can be cut with large slots to allow access to connections made on the inside. Secondly copper strip can be used for the coil/transformer to make it easier to adjust and reduce losses. Electrically, the EH antenna is an autotransformer with a capacitor connected across the ends. The capacitor consists of the cylinders plus coil inter-turn capacitance. Radiation resistance is supposed to increase to 10Ω by Poynting vector synthesis(?).
There has to be an effective common mode choke on the coax cable, otherwise tuning is impossible. Without a good high impedance choke, the feeder can form a tuned circuit with the transmitter ground. I have 3 turns of RG58 through 3 large beads of
type 43 ferrite 2m down the coax cable for the 20m version.
An important realisation is that resistive loss in context of stopping common mode current is good because the combined R + jx impedance is directly in the way of the current. If R >1000Ω and jx is also high, the overall loss current flowing and therefore the overall loss in a 50Ω system is small. A coil of cable is no good as a common mode choke, because R is only large over a narrow range of frequencies near resonance of the coil. Additionally, the resonant frequency of the coil is easily disturbed by nearby objects. This does not happen with a high permeability ferrite choke.
A table showing optimum resistive choking is
given by G3TXQ. A high resistive impedance will stop common mode current sharply, and allow the outside of the cable to radiate effectively. In combination with cable having a good low-resistance outer shield, the system should work better. The distance of the balun down the coax is difficult to determine, but the antenna picks up very little with the balun close to it. That the balun needs to be a distance down the coax is very evident from the received signal strength.
I measured the near E + H fields directly with a loop and stub antenna on a dual channel ‘scope. The position of the probes is very critical. Movement of only a few centimetres affects both magnitude and phase. The loop and stub need to be fixed relative to the antenna. This experiment seemed to indicate moving the tap did not affect the E and H relative phase. However since the original experiment it has been pointed out to me the phase needs to be measured further from the antenna. Eventually I hope to repeat the phase shift experiment.
The next picture is the VSWR and complex impedance result directly at the base of the EH antenna with the feeder calibrated out. The bandwidth is narrower than measured from the end of the cable. The effect of the source coil is to add series inductive reactance on the HF side of the resonance curve. At the 50Ω point it is a conjugate match and gets close to a pure 50Ω resistive result. The red trace is reactance, and it can be seen to dip to the centre line (j0) where the blue resistance curve is 50Ω.
EH SWR 20m

It’s a flat match, but SWR is not the real issue! Does the coax or the antenna itself radiate? Using a field strength meter, it appears some RF radiates from the antenna itself. Moving the meter along the cable (0.5m away) shows nothing below the choke, demonstrating its effectiveness. Above the choke there is some radiation, which drops off alongside the main coil. It increases over the bottom cylinder, but is a maximum over the upper cylinder.
EH proponents say the “Poynting Vector” from a 90 degree phase shift is the most critical thing to make the antenna work. Compared to the OCF dipole described elsewhere on this site, signals are 6-10dB lower on the EH antenna. It receives fairly well because noise is correspondingly lower. By the reciprocity rule the outgoing signal must be down by 6-10dB.
I see no evidence of Poynting vector radiation, despite lengthy testing and research. However the EH antenna did lead to other designs with more successful results as show on later pages.
Published test results of PVA and EH antennas by their promotors and patent holders have been inconclusive. A book is due to be published in 2016 with more information. Further work on PVA or EH antenna awaits release of this work. There are a lot of EH antenna websites, some not in English:
Original EH site from Ted Hart - snake oil warning?
Alpha Cognetics - new (2013) site
EH antenna group on Yahoo (membership required)
A rather mathematical, but recent analysis
Site of WB5CXC
Site of SM6DCO (Conny)
Lloyd Butler - his results agree with mine
Clear instructions from a French amateur
Non-English sites requiring Google Translate:
UA1ACO site for flat EH designs
IW0BZD
Stub matched “metamaterial” dipole
Originating with research on metamaterials is a technique called stub matching. To transmit a signal its necessary to match efficiently to a 50 ohm transmitter. Arranging a shaped conductor in the near field of a vastly shortened dipole gives a very effective matching system. The near fields of the dipole are “loaded” by the secondary element.
Putting additional elements in the near-field of a dipole can affect radiation resistance. With a Yagi it is reduced. It’s also possible to increase radiation resistance. Because a short antenna has low radiation resistance, the parasitic elements must be used to increase it to match a 50 ohm feed line.
There is no metamaterial involved in this antenna concept. Metamaterial is where hundreds of cells create negative refractive index. Its useful when applied to microwave components, but at HF the resonant structures are impractically large. It is nothing more than a new loading method. HF antennas are a whole different game to the UHF and SHF devices shown in theoretical papers. Viewed from the perspective of ham radio, a lot of academic hot-air has been published on this subject.
The resulting antenna is a high-Q device. Nearby objects (such as the ground) make tuning it to a 1:1 VSWR match a challenge. It can be tuned using a lumped inductor, but tuning downwards in frequency reduces bandwidth as predicted but the Chu limit.

The stub match technique produces performance close to a dipole, unlike the PVA. There is no way to break the Chu limit with it. It performs well down to
ka=0.4, which is 1.4m at 28MHz. After several years of intermittent work, I concluded this antenna is in many ways the counterpart of the magnetic loop. It has the advantage of not needing very thick copper tubing or high voltage capacitors.

Local signal tests are within 2dB of a reference dipole. On skywave transmissions it is difficult to tell them apart. Rotating the prototype in the horizontal plane shows a radiation pattern very similar to a dipole. I hope one day to commercialise this design, so no in depth constructional details can be released at this time.
The stub match technique is an obvious pairing with the third concept on this page...
LINK TO TOP OF PAGE
Non-Foster Matching
Non-Foster is the holy grail of antenna designers. Foster’s reactance theorem states passive components never have negative reactance. But it can be can be created using active devices. Applied to antennas, the loading inductor or passive matching circuit is eliminated. Simultaneously defeating the energy storage limitation of the Chu Limit, and allowing tuning without any nasty mechanical components.
There is a general limit on the bandwidth over which a good impedance match can be obtained in the case of a complex load impedance. It relates to the ratio of reactance to resistance, bandwidth over which we want to match, lowest SWR expectations, and is called the Bode-Fano limit.
I can see the basic principle of a negative impedance converter (NIC), as summarised by this fragment of circuit diagram. A floating and balanced feed from a dipole:
Pasted Graphic 4

Removal of energy storage elements (passive components) results in an efficient broadband antenna. It has already been demonstrated in broadband receiving applications. Transmitting is more complicated, but there is an obvious solution. It means having parts of the transceiver at the antenna. Such technology would be revolutionary in MF/HF communications, where the limitation of antenna performance at low frequency outweighs any improvements possible in receivers or transmitters. Any need for expensive mechanical variable inductors or capacitors as used in antenna tuners is also eliminated.
Non-Foster matching would be a paradigm shift, BUT implementing it and even understanding intricacies of the theory are difficult. Stability is the main problem. Simulation only gets so far because objects near the antenna cause its equivalent circuit to shift, introducing a high probability of oscillation. Implementing fixed station systems would seem to be easier than handheld or mobile for this reason.
The lower (HF) frequencies must be easier to implement than UHF/SHF. Equipment based on this technology may never happen if amateurs do not do the research for themselves. AFAIK there are no commercially sold equipments using non-Foster matching, though academic papers abound. A wide open field for amateurs to break new ground, if we can only make it work!