Alternative Techniques in Small Antennas 2
Updated 20170930
In the past few years I looked at over 100 academic papers and books, searching for ideas suitable for small HF antennas. Most professional research now is into designs for mobile communications because that’s where the money is. For MF and HF a small amount of work goes on in military research, which of course is not in the public domain.

Many new ideas in HF and VHF antennas come from the amateur radio community. There’s gradual advancement in design and materials used. Unfortunately few amateurs are aware of academic research in small antennas which they could experiment with. This is partly due to the academics doing research above 1GHz, and partly because their designs are complex to implement.

One paper that jumped out was a 1973 paper by Friedrich Landstorfer and Heinrich Meinke, translated by Karl Fisher, DJ5IL. The translation from the original German is good enough to understand. It presents a theory of radiation resistance of 30Ω from a short monopole, and is above most other academic papers for logical and lateral thinking. Their paper has never been refuted, but does have two clear omissions.
  1. There’s no mention of the magnetic (H) field, which is needed for a E x H vector to equal 377Ω
  2. No method of implementing their 30Ω radiation resistance from a practical antenna

The rest of this page is a critique of the Landstorfer paper. I decided to hand mark up a print and place links to reduce the length of this page.

The first equation is found in several books, but I don’t know where it originally came from. It’s in “Antenna engineering handbook” edited by Richard C. Johnson, formerly Henry Jasik (SK). Perhaps the inspiration for the whole paper originates in the chapter on LF antennas in Henry Jasik’s book by (the oddly named)
Boynton Hagaman, which has not only their opening formula, but sections on equivalent circuits and top loading.

Their premise is the effective height depends on the shape of the feed point which doesn’t make sense to me. The statement effective height depends on conductive or dielectric materials in the vicinity does make sense.

The animation mentioned in reference [3] like the paper itself dates back to the 1970s, so is not available.

Their major step is to define an equivalent circuit dividing the antenna into a radiating (space capacitance) and non-radiating (dead capacitance), with the division depending on effective height. The rest of page 1 deals with justification of their equivalent circuit in terms or moving charges. A few lines are written about the magnetic energy caused by the moving charges, rather dismissive given its equal importance as the electric (field) energy in generating EM radiation.

Some drawings of expanding electric fields. Their evidence of different regions of electric field is based on how the field moves. I have two problems with their ideas. Firstly, electric fields (as with magnetic fields) fall away as an inverse-cube law. So the lines depicted far from the antenna will be very weak. Secondly, an electric field has no mass, therefore difficult to believe it has “inertia”.

The “inertia” theory develops into a dividing line separating energy which flows out into space and back into the generator. This rounds off their premise of dead and space capacitance, accepting that the field can ever have inertia…

They then look in circuit terms at what the space capacitance does. Hence, the derivation through fairly simple maths of the mythical 30Ω. They mention a large number of numerical evaluations, which were presumably done on the
early TR-440 computer mentioned in the references. The calculation was probably for the length of the border line between regains I and II and shown in fig.6, under different rod dimensions.

I drew on a “capacity hat” of a type found on
existing HF antennas. It seems self evident that a normal kind of capacity hat will not encourage expansion of the electric field, because it “confines” the field between its lower edges and the ground plane. A grid of wires arranged vertically gives a more open structure where the E-field is less constrained and can expand further.

Top left has an important graph. The limit of electric charge expansion, point A, is confined to a small part of the total height as it is reduced. At 0.05λ the expansion point is about 85% of the way up. So to avoid becoming dead capacitance any impedance raising structures must be confined to the top 15%, or at least the top 20%. For a 20m band antenna this means the capacitance structure must be <0.2m of a 1.0m high total.

So, if we place a plate at the top of the antenna, efficiency improves. This gives a clue to practical implementation of their ideas.

On the right of page 4, is a statement that electric field lines are able to tear up and develop space waves. This doesn’t make sense in traditional antenna theory, where E x H = 377Ω in the far field. In other words the electric field of itself cannot form electromagnetic radiation. Since 1973 no “electromagnetic wave process” has been documented which discovers how electric field lines can tear up and develop (far field) space waves.

They further develop the equivalent circuit, and the dead/space capacitance division as given in equation 11. Here at last, they give more words to the magnetic (H) field. Also there is perhaps a common error. An electric field in free space does not give rise to an magnetic field. This duality only happens in conductors, and follows from my first point at the top of this page.

On the right of page 5 they justify their 30Ω as being near to the 36Ω of a quarter wave monopole. The rather oddly, go on to look at receiving antennas as somehow different from transmitting antennas. Odd because the two are always reciprocal.

They look at the traditional formula for capture area as equation 14, and incorporate their 30Ω. The final paragraphs talk about the antenna with varying load impedance, and the electric field lines for a dipole realisation. Attempts to look up any of the references given on the last page drew a blank. Not surprising given their age!

Landstorfer/Meinke never followed up their work, and both are dead now. Even after close analysis their paper is still something of an enigma. It can be viewed as they are stating the obvious that the top bit of a short monopole radiates, and capacity hats help short antennas. On the other hand the 30Ω from a short monopole is potentially an advance, if it can be practically realised.

There are two ways to increase the effect of space capacitance. Increasing the surface area of Landstorfer’s monopole in their so-called escape region is one. The other is increasing the charge flow by increasing voltage via a step-up transformer at the bottom of the space capacitance area. These two measures greatly increase charge in the space-capacitance region. The result seems to be (and proving this is difficult) a practical method to access the elusive 30Ω radiation resistance. It’s simply a plate/grid charged to a high voltage. The resulting design is much larger than promised by the Poynting vector synthesis method, in the region of 0.5λ, which is 8m tall at the 160m band for instance.

Accessing the elusive 30Ω, albeit with an associated loss of practical passive components, seems to be how a short antenna can perform so well. Having disproved the Poynting vector theory, this may explain why the EH antenna works, but with large losses. The dead capacitance is large, and the expanding electric field uses the feed line. With such a small area the feed line constrains the electric field. If we increase the area of the feed-line by adding a ground plate/grid connected to it, radiation efficiency increases. I measured this effect, so others should be able to also.

Electric fields are subject to inverse-cube law with distance. But we can increase the charge flowing into the E-field by increasing the voltage, and the space-capacitance is driven by that voltage. If it’s increased by say 10x which is quite easy with a transformer, then the escaping electric field (if such exists) would be extended. Combining a high voltage transformer and “space-capacitance” increasing measures does seem to work as a practical antenna.

Whether electric fields can radiate in the far-field is also open to debate, and linked to the more specific question of capacitor radiation. One view of this is loss of energy
when two capacitors are switched together. Another view is by the Lienard-Wiechert potentials, an extremely mathematical concept with links to relativity theory. Unfortunately dielectric resonators don’t work at low frequencies.

There’s definitely cause to re-evaluate the concepts of Landstorfer/Meinke with modern electromagnetic simulation. Unfortunately this method of radiation will not simulate in NEC2 because it expects antennas to work like a dipole. This lack of simulation evidence needs to be addressed with a different method, like
finite element method (FEM). Some open source FEM simulators are available, but commercial software is extremely expensive.

The value of “C2” in Landstorfer’s equivalent circuit explains the bandwidth effect seen with test antennas. The step-up transformer resonates with the C2 capacitance, giving a similar tuned circuit and a 2:1 VSWR bandwidth of about 2.5% results.

My measurements indicate the resulting antennas perform favourably to conventional designs of comparable size. Taking the necessary ground plane into account, evidence that they challenge the Chu-Wheeler criterion is open to measurement uncertainty. Peer reviewed measurements are necessary to confirm any such challenge. For unilateral results
see next page.