Review of G3LDO STL (Radcom Sep 2010)

(Dodd 2010) describe a small transmitting loop (STL) and gave some meaningful performance measurements. It is rare to see such measurements and he is to be congratulated.

The loop is an octagon of perimeter 4.7m which at 14.2MHz is 0.224λ so although many will consider it meets the requirements of an STL, the common formula for radiation resistance Rr of a STL fail for perimeter above about 0.1λ (see Accuracy of estimation of radiation resistance of small transmitting loops).

Dodd gives calculations of one of the many simple loop calculators which gives Rr as 0.422Ω, it is probably closer to 160% of that value. This is an important quantity as it has direct bearing on calculated efficiency.

Dodd’s NEC model should have used a better figure for Rr, but it seems unlikely that the structural losses were fully included and its bandwidth prediction will be impaired.


Above is Dodd’s measurement of antenna VSWR at 20m. This is most useful as it allows estimation of the half power bandwidth of the antenna. In this case, the antenna is not perfectly matched at its centre frequency, the residual VSWR is 1.07. The graph allows scaling off the VSWR=2 bandwidth as approximately 42kHz.
Continue reading Review of G3LDO STL (Radcom Sep 2010)

Helical loading and Calculate small transmitting loop gain from bandwidth measurement

Several correspondents have asked about the application of Calculate small transmitting loop gain from bandwidth measurement to the helically loaded small transmitting loop.

The helically loaded small transmitting loop appears to be the invention of K8NDS and is described at Stealth Antennas for the Radio Amateur and (K8NDS nd). It may not be a novel idea as it was analysed at (Maclean 1978).

Without getting too much involved in the inventor’s specious arguments which attribute magic properties to his antenna, this article focusses on whether / why the calculator will or will not provide valid results for the antenna.

At Stealth Antennas for the Radio Amateur he makes the statement

A solid copper tube “Magnetic Loop” exhibits a certain inductance per foot of the total circumference of the antenna.

The statement seems to belie a basic understanding of inductance, the inductance of a given conductor formed into a single turn loop is not simply perimeter multiplied by some constant “inductance per foot”. Continue reading Helical loading and Calculate small transmitting loop gain from bandwidth measurement

End fed matching – PA3HHO design review


A correspondent having read End fed matching – design review raised a similar design by PA3HHO which uses a#43 ferrite toroid as part of an end-fed matcher, see Multi band end-fed (English).

The text and diagram  are inconsistent, but to allow him the benefit of doubt, lets consider the FT240-43 with a 3t primary… this is his lowest loss configuration.

Continue reading End fed matching – PA3HHO design review

Designing a Guanella 1:1 balun using the ‘unknown’ ferrite toroid

At Characterising an unknown ferrite toroid an ‘unknown’ ferrite toroid was characterised. This article uses that information for design of a Guanella 1:1 current balun.

The proposed design uses 11t of small coax wound in the Reisert ‘cross-over’ style.

The impedance of a single turn vs freq was used to predict the impedance of an 11t choke. Such a choke exhibits a self resonance that can be represented as due to an equivalent shunt capacitance. This equivalent capacitance is not easily estimated, and can best be determined by calibrating an analytical model of the choke for the same self resonance as exhibited by a real choke.

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Above is common mode impedance from an analytical model of the choke, assuming an equivalent self capacitance of 11pF.
Continue reading Designing a Guanella 1:1 balun using the ‘unknown’ ferrite toroid

Characterising an unknown ferrite toroid

The ‘unknown’ toroid is wound with a single turn and measured with a VNA, an AIMuhf in this case.


Of interest in the first instance is the apparent inductance of the single turn winding at low frequencies where typically permeability µ is fairly constant and core loss is fairly low. Continue reading Characterising an unknown ferrite toroid

A method for initial ground loss estimates for an STL

Over recent weeks, I have run literally hundreds of thousands of NEC models of small transmitting loops (STL) over real ground. The objective was to try to discover some simple methods for initial design of a STL, particularly an estimate of ground loss of STL mounted near natural ground. Continue reading A method for initial ground loss estimates for an STL

Mini60 antenna analyser

There seems a never ending stream of low end antenna analysers appearing.

The Mini60 antenna analyser is one in that vein, and is sure to prove popular because of its low price. As is common, there does not appear to be an English language user manual and the specifications in eBay ads are not very reliable (eg weight: 200kg).

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Above is a screenshot from an online demo of the Mini60 on a 7MHz antenna. Continue reading Mini60 antenna analyser

Silicon Chip Collinear

This article has been copied by request from my web site which is no longer online. The article may contain links to articles on that site and which are no longer available.

(Tester 2013) described a coaxial collinear array for VHF/UHF. Tester describes the antenna a collinear is a vertical antenna whose resonant elements are connected along a common line (ie co-linear) so that each element is opposite in phase to its neighbour.

He is a little confused, in fact, the elements are in-phase with each other so that in the horizontal direction, the contribution of the current in each element to the far field is an additive or reinforcing one.

He goes on to say [i]f you are not into antennas, that mouthful is, fortunately, very easy to achieve... but is it?



Fig 1

Fig 1 is from (Tester 2013) showing the construction. Continue reading Silicon Chip Collinear