This page provides a top-level description of Small Transmitting Loop antennas (a.k.a. "Magnetic Loops"), and my two versions of such an antenna for 80 mtrs and up. My smaller 40-20 loop is described here.


I wanted a small transmitting loop (STL) antenna that covers at least the 80 and 40 meter bands (preferable 80 - 20). Why?

A loop antenna is generally considered "small", if its circumference is less than 10% of the operating wavelength. So in my case (for 80 mtrs), "small" would be a circumference of less than 8 mtrs, e.g., a circular loop with a diameter less than 2.5 mtrs (≈ 8.2 ft). To be more precise, we are talking about a small resonant loop.

Note that a multi-band loop that is "small" in the lowest band, is not necessarily "small" in the higher bands.

To obtain resonance, we need to combine inductive and capacitive reactance. The loop itself is the equivalent of a single winding of a coil, so it has a self-inductance. Resonance is obtained by connecting a capacitor across the ends of the opened loop. The capacitance has to be appropriate for the desired resonance frequency, as in all resonant circuits. Note that the loop-winding not only has inductance, but also stray capacitance. So, even without the additional tuning capacitor installed, the loop has a resonance frequency. It is the maximum resonance frequency that can be obtained with the particular loop.

A loop antenna is basically a simple series-resonant circuit, comprising an inductor with the shape of a closed-loop, a tuning capacitor inserted somewhere in this loop, and components representing losses within the loop + capacitor, and to the environment.

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Simplified equivalent electrical circuit of an STL

"Tuning" the antenna is done by keying the transmitter (at low power setting) and observing the SWR-meter or antenna-current meter while adjusting the tuning capacitor. Alternatively, by tuning for maximum receiver noise level: there should be a sharp increase in the noise level when the antenna becomes tuned to the desired operating frequency.

As with all antennas, "radiation resistance" is very important parameter. This is a fictitious resistance that relates the power that is radiated by the antenna, to the current flowing in the antenna. It is directly related to antenna efficiency. As the graph below shows, the radiation resistance of an STL is very small - we are talking milliohms here! This implies that loss resistances (e.g., loop construction) of the same order of magnitude are also important!

Mag Loop

Radiation resistance of a Small Transmitting Loop antenna

An other important aspect to keep in mind, is the voltage and the current distribution along the loop - at resonance. As shown in the figure below, the voltage is highest at the capacitor, and zero at the point diametrically opposed (in a perfectly symmetrical loop + capacitor + capacitor connections + environment). In some coupling methods, the braid of the coax feedline is actually connected to that neutral point.

Mag Loop

Voltage distribution around a loop antenna

The current is highest at the point opposite the capacitor, and lowest at the capacitor. See ref. 1 for an illustration. Note that the minimum current is not zero! Unlike the voltage distribution, the current distribution depends on the size of the loop (circumference), as a fraction of the wavelength. For a small transmitting loop (circumference < 0.1 λ), the current distribution is nearly constant (uniform) around the loop. Based on the (symmetrical) voltage and the current distributions, the impedance (Z = V / I) varies around the circumference. Both the voltage and the current distribution are symmetrical.

Mag Loop

Current distribution around a loop antenna - a circumference > 0.1 λ is not "small"

Clearly, the resulting impedance (ratio of voltage and current) also varies around the loop. It is highest near the capacitor and lowest at the point opposite the capacitor. E.g., there are two points, left and right of the neutral point opposite the capacitor, where the impedance is 50 ohm points with respect to that neutral point. This property is used in coupling methods such as Gamma Match and Delta Match. The position-dependent impedance can also be used when deciding where to place a transformer coupling to the feedline.

The diagram above shows that the largest current occurs at the point opposite the capacitor. This part of the antenna radiates most. Some operators therefore install their loop with the capacitor at the bottom. However, this effect only becomes noticeable when the loop circumference is at least 0.2 λ. Furthermore, placing the part with the highest voltage closest to the ground, increases losses due to parasitic capacitance to ground...

The radiation pattern of a Small Transmitting Loop antenna is shown below. The kidney-shape of the horizontal pattern (top view) becomes more pronounced ( = deeper minimums) as the antenna circumference becomes a larger fraction of the wavelength. This is the case when operating a multi-band STL on the higher band(s). For large loops, the maximums of the horizontal pattern are actually in the directions perpendicular to the surface of the loop!

Mag Loop

Approximate radiation pattern of a vertically oriented Small Transmitting Loop antenna

(circumference = 0.15 λ, installed 0.08 λ above ground)

Due to the closed-loop shape, this type of antenna can be considered an extreme case of a "terminated folded dipole". A standard loop has a circular single-turn inductor. Of course, other shapes are possible: square, rectangular, octagonal, etc. There are variations such as multiple turns and configurations such as "figure-eight". As with dipoles, directivity and gain can be increased by adding passive reflector and director loops, as in so-called multi-element "Yagi" beam antennas. This is all beyond the scope of this discussion.

Just to get a feel for some basic parameters, I have calculated the characteristics for a circular loop with a circumference of 5 m (diameter =1.6 m, ≈ 5.2 ft), made of copper tubing with a standard 16 mm outside diameter (5/8"). Input power is 100 watt (the limit of my transmitter).

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Calculated antenna characteristics for the given copper loop (KI6GD calculator, ref. 2A)

(no additional losses assumed in the calculation)

Mag Loop

Calculated antenna characteristics for the given copper loop (AA5TB calculator, ref. 2B)

(no additional losses assumed in the calculation)

If the input power is increased by a factor of N, then the maximum voltage across the capacitor is increased by a factor √N. E.g., doubling the power increases the capacitor voltage by ≈1.4. Conversely, the voltage is reduced by a factor of 1.4 when the input power is reduced by a factor 2.

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Capacitor voltage as a function of frequency, calculated for 1.6 mtr Ø loop made of 16 mm OD copper tubing

(calculated with ref. 2B; assumed 5 milliohm loss resistance)

One important thing that the tables show, is the very high voltages across the capacitor at resonance. This is not only important when choosing a suitable capacitor, it is also a SAFETY issue!

Mag Loop

As a rule of thumb, the optimum circumference of a multi-band STL is about 0.15 λ of the lowest operating frequency. With an appropriate variable capacitor, the resonance frequency of an  STL can be tuned over a frequency range that covers at least two octaves ( = factor 4x). However, from an antenna efficiency point of view, a factor of 2-3 is probably the practical limit. E.g., 80-40, 40-20, 30-10.

"Efficiency" is "power radiated by the antenna" divided by "power applied to the antenna". The tables above show that the calculated / predicted efficiency for 80 mtrs is rather low (no surprise), but my other antennas for 80 mtr are (very) short verticals. I do not know what their efficiency is, but I am sure that it is very low. In the end, what counts is performance at my location, for the available space, for the prevailing conditions (proximity to the building, QRM levels, etc), and with respect to other antennas that I can (afford to) install there. The efficiency of STL antennas remains controversial (ref. 3).

I have not looked into the assumptions that the calculators make, regarding installation height (free space?), coupling method, etc. As in all high-Q resonant circuits, calculated and actual performance is highly dependent on the losses in all components (loop, capacitor) and all interconnections. Losses in the milli-ohm range may be significant! In general, increasing the diameter of the tubing will reduce the (inductor) losses - up to a point.

Instead of the standard tubular conductor, loops can be made of a wide, flat conductor strip such as copper "flashing" or foil. The strip can be straight (ref. 4), or helically wound (hence, helically loaded, ref. 5A/B). Of course, helical/spiral "hula hoop" antennas can also be made of a Slinky™ coil. Ref 5C. This too is beyond the scope of this discussion.

Mag Loop

Flat conductor loops (straight & helically wound/loaded (shown without capacitor) and Slinky™ coil loop

(sources: ref. 4 and 5)


A loop with a fixed-value capacitor is resonant at a single, fixed frequency. A well-constructed small loop antenna has a bandwidth that requires re-tuning when changing frequency across a band. Also, the resonance frequency will vary with temperature changes (sunshine/weather). In order to change the resonance frequency to the desired operating frequency, we need a variable capacitor.

The basic choice is between "air variable" capacitor and "vacuum variable" capacitor. Clean air has a dielectric strength of 0.8 kV per mm (at 20 °C and standard humidity). So, an "air variable capacitor" for 5 kV would need an air gap of 6.25 mm (1/4 inch) between the plates. High-vacuum has a dielectric strength at least 10x as high.

A typical air-variable capacitor is "rotary variable". It consists of a stack of stator plates ( = stationary), and a stack of plates that are mounted on a rotor shaft. The rotor plates intermesh with the stator plates. Ideally, the rotor plate "vanes" are welded to the rotor shaft, rather than clamped. There are at least two basic types:

Mag Loop

Single-stator (with sliding contact), split-stator (limited to 180° rotation) and butterfly trimmer-capacitor

Rather than meshing stacks of rotor and stator plates, a variable capacitor can also consist of "trombone-style" coaxial tubes: one tube is slid in and out of a slightly larger "stator" tube. This principle is also used in variable vacuum capacitors, see below.

The tables in the previous section show that the assumed loop should be tunable from 80-20 mtrs with a 15-500 pF high-voltage variable capacitor. Note: a commercial air-variable capacitor for 15-500 pF and 5-10 kV is not necessarily smaller or less expensive than an equivalent vacuum variable capacitor! In 2010, I purchased a Russian-made capacitor. It is marked "10 kB 10-500 πФ" in other words: "10 kV,10-500 pF". I measured 15-510 pF with a simple LCR-meter. This "bottle" is quite heavy: 2.2 kg (≈ 4.8 lbs). It takes 36 revs of the shaft to go from minimum to maximum capacitance.

Mag Loop

My vacuum capacitor

A vacuum variable capacitor uses two sets of plates concentric thin-wall cylinders. On such set can be slid in or out of the opposing set of cylinders ("sleeve and plunger"). Spacing between opposing cylinders is several mm. These plates are then sealed inside of a non-conductive envelope such as a glass or ceramic "bottle", and placed under a high vacuum. The movable part (plunger) is mounted on top of a flexible metal membrane (harmonica-style bellows). The membrane seals and maintains the vacuum. A screw-shaft is attached to the plunger. When the shaft is turned, the plunger moves in or out of the sleeve and the value of the capacitor changes. The vacuum dielectric significantly increases the voltage rating of the capacitor, compared to an air-variable capacitor of the same dimensions and construction.

I did a quick test to verify integrity of the vacuum: put the capacitor in the refrigerator for about an hour. There should be no formation of condensation on the inside of the glass when in the fridge, or after taking it back out (on outside is OK).


The loop antenna will be connected to my transceiver via a coax feed-line. This means that the coax needs to be coupled to the loop, and the coupling must be wide-band enough to cover the tuning range of my antenna. As with other types of antennas, there are several ways to do this:

Note: this is only a overview of the most common methods. The list is not exhaustive.

Inductive coupling

The most common coupling method is using an inductive coupling loop. The main loop and the coupling loop form a (rather) loosely coupled transformer. The turns ratio is fixed (1:1), but there are several coupling loop parameters that affect the coupling: