Large, Horizontal, Loop antennas Fed via Balanced or Coaxial LinesAuthor: R.J.Edwards G4FGQ © 23rd March 2005
Large loops are multiband antennas. They are most useful when the loop perimeter is more than half-wavelength at the lowest frequency of interest. The lowest self-resonant frequency occurs when the loop perimeter is 1/2-wavelength. The SWR on the feedline is very high when the perimeter is near an odd-number of half-wavelengths.
When loop perimeter is not far from a whole number of wavelengths the input impedance is such that a loop can be connected to balanced-twin feed-lines with a reasonably low SWR and hence low line loss. In some cases the performance with a coaxial feed-line may be acceptable.
Operation is not limited to resonant frequencies. A choke balun plus an L-tuner at the transmitter end of the line allows operation at most high frequencies. If a matching problem occurs in an amateur band, change the loop perimeter or feedline length by a small amount to change the impedance at the transmitter end of the line.
The shape of a loop is not critical. Performance is little affected by shape provided the enclosed area is not less than 2/3 that of a circle of the same perimeter. Radiating patterns, over a wide spectrum, can be considered to be randomly crudely-omni-directional. The location of the antenna feed-point around a loop does not change the random nature of the pattern accross the spectrum, but does rotate the radiation pattern at particular frequencies. Experiment and try your luck.
This program is intended to describe only the general behaviour of large loops of given perimeters, which is what one usually is interested in for multiband operation. Large loops are those with a perimeter greater than 1/2 wavelength without loading coils or tuning capacitors. Long loops of the same perimeter and roughly enclosing the same area, have general characteristics in the same ball park. This is intuitively obvious. There are no special characteristics.
The antenna input impedance is usually only of academic interest. The effect of antenna input impedance is on feedline SWR, feed-line, and tuner loss.
Observe how feed-line loss changes between low impedance coax and high impedance open-wire lines of the same length, especially at higher frequencies.
Impedances presented to the choke balun and then to the tuner, which depend to some extent on the height and unspecified shape of a loop, tend to stay near to values which can be handled by a two or three element antenna tuner.
Power efficiency of the system for a given perimeter is relatively independent of loop shape. Shape often depends on pre-existing locations of trees, poles, and antenna supports attached to buildings not under an experimenter's control.
Tuner Circuits to Match to a 50-ohm Transmitter
The antenna tuner consists of two components. Either component is a capacitor or an inductance coil as indicated against "A" or "B" on the data screen. For a given perimeter the two types of circuit are not interchangeable. Type "A" occurs much more frequently than type "B" over the HF range. Other two-component configurations may exist that will match the same impedances.
|Circuit Type "A"|
The series component X1 is connected between the transmitter and the antenna terminal of the tuner.
The shunt component X2 is connected between the antenna terminal and ground.
|Circuit Type "B"|
The series component X3 is connectedbetween the transmitter and the antenna terminal of the tuner.
The shunt component X4 is connected between the transmitter terminal and ground.
For the purpose of estimating loss in the antenna tuner, tuner coils are assumed to have a modest Q of 250. Q increases with physical size. Tuner capacitors, associated wiring, and the choke balun are assumed to have zero loss.
The Choke Balun
A long-perimeter horizontal loop antenna normally is used as a multi-band device, otherwise a completely different one- or two-band antenna would be erected. The component which can restrict a wide operating bandwidth is the choke balun.
The choke balun actually used is a 50mm (2") diameter ferrite ring, wound with 12-turns of stranded 18 AWG twin speaker cable having a Zo of roughly 130 ohms. The useful frequency range of the choke is 1.8 to 30 MHz. Transmission loss through the choke is negligible and for calculating convenience is assumed zero.
It is often not appreciated that besides the choking action, the length of line wound on a choke-balun behaves as an impedance transformer. At the higher frequencies the impedance presented to the tuner can be altogether different from the input impedance of the transmission line. The program will illustrate this. From a choke design point of view it is preferable that the length of line on a choke, whether coax or twin line, should not exceed 1/8 or 1/10 wavelengths at the line's own velocity at the highest frequency of interest. To achieve a satisfactory inductance and choking action at the lowest frequency of interest, and at the same time comply with the line-length restriction, it may be necessary to use a ferrite core with an HF permeability of several 100's. However, the balun choke design is not critical. From 3.5 MHz to 21.3 MHz it is easy.
It sometimes occurs that L-tuner components calculated by this program are both coils or are both capacitors. This is not what is available in commercial L-tuners which accounts for the inability of commercial L-tuners to accommodate all possible variations of antenna plus line input impedances.
In an L-tuner there are eight possible combinations of coils and capacitors. It is always possible to find values of coils and/or capacitors which will match any value of R+jX to the 50 ohm standard. Unfortunately, the number of switches and switch operations required to select a correct L and C combination is excessive.
Because of that, to reduce price, tuner manufacturers, settle for three-element T-tuners that in principle can match anything to anything except for the fact that (again to reduce price) the number of components is reduced to two variable capacitors plus one variable inductance - without switches. Fewer components also reduce stray inductances, capacitances and couplings.
To reduce costs to a minimum, amateurs can use a collection of coils and capacitors plus a set of small alligator clips with short flexible leads. A home-made flexible L-tuner is more efficient than a T-tuner and will work best without a metal screening box. Unwanted radiation will be negligible.
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