Transmitting Loop Antennas with Perimeters Less than 0.75 Wavelengths LongAuthor: R.J.Edwards G4FGQ © 17th December 97
This program assists with the design and estimates the performance of single turn, rectangular, transmitting loop antennas where the perimeter is less than 0.75 wavelengths long at the operating frequency. A 50-ohm feed-line is assumed. Any program input value may be changed after an initial calculation on a "What-if" basis.
The main loop may be formed from solid wire or thin-wall copper tube. Only the external diameter of the conductor is of consequence at radio frequencies, because of the "RF skin effect." The loop is assumed to be in the vertical plane. At very low heights in the horizontal plane, actual earth losses will be greater than computed. The ground under the antenna is assumed to be ordinary garden soil without an intervening earth screen or conducting mat.
RF power can be fed into the main loop via a smaller inductively-coupled loop mounted inside it. The coupling loop can be formed from the coaxial feed-line itself if desired. An exact impedance match may be obtained by fine adjustment of the coupling loop diameter or of its rotation-angle relative to the main radiating loop.
Alternatively, a ferrite ring can be fitted over the loop conductor opposite the capacitor with a multi-turn primary winding on the ferrite ring to match the loop to feed-line. The loop conductor then becomes a single-turn secondary winding. The turns-ratio computed by the program applies to a 50-ohm transmission line. Ferrite core permeability must be high enough to give a primary reactance at least twice the feed-line impedance. The core material should have low hysteresis-loss at the operating frequency.
If the loop perimeter exceeds half-wavelength, the required tuning component changes to an inductance, tuning becomes less sharp, and because of that, the tuning coil need not be variable. The tuning coil can be part of a balanced ATU with RF-power being fed into the loop via a link winding. Feeding and tuning the loop at the same place is structurally simple. A loop > 1/4 wavelength long is very efficient.
The computed series input resistance is the low resistance seen when looking into the loop from the tuning component when adjusted for resonance. Shunt input resistance is the equivalent high resistance seen across the tuning component. These values are of interest when feeding the loop at the tuning point.
RF current is not uniform around the loop. It is at maximum directly opposite the tuning component. Computed values of radiation and loss resistances are with reference to maximum current. It is their sum that the toroidial transformer matches to the feeder. Relative values of the three resistances indicate which physical dimensions to change to best improve efficiency. The cost, size and adjustment of high-voltage variable capacitors should be taken into account when determining optimum dimensions.
When using a coupling loop, it should be fitted eccentric to the main loop such that it is adjacent to the side opposite the tuning component. It can be round or rectangular provided the perimeter length is unchanged. The computed diameter of the coupling loop is an approximation. The precisely optimum size will depend on height above ground and the presence of any nearby structures.
Run this Program from the Web or Download and Run
it from Your Computer
This program is self-contained and ready to use. It does not require installation. Click this link rjeLoop2 then click Open to run from the web or Save to save the program to your hard drive. If you save it to your hard drive, double-click the file name from Windows Explorer (Right-click Start then left-click Explore to start Windows Explorer) and it will run.
Search other ham radio sites with Ham Radio Search