Trapped Dipole Radio Antennas

Author: R.J.Edwards G4FGQ © 21st November 1998

A "trap" is a coil and capacitor in parallel having a very high impedance at its resonant frequency. If a trap is inserted at some point along an antenna conductor, that part of the antenna beyond the trap is isolated from the active part and the feedpoint. When a pair of traps are inserted in a dipole, equidistant from the feedpoint, that part of the antenna between the traps when of correct length becomes a halfwave dipole at F2, the trap resonant frequency.

Below its resonant frequency the trap has an inductive reactance and allows the antenna conductors beyond the traps to become active again. So at a lower frequency F1 there exists a second, longer, coil-loaded resonant dipole.

The ratio F2/F1 can be selected between 1.2 and 4 but the L/C ratio of the trap and trap loss impose limits. A ratio of F2/F1 = 2 is a typical compromise and this program assists with the selection of suitable antenna proportions.

For given F1, F2 and overall antenna length, the program selects best L and C values for the trap. For given coil diameter and length, the program completes the design by computing number of coil turns, wire diameter and wire length. A wire diameter to pitch ratio of 0.71 is used to minimise coil loss and weight.

The tuning capacitor is best fitted inside the coil former. Its computed value allows for coil self capacitance as increased by weather protection materials.

There are three principal RF power losses - in the antenna wire resistance, in the trap winding wire resistance and in the soil under the antenna. Soil loss is the least predictable especially at low heights. Ordinary garden or farmland soil is assumed. Ultimately, trap power dissipation limits transmitter output.

Losses are shown as a percentage of RF power input to the antenna. Trap loss is reduced by increasing size of coil former. This also improves the coil's heat dissipating capability and, for a given Transmitter input power, trap temperature falls much faster than coil size increases. There is an inverse cubic relationship.

At low heights the coupling to ground also detunes the antenna. Resonant frequencies decrease. The effect is not accurately calculable. To include it in this model would not compensate for the extra complication but it should not be forgotten.

Due to the many uncertainties involved no accuracy claims are made on the data produced by this program. Inevitably, if height is less than 1/8-wavelength at the lower resonant frequency, pruning will be needed to meet the objectives. But use of this program can eliminate exasperating and wasteful experimenting in the dark.

Hints & Tips
Do observe wire dia/pitch ratio of approximately 0.71. Otherwise trap losses will increase and arcing may occur between turns. Inductance and stray capacitance will increase. Behaviour of the whole antenna will change. If the coil former is not grooved, wind a length of cord together with the wire. A nylon braided cord can be doped and left in situ.

Use several mica-dielectric capacitors in parallel for trap tuning to improve current carrying capabilities. This also facilitates adjustment of resonant frequency which should be done with a GDO before inserting in the antenna.

A worthwhile objective is to adjust overall length such that trap losses are identical for both resonant frequencies. This is possible because as one loss decreases the other increases. But site size may be an overriding influence.

For F2/F1 = 2, optimum conditions occur when the traps are placed at about 5/8-ths of the way to the ends. This results in a resonant antenna at the lower frequency which is about 80 percent of the size of a full size dipole and for practical purposes just as efficient and with a convenient input resistance.

Use keys "Y" and "U" to vary antenna length for given F1 and F2. Also "Z" and "X" to vary F2 for given F1 and length. Enter all commands and data on bottom line.

Dear Program User
The number of ways in which data can be entered in this program to cause it to abort and eject you unceremoniously into the operating system is beyond belief. To mention just one way: a request to the maths processor to extract the square root of a negative quantity is certain to trigger a general strike.

In general, dear user, you will not be aware you have asked the processor to do the impossible until it is too late. But if you enter sensible, feasible data the processor, with an understanding of what is reasonable to ask of it from a trapped dipole's point of view, it will not let you down very often.

I, the programmer, have taken a few precautions to trap some errors in entered data. In some cases a message appears with a few words of advice. But in most cases it is necessary to allow the processor to attempt to do its job before anything awkward can be detected.

It would be possible to trap most invalid data entries only by doubling size of the program. This would increase its complexity 10 fold - and in the process reduce reliability by the same factor. It would also take 10 times longer to prove the program and make it available from my website.

If aborted into DOS, type the name of this program "trapdip" followed by RTN. Or type "exit" followed by RTN to return to Windows if that is where you began.

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 TrapDip 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.

Also see a newer version of this program.

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