Some Generally useless Ramblings on Tuned Circuits and how they apply to multi band or (Trap) Dipoles!

by KG4BIN David Morgan

The trap antenna, was invented mostly for convenience. It was not invented for maximum efficiency. As with all antennas, trap antenna adherents claim they get good results--and they do. But whether they get better results than they would with other types of antennas of comparable size, is a question very hard to answer. The answer would require that the trap antenna and the alternative be placed in nearly the same position at the same height, and few of us can afford the space, time, or money for such comparisons. However the trap antenna is cheap, easy to build and can cover many bands, without having to resort to a tuner.

The most common type of trap antenna, is the parallel tuned circuit, that is resonant at or just below the edge of the higher frequency band to be covered, with extensions to make up the length of the lower band. These antennas will be shorter than a full-size dipole at the lower frequency, since the trap acts like an inductor at the lower frequency, much like a mid-element loading coil. However, the inductive reactance is not a product of the coil alone, but of the tuned circuit making up the trap.

Virtually all radio communication involves the use of resonate circuits, in one way or the other. Any tuning element, involves resonance; without it we could neither transmit a stable signal, nor could we select the desired signal with our receivers. In the old days, when big spark gaps furnished the transmitted RF and mechanically shaken cohers were the receivers, resonate circuits as such, were unknown-and radio's usefulness was sorely limited.

If you're designing a circuit and your major requirement, is to obtain resonance at some single desired frequency, you have an infinite number of values to choose from! No matter what size capacitor you choose to use, it will have a definite capacitive reactance at your desired frequency, and all you will have to do to achieve resonance is to provide an inductive reactance of exactly the same value at the same frequency.
 
 

Let's look at the more conventional trap antenna and simplify it to just 2 bands, like 80/40 or 20/10. A full size #14 copper wire resonant dipole will have a gain of about 2.1 DBI in free space, but it has this gain only in one ham band. We can use the gain figure, as a standard against which to measure trap antennas for two bands. The first thing we see is that performance of a two band trap antenna of conventional design, depends very heavily on the Q of the trap. There are many trap designs, but here is a table of one pretty good design with coils of various Qs. The gain is for free space.

       Q           High-Band Gain (dBi)      Low-Band Gain (dBi)
       50                 0.7                       1.7
      100                 1.4                       1.8
      200                 1.8                       1.9
      400                 2.1                       2.0
      800                 2.2                       2.0

The Q of a coil, is the ratio of energy stored in the coils magnetic field, to the energy
dissipated in the coils resistance and radiation losses! Long thin coils, require more wire and so have a higher resistance, while at the same time producing smaller magnetic fields.
Large short coils, also have smaller magnetic fields and because of it's size, is more subject to radiation losses.
The "square coil", strikes a balance between large magnetic fields, for energy storage and small enough size, to minimize radiation, and so has an optimum Q.

The voltage drop across any single impedance in a resonate circuit, is affected by the Q as shown here. This circuit has a Q of 100, since 1 amp of current overcomes losses to maintain 100 amps of circulating energy. If the impedance of the capacitor is 1 ohm, a voltmeter will measure 101 volts across the capacitor. Although only 1 amp is flowing in the external portions of the circuit. The meter impedance must be taken into account, since it will load the circuit and effectively reduce Q.
 

Avoid low-Q trap coil designs. It is fairly easy to homebrew airwound coils with a Q of 200, and common coil stock usually meets this figure. Even the best series-wound coaxial trap coils will not have Qs higher than about 400, and most coils with Qs claimed to be higher than 400 will not retain that Q under the influence of a chemistry-lab atmosphere. Nonetheless, a dipole with a gain of 1.8 or so will not yield results noticeably worse than a full size dipole, since a half dB of lost gain translates into less than a tenth of an S-unit.
The sample conventional 80/40-meter trap dipole in Figure 1 uses traps tuned to 6.75 MHz. With a Q of 200, the traps equalize performance on the two bands at just above 1.85 DBI in free space. This is only about 0.35 dB down from a full size dipole for each band.

Coaxial-cable antenna traps are constructed by winding coaxial-cable on a circular form.  The center conductor of one end is soldered to the shield of the other end, and the remaining center conductor and shield connections are connected to the antenna elements.  The series-connected inner conductor and shield of the coiled coaxial-cable act like a bifilar or parallel-turns winding, forming the trap inductor, while the same inner conductor and shield, separated by the coaxial-cable dielectric, serve as the trap capacitor.  The resultant parallel-resonant circuit exhibits a high impedance at the resonant frequency of the trap.

I constructed the traps using RG-58/U coax.  PVC pipe couplings were chosen for the trap forms:  they are very inexpensive and readily available in useful diameters.  12 gauge solid wire was used to form "bridle wires" for electrical termination of the coax and electrical and mechanical termination of the antenna wire elements.  The coax turns were spread slightly until the desired resonant frequency was reached, as measured by a dip meter.  After adjustment the coax turns were secured by coating with lacquer.

The 10 and 15 meter traps, wound on 3/4" PVC pipe couplings.
 
 

I used 12 gauge stranded household electrical wire for the antenna elements.  This wire is very inexpensive when purchased in 500 foot spool quantities at home centers.  The insulated jacket causes the wire to have a velocity factor somewhat lower than that of bare copper wire.

Some amateurs argue that a balun is not necessary when feeding a dipole with coax.  The simple choke balun used here is trivial to construct, and I do not feel it is worth the risk of feedline radiation problems to omit it.

If you are not interested in the 30 meter WARC band, here are the dimensions of the antenna without the 30 meter traps. You may note that the 80 meter end sections are significantly longer in the version without the 30 meter traps: much of that difference may be due to the larger percentage of the 80 meter section length.

Table 3 contains the resonant frequencies and SWR, 2:1 SWR limits, and 3:1 SWR limits of the antenna as measured after the final trimming of each of the elements.
 

Table 3.  SWR vs. Frequency (Measured with MFJ-259 Antenna Analyzer).

SWR	10 meter band	15 meter band	20 meter band	30 meter band	40 meter band	80 meter band
3	27.17 MHz	20.64 MHz	14.00 MHz	10.05 MHz	7.06 MHz	3.56 MHz
2	27.70 MHz	20.83 MHz	14.07 MHz	10.09 MHz	7.09 MHz	3.64 MHz
resonance	28.65 MHz @ 1.0 52 ohms	21.14 MHz @ 1.3 54 ohms	14.16 MHz @ 1.3 44 ohms	10.13 MHz @1.6 82 ohms	7.12 MHz @1.8 35 ohms	3.67 MHz @1.9 39 ohms
2	29.90 MHz	21.47 MHz	14.26 MHz	10.19 MHz	7.14 MHz	3.70 MHz
3	31.40 MHz	21.68 MHz	14.33 MHz	10.24 MHz	7.17 MHz	3.76 MHz