W4ULD 6-1 09
All of the trap antennas I know of in their various forms use the parallel LC trap shown
in Figure 1. For a reasonable antenna efficiency the Q of the trap should be at least 200.
This is easily achieved with an air wound inductor and a 7 to 15 KV ceramic doorknob
capacitor. The ratio of diameter to length of the inductor should be no more than 2.0 and
the wire in the coil should be at least #14 and preferably #12. In the days when trap
antennas were popular (before low loss ladder line and T-network tuners were available
and popular) B&W and AirDux made a wide variety of air wound inductors that were
readily available. These were air wound and held together by four strips of polystyrene.
Figure 2 shows one variety. In practice, in making Q measurements of parallel LC
circuits, the very low RF resistance of the capacitor is ignored and the total RF resistance
of the circuit is assigned to the inductor. This is essentially true as long as the capacitor is
a good grade of ceramic, an air capacitor, or the dielectric is either mica or Teflon. As
reported previously, the Q of a LC circuit is:
: X = the reactance of the coil at the operating frequency
R = the RF resistance of the coil.
Thus R = X/Q.
As an example, we will take the case of a common trap that is used as an end insulator in
a multiband dipole. We will choose the resonant frequency at 7.2 mhz. At this frequency
both the capacitive reactance and the inductive reactance are equal but opposite in sign,
so they cancel.. If we use a 100 pF capacitor which is common practice, the capacitive
XC 221 .048 ohms
2 7.2 100
Since the capacitive and inductive reactances must be equal at resonance, The RF
resistsnce of the inductor must be:
The R value must eventually be added to the total R of the antenna, thus reducing the
efficiency of the antenna.
I mentioned 7 KV and 15 KV doorknob capacitors. These used to be readily available
from surplus electronics suppliers. Because of the very high voltage across most traps, a
15 KV doorknob is recommended but a 7 KV unit will work in most cases with 100 watts
power. Figure 4 shows a 7 KV door knob capacitor. The body is about .75” dia. and
about 5/8”” long. A good substitute for the door knob is double sided Teflon circuit
board. Here, one selects the amount of circuit board for a given capacitance. One must
measure the capacity of a piece of circuit board and calculate the area required for the
desired capacitance. This is not difficult but even Teflon circuit board is hard to find and
is expensive.. Do not attempt to use double sided epoxy or phenolic board for trap
capacitors. The losses will be very high because of the high power factors of epoxy and
I don’t believe that Airdux and B & W inductors are still available. Rather than use air
wound inductors, I have made many traps using #12 wire wound on 1-1/2” PVC pipe.
One of these is shown in Figure 3. These inductors wound at about eight turns per inch,
worked very well for 7.0 MHz. and below on single sideband and CW but heated and
changed characteristics when using high duty cycle modes such as Packet and Amtor..
So they had to be marginal at other operating modes. Although it is cheap, PVC pipe
makes a fairly high Q core for an inductor. The common test of a core’s loss is to place
the core material in a microwave for about one minute. After one minute, if the core is
warm to the touch it is not very good. PVC pipe passes this test with flying colors.
As described above, it is difficult to find commercial components to build a trap antenna.
This is simply because Hams build less these days but especially because trap wire
antennas are not popular as they used to be. They have been replaced by dipoles fed with
low loss ladder line and relatively cheap T-network antenna tuners.
I guess the moral to this story is : It is difficult to build a good trap antenna today. A 135
ft. center fed Zepp fed with CQ-552 to a T-network tuner is hard to beat as a multiband
Figure 2 Figure 3