Simple Tuning of Traps
Simple Building and Tuning of Traps (by DK7ZB)
Traps are useful for multiband operating of all kinds of antennas. It is much easier to build traps as you think and you do not need any special measuring equipment for tuning. Only a transceiver each ham is owning will be needed and a simple homemade measuring circuit.
A trap is a parallel circuit of L and C on the frequency you want to close an antenna segment.
For n working frequencies of an antenna, you need n-1 traps in each part of the antenna. For example a 2-band-dipole needs one trap in each half of the antenna.
The trap can consist of a coil and a separate HV-capacitor. Another method for building traps with coax-cable will be described down.
|Here we see the principle of a 3-band-dipole. If we need 10 m, 15 m and 20 m, trap 1 will be resonant on 28,5 MHz and trap 2 on 21,2 MHz.
How to Tune a Trap?
We need only a simple arrangement of two coils and the trap, a diode, a C of 5 nF and a meter for tuning.
|L 1 = 2-3 turns, 25-30 mm diameter
L 2+C = trap to tune
L 3 = 3 turns, 25-30 mm diameter
D =1N4148 or similiar
The photo shows how to tune the trap. We couple 2-3 watts continous RF from the transceiver into L 1. When changing the frequency, we will find a clear dip of the meter on the resonant frequency of the trap. In that case the RF is coupled from L1 into the trap and then to L2, where it will be rectified. The dip is very sharp, on 10 m we can locate the resonance with an accuracy of +/- 10 KHz!
The picture below shows how I tuned a RG174-coax-cable-trap for a 10/15-m-Vertical on 28,300 MHz. By changing the distance of the windings the frequency can be influencened. In the case of this RG174-trap we come from 27,8 MHz to 29 MHz. After tuning the coil will be fixed with some glue.
|This photo shows an other arrangement of the three coils,
you can make the distance between the coils greater for a sharper dip.
Here it is a 28,3-MHz-Trap with two 60 pF-5KV-ceramic capacitors in series and a coil of enamelled copper wire for the vertical sloping dipole described on another page.
On the european ham-fleamarkets you can get very good HV-capacitors from russian military purposes, here we see two different ones with 15 KV and 20 KV. With these capacitors we can build traps for more than 1 KW continous power.
Another way to build traps is to use coax-cables. The inner conductor and the screen form the C, the screen is the inductivity of the trap. The main problem is that we cannot change L and C individually. Coax-cables have a C of 60-100pF/m, see tables with the data of the cables.
A very useful tool for coax-traps is a program of Tony, VE6YP named "coaxtrap.exe". You can download the program from his website www.qsl.net/ve6yp .
The picture shows a 30-m-Trap with PTFE-coax