Construction manual antenna trap

An antenna trap is used in multiband antennas. The antenna trap blocks a certain frequency. This shields part of the antenna for higher frequencies where a shorter antenna is needed. The entire antenna participates for the lower frequencies on which the trap does not block. This can be used in dipole antennas where a trap is needed in each leg of the antenna, as well as in end-fed antennas where only one trap is needed.

Resonance frequency of the antenna trap

A stage is a parallel circuit of a capacitor and a coil. The frequency at which the antenna trap has a high resistance (so it blocks) depends on the coil and capacitor. The resonance frequency is determined by the formula below. I choose to use an easy program that does the math for you. Download link to Mini Ringkern Rechner: Click here!!! The installation is in German but in the program the language can be changed to English. First we are going to determine the value of the coil and capacitor. In the program, choose: Tools –> Resonant Circuit. HF Kits supplies in the homebrew kit a circuit board where the two copper layers on the board act as high voltage capacitors. Various capacitor values can be selected by grinding through printed circuit boards. A choice can be made for the 10,15,20,30 and 40 meter band. The capacitance in pF corresponds approximately to the band in meters. So the 10 meter setting gives about 10 pF. To now calculate the value of the required coil, the program RingCoreRechner can be used. Enter the value of C and Frequency. I have entered 50 pF as an example here as the capacitor value and a frequency of 28.6 MHz. The antenna trap is pretty wideband so the frequency doesn’t come close to a few kilohertz. If I then click on the arrow, the value of the coil is calculated. In this case 619.35 nH. So we now know that we can start making a stage with a 50 pF capacitor and a 619.35 nH coil.

Soldering capacitors

The PCB has room for 3 capacitors C1, C2 and C3. (and possibly for SMD capacitors C4, C5 and C6). The PCB is designed so that it is possible to use one capacitor, connect two capacitors in parallel or connect two capacitors in series. After soldering the capacitors, I briefly check the value. The capacitors have a 10% tolerance. In this case, therefore, the value falls a bit lower to 46.39 pF. Because the capacitors have a different value in practice, the value of the coil must also be redetermined! result –> 667.55 nH

Determine the number of windings of the coil

Now return to the main program and choose the “Air Cores” tab meaning air coil. Now enter the inductance of the coil as we calculated earlier in “Resonant Circuit” calculator. Next we fill in the diameter of the coil, which in our case is 19 mm. Then I experimentally determine the length of the coil, I choose about 1 mm per winding, so this is a matter of trial and error. I now calculate 4,99 ( so 5 ) windings with a coil length of 5 mm. We use winding wire of 0.75 mm so this should give a minimum coil length of (5 x 0.75 mm) 3.75 mm. So there is still some space to slide the coil in and out which is necessary to bring the trap exactly to the right resonance frequency. This will be covered later when adjusting the stage. Coil assembly Now that the capacitors are soldered, we can start with the coil. The base of the coil is in our case a PVC tube in which the PCB fits exactly. On two sides, cut the PVC pipe so that it forms an easy start and end point of the coil. Solder the enameled copper wire as shown in the photo below. Make sure that the insulation is removed properly otherwise strange phenomena occur, this can be done with a sharp knife or sandpaper. Then wind the coil as in the picture below (number of windings does not match the example) Solder the other end of the coil to the PCB. In the picture above, 7.5 windings were chosen (I thought for the 15 meter band ) so it is also possible to make half windings, then you solder the end to the other side of the board.

Tuning the antenna trap

This requires a grid-dipper or spectrum analyzer. The picture below shows the pass-through characteristic of the trap. Bringing the windings of the coil closer or further apart changes the resonance frequency. Now set the stage to the exact desired frequency, 28.6 MHz in this case. Finishing Now that the stage has been set to the correct resonant frequency, it is advisable to fix the coil so that it does not drift. I do this with some quick drying plastic spray. PVC glue is also a good alternative. After the coil is fixed, the resonance frequency is checked one more time, after which the whole can be finished with the supplied heat shrink tube. Be careful that with heating the heat shrink tubing that the entire stage does not become incandescent hot, this can deform the PVC causing the resonant frequency to go off again.

Mounting the traps in the antenna

There are roughly three ways to assemble stairs.

1. Insert the antenna wire through the outer hole, then solder to the inner mounting point. This is a suitable way for short antennas that are not set up permanently. This way of mounting is by far the easiest because no loops, cable clips and other attributes are needed. I use this for my short end fed antennas that I use vertically in the field. In this case, choose: Antenna trap homebrew kit, ideal for wire antennas
2. Solder the antenna wire into suitable cable eyes M4 or M5 and finish with a piece of heat shrink tubing. Secure the cable lug to the outer fixing holes of the trap using M4 x 10 mm stainless steel bolts. This is a suitable way for medium sized antennas.
3. By far the most solid way of mounting is as follows: Make a loop of wire through the outer mounting hole of the trap by means of a stainless steel cable clamp. Use stainless steel cable thimble to prevent the wire from being damaged (missing on the picture below). Solder the antenna to a suitable wire eye (M4) and finish with a piece of heat shrink tubing. Secure the cable lug to the inner fixing hole of the trap using a stainless steel M4 x 10 mm bolt. In this case, choose: Antenna Trap Kit including cable lugs and cable clamps

Example 1: multiband dipole antenna for the 10 and 20 meter band using antenna traps

Start by making two stages resonant at the highest frequency. That’s the 10 meter band in this example. If you make the trap’s so that the resonance frequency is at 28.6 MHz, then signals of this frequency will be blocked. Now start making a standard dipole antenna for the 10 meter band. So a feed point (preferably a 1:1 BalUn) and two pieces of antenna wire of about 2.5 meters ( 300 / 28.5 * 0.95 / 4 = 2.5 mtr). Start with a bit more length (2.8 meters) so you can still cut something. Once you have the antenna in resonance for the 10 meter band, the traps can be placed at either end. Now add the other two pieces of antenna wire for the 20 meter section. In our example also about 2.5 meters per side. Now cut the two ends to length so that the antenna is in resonance on the 20 meter band. It is also possible to apply this trick several more times to add several more bands to the antenna.

Example 2: multiband end-fed antenna for the 40 and 60 meter band using an antenna trap

Start by making one stage resonant at the highest frequency. That’s the 40 meter band in this example. If you make the trap so that the resonance frequency is at 7.1 MHz, then signals of this frequency will be blocked. Now start making a standard end fed antenna for the 40 meter band. So a feedpoint (1:49 impedance transformer) and a piece of antenna wire of about 20 meters ( 300 / 7,1 * 0,95 / 2 = 20 mtr). Start with some more length (21 meters) so there is still something to cut. Once you have the antenna in resonance on the 40 meter band, the trap can be placed at the end. Now add the remaining piece of antenna wire for the 60 meter section. In our example about 7 meters. Now cut the end to length so that the antenna is in resonance on the 60 meter band. It is also possible to apply this trick a few more times. This makes it possible to add multiple bands.