Build a 2 Element 3 Band Beam
Homemade Antenna for 10, 15 and 20 Meters
At first I tried normal coils with high voltage capacitors. The coil form was nylon that fitted into the aluminum tubes. The traps are tuned without the elements and the resonant frequency is about 21 MHz. After you tune the 21 MHz section, tune the 14 MHz. section. The total length of the antenna will be shorter than a full size 14 MHz antenna because of the loading effect of the coil in the trap. The dipoles are insulated from the boom with PVC, nylon or DelrinÒ material. This is then connected with muffler clamps on a mounting plate to the boom (see drawing, above and below).
The antenna worked well, but mechanically it was not strong enough, so I decided to make the capacitor out of aluminum tubing like the well-known triband beams. After some experimenting, I succeeded. The problem is that you cannot tune the traps unless they are connected to the elements. So it must be done by trial and error. Once you succeed, you can easily duplicate the traps for the other half of the antenna.
To tune the antenna, I used a homemade antennascope. Connect the antennascope to the dipole, and place a grid-dip meter close to the coil of the antennascope. You start with the 10-meter radiator, adjusting the element ends by sliding them in and out of the other elements until the VSWR is minimum
Then connect the 15-meter dipole to the boom, adjust the resonant frequency and then connect the traps with the 20-meter end pieces. The traps can be tuned by sliding the outside capacitor (aluminum tube) in and out of the other element so that you get resonance. You will have to check the 15-meter dipole tuning again, because the tuning adjustments will interact with each other. After tuning the traps, adjust the end pieces for 20-meter resonance. Then connect an SWR meter direct to the dipole and apply low power to the antenna. Don’t connect the SWR meter at the transceiver because you can get misleading readings!
During adjusting of the antenna, place it as high as possible, for example on a ladder or a balcony. Now fine-tune the antenna and put it on the tower. You will find that the resonant frequency will change somewhat because of the height increase.
The bandwidth of the antenna was about 200 kHz between 2:1 VSWR points. If an antenna gives a flat SWR reading over the entire band, something is wrong with it. Also, the length of the cable can influence the reading. You can even shorten the length of the 20-meter pieces by adding so called “spokes” at the end of the antenna. But mechanically it is difficult to make the spokes rigid.
I had a lot of fun using this antenna, in the next article; we will make a 2-element version with a reflector.
Originally posted on the AntennaX Online Magazine by Kees Wiegers, PA3BHS
Last Updated : 16th March 2024