Full Sized Portable Beam for 20-10 Meters
There are times when a beam antenna would be handy to have when operating QRP and portable. Following the project and article on the phased verticals, the next antenna I wanted to make was a portable beam that could be transported in a small car. I had decided to build an antenna that was quite common on the ham bands. This antenna was originally designed and built by John Kraus, W8JK and a great DX antenna. It consisted of a close-spaced pair of dipoles that were spaced 1/8 wavelength, in the case of 20 meters, 8.6 feet (2.62m). These two dipoles were fed out of phase by 180 degrees. This resulted in very low antenna impedance and the antenna was fed with open ladder line and tuned with a tuner. It was found that the antenna would work well on all bands above 20 meters without changes other than to tune it with the tuner to the frequency of choice.
Figure 1 is the electrical diagram of the antenna and shows the connections from the feedline to the antenna and interconnecting harness. You will note the interconnecting harness is twisted one half turn between the two elements. This is what gives the required 180-degree phase shift between the two elements and the resulting directional characteristics. It is a bi-directional antenna, having a figure 8 pattern. This will enable the antenna to be rotated only 180 degrees to cover the world. It also gives some very interesting capabilities to the antenna.
The almost instantaneous break-in capabilities of the solid-state rigs on the market now allow you to send a dit and listen to see if you can hear your signal return to you on the reciprocal bearing of the beam. In other words, if you are pointing the beam to the west, your reciprocal bearing will be east. If you can do this, you have found an around the world path from west to east. If not, rotate the antenna some and repeat the process, identifying yourself as you make a transmission. If there is a signal path around the world from your location, you will find it when you can hear your signal come back. This will allow you to do DX hunting easier than with a conventional beam as you will be able to hear signals off of the back of your antenna as well as the front. And that is an advantage; believe me, as it will not be necessary to turn the beam to make contact.
Figure 2 is a side view of the complete boom showing the center-threaded connector at the T connector that allows the boom to be broken down. Total boom length of the 20-meter version is 8.36 feet, (2.55 m). (Click here for metric conversion chart)
The center section has to have a threaded adaptor on one end to allow the boom to be disassembled for transport. This will reduce the length to approximately 4.5 feet (1.4 m) for the boom. You could put two more threaded connectors in the boom and reduce the length more if desired. However, the telescoping elements will be between 3.79 and 3.88 feet (1.15 and 1.18 m) in length when collapsed, so not much would be gained by doing this. Foto-1 is a picture of the PVC fittings used. There are 4 threaded female adapters, 3 T connectors and one male and female connector, which are screwed together for the photo. The 4 male threaded fittings are epoxied to the ends of the telescoping elements. One more male and female connector is used to fit the boom to a short mast used for mounting to either a rotator or another mast. These are not shown.
For Some reason Richard posted “Links” to the rest of the Photo’s and because of this page was not checked and the ‘Photo’ are therefore missing from Photo 2 onwards, Hopefully one day someone will send me a copy of the CD so that I can update these pages. – MD0MDI
The elements of the beam are made of the same telescoping fiberglass fishing poles used in the phased vertical array described in the August 2000 article of antenneX.
The 4 fishing poles need to be assembled carefully as well. Keep the male adaptors and the poles as straight as possible so as to not have the elements skewed at an angle when the antenna is assembled. I did not glue my connectors to the boom with the exception of the center-threaded adaptors. Instead I used self-drilling and tapping screws. This will allow any modifications needed to be done with ease.
Figure 4 shows the two eyebolts used on the two end T connectors. These two eyebolts are where the wires are connected to make up the radiating elements. These eyebolts should be made of either brass or bronze. The radiating elements are made of wire and they are cut to the length equal to the normal half wave dipole length for 20 meters, 33.43 feet (10.19 m). By connecting them to the two eyebolts at each T connector you have a connection point for the feedline as well. The size of the wire used is not too critical, but I would recommend at least number 12, either solid or stranded.
After the beam is constructed and assembled at a place of operation, all that remains is to connect the inter element feedline and the driven elements to the eyebolts. You must connect the wires to the eyebolts first, and then spiral the wire outward towards the end of each rod. The wire is spiraled around each element about 3 times over the entire length of each element. This is to prevent the wire from flopping around in the wind and possibly breaking off. The far end of the wire could either be taped to the element or connected to the end eye on each rod by using monofilament line and a fishing snap swivel.
The feedline may be coax if only to operating on 20 meters. Use a step down balun, as the impedance of the two elements are 8.34-j22.3 ohms and 8.4-j22.2 ohms and are in parallel. If open line feeders are used and a good tuner then you have it made, as you can match the antenna on all bands from 20 down and even on 40 and 30 meters with reasonable efficiency.
Bear in mind this antenna is not going to handle a lot of power and is intended for no more than 200 watts. It is meant for portable operation and was designed with this in mind and not high power.
Now if you would like to operate on 40 meters, the antenna impedance will be very low on 40 meters, 0.159-j920 ohms and 0.156-j920 ohms for each of the elements. Your feedline will have to be twinlead and this will definitely require some sort of tuner. The antenna can be loaded up on 75 meters too, but it may or may not do well on that frequency.
Operation on 10 meters causes the impedance to go up to 3729-j5091ohms and 3729-j5091ohms at each element. This is because the antenna is now a 4-1/2 wave end fed antenna, so again, a tuner is needed.
Figures 5 and 6 are projections of the 20-meter patterns with Figure 5 being the azimuth.
Figure 7 shows the vertical pattern of the antenna on 10 meters and the minor lobe at 65 degrees can be seen. Figure 8 is a 3D representation of the 10-meter pattern and it shows the bi-directional characteristics of the antenna in an easy to see manner. The small minor lobe is also visible.
Foto-4 is a picture of the boom middle connection. Threaded connections can be seen for the other end of the boom and the connector for the mast section at right angles to the boom In Foto-5, you can see one end of the boom and this shows the threaded connectors that the telescoping elements are screwed into. The eyebolts for the antenna wires have not been installed yet.
The individual fishing pole elements are shown in Foto-6 and are identical to the ones used in the article on the Phased Verticals. Foto-7 is a close up of the labels on the poles. You can see the dimensions on the poles as 16.5 feet (5.02m) when extended. The center of the mast is shown in Foto-8 and you can see the mast mounting connection.
In Foto-9, the short mast has been screwed into the boom. This is only a 5 feet (1.5 m) long section and should be adequate for mounting to a small TV type rotor, if desired. The assembled antenna is seen in Foto-10, before the elements are extended to their full length. Finally, the antenna with the elements extended is shown in Foto-11. Yes, it is large with the poles extended, nearly 34 feet (10.3 m). The one end of the element visible ends just above the fender of the red truck on the left and the other end can’t be seen as it ended in the middle of the window on the right above the hood of the small red car. The elements on the bottom cannot be seen very clearly as they were sitting on the grass next to the sidewalk.
The type of construction used here can be adapted to other antennas if needed. I would suggest that considerable forethought be given to your construction project before you finalize the design. This particular design here can be applied to a two-element yagi if desired by making some slight dimensional changes and cutting the wires that make up the elements to match the design parameters of a two element yagi. The question may has crossed the minds of some reading this article as to why I did not use small diameter aluminum or copper tubing instead of fiberglass telescoping fishing poles. Weight and cost were the reasons. Aluminum in this area is very costly and copper in 1/2-inch (1.27 cm) diameter is heavy. The collapsible fishing poles are light and if the bands are all dead, you can use them for the job they were intended for, catching fish if you are near a lake or river.
Originally posted on the AntennaX Online Magazine by Richard Morrow, K5CNF
Last Updated : 23rd May 2024