A cheap, compact and simple-to-construct tube-yagi is described that is ideal for low power backpack portable operation. The antenna elements pack-up by sliding into the boom tube for easy storage. The antenna is light weight as well as being quick and easy to assemble.
A 177cm (69 �") length of 19mm diameter plastic waste water pipe was used as the boom for the tube-yagi. This needs to be carefully marked and drilled to make sure the elements will be parallel when they are inserted.
Start off by drawing a straight line down the tube. There might be a seam on the tube that you can use otherwise try laying it on a flat surface and run a marker pen alongside using the surface as a guide. Next wrap a piece of paper around the tube and use the straight edge as a guide to mark an accurate circle around the tube where the elements will go. Then cut the paper to the length of the circumference and find the halfway mark. By aligning one end of the paper with the line the middle point will act as a marker for the holes on the other side of the boom.
Don't try and drill through the whole boom in one go. Instead drill the two holes separately. In this way you can correctly mark out, and then drill, the 10 holes on the boom that will take the 5 elements. You might need to open up the holes a tiny amount but try to make the fit tight enough so that the elements slide in with a bit of friction.
The reflector and director elements are made from 3mm aluminium rod (welding rod or similar). An off-center blob of epoxy resin (or small circular push-on metal clip) acts as a stop for the rods so that they will be correctly positioned when inserted. A label can be added (to clearly tag each element for easy identification in the field) to this side of the element so that you will never try to put the element in the wrong way. Once the rods have been fitted into the boom a small rubber grommet is pushed on from the other side to fix the elements in place. 6mm diameter tube was used for the dipole element and a few turns of tape were used on one side to locate this.
The antenna is matched to the coax using delta arms and a 4:1 coax balun to the dipole. The balun was constructed in the standard way (see diagram) using a loop of about 71cm. All the screens are soldered together at one point. The balun joints were covered in rubber solution glue to water proof and wired to solder tags to attach to the 15cm delta arms (which should be made of the same thickness tube as the dipole). The balun and delta arms were fixed to a sheet of plastic to provide support. The other end of the delta arms were attached 11.5cm either side of the dipole center using 3M bolts and butterfly nuts (for a really quick and easy connection I have successfully used large crocodile clips instead of bolts and butterfly nuts (see picture) however this is only really suitable for low powers).
The half wave balun
The balun consists of the main length of coax connected to a half wavelength piece of coax. The power into the balun is composed of V volts and I amps (where P=VxI). The balun has two outputs; one is connected directly to the main length of coax while other output connection is after the half wave of feeder. The second output connection will thus be 180 degrees out of phase with the first. So if the first output has V volts the other will have �V volts. The voltage difference on the output will be V(out) = V - - V = 2V. The balun acts as a voltage doubler. As the power through the system must be constant (ignoring losses) the output current must drop to a half, I(out) = I/2. Ohms law on the input is R = V/I so we can work out the impedance change as R(out) =V(out)/I(out) = 2V / I/2 = 4R and so we can account for the 4:1 impedance transformation of this type of balun.
Setting up and adjusting the SWR On unpacking, the elements are fixed to their correct place on the boom and the rubber grommets used to secure the passive elements in place. The delta arms attach via the bolts and butterfly nuts either side of the dipole center. When using the beam for horizontal polarisation let the balun hang down away from the plane of the yagi and tape the cable to the mast as shown in the picture. Adjustment of the SWR is possible by changing the lengths of the delta arms and where they attach either side of the dipole center. I found that a good SWR was achieved over the whole of the band when the 15cm delta arms were attached 11.5cm either side of the dipole center but some experimentation may be needed here depending what materials you decide to use.
Shown are typical measurements for the SWR for the beam mounted horizontally. I used about 7m of RG58A/U coax for this beam, lower loss cable will probably change the curve slightly. The yagi-delta match and also the balun are each resonant devices so two peaks in the SWR can occur if the two resonances don't coincide exactly. A good SWR over a part of the band is easy and with experimentation a SWR of less than <1.6 over the whole144MHz band is possible.
For vertical polarisation its best to use a non-conducting mast. There is no natural position for the delta match / balun in the vertical position. I tried them fixed along the boom (toward the first director) and also perpendicular to the beam and each gave slightly different SWR results but I found the whole assembly still gave very usable SWR results over the whole band, even with a metal mast.
The antenna gain of the yagi was determined in a very simple manner. The GB3VHF beacon in Kent (JO01DH), roughly 60km distant from the test site (IO90WU) was used as a reference signal and received i) on a dipole and then ii) on the 5 element yagi (in the same height and with similar coax feeders).
An VHF RF attenuator (0-40db in 1dB steps) was fitted between the antennas and the receiver. The S-meter was used to measure the signal on the dipole. When the yagi was measured the attenuator was adjusted until the S-meter read the same as that obtained using the dipole. The change in attenuation thus gave a guide to the yagi gain compared to the dipole. These experiments gave a gain of 7-8 dB with a front to back ratio of 10-11 dB.
Note: I used about 7m of RG58A/U coax for convenience as it is much easier to pack away. However, less loss would be obtained using the more bulky RG213 but it is a heaver load for the antenna to support and so some support (tape) for the cable needs to be provided.
To pack-up, the elements of the yagi slide into the plastic boom. Rubber bungs stop the ends � you have a yagi-in-a-tube. I found that it was also possible to slide the tube-yagi within the center of my portable telescopic mast so that only one item then needed to be carried besides the backpack.
A standard TV / FM aerial mast clamp was used to attach the yagi to the mast. It is worthwhile replacing the nuts on these clamps for (butterfly) wing nuts, using them is quicker and cuts down on the number of tools you need to carry when out-and-about on the hills.
The antenna is based on an old ARRL design that I have used with excellent results over many years. The antenna seems to have a good combination of gain and bandwidth for its size. I have used the antenna singly, in two's and even in four's using a light weight open-wire phasing harness and universal stub, as described in the original article (see below). Because the packed-up tube-yagi is so compact and light weight it opens up the exciting possibility of being able to get further gain by using 2 (or perhaps even 4!) of these antennas for backpack portable work.
The Radio Amateur's Handbook, 1972, Chapter 22, VHF and UHF antennas
plastic pvc_u waste water tube (e.g. osma 5) 19mm (3/4�)
one ~1m, 6mm diameter aluminium tube (for dipole, see table)
four ~1.1m, 3mm diameter aluminium rod (for passive elements, see table)
two rubber bungs (to fit the ends of the boom)
one TV / FM antenna clamp + wing nuts
labels for element identification
four small rubber grommets + four cable ties
two solder tags and nuts and bolts
two 3M bolts and butterfly nuts (or car battery size croc-clips, see text)
50 ohm coax (RG58A/U or CU convenient for short say 10m lengths) and RF connector for radio
|Reflector - dipole distance||43cm (17")|
|Dipole - 1st director||33cm (13")|
|1st director - 2nd director||41cm (16")|
|2nd director - 3rd director||53cm (21")||Delta arm length||
same diameter as dipole tube
depending on velocity factor of cable
|dipole||98.8cm (38 7/8")|
|1st director||96.8cm (38 1/8")|
|2nd director||94.0cm (37")|
|3rd director||93.0cm (36 5/8")|
link to 2 x 5 element yagi page
link to 4 x 5 element yagi page
back to G1EXG's Radio page
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