John's homebrew pages
Portable 12 element 70cm yagi antenna
Following on from the 2m 7 ele Yagi, I want to start moving up in frequency. The FT-817 is a bit short of power but maybe a decent portable antenna will help!
Both the 5 element and 7 element DK7ZB design antennas for 2m I have built recently seem to work well. So I thought that would be a good starting point for 70cm as well. The three section plastic tube boom works well, and I had one already for my first 6m 3 ele Yagi; so I found one of the DK7ZB designs that would fit on that length, which is a 12 element design. As with the 2m antennas, it can be easily dismantled and carried attached to a backpack up a hill for portable operation. This design is basically the DK7ZB 70cm 12-ele Yagi, 28 ohm, 2.5m long, with the 28 ohm DK7ZB match.
The boom is made from plastic tube (nice and light weight to carry up hills), built in the same way as for the 7-ele 2m design.
The elements are made from 6mm and 8mm aluminium tube obtained from B and Q. These come in 1m lengths, so it's not too wasteful to cut them for a 70cm antenna. All elements except the driven element are 6mm; I had to do a bit of extrapolation of lengths from DK7ZB's tables.
Here's the driven element and its connectors:
The driven element is made from 8mm tube, which is a tight fit into a plastic 10mm tube from the same source. Once in place, with a 10mm gap in the centre, holes can be drilled to take the electrical connections. This time, instead of using screws, I used 4mm holes and banana plugs.
Here's the reflector end of the boom, showing elements inserted into the boom, with rubber grommets to hold them in place.
Initial tests on air showed the antenna with a VSWR too high for my liking; the wavelength is much smaller than for 2m, and I think the banana plug connectors are a bit long. They have now been modified so that the insuating sleeves have been removed, and the plugs connected directly on to short ends of the DK7ZB matching section; then end wires are probably half the length they were before. Tests at home suggest that the VSWR is greatly improved now; a couple of cm makes a big difference at 70cm!
Here's the completed kit laid out on the floor ready for packing up. As for the 2m designs, the straight bits all bundle together and then are held tightly by reusable plastic cable ties, which are also used to attach the bundle to a rucsac. The feeder is URM67 - even more than for 2m, the precious dB ought not to be thrown away in a length of RG58 - at 70cm losses are quite significant. The antenna design is a 28 ohm impedance design, matched to the 50 ohm feeder using a quarter wave balun as on the DK7ZB web pages, made from two pieces of RG59 (75 ohm) in parallel.
Here's the antenna on its first outing. It certainly works, but needed a bit of tweaking - the VSWR was not wonderful, but has been improved as described above. Note the improved boom fixing developed for the 2m antenna is used here as well - fewer bits to carry up the hill! More trials needed yet to prove the worth of this antenna - and I still think a little 70cm amp is in order as a winter project.