John's homebrew pages
Portable 7 element 2m yagi antenna
Now the 2m portable linear amp is working, the next improvement worth having a look at is the antenna.
I have been very pleased with the 5 element DK7ZB design I have been using recently (see the antennas page). I thought about trying one with a bit more gain, to see if it makes much difference; later I'll combine this with a preamp as well. The problem is finding something that can be built in such a way that it will easily dismantle to be carried attached to a backpack up a hill for portable operation. This tends to limit the length in that too many sections to the boom would be a nuisance. I decided that three is reasonable, so worked from that assumption. More on the design to follow ... for now I'll note it's basically the DK7ZB 2m 7-ele Yagi, 28 ohm, 3.3m long, with the 28 ohm DK7ZB match.
The elements are supported by plastic tube (nice and light weight to carry up hills). I found that white plastic 22mm plumbing tube fits nicely inside 25mm electrical conduit tube, making a good way of connecting pieces together to make a long boom. It's a bit flexible, but works fine without any additional support for the 5 element version.
The 5 element beam I built suffers from the elements not being too well aligned, since I drilled all the holes to take them using a hand drill and my eye to line the drill up vertically. To improve this I decided to dig out my ancient drill stand with old drill that still works (no gearbox so a bit fast for plastic, but it works). I made a V channel jig to hold the antenna support tubes for drilling.
The elements are made from 6mm aluminium tube obtained from B and Q. These come in 1m lengths, so are mostly ideal for a 2m antenna, except that the reflector is a bit longer than 1m; this is extended by using a piece of screw internally to attach a short extension section of the same tube material. Here are the five directors, labelled with rings of PVC tape to make it easy to identify them on a windy hill. They are held in place in the boom using plastic grommets, seen here on the side next to the PVC tape. The marks on the elements show the boom edges and the centre line.
Above the ruler in this photo are one black and one white support tube.
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 rilled to take screws to make the electrical connections. The holes in the outer 10mm plastic tube are larger to let the screw heads and nuts make electrical contact with the element.
Where the element enters the plastic support tube, a bit of vinyl tape is used - more for decoration than anything else really, just that the element with red and black on it is the driven element; the reflector has a red band; and the directors have black bands as above.
Here's the completed kit laid out on the floor ready for packing up. The straight bits all bundle together and then are held tightly by reusable plastic cable ties (left of the feeder), which are also used to attach the bundle to a rucsac. The feeder is URM67 - I decided that these precious dB ought not to be thrown away in a length of RG58 - at 2m losses are not insignificant - so the whole thing is an experiment. 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.
This antenna was put into action on the top of Cairnharrow on 10 May; signal reports were pretty good, with conditions not too wonderful, so I now need to do a proper comparison of the 5-ele and 7-ele designs when I have a bit of time. The photo below shows how the antenna is kept horizontal with a bracing bit of string fed through a platic bottle cork stuck into the top of the fishing pole.
The antenna is very pointy - I found that people's signals peak up wonderfully providing I point it at them. However the wind loading is non trivial; the next picture shows this in practice in the first RSGB Backpackers' contest, with bits of string to keep the beam pointing in the right direction.
The wind loading also showed up another problem; the simple pipe clip used to fix the boom to the pole means that it is easy for the wind to push the antenna around, and also puts a lot of strain on the fairly lightly built pole (this would not be a problem with the heavier Spiderbeam pole). In fact, the pipe fitting has a couple of sharp corners which dig into the support pole and damage it if the fixing is pushed too far down the tapered pole. I decided a better fixing was needed.
This shows the improved boom fixing. The boom is fixed to a perspex plate with a couple of screws (this can also be improved once I find some correctly sized fixings for the black electrical conduit tube). Then at right angles, a piece of tube is held using the pipe fixings; this tube slots over the pole, and is held quite securely by friction. I did thing of putting a slit in it to allow for the pole taper, but it seems to fit quite well without.
Here's the improved boom fixing in use (actually with the 5-ele DK7ZB Yagi). The vertical tube on the fixing fits tightly onto the slightly tapered pole, allowing the antenna to be rotated by rotating the pole. The antenna boom fixing cold be better, maybe with a couple of "half moon" pipe supports - still to be investigated. Overall it is much more solid than the original single pipe fitting, and doesn't damage the support pole. (Just at the bottom of the photo is a pole joint stuffed with cable ties - the pole had collapsed due to its maltreatment using the previous design of fitting with these heavier Yagis.)