brainwagon "There is much pleasure in useless knowledge." — Bertrand Russell

3Apr/103

Weekend Project: A Cheapie 70cm Yagi for Moonbounce Day

Okay, okay. The postings about computer checkers haven't exactly been all that popular with you guys, so I thought I'd write up something that I am currently working on. Moonbounce Day is coming up April 16, 17, and 18, and I thought it would be a good opportunity to try to hear the station operating from Arecibo, Puerto Rico. I don't have any equipment that does 70cm SSB/CW except for my trusty FT-817, so my idea is to just listen and record, and to use the minimal antenna that might conceivably work.

I of course continue to have my trust Arrow antenna, but I thought something a bit longer could be better. So, I have designed a DL6WU yagi consisting of a six foot insulated beam with 9 directors tuned for 432 Mhz and matched with a folded dipole/hairpin match. A trip to a local welding supply house netted me enough welding rod to make all the elements for $5, and another $3 netted me a nice six foot length of 3/4" square hardwood to serve as the base. A few quick experiments last night showed that a miter box and a hacksaw could result in reasonably accurate cuts, but that maybe I should cut them 1mm long, and then trim them using my benchtop belt sander.

I'll try to nab some pictures as it progresses. With any luck, by the end of the weekend I'll have this mounted atop an old camera tripod, and I'll be able to test it out a bit by maybe aiming it at some LEO sats and seeing how well it works. I'll also have the complete dimensions later.

For now, coffee and breakfast.

Update:

Okay, here is the design that I came up with using VK5DJ's Yagi Calculator. I specified an insulated boom 0.75" in diameter, round elements 3.2mm in diameter, and a design frequency of 432 Mhz. One slightly annoying thing is that the design software only produces metric measurements, so I took them, wrote a little program to convert them into properly rounded values expressed in 1/32nds of an inch, and then dumped them all out. Here we go.

 338mm |  13 5/16" | Reflector
 327mm |   12 7/8" | Radiator (single dipole)
 305mm |       12" | Director #1
 302mm |   11 7/8" | Director #2
 299mm | 11 25/32" | Director #3
 296mm | 11 21/32" | Director #4
 294mm |  11 9/16" | Director #5
 291mm | 11 15/32" | Director #6
 289mm |   11 3/8" | Director #7
 287mm |  11 5/16" | Director #8
 285mm |  11 7/32" | Director #9
 686mm |       27" | Total folded dipole length
 343mm |   13 1/2" | Center of the dipole
 154mm |   6 1/16" | Center -> bend of the dipole 
 149mm |    5 7/8" | End -> bend of the dipole 
  25mm |  0 31/32" | Bend Diameter
  30mm |   1 3/16" | Boom position of Reflector
 169mm |  6 21/32" | Boom position of Radiator
 221mm |  8 11/16" | Boom position of Director #1
 346mm |   13 5/8" | Boom position of Director #2
 495mm |   19 1/2" | Boom position of Director #3
 668mm |  26 5/16" | Boom position of Director #4
 863mm | 33 31/32" | Boom position of Director #5
1071mm |  42 5/32" | Boom position of Director #6
1290mm | 50 25/32" | Boom position of Director #7
1519mm | 59 13/16" | Boom position of Director #8
1758mm |  69 7/32" | Boom position of Director #9

I cut the elements by marking the lengths, then putting them into a little plastic miter box and cutting them with a hacksaw. A few are very slightly long, so I'll go back and fine tune those by a little judicious use of my benchtop belt/disk sander. I then turned to the boom. I must admit that on second glance, I'm not very happy with the hardwood boom: it's got about 1/4" of wave/twist in it as you sight down the length. But oh well, this is an experiment anyway. I marked all the positions, and drilled them out with a 1/8" drill bit. This proved to be too tight of a fit for my elements, so I dug up a 9/64" bit, which is very slightly too large, and allows some wobble. When I made a cheap yagi before, I secured these with hot glue, which is what I suspect I will do later.

That's where the antenna stands now. I have to work on the driven element a bit more. Bending this by hand is a bit imprecise.

I'll have pictures up when there is something to see.

Filed under: Amateur Radio 3 Comments