Super-sizing the “cheap Yagi” (Part 2)
The two most expensive parts of a VHF/UHF Yagi are the boom hardware and the feedpoint. So, I set about eliminating these costs, keeping in mind that I may only have the antenna installed for a year or two at this QTH.
The feedpoint mechanical construction has been addressed in a previous note. However, I should back up and discuss changes from the K1FO Yagi. In its original configuration, the K1FO antenna is fed with a T-match. This is mechanically complex, although some might argue that it’s sturdier than my solution. I elected to feed the antenna with the WA5VJB hairpin design (38-inch element with harpin 1/4-wave stub spaced 1 inch for 19 inches—this is just a convenient and inexpensive ruse for direct feed without splitting the driven element) for the moment. Yes, I am aware that the K1FO antenna has a natural input impedance considerably lower than 50 ohms, but this is just the first (essentially mechanical) prototype. I’ll do some modeling eventually and determine if I can or should optimize it further.
There are four choices for a boom: PVC pipe, fiberglass, aluminum, or wood. PVC is heavy and too flexible for anything longer than two or three feet. (I see people asking questions in forums all the time about building antennas out of PVC. Why bother when wood and aluminum are so readily-available?) Fiberglass is light and strong, but unless you have access to a lot of it, it’s the most expensive of these options and the most difficult to work. That leaves aluminum and wood. Aluminum is hard to beat for strength-to-weight ratio and ease of working. But, wood will give it a run for its money on cost for a reasonable strength up to a point. Since I had a bunch of wood readily available, I elected to build the boom from wood. This is probably pushing the practical upper limit for a wood-boom antenna.
For the boom, I used three pieces of 1 x 2 select pine that was weatherproofed with a clear lacquer:
The 0″ reference point for the element position measurements is at the left end. Don’t forget to leave a couple of inches at the end.
The original WA5VJB designs were optimized for 1/8-inch diameter elements, which is fortunately quite inexpensive (part #8974K14, $2.11/each, working out to a $12.66 antenna, plus about $5 for shipping…you can buy a lot of Al rod and still ship it for $5.) from McMaster. The K1FO designs are provided for 3/16- and 1/4-inch elements, costing $4.13 or $6.03 for six-foot pieces respectively. That was a little rich for my budget, but then I looked at the Metric-dimensioned 6061 aluminum rods. 5 mm is a little more than 3/16 inches and these rods are only $1.64/each ($19.68 for 12 six-foot lengths; examining the element length table below should give you an idea how much savings there is if you get some buddies together to build a few of these…forget buddies, build an EME array)! So, I built the antenna out of 5-mm diameter rods using the 3/16-inch dimensions.
The table contains initial element lengths and offsets in inches. Do not build this antenna! (Do as I say, not as I do.) It appears to be a good performer, but it has not yet been optimized as discussed above. The columns “Dist (in)” and “Dist (in/16)” refer to the integer and fractional portions of the distance, respectively. Likewise, “C. Len. (in)” and “C. Len (in/16)” refer to the element lengths.
Using a cheap Dawia SWR meter at the end of the feed cable, I can tell that the SWR is less than 1.7 across the low portion of the band. Actually, it’s relatively flat around 1.5-1.7 all the way up to 144.5 MHz where I quit measuring. The pattern is apparently good. My “local” beacons that I can pretty much always count on are WA1ZMS (to the southwest) and W3APL (to the northeast). WA1ZMS runs a lot of gas to an excellent antenna system from an even more excellent QTH. I can fade either of them into the nulls when listening to the other. When I turn the antenna, they fade pretty rapidly into the noise, as well. Good F/B, F/S, narrow forward lobe, etc.
So, the upshot is: I built the unmodified K1FO-12 design for 144 MHz on a wood boom for $30 and about 5 hours of tinkering with basic hand tools. I can turn it and my 3-element 50-MHz Yagi with a 60-year-old CDE TR-2 TV rotor. My TS-700S happily blasted 10 watts into it even at SWR of 1.7. I’ll need to verify the cable loss and determine if my newly-acquired Mirage B3016 will tolerate it.
I will post models and photos eventually (once I find the files again…oops) for the 11-element disaster and the 12-element one I built. Yagis are tricky to optimize well. So, I’m somewhat disinclined to mess with the K1FO design and more likely to switch from the WA5VJB driven element to the T-match if I decide that the SWR matters that much.
Has there been any further progress on this?