Invisible airwaves crackle with life
Stretched WA5VJB Cheap Yagi for 2 Meters
Ok,so when two meters was really REALLY open,my little 6 element yagi up at 25' did ok,but I definitely wanted more gain and more side rejection. I got to looking over the WA5VJB Cheap Yagi article again,and found a mention of being able to "grow" the designs using more of the same parasitic elements. Considering the 6 element 2 meter design I'd built,the D2 and D3 elements are both 36.5" long,and each is spaced 18" from the next director element "forward" on the antenna. So,I deduced from this that I could add more 36.5" long elements 18" apart,maybe even out to the practical size limit under which I was operating (about a 13' beam was the longest that would clear the trees at 25'). The antenna would end up with elements like this:
Since this was based on the WA5JVB Cheap Yagis,it was to be built in that spirit,of pretty much whatever I found laying around the place. That meant that the boom was constructed of hunks of 1x1 leftover from a window trimming project spliced together,the driven element was fashioned from #8 solid copper ground wire (just like the 6 element antenna that it replaced),and the parasitic elements were thin galvanized steel wire intended for hanging grids for drop ceilings. I had a bunch of those out on the carport,left over from putting ceilings in the laundry and storage rooms at the old house. A pair of U-bolts through the boom at the splice point (where it's thicker) did nicely for a mast mount.
I'd intended to varnish the boom to protect it from the Florida winter,but the January 2005 VHF Sweepstakes came up,and I had to put up a 70cm beam anyhow,so I pulled the mast down and stuck the 2m beam up there. Turned out a single U-bolt with a notched plate that fit against the mast (to keep the boom from tilting) wass plenty for securing it. The few folks I contacted on 2m gave me good reports,but band conditions flat out stunk. Later experience showed that the antenna seemed to perform significantly better than the 6 element version,with an S-unit or so better hearing,and much less tendendcy to pick up stuff to the sides and rear.
A new twist...
The problem that has surfaced after several years of use is that that unvarnished boom has developed an alarming spiral warp from about the third director clear back to the end of the boom. The driven element and reflector are at nearly 45 degress off horizontal. One wag has wondered on-air if maybe the thing is now circularly polarized. It wouldn't surprise me.
That twisted boom cannot be left uncorrected. Also,those elements have proven to be too flimsy,subject to being bent by anything from falling branches and pinecones to large cardinals landing on them. Hence,I've come to the conclusion that replacing the antenna entirely is in order. I've come up with what I expect will be an effective means of making a lightweight,rigid boom out of 1/2" PVC pipe. I first had this brainstorm while considering how to build a 6 meter 4 element quad out of PVC pipe,The approach I came up with is to use two parallel runs of the pipe,one above the other,periodically tied together by tees and short sections. The resulting "ladder" structure ought to be plenty rigid. The driven element will once again be bare copper wire,so that the feedline can be soldered to it. The parasitic elements will be cut from straightened out wire coat hangers. I picked up enough hangers to build two 10 element antennas for just $3.00 at WallyWorld.
The PVC ladder boom idea will be put to the test shortly; as this is being written it's just over a week until the June 2009 ARRL VHF QSO Party. I'm going to build one or two ladder boom yagis and put them on the wooden VHF tower I'm building. It'll be out in the middle of the yard more than the current mast,allowing more room for the long booms to swing clear of any obstructing tree limbs.
The situation has become critical...
It's now early 2010. I happened to notice the other night what looked like a fallen tree branch snagged on the 6m Yagi. Considerable effort went into trying to shake it loose to no avail. Next morning I discovered that what I had taken to be a tree branch was actually about half of the 2m Yagi. Seems that the twisted wooden boom finally rotted through and broke. The PVC ladder boom experiment has now risen dramatically in priority. And since the whole antenna was so cheap to build,if it pans out as expected I'll build two of them and stack them.
With the arrival of the 2010 ARRL June VHF QSO Party,it was high time to crank out another iteration of the 10 element antenna. This was duly accomplished.