Homebrew 4-element 12m Yagi

Reworking a Hygain 15m Long John for WARC-12m Duty

I had not planned to put up a 12m beam but when a big dxpedition in 2018 looked like it would be workable on 12m, it was a good enough excuse to cook up a quick redesign of a Hygain 15m LJ105. 

The construction method used on the Hygain makes it very easy to change both tip length as well as element location along the boom.  I've reworked perhaps 8 of the LJ series over the years and not only are they easy to reconfigure, most of the time they will also hold together even in serious winds.

The beam as it sits now is shown in the photo at right.    

I can't say much about it's performance.  The dxpedition failed to materailize and I've only made perhaps 2-3 QSO on it since I put it up.  Looks nice in any event!

The Model

The antenna is simple in configuration - a 4-element type using the available boom length of the OEM LJ (24'). 

With this kind of old vintage beam rebuild often the OEM used too many elements and without a specific added goal (broadbanding, etc) as part of the rebuild, then the first trick is to determine the right element count.  I normally do that by looking to see what the best gain/pattern is with (for example) 4 elements to serve as the baseline. Then adding a 5th element, pushing the 5 elements together so they fit on the boom, and reworking the optimization is next.  If the added element case does not add significantly to the pattern or the gain compared to the earlier baseline, then I use the lower element count.  More elements just for the sake of having a higher element count alone is a bad choice here in wind & ice country.  

Such was the story with this boom - 4 elements was the right count.

The 12m band is narrow and a hairpin match is fast to implement and would work fine allowing me to push the optimizaiton a bit more to squeeze additional gain.

The beam is mounted on a tower near the house that I normally use for testing antennas but it's otherwise free of antennas.  Unforutnately at 33' it's pretty short.  It works ok in this case because 33' above ground for 12m is not bad at all.

Model Predictions

Patterns were very nice with a tight rear and a thin front azimuth profile.

The plot below is a feature from the AC6LA AutoEZ EZNEC tool.  Shown here are the R/Xj trend vs. frequency for the antenna, the SWR and lastly the gain + F/R plots.  Very handy!!!