YU7EF 6 element 2m Yagi
Another Ham had trouble with a YU7EF 6 element 2m band Yagi that he had built. The SWR dip was off the design frequency and the dip did not go as low as expected (1:1) even after significant adjustment of the driven element (DE) length. Normally YU7EF Yagis "work as advertised" as long as the appropriate correction factor for the element mounting method is observed, an effective 1:1 balun is used, and the connections to the DE are as short and direct as possible.
To try and find the trouble I first checked the 'free space' dimensions as given on the YU7EF web site with the 4NEC2 antenna analysis program, and there were no surprises there ...
EF0206 "free space" antenna parameters as shown in a 4NEC2 computer software simulation
Then I built the Yagi using the free space dimensions with the same 1/4" diameter element material, element mounting blocks (Stauff PP hydraulic pipe clamps), and boom material (30mm square aluminium) as used by the other Ham. Some ferrite sleeves were slipped over the feedline near the feed point to act as a 1:1 choke balun. As expected, due to no boom correction factor being added to the element lengths to compensate for the effect of the aluminium boom, the SWR curve was high in frequency and did not dip to near 1:1 ...
SARK110 antenna analyser plot of the uncompensated EF0206 Yagi
Using the frequency shift of the SWR dip as an indication (145.2 MHz rather than 144.6 MHz), it seems that the elements were 4mm short ... this was supported by the "Boom Correction" diagram at the bottom of the YU7EF "Boom Correction" web page. Therefore I cut another set of elements 6mm longer (to bring the SWR dip down even a bit lower to 144.2 MHz) and rechecked the SWR ...
SARK110 antenna analyser plot of the EF0206 Yagi with the boom correction factor added
Here are a some photos of the Yagi on test during a very dark, heavily overcast day.
Note the slight upward bowing of the DE ... not enough to be of any consequence
Close up of the split dipole driven element and 1:1 ferrite choke balun
In conclusion, the YU7EF EF0206 2m Yagi SWR curve does perform "as advertised" when the proper boom correction factor is applied to the element lengths ... now it is up to the other Ham to find out where he went wrong in his construction.
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