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Antenna No 68


7 element Yagi for 70cm, Make II


I built this antenna a few years ago (Antenna No 15) and used it on SOTA in Wales and Ireland.  It is fascinating being on top of a hill near the coast, pointing the Yagi across the sea and getting a reply from the "other side"! 


Problem was that I occasionally got a high SWR warning on my FT-857 dependent on the coax length and position of the coax used on the day for my field portable set-up.  Additionally, using very small self tapping screws was not handy for field portable work.


I experimented with a coax RFC with mixed results, can't say it cured the occasional high SWR problem.



Unsure how to dimension a coax RFC for 440MHz ...


Another experiment using a folded dipole and 4:1 coax balun was quite positive (SWR wise) but construction was flimsy and would not withstand rough transport for SOTA.



The folded dipole was OK to build but the 4:1 impedance match was difficult to fit onto a round boom.


I recently read about a sleeve balun using a quarter wave copper tube and decided to investigate.



The principle of the "sleeve balun".


According to literature it is recommended that the ratio of diameters D/d >= 3.  For the sleeve I used copper tubing 15mm O.D. / 12mm I.D. and for the coax semi-rigid 50 Ohms with an O.D. of 3.6mm, yielding a ratio of 12.0/3.6 = 3.33



The following pictures illustrate construction and testing of the sleeve balun and Yagi.



Hand made support bits, semi-rigid coax, copper tube with adapter, N-connector mounted on a flange.

N-connector and semi-rigid with concentric PE pieces properly spaced.

Plumbing is completed ...

Ugly mounting method on a round fiber boom.  It works, don't touch it!

The 7 element 70cm Yagi on a fiber mast (about 5m agl) at initial testing.

No more SWR problems caused by the coax.  The original Yagi dimensions were all OK.

Apply some black gloss paint ...



I am looking forward to using this antenna on my upcoming SOTA activities in Belgium and abroad.




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