J Beam aerial

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g4bbh
Posts: 72
Joined: Thu Mar 13, 2008 7:19 pm

J Beam aerial

Post by g4bbh » Sun Feb 26, 2012 2:28 pm

Picked up an old J Beam slot fed 23cm yagi at the rally today (15/15 I think). Does anyone have exoerience of this aerial? Is it supposed to be 50 or 70 ohm? Got to be better than certain others I've seen.
Dick G4bbh

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G8ADM
Posts: 115
Joined: Sat Feb 23, 2008 7:14 pm

Re: J Beam aerial

Post by G8ADM » Wed Feb 29, 2012 10:04 am

I used one in the 1980's. Compared to a modern 24 element Yagi it is not very good.
Partly due to a lossy balun. Not recommended.
I hope this helps,
Dave.

g4bbh
Posts: 72
Joined: Thu Mar 13, 2008 7:19 pm

Re: J Beam aerial

Post by g4bbh » Wed Feb 29, 2012 2:17 pm

I spent last night feeding in all the dimensions to MMANA-Gal. The resulting model showed that it has maximum gain around 1340 so may have been cut for the high end of the band. Calculated gain is not very good. A few quick tests confirm the MMANA-GAL simulation. Still, it is rugged and only cost me three quid. Only thing I am unsure about is the location of the directors which someone may have altered at some time (or reassembled the booms upside down). These are all on the inside of the booms. My old 70cm version had them on the outside of the booms. Perhaps it was just designed this way.
I get almost the same gain from a double tri-quad (six quad loops with a plane reflector).
Dick G4BBH

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