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 ANY IDEAS PLEASE KJ5VW MINI YAGI 
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Post ANY IDEAS PLEASE KJ5VW MINI YAGI
ANY IDEAS PLEASE , I HAVE JUST BUILT THIS KJ5VW AND FOUND IT TO HAVE , LITTLE OR NO DIRECTIVITY I WAS THINKING OF REPLACING THE BACK ELEMENT WITH A QUAD LOOP ANY IDEAS OR HELPFULL SUGGESTIONS ,, MIKE

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Sun Jun 13, 2010 5:01 pm
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 ANY IDEAS PLEASE KJ5VW MINI YAGI 
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Post Re: ANY IDEAS PLEASE KJ5VW MINI YAGI
Mike,

Can you provide some details on what you have built - Driver/Reflector dimensions, loading coil positions and inductance etc?

Steve G3TXQ


Sun Jun 13, 2010 6:26 pm
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 ANY IDEAS PLEASE KJ5VW MINI YAGI 
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Post Re: ANY IDEAS PLEASE KJ5VW MINI YAGI
sorry steve i put the designers callsign as a reference , wire beam fitted to fishing poles the driven elements are 4 feet of wire then an inductive coil , 7 foot six is wound onto this coil and then a further 4 feet is added , taped to fishing poles in dipole fashion,The reflector is built the same but 4foot 6 is added after the inductive coil to make the reflector longer , boom is 8 foot 6,, hope this heps mike

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He thrusts his fists against the posts and still insists he sees the ghosts, Stephen kings IT ,,


Sun Jun 13, 2010 6:57 pm
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 ANY IDEAS PLEASE KJ5VW MINI YAGI 
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Post Re: ANY IDEAS PLEASE KJ5VW MINI YAGI
KJ5VW TWENTY METER MINI YAGI , IS ON DX ZONE STEVE

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He thrusts his fists against the posts and still insists he sees the ghosts, Stephen kings IT ,,


Sun Jun 13, 2010 7:03 pm
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 ANY IDEAS PLEASE KJ5VW MINI YAGI 
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Post Re: ANY IDEAS PLEASE KJ5VW MINI YAGI
Mike,

I guess I probably asked the wrong question - perhaps I should have asked how you have tuned it?

It's very heavily loaded and so will be quite narrow band; that makes the tuning critical if you are to achieve good directivity. Any departure from the original design in your construction will make a significant difference. That includes: conductor diameter, insulation thickness, inductor winding diameter/pitch etc.

If I were trying to tune it, I think I would first remove the reflector and feed just the driver to find where it was resonant. Then I'd do the same with the Reflector on its. That way you can check that the Driver and Reflector self-resonant frequencies are somewhere close, and if not make adjustments.

73,
Steve G3TXQ


Sun Jun 13, 2010 7:28 pm
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 ANY IDEAS PLEASE KJ5VW MINI YAGI 
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Post Re: ANY IDEAS PLEASE KJ5VW MINI YAGI
Mike,

I just found the article and I see the author recommends the same tuning method I did. What resonant frequencies did you measure for the Driver and the Reflector?

Steve G3TXQ


Sun Jun 13, 2010 7:33 pm
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 ANY IDEAS PLEASE KJ5VW MINI YAGI 
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Post Re: ANY IDEAS PLEASE KJ5VW MINI YAGI
Steve i was critical with my measurements and it tuned very well indead with a good bandwidth, 14.200.for the driven and 14.000 for the reflector , I was dissapointed at the directivity of the aerial , And obviously the power handling of the aerial because of the loading coils ,,, any thing above 200 watts caused the swr to creep up before my eyes , , i assume because the coils were maybe heating up ,However the power handling was never an issue , just something else i threw in the tests ,The overall size suits my location , i just wanted to try again to get it a little more directional, i tried it as a rotary dipole first with better results, as soon as i added the reflector, it all went to pot , so to speak so obviously somethings amiss, with that element ,, regards mike ,

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Sun Jun 13, 2010 8:25 pm
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 ANY IDEAS PLEASE KJ5VW MINI YAGI 
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Post Re: ANY IDEAS PLEASE KJ5VW MINI YAGI
Mike,

I tried modelling it in EZNEC. I found it very critical on the ratio of Driver/Reflector resonant frequencies. I got the best results with the Driver resonant about 100kHz above the Reflector resonance. Assuming a Q of 100 for the loading coils it showed about 0.5dB gain over a dipole and about 10dB Front/back.

At the 200W level the dissipation in each Driver loading coil would be 26W; that would certainly be enough to shift the inductance and the tuning. The coil losses are reflected in the poor forward gain figure. I'm afraid you really need much better loading coils in a beam that small.

Steve G3TXQ


Sun Jun 13, 2010 8:36 pm
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 ANY IDEAS PLEASE KJ5VW MINI YAGI 
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Post Re: ANY IDEAS PLEASE KJ5VW MINI YAGI
THANKS FOR YOUR ADVICE STEVE I THINK THE OTHER GENTLMAN WHO BUILT THIS BEAM IN THE UK MAY HAVE OVER EXAGERATED ITS PERFORMANCE. REGARDS MIKE

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He thrusts his fists against the posts and still insists he sees the ghosts, Stephen kings IT ,,


Mon Jun 14, 2010 6:02 am
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