At 06:22 AM 2006-12-03, Rick Dougherty NQ4I wrote:
>Hi all...I am new to this list, but I am hoping that some members might be
>able to assist in an antenna problem I think I have...first off I have
>recently installed a KLM 3 el 80m yagi....its at 175 ft and I spent most of
>the past summer re-building the antenna and installing hardware etc...the
>antenna was built exactly according to KLM spec's in the manual...lengths
>were measured and are within less than 1/8 in from spec.....here in lies the
>problem.....the antenna has a feed point impedance of 100 ohms...almost
>every 3 el design I can find in YO, AO, YW and any other yagi design program
>has antennas with 25 ohm impedances for 3 ele designs....The original KLM
>design used a pair of 75 ohm Teflon coax cables 1/4 wavelength in length in
>parallel to a 1:1 balun at the feed point to provide a nice match....well I
>had to make a single 75 ohm 1/4 to transform the 100 ohm to a 50m ohm
>match....
>
>I plan to make another trip to the top of the tower next week to plot x and
>y values from 3.2 mhz to 4.2 mhz to see if I have overlooked
>something....the antenna as it is now has a W shaped resonance curve...its
>1.2:1 at 3523 and goes slightly up to approx 2:1 and then dips back down to
>1.1:1 at 3799 mhz...so it has a very acceptable match....
>
>The actual performance is far less than I would expect though....there are
>many times at sunset that the 4 square I have will beat this yagi into
>EU....there have only been a few times that the yagi has beaten the 4 square
>in EU or JA....but it sometimes does occur.....the yagi shows approx 20-25
>db f/b.....
>
>Are 80m conditions so flaky that the yagi will at times not equal the 4
>square?? I am puzzled by the 100 ohm feedpoint impedance also??
Is the 100 ohm feedpoint impedance a measured value? The parallel 75
ohm lines in the original design would form a 37.5 ohm line and one
quarter-wavelength of this would transform a 28 ohm feedpoint to 50 ohms.
Maybe you could clarify this. (I'm not familiar with the design
specifics of the KLM.)
At 175 feet, the peak signal would be expected at a 20 degree
elevation over flat earth at 3.8 MHz. I have a model of a 3-element
with an 89.4 foot boom and it provides 13.3 dBi gain (including
ground reflection gain). A typical 4 square over average ground
would have about 5.5 dBi gain at a 25 degree takeoff angle. I'd take
the yagi unless you have salt water! The 4 square elevation pattern
only exceeds the yagi around the null between 39 to 53 degree takeoff angles.
73, Terry N6RY
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