Thanks for sharing your design, Stan. It is a very unique way of shortening
a 40m element. The design looks great to me and this idea could be useful
on other bands well. It would be nice to see photos of the 2el version that
Kevin built. For those that don't remember, Stan and Kevin owned and
operated the Cycle 24 antenna company which supplied a great lightweight
tribander for WRTC in 2014.
73,
John KK9A
Stan Stockton K5GO wrote:
A few months ago I designed and my son built a 2L 40m Yagi using this type
of design for loaded elements inspired by the W6NL style Moxon since that's
what he was going to build. The results are virtually the same in terms of
gain, pattern, etc., but it is not as unwieldy and reduces the weight out on
the element that causes it to sag excessively. TBD, but I believe it is less
likely to interact with a 15m antenna.
I have the need to put up a 40m dipole between a couple of towers here at
ZF9CW. If the towers were about 20 feet farther apart I would just put up a
flat dipole but they are a little close spaced for that. The element is
going to be less than 50 feet long and it will be 50 ohm direct feed as
opposed to about 75 ohms if it were just a normal dipole.
For those who want to put up a dipole for any band, do not have the room for
a flat dipole and do not want to either do some impedance matching with a
12th wave transformer or live with 1.5-1 SWR at best, here are the details
of what I am going to find the time to do on 40 meters. I will be limited
to using the materials on hand. I would rather use #14 wire.
49 feet total (24-1/2 feet either side of center) of #16 wire fed in the
center. At 15 feet out from center on either side there will be a 6 foot
piece of 3/8" aluminum tubing that will be electrically connected in the
center to the wire which will continue out for another 9-1/2 feet. So there
will be 3 feet of aluminum tubing either side of center. From the ends of
the aluminum tubing there will be #16 wires that will connect to the end of
the wire (24-1/2 feet out). Those two wires should be ever so slightly
slack so when the dipole is pulled up to its flat position there won't be
any significant stress on the ends of the aluminum tubing.
If you just draw this on paper it looks like a dipole that is 20 feet
shorter than a normal dipole and near the ends you have what looks like an
arrow.
I have this skewed a little toward the bottom end of the band, but about 4
inches shorter overall length would give you less than 1.5-1 from 7.0 to
7.225.
The gain of this dipole is about 1/4 dB less than a full sized dipole. Its
bandwidth between 1.5-1 points is about 25 khz less than a full sized dipole
but only if you do a perfect job of matching your 50 ohm coax to the higher
impedance of the full sized antenna.
I probably won't do anything other than write this but if I get inspired I
might sometime write it up for a magazine. I can see great benefit for those
with limited space. A flat dipole is so much better than what's commonly
referred to as an inverted V.
I'll be glad to send an EZNEC file to anyone who wants to get a better
picture of the design. Just send me an email.
73 and Merry Christmas.
Stan, K5GO/ZF9CW
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