----- Original Message -----
From: "Tom Rauch" <w8ji@contesting.com>
To: "jbattin" <jbattin@msn.com>; "Dave NØRQ" <n0rq-1@dfwair.net>;
<topband@contesting.com>
Sent: Monday, December 08, 2003 8:56 PM
Subject: Re: Topband: Cone of Silence
> > lengths in case there is such a beast. My impression is that when we
> adjust
> > the exact length of the beverage the most predominate effect is to
adjust
> > the phase of the signals being picked up by the vertical feeds so that
> they
> > differ by 180 degrees.(in some direction)
>
> I'm not sure how important the ends are anyway. The amount of signal they
> contribute is very tiny, and the whole Beverage responds to vertical
signals
> anyway.
>
> I do have one suggestion......
>
> I sweep the Beverage feedpoint over the desired operating range with an
> antenna analyzer. I have the antenna somewhat misterminated (usually a
> thousand ohms or so), and use the peaks and nulls of impedance to
determine
> the electrical length. The frequency difference between nulls is the
> frequency where the antenna is 1/2 wl long! You can measure the true
> electrical length that way using the 492/F=feet formula. If you saw an
> impedance dip at 1.8 and another at 2.4, the antenna's electrical length
> would be 2.4-1.8=.6 for 492/.6 or 820 feet.
>
> I added the same method in the MFJ antenna analyzer firmware. They read it
> out directly in the "distance to fault" mode.
>
> If you like the cone of silence lengths (like John I don't find them
> especially useful) then you can get it exact. This method is great for
> finding the actual electrical length.
>
> 73 Tom
>
I believe the pick-up in the vertical feeds is understated rather than
overstated. This summer I experimented with a 580 foot beverage. The signals
off the side were only down -15 db. I went out and cut the feed away from
the beverage and the vertical feed was by itself only down -18db. Figuring
that the beverage was "top loading" the vertical feed and making it 3db more
efficient (like an inverted L) I put a T of 1/2 wave wires at right angles
to the beverage and put a 450 ohm resister between the top of the vertical
feed and the T. Presto, the signal from the vertical feed went up 3db just
like it was with the beverage connected. I then terminated both ends of the
beverage against elevated "T's and presto the pattern was 23db off the sides
and agreed with the computer model instead of bottoming out 15-16 db as it
did before. In all of these case I kept a duplicate of the original antenna
in service in four directions so I could listen on the band and see if I
could hear the difference. I could hear the pattern difference both daytime
and night. I am convinced that most of our beverages are "beverages off the
front" and "inverted Ls" in other directions. By the way, I did not go the
raised feed .... having 270 feet of wire at the end of each beverage is
unworkable. The improved pattern did not seem to help the antenna hear
better. The background noise seems to come primarily from the front lobe.
John K9DX
John K9DX
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