Heres one for all you radial efficiandos. I am involved with a site (workplace
club
station) where the antennas sit near the edge of a 400 foot mesa. The mesa
drops
off in the direction of our longpath heading to Europe suggesting that the site
should
be favorable for this path. On the air experience on 80 meters suggests that we
do
"okay" compared to other well equipped stations in the area, but my gut feeling
is
that there is significant room for improvement. The 80 meter transmit antenna
is a full
wave delta loop hung off a 60 foot tower and fed at the bottom corner. The
bottom leg
of the delta loop is only about 8 to 10 feet above ground. The antenna is
probably
about 100' from edge of the mesa (the terrain drops at > 45 deg beyond this
point). On
160 meters, we are using a inverted vee which is on another tower (telephone
pole with
metal mast extension). The apex is about 50 feet above the ground with the
antenna
broadside to the longpath heading. This tower is even closer to the edge of the
mesa
with the base being roughly 50' from the "cliff".
I have been trying to lobby my colleague who built all this stuff that we would
probably do better on transmit using a vertical with an extensive ground
system, but
because of the logistic difficulties involved with doing this, he has been
somewhat
resistant to my suggestions arguing instead that we do pretty well already and
that
the 80 meter delta loop is fed at a point that is optimum for long angle
radiation.
My argument is that delta loop is probably inefficient since at least one of
the current
maxima's is close to the ground and there are no radials under the antenna. For
160
meters, I am wondering what is the effective height of the inverted vee
antenna? From
a far field perspective its probably 400 to 500 feet above the surrounding
terrain (at
least to the south), but in terms of near field coupling, its very close to the
ground
immediately underneath it, so I would expect a lot of induced current that
would
decrease efficiency, and also possibly negatively influence the low angle
performance
of the antena towards the south (I would expect the induced ground current to
be
out-of-phase with the antenna current).
Anyone have any suggestions. Would it be worth the trouble to switch over to
verticals and start adding ground radials?
Thanks,
Mike, W4EF.........................
----- Original Message -----
From: "Maurizio Panicara" <i4jmy@iol.it>
To: "Bill Coleman" <aa4lr@arrl.net>; <W8JI@contesting.com>;
<towertalk@contesting.com>; <K3BU@aol.com>
Sent: Thursday, January 11, 2001 10:53 AM
Subject: Re: [TowerTalk] Re: Radials over salt water
> Space between radials has to be equal or less than .025 WL.
> The tables consider .5WL radials and this numbers lead to the famous 120
> radials (>90% efficiency).
> Unless the radiator is longer than 1/4 WL there's no need to extend radials
> to .4-.5 WL.
> Halving the radials lenght and keeping the same .025 distance, the required
> number for .025 spacing is 60 .
> In theory and in case of a full size quarter wave radiator, with 30 radials
> the efficiency drop from 90% to 86% that's an attenuation of 0.25 dB if
> compared with the 60 radials case.
>
> 73,
> Mauri I4JMY
>
> ----- Original Message -----
> From: "Bill Coleman" <aa4lr@arrl.net>
> To: <W8JI@contesting.com>; <towertalk@contesting.com>; <K3BU@aol.com>
> Sent: Thursday, January 11, 2001 4:39 PM
> Subject: Re: [TowerTalk] Re: Radials over salt water
>
>
> > On 1/10/01 15:35, Tom Rauch at w8ji@contesting.com wrote:
> >
> > >> Thinking and analyzing the situation, there should
> > >> be improvement if using many (>60) elevated (or on ground) radials vs.
> few
> > >
> > >Fact Yuri. When the radials are less than .025 to .05 wl apart at
> > >the open ends, they look like a solid screen. Using more radials
> > >than that is a waste of wire.
> >
> > Hmm. 1/4 wave radials means a circle of 1/2 diameter. That's a perimeter
> > of pi/2 wavelength, which divided by .05 yeilds about 31 radials.
> > Similarly, .025 spacing at the ends is about 63 radials.
> >
> > If >63 radials is like a solid screen, I wonder why there's so much in
> > the amatuer literature about installations with 120 or so radials.
> >
> > Bill Coleman, AA4LR, PP-ASEL Mail: aa4lr@arrl.net
> > Quote: "Not within a thousand years will man ever fly!"
> > -- Wilbur Wright, 1901
> >
> >
> > --
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> > Submissions: towertalk@contesting.com
> > Administrative requests: towertalk-REQUEST@contesting.com
> > Problems: owner-towertalk@contesting.com
> >
>
>
> --
> FAQ on WWW: http://www.contesting.com/FAQ/towertalk
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>
>
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