See interspersed.
73, Guy
-----------------
----- Original Message -----
From: "Tom Rauch" <w8ji@akorn.net>
Subject: Re: [TowerTalk] vertical antennas
>
> > The biggest advantage of a buried dense radial system over an
elevated
> > one, is that it's buried and you can grow grass on top of it. Up in
> > the air gets ice, falls down, and you can see it, etc, etc.
>
> Other problems are with small elevated systems, even at great
> height, you need to choke RF off the OUTSIDE of the feedline or
> efficiency suffers.
Same problem as with a triband beam. Though if the elevation is 8 feet
for a shortened 160 meter vertical, not sure that is % wise enough to
worry about. If it's for 80 and above, or a multibander, then use one of
the choke bead "baluns". Actually I have used those isolators from Radio
Works on all my vertical systems.
> You can NOT have a path to earth for RF, and that means you
> can't have a direct connection to earth for lightning. Not a good
idea.
Actually you can ground the shield of the coax to the ground screen at
the bottom of the support, the same as with a triband beam and grounding
the coax shield at the base of the tower. With a shunt fed tower and
elevated radials, there's no grounding problem.
> Also, no matter how well you "balance" currents, the radials will
> radiate quite strongly in the area around the radials. That means
> interaction and unwanted coupling.
Yes, for sure, but better than using a pitiful handful of buried
radials. Or trying to explain to the wife why the four year old has an
RF burn on his hands. In this thread, the original post's vertical would
probably be a quarter-wave from the house at least. The use of the
full-size ground screen tends to gain separation from the interactive
possibilities. At a quarter wave distance the cancellation is mostly
formed.
> Small resonant systems are also single band systems, and they
> even narrow the antenna bandwidth on the desired band!
Yea, full size on 160 if you can, and narrow-banding, tuners, etc, if
you can't. Something to be said for that 160 1/4 wave inv el over a
dense ground screen.
> All the above, even if we ignore the reduction in efficiency and
> mess of wire in the air, is enough that I won't use them!
And we all know how small your plot of ground is. REALLY confined for
space down there.... The only thing you are missing is your own private
20 square mile inland salt-water marsh.
> > In the ground screen you suggest, you need to pay attention to
> > "dense". Depending on who you talk to, dense starts somewhere
between
> > 60 and 120 radials.
>
> RCA spent a fortune on studying this, and they concluded 50-60
> 1/4 wl radials acted like a solid plate. The key is to not let the
open
> spaces exceed about .025 wl between wires. That even works with
> a screen reflector antenna!
>
> The 120 radial wires comes from the FCC's recommended ground
> of 120 1/2 wl radials for BC stations, where they want a LOT of
> headroom on the conservative side.
Hmm... If you talk to JI, you get 60, and if you talk to the FCC you get
120?
There was one other advantage to 120, that over time some of them may go
bad, for any number of reasons. Even if you lose half of them, you still
have 60 and still are dense according to JI, unless they all go bad in
the same place.
>
> 73, Tom W8JI
> W8JI@contesting.com
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