On 24 feb, 15:15, "Francesco L." wrote:
Hi Francesco,

On the ground, or elevated?

Both cases please. I did a reasearch on arrl antenna book, low band dxing
and newsgroups and got a bit confused, so I need more tips. There are too
many options, I mean: at a certain height above the ground, atop the roof,
on the ground and so on, so I'm trying to collect as much info as possible
in order to get the overall picture.
Thanks

Hello Francesco,

It depends heavily on the antenna. When your antenna is an
electrically half wave radiator, the input impedance is in the kOhm
range and "ground" becomes less important. Many CB 27 MHz antennas are
half wave antennas (without any radials). For other lengths, "ground"
is important.

The smaller your antenna the lower the radiation resistance (=higher
feed current), and the more important the "ground" system.

For fertile wet ground and buried radials, the 0.25lambda is no longer
a magic number as the attenuation is very high. For dry Rock/sand,
dielectric properties dictate and some standing waves appear in the
ground conductor. As long as the real part of the ground impedance
is less then the real part of the input impedance of the radiator, it
is OK.

If possible, I prefer elevated radials. In many cases, they can be
shorter than 0.25 lambda (add more of them). The disadvantage is that
your floating ground becomes capacitive and you get a common mode
voltage on the cable screen, so you should add a common mode choke.
When you make them very short, the electrical situation is upside
down: your radiator functions as ground and your floating radial
network is the antenna.

An advantage of floating radials and the high end of HF is that your
radiation center is higher and ground properties become less
important, so you can have less loss (also in nearby structures) and
less noise from electrical equipment.

To give a more precise answer, one need to know your local conditions,
structural limitations, frequency and antenna type to be used, etc.
For low frequency, "Ground systems as a factor in antenna
systems" (Brown, Lewis Epstein, 1937) maybe interesting for you in
case of buried radials.

I know this doesn't answer your question, but I hope it will help you
a bit.

Best regards,


Wim
PA3DJS
www.tetech.nl (Dutch).