On 10/28/16 10:43 AM, Gary Schafer wrote:
Hi Ian,
I am wondering why even bother with mesh wire. Radials provide a return
current path back to the feed point so current should mainly flow along the
length of the radial wires. Having shorts between the radial wires as a mesh
provides, would seem of little benefit. Any current flowing between radials
via the shorts would seem to result in loss current.
A ground screen also provides a "shield antenna fields from the lossy
soil" function as well as a "provide a return path for half a dipole
sticking up"
That is, you could run a vertical dipole above a screening layer laying
on the surface, with no connection between dipole and ground screen,
with the idea that you'll have less ohmic and dielectric loss because
whatever the losses in the screen are, they're less than they'd be in
the soil.
As to using any kind of steel material for radials the loss due to eddy
currents is going to be rather high, the same problem that steel antenna
wire presents.
But is the eddy current loss all that high.. compared to eddy current
losses in soil? Are you talking about what would be called "core loss"
in a transformer which is a combination of eddy current and hysteresis?
Skin depth at 4MHz in Iron (assuming mu=1000)is about 0.1 mil (2.5
micron), so the current is going to be flowing in the surface. (soft
iron has a mu around 4000, cold rolled steel seems to be in the
1000-1500 range, depending on the alloy, heat treatment, etc.)
I think hysteresis loss would be negligible - the flux is really really low.
https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19660001049.pdf
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