Dave
Your comments have been very helpful, THANKS!
I must admit that part of my reason for asking this question was to get some
discussion going on this under-utilized reflector. While I am certainly
intensely curious about the subject, I will build the biggest counterpoise
practical under my new lowband antenna system no matter what the answer!
Back in 1978-1985, I worked in broadcast consulting and designed, tuned and
measured a bunch of AM non-directional and directional stations. I've seen
first-hand how the ground wave diminishes with reduced conductivity. I've
made field measurements in the U.P. of Michigan where the conductivity is
lower than the lowest curve on the FCC charts (0.5 mS/m). Up there, signals
die off astonishingly fast with distance! Now I need to go back to the
references you noted and review their techniques and results for skywave
performance.
The question I have is similar to the elevated radials situation. There, a
layer of air with conductivity of nearly zero and dielectric constant of 1
appears to lower the losses in the radial return currents, therby improving
the efficiency of the counterpoise system and requiring fewer radials.
Granite has a conductivity of perhaps 0.01 mS/m and a dielectric constant in
the 6 to 8 range, and at my QTH is overlaid with a thin layer of clay that
is maybe 2 mS/m conductivity and dielectric constant of 3 or so. Do these
low numbers have a similar effect to the air space under elevated radials in
reducing localized losses (not groundwave propagation losses). The answer to
this would determine whether a smaller counterpoise would perform
adequately, as with elevated radials.
73, Gary
K9AY
-------------------
>
>Gary,
The best reference material available indicates that both conductivity and
dielectric constant are very key factors in the performance of a vertical
raidator and the associated ground system, either burried, on the ground, or
elevated. This original work was published by K.A. Norton in 1936 (The
Propagation of Radio Waves Over The Surface of the Earth and in the Upper
Atmosphere, Proc. of the IRE, Vol. 24, No. 10, October 1936, and part 2,
Vol. 25, No. 9, September 1937). This work has provided the fundamental
basis for the FCC's engineering practices for AM broadcast antenna systems,
and much of the material appears in Part 73 of the FCC's regs., and may aid
you (avalible on-line). You may be able to obtain a copy of the paper
through inter-library loan (ask them to contact the archives at Rensselaer
at Hartford (formerly The Hartford Graduate Center), Hartford, CT. This
work also provides an excellent explanation of Sommerfield's (1909) integral
and his concept of the effects of ground conductivity upon radiation from a
grounded vertical radiator. It is today known as the Sommerfield-Norton
ground and found in most higher capability implementations of NEC modeling
software.
In general summary of that paper with applicaton to your question, both
conductivity and dielectric constant in the antenna region (near field) have
a very pronounced effect upon field strength in the outter region (far
field). The effects upon FS are inversely dependent upon frequency at any
specific distance outside the near field, and are quasi-logrithmic in
nature. However, there IS NOT a point at which poorer ground C and DC
constants cause losses and overall system performance to reverse and become
better. As C and DC continue to degrade and approaching infinitely poorer
characterstics, the far field FS decreases quasi-logrithmicaly.
BTW: the data and principals discussed in the paper were based upon acutal
measurements.
I hope this helps to answer your question.
73, Dave, K1FK
Fort Kent, ME
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