On Sun, 9 Jan 2005, J. Allen wrote:
The antenna is in a flat mountain valley far from any trees, but it has a
long low house running from about 7 meters to about 27 meters from the base
of the vertical tower. Other buildings and wiring are rare and farther
away. The site is very "open" other than the one close house. There are no
other towers or antennas on the site.
It is exelent condition! Seems in such a condition it is posible to get
effectivity of few (2-5) percent! If use large and optimal loading coil
with good Litz wire and good elevated conterpoise system.
Indeed capacitance of Your antenna is of order 1500 pF. Then reactance of
loading coil is of order 700 Ohm. Loading coil of 1 m diameter with good
'fat' Litz wire can give Q-factor of order 1000 or may be 2000 (RU6LA
wrote me his Q-factor of such a coil was of order 2000). Then coil active
resistance is about 0.5 Ohm only! If there is no ground loss then
effectivity is 0.2/(0.5+0.2)= 0.29 or 29 percent!!! Certanly in this case
main quation is ground loss. Seems if build good conerpoise system it is
posible achive 1 Ohm of ground loss. It means 0.2/(0.2+0.5+1)=0.12 or 12
percent of effectivity. It is not easy of cose! May be one
need to 'screen' the nearest building by few wires, connected to
conerpoise system. Also ineresting quation is should or not be conterpoise
system connected to the ground. I do not know definitely. But though it
should be isolated from the ground. Then one need use transformer betwin
TX and antenna.
Any way with such a rare condition You can do very interesting and usefull
experiments! Try to do some experiment with conterpoise system trying
achive as less input resistance (then You get maximum of current) as
posible. Worth quation is soil type of cose...
I understand that the effect of the environment is impossible to calculate,
but is there a general direction that the environment moves the
resistance... does it normally move the resistance up from the calculated
values or its it sometimes up and sometimes down...
I can not imagine enviroment moves resistance down. Certanly I meaan not
metalic enviroment. Up only, this is loss. More loss - more resistance.
What is the experience
that most LF operators have found?
Usualy few tens of Ohms. But this is for bad enviroment. For Your
good (and rare) conditions I hope it will be less then 10 Ohm. May be (?)
not significant at all.
Ground resistance can be calculated by NEC-2 (eznec-2). But ONLY for
elevated conterpoise and one SHOULD use Sommerfeld-Norton method for
ground description (there is no such a method in elnec).
I am only curious about this.... My intention is to measure the installed
antenna with its upper loading coils in place, and then add the amount of
fixed and variable base loading necessary to make the antenna resonant. I
understand that this is the normal method... Is this correct?
Shure!
73 de RA9MB/Alex
http://www.qsl.net/ra9mb
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