Hello Jürgen,
ground losses (or better environmental losses) depend on a number of things:
- frequency : losses are increasing at lower frequencies (so measuring DC
resistance is not very meaningfull)
- soil conductivity : the better the soil conductivity the lower the losses
- objects near the antenna: objects, such as buildings and trees, can increase
the losses (in addition they can also decrease the effective height of the
antenna).
- seazon: losses can depend on seazon and temperature
- ground system : such as rods and radials can decrease the ground loss. It
depends on many things wether rods or radials or a combination of both perform
best.
To give an idea:
at my antenna location there are many 100's trees very close to the antenna (so
far from perfect).
The antenna is a "lazy-L", 11m high and 25m long.
The ground system in a combination of rods and radials: one rod at the antenna
base and from there 6 buried radials (15 to 30m long) with rods at the end of
each radial. All rods are 2.5m steel pipes (15mm diameter).
On 137kHz the loss resistance is 120-140 Ohm in winter (temperature dependent)
and up to 170 Ohm in summer.
On 502kHz the loss resistance is 30-45 Ohm in winter (again temperature
dependent) and up to 70 Ohm in summer.
Disconnecting the radial system makes the loss increase by about 20 Ohm (on
both bands). But as the radials and buried I cannot remove them, so I guess
that even disconnected they play a role.
73, Rik ON7YD - OR7T
________________________________________
Van: [email protected] [[email protected]]
namens Jurgen Bartels [[email protected]]
Verzonden: donderdag 10 februari 2011 9:23
Aan: [email protected]
Onderwerp: LF: how to get ground rod ressitance low
I just discovered that my ground rod isn't ideal by having the assumed 0 ohm to
earth. Measuring with 10VDC the current from neutral/waterpipe to the rod gives
about 140 ohms.
I use a 8mm rod 1m long in wet soil. I put in another one and tried several
places nearby and got it a bit lower, but far away from ideal.
So I assume the surface size is too small, one the other hand I want a small
rod that can be pulled out with ease in spring when the season is over.
What resistance do you get usually and what size do you use?
Stefan, you wrote yesterday that you got 65 ohms with your rod on sunday.
That's still pretty high
Jurgen Bartels Suellwarden, N. Germany
hor. Antennas: 11-ele 45-87MHz 13-ele Band-III, 48-ele UHF, FM: 15.11
TV: Winradio G305 / Fly2000 + video noise filter & variable IF BW
FM: Downconverter + Perseus + Speclab as WFM demod.
MW: staggered 300m Beverage 320°, 30 x 4m EWE 320°, Winradio & Perseus
http://dx.3sdesign.de/tv_offset_list.htm
http://dx.3sdesign.de/station_list.htm
http://fewo.3sdesign.de - Vacation home: DX right at the Northsea coast
|
