> That Fact of physics is called RF Depth of current
> penertration or skin
> depth. What this basically means is that as you sink RF
> current deeper into
> any material the loss increases. There is a transitional
> zone where the loss
> is considered acceptable equating to 37% of RF current
> loss this is the skin
> Depth. The SKin depth of Salt water is fairly uniform and
> is about 7 inches
> at 1.8 mhz and 2 inches and 30 mhz. Once you go below 7 or
> 2 inches its like
> installing your ground system under a copper plate the
> losses are so great
> and none of your RF current will return to the antenna
> feedpoint to be
> radiated. Its that simple.
It's actually like you are sticking the wire below a few
feet into "nothing". Current cannot follow the wire down
into the saltwater. If you want to see how a copper sheet
behaves look at his link:
http://www.w8ji.com/skindepth.htm
Saltwater would be the same, it just has significantly
deeper skindepth because it is much more resistive than
copper. People like to think saltwater is lossless, but
every company that has made saltwater jumper cables for cars
has not won over very many consumers.
Power loss is directly related to current density in a given
volume of material. If we have a very wide surface area with
current spread all around uniformly the losses can be very
low. If the current is concentrated in one area of a media
then the loss can be pretty high. We can bounce a signal off
the moon even though it is a horrid conductor because the
current density is low when the entire surface is
illuminated with RF. Saltwater has very low loss compared to
soil for distant sources that illuminate the saltwater
because the current density is low and the salt water is
much less resistive than dirt, but it is still very poor and
lossy for direct connection.
Without speading the field and current around to keep field
and current density low, the connection would be lossy. As a
matter of fact saltwater can make a very good dummy load.
It's actually a common use for saltwater in the lab. I've
used saltwater loads (with much higher salt concentration
than the ocean) to terminate loop antennas when doing FDA
measurements on prototype medical equipment.
The key is to spread the "connection point" out over a wide
uniform area. Not to drop a wire or two in the water where
you have a two foot long connection that gives you a couple
square inches of connection point spreading at best.
A small elevated screen or multiple spread out radials above
or even into the water would probably be best. But then
there is the SWR change with tides.
73 Tom
_______________________________________________
Topband mailing list
Topband@contesting.com
http://lists.contesting.com/mailman/listinfo/topband
|