In article EBJ5j.5593$gi7.4456@trnddc04,
"Dale Parfitt" wrote:
"Telamon" wrote in message
...
In article KCy5j.2781$md.534@trnddc06,
"Dale Parfitt" wrote:
Yes, for shortwave a 10-20 foot piece of wire can be used. You
do not need to spend a lot of money to get started. If noise is
an issue, google for 'shielded loop antenna'. You can also buy
more expensive antennas. Try,
http://www.universal-radio.com/catalog/sw_ant.html
Why would a shielded loop receive less noise than, say, a classic
wire turn loop?
Shielded loops are less sensitive to local inductive E fields.
I thought there was enough math/data out there that no one any longer
believed this.
From the W8JI page- http://www.w8ji.com/magnetic_receiving_loops.htm
"Folklore claims a small "shielded" loop antenna behaves like a
sieve, sorting "good magnetic signals" from "bad electrical noise".
Nothing is further from the truth! At relatively small distances a
small magnetic loop is more sensitive to electric fields than a small
electric field probe.
I would not be quoting some hamıs web site as an authority on
electromagnetic theory or folklore.
There is a lot of bad information on the net. The areas of electronics
and electromagnetism are easy to misunderstand. Part of the problem is
that it is also easy to leave questions more open ended than intended
and so the answers tend to be ambiguous.
So now, I'll be more specific. Maybe it will help.
Small loops that have a shield that is split half way around are only
sensitive to magnetic fields. The split half way around ensures the E
field is canceled for far field and it works pretty well for near
inductive fields. I have used this type of probe along with small E
field probes in EMI and RFI work so I know they operate as they were
designed or I would not have been able to solve problems.
Another situation occurs when the shield is not split evenly around the
loop and grounded on one end. Here the outer shield picks up the E
field and the shielded wire picks up the M field and this configuration
will generate about two times the signal the center split shielded loop
will generate.
So it depends on a number of parameters not least of which is physical
size and electrical size for the application.
--
Telamon
Ventura, California