Antennas guilty of RFI?
 

I believe antenna feedlines can be a source of rfi.The radiation from an
antenna should be much greater than any feeder radiation_why the emphasis on
feeder radiation? 73 de ZL2DG

Antennas guilty of RFI?
 

"ZL2DG Dave" wrote in
:

I believe antenna feedlines can be a source of rfi.The radiation from
an antenna should be much greater than any feeder radiation_why the
emphasis on feeder radiation? 73 de ZL2DG

Dave,

"should" being the operative word.

It isn't always the case that feedline radiation is insignificant. Keep in
mind that the transmitter feedline might also be much closer to the
affected device, its power line and its feedline.

It is also fair to say that there is a whole family of ham myths that
revolve around feedlines being the cause of RFI, TVI, generation of
harmonics, causual links between open wire feedline and interference, etc.

Owen

Antennas guilty of RFI?
 

"ZL2DG Dave" wrote in
:

I believe antenna feedlines can be a source of rfi.The radiation from
an antenna should be much greater than any feeder radiation_why the
emphasis on feeder radiation? 73 de ZL2DG

Because feeders often pass near consumer electronics.

If the feeder has no antenna currents on it (like my feeder years ago when
I had a vertical 250 feet out from the house), then the antenna, while
radiating much more than the feedline is far enough away that the inverse
square law protects the consumer electronics.

Of course this depends on antenna placement.



--
Dave Oldridge+
ICQ 1800667

Antennas guilty of RFI?
 

Dave Oldridge wrote:
"ZL2DG Dave" wrote in
:

I believe antenna feedlines can be a source of rfi.The radiation from
an antenna should be much greater than any feeder radiation_why the
emphasis on feeder radiation? 73 de ZL2DG

Because feeders often pass near consumer electronics.

If the feeder has no antenna currents on it (like my feeder years ago when
I had a vertical 250 feet out from the house), then the antenna, while
radiating much more than the feedline is far enough away that the inverse
square law protects the consumer electronics.

Of course this depends on antenna placement.

The improvement noted by increasing the distance between the radiating
conductor and the consumer electronics (or power or other wiring they're
connected to) can be much greater than an "inverse square law" would imply.

That's because a lot of the problems with interference occur in the near
field of the radiating conductor. In the far field, the field strength
is inversely proportional to the distance (the power density follows the
"inverse square law"). But in some parts of the near field, the field
strength varies as the inverse *cube* of the distance. So even a small
increase in distance can often have quite a dramatic effect on
interference level.

Roy Lewallen, W7EL

Antennas guilty of RFI?
 

Roy Lewallen wrote in
:

....
The improvement noted by increasing the distance between the radiating
conductor and the consumer electronics (or power or other wiring
they're connected to) can be much greater than an "inverse square law"
would imply.

That's because a lot of the problems with interference occur in the
near field of the radiating conductor. In the far field, the field
strength is inversely proportional to the distance (the power density
follows the "inverse square law"). But in some parts of the near
field, the field strength varies as the inverse *cube* of the
distance. So even a small increase in distance can often have quite a
dramatic effect on interference level.

Roy,

The NTIA's long awaited second report on BPL has some interesting results
of NEC simulations to support the Part 15 distance extrapolation factor
of 40dB/decade on slant distance below 30MHz. NTIA have models at a range
of frequencies that explore the validity of the above factor, part II
which contains the graphs of the simulations makes an interesting read.

It seems to me that the issues to do with field strength vs distance are
quite applicable to a ham antenna as the radiator.

The report is at
http://www.ntia.doc.gov/osmhome/repo...7/bpl2007.html .

Owen