On 1/14/2015 7:07 PM, Jim Brown wrote:
On Wed,1/14/2015 12:28 PM, Roger (K8RI) wrote:
It appears from what you have said that there may be Knob and Tube
connected to regular wiring. The biggest problem with Knob and Tube
is one of polarization.
Probably a much larger issue with RFI is the wide spacing between the
conductors. This increases the area of the magnetic loop, and
magnetically induced current is proportional to the loop area. In the
near field of antenna, the magnetic field is dominant, so magnetic
coupling can be a very big deal.
Agreed.
I was referring to the AC voltage at the equipment and computers. With
a possible polarization issue between computers, plus the CAT5, there is
not only the issue of the wide spacing, but that entire run including
the equipment and parallel line in the near field (really close) and
possible polarization issues could act as an antenna making for some
really high voltages. It's impossible to know from here the phase
relationships, but the safe rout would be to assume a worst case where
they all add.
Still, it doesn't account for the intermittent failures. I'm beginning
to think it's either a polarization issue, or a combination exacerbated
by some as yet unknown combination of loads that could be elsewhere in
the house.
I wounder if there is any commonality in the Weather, time of year, day
of the week, or time of day for the failures?
It takes very little voltage to wipe out the logic chips and the control
chips use smaller components, more densely packed, although that doesn't
necessarily mean they are more susceptible to failure from RF.
Was there a standard for the spacing on Knob and Tube? "It seems" like
I've seen spacings from 2" up to about 6", using insulators where the
nail through the cap put pressure on the cap to hold the wire.. but I
hasten to add that was over 60 years ago and a 14 year old kid has
things other than house wiring on his mind. While 60 years does fog the
memory a tad.
73
Roger (K8RI)
At my Chicago QTH, I fed my 80/40 dipole with some old 75 ohm KW
twinlead, but I also tied the two sides of the feedline together to
feed it as a top-loaded quasi-vertical on 80 and 160M. This put a
current peak in the shack, and the magnetic coupling to the parallel
wire RS232 cable for the K2 caused the CW generation from the logging
program to lock up with as little as 15W. I replaced that parallel
wire serial cable with unshielded CAT5, using one pair for each
signally circuit, and with the returns going to the DB9 shell on both
ends. This allowed me to run my Titan at full power.
As an experiment, I loaded that antenna to full power all the way up
to 10M, with no RFI issues until I got to 15M. At 15M and above, I
needed shielded twisted pair. (I never USED that long wire on the
higher bands -- I was simply doing this to learn the conditions under
which un-shielded cable was good enough. In that shack, the cable from
the computer, a Thinkpad, ran within several feet of the antenna.
73, Jim K9YC
_______________________________________________
RFI mailing list
RFI@contesting.com
http://lists.contesting.com/mailman/listinfo/rfi
---
This email has been checked for viruses by Avast antivirus software.
http://www.avast.com
_______________________________________________
RFI mailing list
RFI@contesting.com
http://lists.contesting.com/mailman/listinfo/rfi
|