Gentlemen,
I have had some success in reducing the level of "unwanted" signals heard in
my receiver by isolating my antenna from its feedline using common mode
chokes. One method of propagation of the signals is up the feedline from my
house wiring to the antenna.
You should be able to find an article on the net by W1HIS discussing this in
more detail. The article is about Common Mode Chokes.
Robie - AJ4F
----- Original Message -----
From: "Alan Biocca" <[EMAIL PROTECTED]>
To: "Elecraft List" <elecraft@mailman.qth.net>
Sent: Wednesday, August 15, 2007 9:02 AM
Subject: Re: [Elecraft] 14.03 MHz Continuous Tone/[EMAIL PROTECTED]
My experience with network gear is that different designs use different
frequencies for the oscillators and such, so it is likely that both of you
are correct! Newer gear tends to have higher frequencies and lower power
in the general trend, so changing or upgrading gear may help the level of
interference. Interestingly, lower priced gear generally uses higher
levels of in-chip integration and has less effective antenna area. Of
course really low cost gear may skip some filtering, so testing is really
the only way to tell. Perhaps we should collect info on which switches
seem to be better or worse and see if there is a correlation between
different people's experience and maybe provide some useful info.
If the birdies were net traffic related they would vary over time, and
probably would not be referred to as 'birdies' which implies a CW note.
The thing we are most likely to hear is the oscillators feeding the
chips - elements with higher power and more radiative capability over
multiple traces on the pcb, etc. The chips that do the encode/decode
generally develop the different frequencies for different network speeds
internally, so those signals would be lower power and have negligible
antennas and not tend to be radiated. 10 and 100 megabit twisted pair
network signals are transformer coupled, and with the FCC certification
requirements of this gear those transformers are likely designed to help
reduce EMI at HF, I don't know the precise characteristics, but common
mode EMI is something they are designing to avoid. I have not looked at
the details of Gig hardware interfacing yet, but the requirements of
network ground independence generally demand isolated connections, so they
are likely transformer coupled as well, and are also designed to reduce
EMI to meet specs.
None of the noises I hear seem to be net traffic related. Those would tend
to be very brief most of the time.
The design of these network system components varies with their bitrate,
manufacturer, model and age. Higher bitrate capable devices tend to have
different basic oscillators. In general the higher speed devices have
higher frequency oscillators which are often above HF.
Each link has multiple speed capabilities in modern switches, independent
of the other links.
As Leigh hints, Many stations have inadequately balanced feedlines
(whether coax or balanced lines), so noise and carriers in the shack
conduct out to the antenna and come back into the receiver rather easily.
An effective balun can help here, and ferrites on the lines coming out of
the switch.
Distance to the antenna and radio all matter to some degree, as both
radiated and conducted RFI are attenuated by distance. Distance to the
radio should not matter, but many stations use inadequate baluns and so
are quite susceptible to common mode noise and EMI at the radio. (Note
that no balun at all is also an inadequate balun). Jim's paper, link
below, has a lot of good information in it, and agrees that most baluns
are inadequate (less than 5,000 ohms choking Z).
.
In my shack these birdies are all extremely weak and readily covered by
minor band noise. Two switches and the cable modem and cable entry and
several computers plus the wifi are all within a six foot sphere including
my HF station, and the antenna is 30 feet away fed by 6 feet of coax to a
substantial balun to 30 feet of balanced line to the lower leg of the
inverted L fed against radials. Just a data point, each station will be
different. Network gear here is Linksys and Netgear both of 100 megabit
fairly recent vintage. Cable modem is probably the only 10 megabit device,
though perhaps the networked printer is also 10, also within this six foot
sphere.
As Leigh mentions, the best way to force things to a certain speed is to
use a 10 megabit hub. However this does force to half duplex which is a
pretty low level of performance. Adequate for your internet connection but
it will take a long time to do your backups or transfer large files over
it. Also note that hubs that do 100 megabits often have a funny mode where
different ports run at different speeds and the hub speed shifts, so you
will have a real mix, and of course switches negotiate each port
independently. The net speed is 10 megabits when these hubs have at least
one 10 megabit device connected, and the 100 megabit devices are forced to
wait 90% of the time so the hub can translate their data to the lower
speed.
Another technique is to put things on wifi and reduce or eliminate the
wired network. 2.4 GHZ doesn't generally bother HF.
73,
-- Alan, wb6zqz
At 11:52 PM 8/14/2007, Leigh L. Klotz, Jr. wrote:
Jim,
I know you and I have a different understanding, that he birdies come from
100Mb (my belief) and that they come from 10Mb (yours), but I would like
to straight out a couple of issues, perhaps minor ones.
1. I didn't write the second quoted paragraph below about 6' distance from
the radio; someone else did, even though your message has my name above
it. You are of course correct on this point, but I didn't make the point.
(I suspect your RFI-Ham PDF file does say, though, that if you have not
taken care, your rig's coax and other surrounding objects may be part of
the antenna and so noise from something in proximity to the rig may be an
indicator of other problems.)
2. Regardless of whether the birdies come from 100BaseT or 10BaseT, if you
force your equipment all to the same, your switch will be forced to use
the same. And better yet, if you use a hub, forcing one jacked piece of
equipment to 10MB (or 100MB in your view, doesn't matter for this case) is
enough to force all to the same.
I've asked a number of ethernet luminaries to explain to me the source of
the birdies, and gotten blanks.
Leigh/WA5ZNU
Jim Brown wrote:
On Tue, 14 Aug 2007 15:25:06 -0700, Leigh L Klotz, Jr. wrote:
Also 21.060. It is from 100 megabit ethernwt devices. Switch your
shack to 10 megabit ethernet and it will be only your neighbors QRP
transmitters with end-fed long wire antennas (CAT5 wiring) that you
hear.
Not that easy. If your 100MBit system is connected to a 10 MBit device
(like most Internet modems), it will carry that traffic as 10 MBit
traffic, and you'll hear the birdies.
See my RFI tutorial for more details and fixes.
http://audiosystemsgroup.com/RFI-Ham.pdf
I have two 100 megabit switches within six feet of the radio, and can
barely hear the spur on 14.030, so experience varies.
No, experience doesn't vary, it has NOTHING to do with proximity to your
radio. What matters is proxmity to your ANTENNA, the degree to which that
RF trash is suppressed by the router, and the ANTENNAS connected to the
router (the Ethernet cables and the power cable)!
73,
Jim Brown K9YC
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