Martin:
I have a few thoughts on your EMI problem.
First, keep in mind that an antenna produces three different kinds of
fields, the induction field, the near field and the far field. The
induction field vanishes within inches of the antenna. The near field, the
cause of many emi problems, has non-trivial strength out to about 1/6 of a
wavelength. The far field is the one that "gets out," and behaves like a
nice predictable travelling wave. The near field turns out to be extremely
sensitive to boundary conditions, and simple acts like opening or closing a
door can change the boundary enough to change the pattern of the near
field. This is one reason for the seemingly "magical" properties of emi.
My reason for making this disclaimer is to make you appreciate the fact
that any of the very sound advice you get on the reflector may or may not
help you to overcome your particular emi problem. I would encourage you not
to give it up; your problem (or set of seemingly unrelated problems) can be
solved.
Second, you have not mentioned what bands give you the emi problem. You
should check for all the symptoms on all the bands. There is a good
possibility that part of your house wiring is perversely resonant in one of
the ham bands, and is acting like an antenna that captures and reradiates
your near field. Of course, if the resonance occurs on 80 meters, you might
also be resonant on all the harmonically related ham bands. Anyway, it is
worth checking which bands cause which bad effects.
Third, having no RF ground is a big nono. The problem is that the RF that
is on the chassis of your rig has to go somewhere. If you do not provide it
a low impedance path to ground (where RF is converted to heat) it will find
its own path, and the one that it finds will not make you happy. The power
system ground has very high inductive reactance (A straight wire has an
inductance of 10 nanohenries per inch. A 10 foot length of wire has an
inductive reactance of 26 ohms at 3.5 MHz in the absence of mitigating
capacitive effects. ) In other words, from your rig to the power system
ground rod the overall impedance is probably on the order of hundreds of
ohms, and virtually none of your RF energy is finding its way to the ground
rod. If you're lucky (in your case you're not) the energy will be turned
into heat in the losses of the wiring. Otherwise, it gets coupled into your
other equipment.
Since you are physically at ground level, I would strongly recommend that
you install an 8 foot copperweld ground rod as close as possible to the
rig, and feed through the shortest possible number 6 solid wire. (Solid has
lower inductance than stranded or braid.) This is not an expensive
solution, but it may solve your problem. Going from the shield of your
cable direct to the ground rod may solve your problem and it may not. The
emi problem may not be caused by radiation emanating from the outside of
your shield. To avoid the creation of ground loops, and new emi problems,
it is considered best practice to use a single point RF ground. Connect the
rf grounds of your auxiliary equipment to your antenna tuner ground if you
have one or to your transmitter ground otherwise. Make sure the ground
connections are done in a star configuration (no loops in the ground wire),
with short direct connections of solid wire (braid has higher inductance;
avoid it) with your rig or tuner ground being the center of the star. To
that single point at the center of the star connect the lead to the RF
ground. This is a compromise configuration that tries to balance many
conflicting tradeoffs, but it is the one that typically results in the
fewest emi problems.
The 8 foot copperweld ground rod is not really that good an electrode at
HF, but it is better than nothing and vastly better than what you have now.
In fact, there is no good ground electrode at HF. All of them have
non-trivial inductive reactance. The least electrically bad ground
electrode is a long solid copper wire (I prefer number 6; it has good
physical strength and is not too hard to handle) at least a half wavelength
at your lowest frequency and buried just deep enough that your lawn mower
does not hit it, but preferably less than an inch deep. Every 8 feet or so
along the wire you should connect the wire to a 12 inch ground rod. The
wire does not need to be in a straight line. It can meander all over the
yard, turning corners as necessary, but no corner should be tighter than 90
degrees. Obviously this is not practical to install during the winter.
Nevertheless, when you can install such a ground, it is worth the trouble
doing.
BTW, a cheap trick that often works in place of an RF ground is to use an
MFJ (or equivalent) artificial ground to a length of wire running along the
floor as an RF counterpoise. The idea is that the reactive elements in the
box tune out the reactance in the counterpoise and create the effect of a
low impedance RF ground. You can find artificial grounds pretty cheap on eBay.
You note that "these are all in different rooms of the house and all at
least 15 feet away from the rig." Since the emi is by your own observation
being coupled into the house wiring, distance from the rig is irrelevant.
If you're lucky, the losses in the power company's equipment where your
house connects to the system is attenuating the emi, and probably none of
the energy is going out on their transmission line and into other peoples
houses.
Personally, I would be looking for a systemic cause and cure rather than
cleaning up individual symptoms. You might clean up everything you have,
but the next new piece of gear that you bring into the house may cause you
a new set of problems.
Some comments on your proposed fixes:
"Make a coax choke balun in the feeder to reduce out of balance currents
(quick and cheap)
Fit a commercial balun (heavy and pricey)"
Unless you're feeding the dipole off center, or your transmission line is
severely off perpendicular to your antenna, or you have some very gross
asymmetry in the boundary conditions (like your house having a metal roof),
your configuration does not look like it is especially vulnerable to the
problem. Unless there is something causing you have RF on the outside of
your coax sheath, the baluns probably will not buy you anything.
"Construct an RF ground in the garden and connect to feeder sheath at entry
into the house. (time consuming)"
Time consuming, but probably worth it. However, as I already noted, you do
not appear to have a compelling reason to believe that you have a line
radiation problem. I expect that you'd be better off using the single point
connection scheme I described above.
"Fit a ferrite ring choke to the PA mains lead"
This is a cheap trick, but it might work. There are a couple of gotchas.
First, in the absence of an RF ground connection, you have no low impedance
path that takes the emi energy to a place where it is converted to heat.
You're simply redirecting a bit of bad energy, and the practical result may
be to trade one emi problem for another. However, you may find that you
need both the ferrite and the RF ground working in concert to solve the
problem. If by a ferrite ring you mean those little rectangular jobbies
that you get from MFJ or Radio Shack, be aware that they do not provide
much attenuation, about 3-5 dB per turn at HF. You need to go 7 times
through the core to get significant attenuation, maybe 7 turns through one
core, or a stack of 7 cores on a straight wire, or some other combination.
"Build an inductively coupled ATU to avoid connection of feeder sheath to
the mains ground."
You'd only avoid a DC connection. If you do not provide an RF ground, and
if you do have feedline radiation, the system will probably find some other
coupling mechanism to couple energy from the feedline to your house wiring.
"Fit ferrite ring chokes to the affected equipment"
Again, it might work. However, those rings are about four dollars each, and
you might need quite a few on each piece of equipment. The price could grow
in a hurry. You're probably much better off trying to find the mechanism
that's coupling RF into your power system and breaking that up.
"Fit a proprietary RFI filter to the telephone line (to cure the modem
problem)"
This will only work if the problem is that RF on the phone line is being
coupled into the modem and being demodulated by the modem to create the
local emi effect. You already know that you've got emi on the power line,
if your emi is coming in through the power lead, the telephone filter would
not do you any good; you'd be locking the wrong door. You might want to put
the scope on the phone wires and see if you have RF there. Those telephone
filters are about 11 dollars each. You do not want to make a big investment
in them unless you have reason to believe they will actually solve your
problem.
Good luck with your problem, and please keep us posted on your progress.
73,
Steve
AA4AK
_______________________________________________
Elecraft mailing list
Post to: Elecraft@mailman.qth.net
You must be a subscriber to post to the list.
Subscriber Info (Addr. Change, sub, unsub etc.):
http://mailman.qth.net/mailman/listinfo/elecraft
Help: http://mailman.qth.net/subscribers.htm
Elecraft web page: http://www.elecraft.com
