Posted by
Ron D'Eau Claire-2 on
Jan 23, 2007; 1:05am
URL: http://elecraft.85.s1.nabble.com/Rf-in-the-shack-distorting-audio-tp443216p443226.html
Don, W3FPR wrote:
I wonder why you are using a 9:1 balun? You may be better off using a 1:1
balun instead. Yes, I know that 50 times 9 is 450, but that is only the
characteristic impedance of your feedline - the feedpoint impedance will be
quite different than 450 ohms, and may vary from very low to very high
depending on the frequency. It just may be that your 9:1 balun is creating
more RF in the shack. If you have no 1:1 balun available, try removing the
balun and connect the feeders to the center conductor and shield of your
coax - and connect your counterpoises to the shield at that point too - you
may find your RF in the shack will drop dramatically.
------------------------------------
To follow up Don's comments, I've fed open wire line from a so-called
"unbalanced" output with *no* balun and RF ammeters in each leg of the
feeders showed that the currents were still balanced. The currents *inside*
a coax line are balanced. The current flowing inside the shield is equal and
opposite the current on the outer surface of the center conductor. Where
things go awry is where the coax shield ends and RF can flow around the end
onto the outside. Now there's two loads on those currents: one is the
antenna and the other is whatever is connected to the outside of the coax
shield, including the rig.
If that sounds confusing, remember that RF does not flow *in* a conductor,
but only along the surface. So the RF currents flowing along the inside of
the coax (or inside your rig near the antenna connector) are totally
independent of any RF currents flowing on the outside of the coax (or on the
outside surface of the rig). It's those outside currents that tend to cause
most of the problems. It's those outside currents you upset when you touch
the rig and so detune the system or give yourself an RF "bite" (Ouch!). It's
those outside currents that get inductively coupled into things like speaker
wires, telephones, microphones, etc., and cause mischief.
A balun is just a length of transmission line, sufficiently long that any
unbalance at one end is "smoothed out" by the interaction between the RF
fields around the wires before RF currents get to the other end. Years ago,
we made a balun by winding up transmission line in a big coil. Sometimes we
wound each leg into its own coil and sometimes both legs were wound next to
each other in one huge 'bifilar' coil. For HF an open wire balun might be 4
to 6 inches in diameter and a foot or two long. (It was common to mount them
on standoff insulators on a large board and screw it to the wall!) Nowadays
we use ferrite cores that give us the inductance needed in a much smaller
space, but the operation is the same. If you look at a 1:1 balun schematic,
it's easy to see how it's a coiled-up two-wire transmission line. Other
baluns, offering various impedance conversions, are also simply coiled-up
transmission lines; usually two or three interconnected to provide the
impedance conversion wanted. You could achieve the same impedance
conversions with sections of open wire stretched out and interconnected. It
would work exactly the same way, but take up a fair bit of space!
So all you're doing when you add a balun to the end of a piece of coax (or
at the "unbalanced" output of a rig) is you're isolating one end from the
other by imposing a long section of transmission line between them. The
simple fact is that, if it's long enough, the open wire transmission line
itself will do the same thing *if* the antenna is balanced. If it's an off
center fed doublet or other inherently unbalanced antenna, the feeder
currents won't be balanced no matter what you do. That doesn't make the
feeder inherently lossy. The worst is that the feeders will tend to radiate
a bit, depending upon the degree of unbalance.
The solution to unwanted RF on the outside of the rig is to stop the
currents from flowing on the outside of the coax shield and the rig. Usually
they get there by flowing out of the open end of the coax at the transition
to the open wire line and around the edge right onto the outer surface.
Sometimes a balun will provide enough isolation that the surface currents
don't cause trouble, but sometimes not. Another way to reduce those currents
is to put some ferrite beads over the coax. A number of companies sell
ferrite beads for just that purpose. Some even provide a complete cable
assembly with the beads installed. Those beads show a very high impedance to
any RF currents on the outside of the shield, stopping or reducing them
drastically. Your rig still is not "grounded" for RF, but you don't care.
There's no RF to cause trouble on the rig anyway.
Another approach many installations use is to have a long section of coax
coiled up to act as an effective balun because the currents on the outside
experience the coil as an inductor with reactance that stops them while the
currents on the inside of the coax only "see" the transmission line and are
unaffected by the fact it's wound up in a coil. (It's important the coil be
solenoidal and single-layer so the input end is well isolated from the
output end. A jumble of coax is not very effective). That's *not* a good
idea for you because you're operating the coax at a high SWR, so you have
significant losses for every foot of coax you have in the system. Keep the
coax as short as possible!
Ron AC7AC
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