[SOC] G5RV

[JMcAulay]

At 05:23 PM 06/11/2003 +0100, Paul M3CRQ wrote:

>Hi Guys,
>
>Well I've done a little research and it seems that most G5RV manufacturers
>recommend a choke balun comprising about 10 turns of the co-ax feed close
>wound with a diameter of four to six inches fitted, as Chris suggested,
>right at the point the phasing line connects to the feed.

Makes sense to me.

>One supplier suggested a second, similar balun where the feed enters the
>shack.

Probably unnecessary, but might help in case of problems.

>I concede that for RX only installations there's likely to be little gained
>but when transmitting, I gather this helps to keep RF out of the shack.
>
>Also, I've got a 17" monitor about three feet from the K2 and the mess
>picked up from that is causing problems. I hope that when the G5RV is
>installed (with a 30m run to the base of the antenna) these problems will
>diminish. I also get a massive hum from the speakers with key down. I put
>that down to the currently unmatched long wire causing massive radiation
>from the feeder.
>
>Do these observations make sense?

You bet they do.

Generally speaking, you do not want your transmission line to radiate.
That's what the antenna is supposed to do.  After all, the antenna is 'way
out there, and part of the transmission line is in the shack.  So to avoid
"RF in the shack," you must keep the line from radiating.  And a line which
does not radiate won't pick up noise, either.

With two-wire feeder, any noise picked up on the two feeder wires is
"common mode."  That is, it is equal and in phase on both wires.  Thus it
is canceled out by the balanced connection at each end.  Either side (at
both the source and load) looks just like the other, only opposite.  Also,
radiation from the feedline should not occur because the radiation from
each wire of the pair is identical -- except of opposite phase, so
radiation from the two wires will be self-canceling.

Coaxial line, on the other hand, doesn't pick up noise because the shield
is grounded, and it won't radiate because the shield of the cable is
maintained at ground potential all the way, so no external RF currents can
flow.  To maintain this situation, it must be connected to an unbalanced
load.  

By the way, neither of the two -- balanced line or coax -- is superior to
the other with regard to undesirable radiation, if properly installed.
There are other advantages to each.

If anything happens to alter either of these scenarios (good balance of
two-wire line or total unbalance of coaxial line), your line *will* radiate
when you are transmitting.  So to work properly coaxial line has to be
completely unbalanced, and two-wire line has to be perfectly balanced.
Connecting the two right together as in the G5RV design thoroughly defeats
both requirements.  Driving balanced line from an unbalanced feeder, or
vice versa, just plain messes up the entire situation.  The "balanced" line
is no longer truly balanced, and the "unbalanced" line isn't really
unbalanced, either.  So the two-wire line will radiate, and the coaxial
line will radiate, too.  If you don't care, living alone on a 640-acre
estate where there's no one to complain, good for you.  But sometimes, this
is a problem.

The solution is to use a balanced-to-unbalanced transformer (balun) where
the two types of line are joined.  This can be both costly and hard to
support, although the cost and size of a good toroidal balun are both much
lower than those of the bifilar multiple-solenoid types of the dim past.
Alternatively, the coax can be wound into a coil at that point, so
commmon-mode currents have great difficulty getting past the inductance.
They are, it's said, "choked off."  If they don't get past at all, life is
good.  This device is sometimes called a "choke balun" (not quite an
accurate title, but so what -- it gets the message across).  If all this is
done well, neither line should radiate.  And if they won't radiate when you
exhale, they won't pick up noise (or anything else) when you inhale, either.

A coaxial choke at the shack will help keep any remaining RF current on the
shield of the coax out of the shack, and it will also keep any unbalance in
the transmitter output from causing RF currents to flow on the outside of
the coax (What?  You say your transmitter is perfect?  And it's grounded?
Really?).  If the rest of the system is working well, this choke should not
be required. 

That's not the whole story, but for practical purposes it should do.  

73
John WA6QPL  SOC 263