[Antennas] Dipole Matching problema

[Robert Lay]

Dear Dennis,

Without doing a careful check on the arithmetic, it looks like the total
length of line is 119 feet of flat top plus two lengths of 40 feet of
feedline. That's about 190 feet total, giving you an impedance on 40 meters
at the feed point that is extremely low. In fact, the maximum current loop
will occur somewhere between 35 and 40' of feedline length, depending on the
actual feedline impedance.  This is all shirtsleeve engineering, but  it
looks like that's why your tuner is having a problem on 40 meters - the
impedance is too low for the tuner to handle.

You need to get that impedance up higher in order for your tuner to be able
to deal with it. There are at least two ways to do that. One is to use a
longer feed line - yes, I know - where will you put it?  Hi!

The other way is to use a 4:1 balun, backwards. That is, you want a 1:4
balun in order to get the impedance seen looking into the feedline
transformed from whatever it is up to 4x that. Yes, I know that most baluns
are 4:1 the other way. That's because some clown decided that since balanced
feed lines are typically in the 300 to 450 ohm range, you need a step down
transformation of 4:1 in order to get anywhere close to 50 ohms. WRONG! (By
the way, why don't they make 6:1 baluns? According to conventional wisdom
that would get you much closer to 50 ohms.)

That reasoning is in error, because we have NO IDEA what the impedance is
looking into a feed line unless we know what is attached at the other end.
The characteristic impedance of the line itself is just one of several
important numbers that define the system.

Anyway, let's get on with fixing the problem of RF in the shack. Since you
were so clever in getting your open wire line converted to a balanced double
coax system, do the right thing. Take it down near mother earth before you
lead it through the wall and get those two shields tied to the biggest
earthing system that you can make.  Don't use any connections longer than 3
inches and get those shield connection into the SOIL! Two or three 8 ft
copper rods
will do for starters. Just kidding about the 3 inch connections, but
seriously, the shorter the better.

Once the balanced double coax cabling is earthed and through the wall and
connecting to the back of the tuner, what do you do with the coax shields at
that end? Well, that's more controversial, and I won't make a big deal of
it, because it gets into about 3 or 4 touchy areas - not the least of which
is the National Electrical Code, 75 years of Ham lore, and a whole bunch of
lightning protection theory and witchcraft. I connect mine to the ground
stud on the tuner, but I can't recommend anything to others, because it's a
personal thing, and you have to do what's best in YOUR case. (I don't think
it makes a damn bit of difference!  If you get a direct strike from
lightning, nothing less than a completely professional installation of >
$10000 in lightning protection equipment is going to save your stuff,
anyway.)

That should take care of the RF in the shack and make your tuner work a
little less erratically!

Bob Lay (W9DMK), Dahlgren, VA
w9dmk at crosslink.net
Home Pages: http://www.qsl.net/w9dmk
http://zaffora.f2o.org/W9DMK/W9dmk.html
http://meltingpot.fortunecity.com/albania/784