[Elecraft] Re: KAT100 with Ladder Line

[Don Wilhelm]

Peter,

Antenna and transmission line behavior is a strange 'animal' that does 
not mesh with our normal thoughts of electron flow in wires.   It is 
indeed a complex subject that must be considered in 4 dimensions to be 
fully analyzed, and our brains think in only 3 dimensions.  How an 
electromagnetic wave is launched from an antenna is at the heart of the 
full answer to your question.  The attempt to get the RF to the antenna 
is the task of the transmission line.  A transmission line (hopefully) 
keeps the RF contained until it reaches the antenna by maintaining a 
balanced condition (equal and opposite RF currents) all along its length.

I like to think that the major thing that determines the balance of an 
antenna system more than any other factor is the ends of the antenna - 
there the current must be zero and the voltage must be high - the laws 
of physics demand that.  With a center fed antenna, balance is carried 
on the antenna wires to the center feedpoint if the antenna has symmetry 
with its environmental surroundings.
If you connect a balanced feedline at the antenna feedpoint, that 
balance should be preserved throughout the system until you get to the 
balanced transmission line end. 

Note that even coax is a 'balanced line' - if one considers only the 
center conductor and the *inside* of the shield - the RF currents inside 
a coax line are equal and opposite which is the definition of balanced.  
It is only the outside of the coax that makes it 'unbalanced' - in other 
words, the outside of the coax shield can be an extraneous conductor in 
space and if steps are not taken to prevent the RF current from getting 
onto the outside of the shield, it too will act as part of the antenna - 
this is the main function of a feedpoint balun used on a coax fed antenna.

If you follow those thoughts, you can see that balanced RF current *can* 
and does flow out of the output of the tuner SO239 jack - so connecting 
ladder line directly to that jack *is* a workable situation.  You may 
have to experiment with the line length a bit to assure that you are not 
feeding the line at a point of high RF voltage where leakage may be 
significant, but that is all that may be required.  Most folks do not 
want to go to the trouble of refining the transmission line length to 
create the proper conditions, and for those situations, the balun is an 
easy way out.

In other words - I believe that RF balance is a condition of the 
physical and electrical nature in the antenna system, no matter what the 
driving source (transceiver and ATU) might be.

Consider also that a ground wire to mother earth may not be a very good 
RF Ground and the situation becomes more complex - in other words, RF 
voltage can 'float' on a DC ground with no problem.  Connecting a ground 
wire to the tuner may not change the amount of RF in the shack 
significantly unless that ground wire is also an RF ground.  The 
function of a balun at the point where the balanced line is attached is 
similar to the function of a balun used at the antenna end of the coax 
feed - it is there to keep the RF voltage from flowing back onto the DC 
grounded part of the driving system, but if the RF voltage at the 
attachment point is already zero (or very low), the balun will not have 
much to accomplish and could even be eliminated.

So if you do not have RF in the shack (due to RF voltage floating on the 
DC Ground), you have an installation where the balun at the tuner can be 
eliminated.  You may or may not be able to achieve all the right 
conditions in a multiband antenna/feedline setup, but you can make an 
attempt.

I would suggest that you measure the RF current in each leg of your 
feedline to determine if the direct connection to the KAT2 changes the 
line balance situation.  If it changes the line balance significantly, 
then use a balun or alter the feedline length to reduce the RF voltage 
at the shack end.

Using a balun at a point where the reactance is high can cause losses, 
but where the reactance is low, inserting a good balun will not create 
significant loss.

Many 'old wives tales' and myths come from using baluns arbitrarily and 
trying to make it a 'cure-all' for all antenna system ills.

73,
Don W3FPR

Peter Wollan wrote:
> Please, I'd like some more discussion of this question.  I've seen 
> before that baluns on ladder line can be lossy, but I'm not about to 
> get a balanced tuner, so it doesn't really help.  What happens if you 
> connect the balanced line directly to the KAT100?  Or, in my case, to 
> the KAT2?  I haven't had any problems so far -- maybe low power makes 
> a big difference, or maybe I've just had antennas which were 
> reasonable in some way.  I gather it's possible to get unpleasant 
> currents on the case, mic, or paddle.  Is that exactly the condition 
> in which the balun will be lossy?  Does connecting the ground screw on 
> the KAT2 (or the one on the balun?) alleviate the common mode current 
> problems?
>
>