[Antennas] why is this?

[Robert Lay (W9DMK)]

Dear RB,

Your only problem is with your assumptions. You say that, "Supposedly the 
1:4 balun should give better results."

But that is dead wrong, and I 'll bet that I know exactly what you're 
thinking. You're thinking that a 450 ohm transmission line needs a 4:1 
transformation in order that it match up with something like 100 ohms or 
less.

Right there is your mistake. The impedance seen looking into the 450 ohm 
line could be any value - no one knows until it's measured. You can make an 
educated guess if you know exactly what the impedance of the antenna is at 
that frequency and you know the exact length of the ladder line and its 
velocity factor and all that. But in all probability most of that would be a 
big guess too, and I'm sure that you don't want to go out and by an RF 
impedance bridge for Christmas at a price that will make you blink twice.

Let's say you're real lucky and you are right at resonance with the antenna 
itself, so you could guess something on the order of 72 ohms at the feed 
point. Now you have to translate that down the 75' of ladder line in order 
to see what it has been transformed into at that end. Not an easy 
calculation , but since it may be around a quarter wavelength, just for 
purposes of discussion, then you would be transforming that 72 ohms into the 
value "X" in the following formula:

450 = square root ( 72 * X)

in which case X is about 2800 ohms. Now what in the world are we going to do 
with 2800 ohms?

Are you sorry yet that you got into this in the first place?

73 de W9DMK - Bob Lay, Dahlgren, VA
http://zaffora.f2o.org/W9DMK/W9dmk.html
w9dmk at crosslink.net

I was fiddling around with my ant and tuner setup this pm.  I have a 140' 
dipole fed with approx 75' of 450 ohm ladderline.  I normally run a 1:1 
choke balun to go from the unbalanced tuner to balanced feedline.

I got a surprise and seemingly illogical data when I swapped a 4:1 current 
balun in place of the 1:1 choke balun.

The first table below gives readings for the 1:1 choke balun.  The next 
table gives readings for the 1:4 current balun.

Supposedly the 1:4 balun should give better results.  But, the data make it 
seem just the opposite.  So, curiosity is compelling me to ask if you guys 
see anything amiss here, and the associated cause.  My rig is capable of 
kicking out 100 watts CW.  That power output level is what the below 
measurements were made at::

                            1:1 (choke balun)

FREQ        LOAD         SWR
                 POWER

1,8            ----               ----         (no match point)
3.9            100              1.6:1
7.2            15                1.7:1
14.2           100              1.2:1
21.3           100              1.6:1
29.5           100              1.5:1


                               1:4 (current balun)

1.8              ----                ----           (no match point)
3.9              ----                ----           (no match point)
7.2              ----                ----           (no match point)
14.2            100               1.2:1
21.3            ----                -----           (no match point)
29.5            ___               ___          (no match point)