In a message dated 06-17-97 JEFF EDMONSON wrote to LARRY KUNZ:
 JE> A DB-224 has 4 elements, and as I remember they are all fed
 JE> simultaneously, meaning that the Z would be somewhere around 12.5
 JE> ohms, per.
  Jeff & Larry-
     Antennas like the DB -224 are typically driven through a phasing 
arness,
 that combines the individual dipoles' 50 Ohm impedance to match a 50 Ohm
transmission line.
     Granted that the individual dipoles may not be 50 Ohms, but the
principle is the same.  From each pair of dipoles, you could connect an odd
number of quarter wavelengths of an intermediate  impedance co-ax, such as 72
ohms.  The impedance at the open end of each such cable, is approximately 100
ohms.  When connected in parallel with a second cable, the result
approximately 50 Ohms.  Another pair is used to combine the other two 
ipoles,
 and a third pair of cables is used to combine the two combinations.
     The principle of this quarter wave impedance transformer, is that the
product of the input impedance and the load impedance, is equal to the square
of the characteristic impedance of the co-ax.  In the case above, the square
of 72 Ohms is 5184.  The resulting input impedance is equal to the square of
the co-ax impedance divided by the load impedance, 5184/50=103.68.  Two in
parallel is 51.84.  If you stopped there, the resulting SWR would be 
1.84/50=
1.04.  Going through the third pair of cables, 5184/51.84=100 Ohms.  Two in
parallel is 50 Ohms, with SWR=1.000.
     This may not be the only way to combine the dipoles.  It could get messy
to combine an 8-dipole array using this technique.  Does anyone know how the
"big boys" do it with 8?  My only thought is that they use folded dipoles
that combine in parallel to have a 50 Ohm impedance.  If so, the 4-dipole
harness would work for 8-folded dipoles.
     73, Fred, K4DII
 -> Alice4Mac 2.4.4 E QWK Eval:30Sep95 
Origin: Alice strikes back @
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