wrote in
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oidnRiueftdl13YnZ2dnUVZ_rXinZ2d@hawaiiantel. net:
I recently built a modified vertical dipole
from wire and using a pole and trees as supports.
I had hoped it would work on 2 bands 75 and 40 but it only
works on one 75 where the VSWR is 1.3:1 and the bandwidth is 85 khz
for the 2:1 points.
On 40 meters the VSWR is over 10:1 so I thought I could use a 1/4 coax
matching section 50 ohms but this doesnt solve the problem either as
the VSWR doesnt change much with the extra coax section.
The feedpoint impedance of a full wave dipole is likely to be in the
region of 4000 ohms.
A quarter wave transformer needs to be of Zo (50*4000)^0.5 or 447 ohms.
You will not find a practical coax line with such Zo.
If you for instance used a quarter wave of RG58C/U for a transformer, the
input impedance would be 2 ohms (VSWR=25) (taking into account the 5+dB
of loss in the transformer.
Both the length and the Zo of a quarter wave transformer are important to
its operation.
Does this help to explain the results you observed?
Owen
I did cut the section properly as the velocity factor for the coax is
.78% which gave me 26.825 ft.
Not sure what I am doing wrong or assuming - but looks like I could
use some input.