Roy Lewallen wrote:
Highland Ham wrote:
With the velocity factor of 300 Ohms ribbon being 0.99 (take it as 1)
and using a symmetrical dipole (hence not a Windom type of antenna),
to get a low impedance at the matching unit (tuner if you like) the
length of half the dipole + the length of the feeder should be about
an odd number of quarter wavelengths.

The velocity factor of 300 ohm ribbon twinlead is approximately 0.80,
but it varies from brand to brand and lot to lot.

Roy Lewallen, W7EL
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Tnx Roy , I thought is was about the same as for 450 ohms twin feeder.
That's what I use. The 'thumb rule' of the length of half the dipole +
length of feeder being about an odd number of quarter wavelengths for a
low impedance at the matching box (mine is an E-zee match unit) works
well for me. With 450 Ohms twin feeder you can sense the current in 1
wire with a clip-on relative RF current sensor/indicator to find a low
impedance point ,because of the large open slots. If right for a lower
freq band , the matching box takes care of the higher freq bands (also
non-harmonic bands) ,without getting 'warm'.
However with the current feeder ,for 1.8 MHz I would have to increase
the feeder length (coiled-up inside shack and kept away from metal)


Frank GM0CSZ / KN6WH