Dave Platt wrote:
snip
You might be able to reduce the losses and heating by changing the
length of the 300-wire line you are using, so that the antenna's
feedpoint impedance is transformed into an impedance at the tuner that
doesn't require extreme impedance transformation, and requires the use
of lower amounts of reactance during tuning.
========================================
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.

Frank GM0CSZ / KN6WH

Highland Ham wrote:
... 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.

That rule-of-thumb is a linear approximation but the
actual function is not linear. Here is a graph (generated
using EZNEC) that is more accurate than the-rule-of thumb.
Using this graph, quite often one doesn't even need a tuner.

Of course, half-wavelengths of feedline are added to
the matching section length to achieve a reasonable
total feedline length.

http://www.w5dxp.com/majic.gif
--
73, Cecil http://www.w5dxp.com

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

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

I measured the VF of 300 ohm and 450 ohm window-line from
The Wireman. The 300 ohm stuff was very close to 0.8 while
the 450 ohm was closer to 0.9
--
73, Cecil http://www.w5dxp.com

Cecil Moore wrote:
Roy Lewallen wrote:
The velocity factor of 300 ohm ribbon twinlead is approximately 0.80,
but it varies from brand to brand and lot to lot.

I measured the VF of 300 ohm and 450 ohm window-line from
The Wireman. The 300 ohm stuff was very close to 0.8 while
the 450 ohm was closer to 0.9

Incidentally, the Z0 of the "300 ohm" window-line
was considerably lower than 300 ohms and the Z0 of
the "440 ohm" window-line was about 388 ohms.
--
73, Cecil http://www.w5dxp.com

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
==========================
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