In article ,
D. Huizinga wrote:

Hi there.

My 40 mtr doublet is fed with a balanced line of 300 Ohms. Nicely tunable
to 1:1 . But after twenty minutes or so the coil gets rather hot! It is
a quality
coil of 3 mm and shiny silvered or chromed on a ceramic body of 1 inch.

Not at all unusual.

An antenna tuner (or "transmatch" as some prefer) is a tuned circuit,
which can develop some very high circulating currents which flow
between the inductive components (e.g. the coil) and the capacitive
components (the internal capacitors, or the capacitive reactance of
the load). These high currents can result in a substantial amount of
loss in the tuner components (mostly in the coils).

How are your experiences with a 100 W tx ? I calculate just 0.3 Amps thru it.
The mind boggles on this..!

If I recall correctly, 100 watts is developed into a 50-ohm load via
70.7 volts at 1.4 amperes.

Those are the voltages and currents at the transmitter output. The
voltage and current relationships will be different at other points in
the circuit... at the antenna itself, and at various points inside the
tuner... because the impedances will be different.

In the case of extreme impedance transformations (e.g. transforming 50
ohms down to match a 5-ohm load, or up to a 1000-ohm load), an
antenna tuner can have to handle either very high currents (leading to
high I^2*R losses) or high voltages (leading to arcing).

The popular "T" antenna tuner configuration can be a bit tricky...
it's notorious for being able to "tune" some very difficult loads,
even down to a short circuit in some cases. In this latter situation,
the tuner is actually "tuning" its own internal losses (e.g. in the
coil). There are plenty of tales around about peoples' antenna tuners
"burning up" when this occurred - all of the transmitter power went
into heating up the coil, and melted the coil form!

Where does that heat come from ?

From the transmitter! You may actually have several amperes of RF
flowing through the coil. The coil's series resistance at RF is
higher than it is at DC, due to skin and proximity effects.

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.

If I recall correctly, the ARRL Handbook has some tables (and
software?) which can let you calculate a tuner's efficiency and
losses, based on the impedance of the load and some reasonable
assumptions about the Q of the inductor. Lower efficiency == more
loss == more heating.

--
Dave Platt AE6EO
Friends of Jade Warrior home page: http://www.radagast.org/jade-warrior
I do _not_ wish to receive unsolicited commercial email, and I will
boycott any company which has the gall to send me such ads!

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