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.

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

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.

How are your experiences with a 100 W tx ? I calculate just 0.3 Amps thru it.
The mind boggles on this..! Where does that heat come from ?
-================================
What is the configuration of your antenna matching unit , if with more
than 2 inductors , which one gets hot ?


Frank GM0CSZ / KN6WH

Highland Ham wrote:
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.

How are your experiences with a 100 W tx ? I calculate just 0.3 Amps
thru it.
The mind boggles on this..! Where does that heat come from ?
-================================
What is the configuration of your antenna matching unit , if with more
than 2 inductors , which one gets hot ?
============================
Sorry , I meant : if more than 1 inductor.

Frank GM0CSZ / KN6WH

Where does that heat come from ?

It comes from losses. Can you try different combinations of coil/capacitor
settings to still get a low SWR?
Only 1 db of loss is still about 20 watts. And 20 watts is quite a bit of
heat.

There will always be some degree of heating. In the best of cases,
it won't be noticeable, but that heat is from losses, and there's
always 'some'. In general, it sounds as if the tuner isn't tuned
correctly. The least amount of inductance necessary is a very good
idea! You might also check things in general, connections are good,
component 'size' is sort of at least in the 'ball-park', that sort of
thing. Keeping that feed line away from things is another idea.
Nothing 'extreme', just 'good sense'.
Not much help, but...
- 'Doc

D. Huizinga wrote:
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.

I once had a similar problem using a 130 foot dipole
fed with 450 ohm ladder-line on 40m. The SWR on the
ladder-line was about 15:1 and the 4:1 balun was
looking into 30 ohms. The balun presented 7.5 ohms
to the tuner and the coil got hot. That's when I
decided to come up with my no-tuner approach at:

http://www.w5dxp.com/notuner.htm

Is your 40m dipole 66 feet long fed with 60 feet
of 300 ohm twinlead through a 6:1 balun? If so,
your tuner is installed in a lossy configuration
and heating is to be expected. The simulator at:

http://fermi.la.asu.edu/w9cf/tuner/tuner.html

says that 16% of the power is lost in the tuner.
16 watts is enough to heat up a coil. Ever touch
a 15w appliance bulb when it is on?
--
73, Cecil http://www.w5dxp.com

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