I am thinking of homebrewing a balanced line tuner per the schematic of the
new MFJ balanced line tuner....but am wondering how efficient that
particular schematic is compared to a Johnson Matchbox.

If a tuner doesn't burst into flames with a 1 KW transmitter then it must be
resonably efficient. There must be a lot less than 1/2 S-unit loss in
signal strength.

Why all the concern about efficiency of tuners? Old wives' ?
---
Reg.

If a tuner doesn't burst into flames with a 1 KW transmitter then it must be
resonably efficient. There must be a lot less than 1/2 S-unit loss in
signal strength.

Why all the concern about efficiency of tuners? Old wives' ?
---
Reg.

Reg Edwards wrote:
If a tuner doesn't burst into flames with a 1 KW transmitter then it must be
resonably efficient. There must be a lot less than 1/2 S-unit loss in
signal strength.

Why all the concern about efficiency of tuners? Old wives' ?

No, because coils actually do melt and switch contacts do arc, even at
the 100W power level. This is because commercial tuners are notoriously
over-specified, and always use components that are at least one size too
small.

Rule of thumb: divide the rated power level by at least 3 - and you
still may not be entirely safe.

The T-match tuner can match a very wide range of impedances, but it is
also prone to operator error - specifically, using too much inductance.

For the lowest possible stress on the components, you must use the
*lowest* value of inductance that will allow the two capacitors to be
adjusted to give an impedance match.

You can also get a match using higher inductance, but the internal
voltages and circulating currents will be unnecessarily high. That's
where the inefficiency problems arise. With T-tuners, these problems are
especially acute when trying to match low impedances at low frequencies.

The ARRL Antenna Handbook has a lot more to say about this, and the
CD-ROM includes a program that lets you map out the efficiency as a
function of load impedance and frequency. Some of the numbers down in
that low-frequency, low-impedance corner are plain scary - most of the
power is going into the tuner itself, and it comes out as smoke.

This situation is not entirely the fault of manufacturers. It's we
customers who demand a tuner that claims to match every impedance, at
every frequency, with the highest possible power rating, and all for
less than the lowest possible cost. The T-tuner probably comes closest
to this ideal, which is why they choose that configuration... but it
isn't always the right choice.


--
73 from Ian G3SEK 'In Practice' columnist for RadCom (RSGB)

http://www.ifwtech.co.uk/g3sek

Reg Edwards wrote:
If a tuner doesn't burst into flames with a 1 KW transmitter then it must be
resonably efficient. There must be a lot less than 1/2 S-unit loss in
signal strength.

Why all the concern about efficiency of tuners? Old wives' ?

No, because coils actually do melt and switch contacts do arc, even at
the 100W power level. This is because commercial tuners are notoriously
over-specified, and always use components that are at least one size too
small.

Rule of thumb: divide the rated power level by at least 3 - and you
still may not be entirely safe.

The T-match tuner can match a very wide range of impedances, but it is
also prone to operator error - specifically, using too much inductance.

For the lowest possible stress on the components, you must use the
*lowest* value of inductance that will allow the two capacitors to be
adjusted to give an impedance match.

You can also get a match using higher inductance, but the internal
voltages and circulating currents will be unnecessarily high. That's
where the inefficiency problems arise. With T-tuners, these problems are
especially acute when trying to match low impedances at low frequencies.

The ARRL Antenna Handbook has a lot more to say about this, and the
CD-ROM includes a program that lets you map out the efficiency as a
function of load impedance and frequency. Some of the numbers down in
that low-frequency, low-impedance corner are plain scary - most of the
power is going into the tuner itself, and it comes out as smoke.

This situation is not entirely the fault of manufacturers. It's we
customers who demand a tuner that claims to match every impedance, at
every frequency, with the highest possible power rating, and all for
less than the lowest possible cost. The T-tuner probably comes closest
to this ideal, which is why they choose that configuration... but it
isn't always the right choice.


--
73 from Ian G3SEK 'In Practice' columnist for RadCom (RSGB)

http://www.ifwtech.co.uk/g3sek

On Wed, 10 Mar 2004 01:17:28 +0000 (UTC), "Reg Edwards"
wrote:

If a tuner doesn't burst into flames with a 1 KW transmitter then it must be
resonably efficient. There must be a lot less than 1/2 S-unit loss in
signal strength.

Why all the concern about efficiency of tuners? Old wives' ?
---
Reg.


Reg, you must be joking ........ 1/2 S unit isn't much when an S-unit
on most of those plastic boxes is around a few decibels, suppose we
are not talking about real S-units any more, then you would get too
many complaints of reporting 55 when everybody else give 59+. Those
modern American tuners (unlike those good old KW tuners) are not
really balanced, they use a balun on the output, and forget about
earth current

73

----
Jan-Martin, LA8AK, N-4623 Kristiansand
http://home.online.no/~la8ak/

On Wed, 10 Mar 2004 01:17:28 +0000 (UTC), "Reg Edwards"
wrote:

If a tuner doesn't burst into flames with a 1 KW transmitter then it must be
resonably efficient. There must be a lot less than 1/2 S-unit loss in
signal strength.

Why all the concern about efficiency of tuners? Old wives' ?
---
Reg.


Reg, you must be joking ........ 1/2 S unit isn't much when an S-unit
on most of those plastic boxes is around a few decibels, suppose we
are not talking about real S-units any more, then you would get too
many complaints of reporting 55 when everybody else give 59+. Those
modern American tuners (unlike those good old KW tuners) are not
really balanced, they use a balun on the output, and forget about
earth current

73

----
Jan-Martin, LA8AK, N-4623 Kristiansand
http://home.online.no/~la8ak/

Ian,

Quite rightly, you blame the melt down of the tuner not to the tuner's
intrinsic inefficiency (which none of them have) but to the abuse inflicted
on it by the user with the help of a transmitter.

All tuners are high-efficiency devices when operated within their ratings.
They can't help being otherwise. They have only a length of wire in the form
of a coil and a condenser.
---
Reg.

Ian,

Quite rightly, you blame the melt down of the tuner not to the tuner's
intrinsic inefficiency (which none of them have) but to the abuse inflicted
on it by the user with the help of a transmitter.

All tuners are high-efficiency devices when operated within their ratings.
They can't help being otherwise. They have only a length of wire in the form
of a coil and a condenser.
---
Reg.

Er...
Well. After a couple of Years, the resistance of the coil roller contacts
reaches
"irritating" high values, destroying coil Q.
Keep it clean! (tomato ketchup is a good cleaner!)



Reg Edwards wrote:

Ian,

Quite rightly, you blame the melt down of the tuner not to the tuner's
intrinsic inefficiency (which none of them have) but to the abuse inflicted
on it by the user with the help of a transmitter.

All tuners are high-efficiency devices when operated within their ratings.
They can't help being otherwise. They have only a length of wire in the form
of a coil and a condenser.
---
Reg.