On 26 abr, 02:59, "Ralph Mowery" wrote:
"Sal M. Onella" wrote in ...



This group has presented members with valuable lessons in antennas and
transmission lines, like how to measure, how to match, etc.

Something I haven't seen is a discussion of the source impedance of
the transmitter. *My curiosity was piqued today as I took some baby
steps into EZNEC. *A particular antenna had such-and-such VSWR if fed
with a 50-ohm cable and a different value if fed with a 75-ohm cable.

While this is hardly news, it got me wondering whether a 75-ohm cable
will load the transmitter the same. *Doesn't seem like it.

My point: *Using 75-ohm cable to improve the match at the antenna
won't help me *... IF ... I suffer a corresponding loss due to
mismatch at the back of the radio. *My HF radios, all solid state,
specify a 50 ohm load. As necessary, I routinely use an internal
autotuner and either of two external manual tuners. *(I'm aware of the
published 1/12 wavelength matching method.)

Wisdom in any form would be appreciated. *Thanks.

"Sal"
(KD6VKW)

A transmitter output impedance is designed for maximum power transfer at a
specific impedance. Most of the *the older tube transmitters impedance was
tunable within a *range.

In simple terms the impedance of the transmitter tube is the plate voltage
devided by the current. *This impedance is then transformed to the nominal
50 ohms of the antenna system. *If the transmitter has the usual tune and
load controls, the exect impedance will not mater as you adjust for maximum
transmitter output.

Most of the transistor transmitters are not adjustable so the output
impedance is usually fixed at 50 ohms for maximum power transfer. *If the
impedance of the antenna system is not 50 ohms, then the output power will
be less than the designed output. *You can use the antenna tuner to adjust
for a match.

Hello Ralph,

The actual output impedance can be anything, but is mostly not 50
Ohms. If you want it close to 50 Ohms, you have to spend money in
components and design time. As 50 Ohms isn't mostly required, one will
not design for that.

Just as an example, take a hard-driven totem pole or push pull stage
with only a series tuned circuit to suppress harmonics (so the LC
circuit shows zero ohms at the carrier frequency). As the active
devices are used a switches, the output impedance of this arrangement
is almost zero (at least far below 50 Ohms).

When you connect a 50 Ohms quarter-wave cable between the output and
the 50 Ohms load, the amplifier-cable combination has very high output
impedance (quarter wave transformer formula).

For power amplifiers, there is no relation between actual output
impedance and efficiency. When an amplifier is designed for 50 Ohms,
it only means that the amplifier will work correctly when terminated
with 50 Ohms. When you deviate from that, output power may decrease
or increase. This may result in more or less stress on the amplifier's
components.

With kind regards,

Wim
PA3DJS
www.tetech.nl