On 27 abr, 18:40, JIMMIE wrote:
On Apr 25, 9:07*pm, Jim Lux wrote:



Sal M. Onella wrote:
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)

I suspect that most ham transmitters do NOT have a 50 ohm output
impedance. *What they do have is a specification that they will
adequately drive a 50 ohm load (and some sort of internal circuitry that
detects an "unacceptable" output condition and turns down the drive).

After all, your transmitter could have an output impedance of zero ohms
(a "stiff" voltage source), and adequately drive your transmission line
and antenna at 50 ohms (yes, this is not the optimum power transfer, but
nobody ever said that ham transmitters are designed for optimum power
transfer... maybe they're perfectly happy with less transfer, but still
operating within their safe area)

ON9CVD made some simple measurements using a couple of resistors and
foudn that a TS440 has a Zout somewhere around 15-40 ohms (depending on
frequency and output power).http://sharon.esrac.ele.tue.nl/~on9c...impedantie.htm

Grant Bingeman also has words on this:http://www.km5kg.com/loads.htm

Being aware that this was the nature of my solidstate *transceiver I
attempted to use a tuner with one to improve my match to my antenna
system. While I didnt damage my transmitter I did notice that the best
settings of the tuner for TX and RX did not coincide. I was wondering
if anyone else has observed this .

Jimmie

Hello Jimmie,

I noticed this also when I was experimenting with CB equipment and
simple antenna experiments.

For several CB transceivers I could get more output by slightly
mismatching the load as seen by the PA (but in many cases with too
much increase in current consumption).

I tried to use a matcher/tuner (and later a high Q resonator) to
reject other stations close by and then I figured out that when
applying mismatch to the receiver, the S-meter moved further.

The above isn't strange. As PA's are mostly not designed to show 50
Ohms, many receivers are also not designed to show 50 Ohms. I am not
discussing wide band receivers (for example digital or analog video).

Most active devices have lowest noise figure when driven from a source
impedance that is far from the input impedance of the active device.
If you want them to be equal, you need to use feedback and that
complicates the design.

Also filters with significant pass band ripple show, even when
designed for 50 Ohms, significant input reflection when referenced to
50 Ohms

With kind regards,

Wim
PA3DJS
www.tetech.nl