On Tue, 30 Jun 2015 17:21:08 -0700, "Wayne"
wrote:

# Since only the power that actually reaches the antenna can be radiated I
have a hard time seeing the point of matching the transmitter to the feed
line. Matching at the feed line connection point will prevent damage to the
transmitter but if # that were the main objective a dummy load would
accomplish that.

Yep. Note that the electric utility companies do not bother to match
the transmitter (generators) with the impedance of the transmission
lines and the load. That was one of the reasons that Edison and
Westinghouse has so much trouble with the experts when they proposed
electric power transmission. The experts assumed that the source had
to be matched to the load, which would cause the generators to
dissipate as much power as is dissipated in the load. Incidentally,
one reason Tesla/Westinghouse eventually went with 60 Hz instead of
133Hz, 400 Hz, or higher frequencies (which use less iron in the
xformers) was the danger of creating standing waves on the
transmission lines because of the mismatch. When the wavelength of 60
Hz (3100 miles or 5000 km) is longer than the width of the country,
it's a safe bet that there aren't going to be any standing waves.

So, why don't we run transmitters with lower than 50 ohm output
impedances? Well...
1. The gain of the PA stage would be reduced possibly requiring an
additional gain stage.
2. The current in the PA stage would increase, possibly causing the
power supply to complain.
3. The low pass harmonic filter will require physically larger parts.
4. The coax cable between the PA stage and the RF connector will need
to have a very thin dielectric to work at low impedances. Same with
the RF output connector.

Instead of dealing with these aforementioned hassles, it's probably
better to run the transmitter at some impedance that provides a
benefit and let everything else conform to that standard. That's
where the maximum power at 50 ohms for transmitters, and lowest loss
for 75 ohms (air dielectric) for CATV were derived. The rest of the
connected devices (PA, filter, antenna) simply conformed to these
standards.


--
Jeff Liebermann
150 Felker St #D http://www.LearnByDestroying.com
Santa Cruz CA 95060 http://802.11junk.com
Skype: JeffLiebermann AE6KS 831-336-2558

On Thu, 02 Jul 2015 06:12:33 -0700, Jeff Liebermann
wrote:
(...)
where the maximum power at 50 ohms for transmitters, and lowest loss
for 75 ohms (air dielectric) for CATV were derived. The rest of the
connected devices (PA, filter, antenna) simply conformed to these
standards.

Continuing from where I accidentally hit the "send" button...

It might be interesting to measure the output impedance of your HF
xmitter. All you need is a dummy load, and an RF voltmeter, RF probe
and voltmeter, or oscilloscope.
1. Turn down the xmitter RF output to some level where you won't blow
up your test equipment and so that it doesn't go into high VSWR
protect mode. My guess is about 10 watts is about right.
2. Measure the RF voltage across the output connector both with a
load (Vload) and without a load (Vno_load).
3. If measuring peak voltage, convert RMS by multiplying by 0.707. If
measuring peak-to-peak, divide by 2 and then multiply by 0.707.

Output_Impedance = 50 ohms (Vno_load - Vload) / Vload

It's been many years since I've done this, so I can't recall the range
of values that I obtained. I do recall that it was surprisingly large
and precipitated a few heated discussions in the lab. Also, the
output impedance will change with output power level but I don't
recall how much.


--
Jeff Liebermann
150 Felker St #D http://www.LearnByDestroying.com
Santa Cruz CA 95060 http://802.11junk.com
Skype: JeffLiebermann AE6KS 831-336-2558