Jimmie D wrote:
"Dan Bloomquist" wrote in message
...

billcalley wrote:


We are all told that VSWR doesn't matter when using low loss
transmission lines, since the RF energy will travel from the
transmitter up to the mismatched antenna, where a certain amount of
this RF energy will reflect back towards the transmitter; after which
the RF will then reflect back up to the antenna -- where the energy is
eventually radiated after bouncing back and forth between the
transmitter and antenna.

As pointed out, VSWR does matter. A lot of bouncing means you heat the
transmission line with the power instead of radiating the power. 'Doesn't
matter', really means it can be tolerated if need be.


I understand the concept, but what I don't
quite understand is why the reflected RF energy isn't simply absorbed
by the 50 ohm output of the transmitter after the first reflection?
For the RF to bounce back and forth, wouldn't the transmitter's
impedance have to be very, very high (or low) when the reflected RF
energy hit its output stages? I know I'm missing something vital
here...

Here is what you are missing. In the case of the output, (real/resistive
component of the transmitter), seeing the reflected wave, it is _not_
reflecting that power back up the transmission line as you think it is. It
would go back to that real impedance and heat the transmitter. Here is
what is done with a miss match in the real world.

trans-output - match - line - antenna

The 'match' is where the magic happens. All the energy coming down the
line that got reflected from the antenna 'sees' the 'trans-output -
match' as a perfect reflector and gets bounced back[*]. On the other side
of the match is the trans-output. There the trans-output sees a perfect
impedance, (technically, the conjugate of the trans-output), so that all
the power travels through the match toward the antenna.

The magic is that when the match is tuned, both of the above conditions
are satisfied.

*The reflected wave sees a purely reactive reflector not just because of
the network but also because of the output power of the transmitter.
Without transmitter power the impedance as seen from the load will
dramatically change.

Best, Dan.



Saying that SWR doesnt matter is a rather broad statement(like saying never
or always) but I have know of antenna systems having an SWR of 30:1 and his
was normal. The feedline was balanced line made of 1 inch copper. Of course
an SWR lie this on coax could be fatal to coax and equipment. A more common
example of this is the 1/4 wl matching section on a J-pole antenna. It
matches 50 ohms to a few Kohms so an SWR of 60: 1 or so would not be unusal
here.S oas long as the feedline can handle the current and voltage peaks
without much los it doesnt matter much as long as the source impedance is
matched to the impedance at the input to the transmission line.Im sure there
is a practical limit though.

Hi Jimmie,
Keep in mind I'm answering in the context of the op's post. And the
theoretical SWR on a stub is infinite. The point of the stub at the
antenna is to keep the SWR on the transmission line in a reasonable
range, to make a match if you will. To put high SWR on the feedline
instead of matching at the antenna isn't a great idea in my book.

OTOH.
I finally did some sidebanding a couple of months ago. (First time on
HF.) I got my hands on an old swan 500c. After changing the 6je6's and
supply caps, I had to find out what it was like to get on the air. I ran
outside and hung a wire between the lab and the shop. 40-50 feet. Put a
couple of alligator clips on the end of a chunk of rg-58 and into the
window. I started looking for the antenna through the trans-match with
an antenna bridge. The tuning was very sharp, lots of Q. I don't know if
I could have found it without the bridge I was willing to tolerate
the miss match to get on the air.

Well, it worked out. I made some great QSLs across the mid west and into
northern CA. I live in Vernon AZ. I'm pleased this turned out to be as
great a radio location as I thought. It shouldn't be long before I get a
beam on a tower. By then I'll look to match at the antenna and keep the
SWR off the feed line as much as possible.

Best, Dan.