RadioBanter - View Single Post - An antenna question--43 ft vertical

Wayne · July 1st 15, 06:01 PM posted to rec.radio.amateur.antenna

"rickman" wrote in message ...

On 7/1/2015 10:20 AM, wrote:
On Tuesday, June 30, 2015 at 2:36:51 PM UTC-4, rickman wrote:
On 6/30/2015 12:40 PM, Tom W3TDH wrote:

I know that what I am about to say is provocative to some but I
still think it is worth saying. If you look at the way that
commercial and military radios are matched to antennas you will
notice that most of the matching is done as close to the feed
point as practical.

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.

When you couple the antenna to the load at the feed point you
can have extremely low losses in the feed line. When you do the
matching at the feed point you will transfer the most energy
possible to the antenna and will get the highest available
effective radiated power. Since the objective is the transfer of
the highest practical amount of power to the antenna the place to
do that is at the feed point were possible.

I do realize that it is often simpler and easier to match at the
feed line connection but I felt obliged to point out that is is
not the most effective place to do the job.

Has it occurred to you that it might be important to match
impedance both at the transmitter and at the antenna? When the
feed line is not impedance matched to the transmitter output the
maximum power is not transferred into the feed line. Then you have
already lost power that can't be recovered by the matching at the
antenna even if it is perfect.

Your statements are not really provocative, they are just
incomplete and/or wrong.

--

Rick

Rick

OK I'll buy incomplete and therefore wrong.

Now given a Fifty Ohm feed line connected to a transmitter that is
designed for that impedance at the antenna connector does not the
actual mismatch occur at the antenna feed point? Certainly that can
be compensated for at the transmitter but isn't there a likelihood or
at least a risk that you will loose significant effective radiated
power in spite of adjusting the apparent feed line impedance to the
transmitter? If I do the matching at the feed point will I not
maximize the effective radiated power of the antenna by installing
the tuner at the feed point.

# I have to plead ignorance. How can you deal with impedance mismatch of
# the antenna and feed line by a network at the transmitter?

If there is a mismatch at the antenna (and there is no matching at the
antenna), then maximum power transfer will occur when the conjugate match is
applied at the transmitter end of the feedline.

Loss in the feedline will include the normal loss of a matched line, and
additionally the loss caused by a SWR other than 1:1.

With low loss feedline, and SWRs under perhaps 5:1, the additional loss is
small and often quite acceptable.

Why do it that way? You can have control in the shack, and you throw less
money at the problem.

In my own particular case, an automatic remotely tuned ATU would be a pain
to install/maintain.