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

Ian Jackson[_2_] · July 1st 15, 03:34 PM posted to rec.radio.amateur.antenna

In message ,
writes
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 already conceded that it is not as convenient to do the matching
at the feed point. I do not allege that doing the matching at the
transmitter end of the feed line is inherently ineffective only that
there is a greater likelihood of loosing ERP needlessly and invisibly
if the matching is done at transmitter end of the feed line. By this I
mean to ask if I may well deceive the power meeter into showing more
power out then I am actually getting. If any power lost is very likely
to be insignificant at a practical level than help me to understand why
that would be true and I will sell off my Icon AH-4, together with the
control converter that allows my Yaesu FT-857D to control it, and my
SGC SG-235 and go back to using the Yaesu FC-30 tuner with my FT-857D
and the built in tuner on my Yaesu FT-1000.

This is especially important for me to get right with my FT-857D since
it is the transceiver that I use for my personal go kit. If putting
the Icon AH-4 on the mast and running the control line in addition to
the coaxial cable is a waste of time I would really appreciate knowing
that.

Thank you for helping with my education on this issue.

It's indeed surprising how well things still work if you use the lowest
loss coax you can beg, steal or borrow, and do all the matching back in
the comfort of the shack.
--
Ian