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

rickman · July 1st 15, 05:30 PM posted to rec.radio.amateur.antenna

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 you assume the transmitter is matched to the feed line, then yes, you
should only need matching at the antenna if that is mismatched. This
seems like a bit of a silly strawman, but maybe I am just not informed
and this is the typical scenario.

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.

--

Rick