Certainly the SWR on the line adds loss to the system, which is why it
can be desirable to do the matching at the antenna to avoid it. But
doing it at the rig is typically much more flexible, is typically less
expensive or complicated, and depending on the situation might be just
as effective in practical terms. As WA4LAV says, that transmission line
loss is not always hugely significant since it depends upon the matched
loss in the line itself (which varies with frequency and the quality of
the line), the length of the line, and the magnitude of the SWR. Using
a transmission line program like TLW (available free with the ARRL
Antenna Book) will pretty much tell you exactly what you have instead of
you having a false sense of anything.
73,
Dave AB7E
On 4/27/2015 9:36 AM, Tom Osborne wrote:
One thing I have noticed with the built-in tuner on my radio is that
all it does is make the radio happy.
The radio shows a 1:1 SWR, but the meter past the radio still shows a
high SWR, so if there is loss on the line, even though the radio is
happy, the loss is still there. I think the tuners just give us a
false sense of security. 73
Tom W7WHY
On 4/26/2015 9:23 PM, Fuqua, Bill L wrote:
Not everyone knows that a tuner only protects the rig. That is
the job of a dummy load.
In fact the tuner matches the impedance of the antenna system to the
rig and not only that,
by doing so providing the antenna with a conjugate match, that is to
say the antenna's radiation
resistance is matched and its reactance is also matched by the equal
and opposite reactance thus
making the antenna system resonant. If the antenna has a radiation
resistance of 10 Ohms and
a capacitive reactance of 100 Ohms the antenna tuner matches it with
10 Ohms real resistance and
an inductive reactance of 100 Ohms thus providing maximum energy
transfer. While at
the same time it provides 50 Ohms resistive load for the transmitter.
Resonating the antenna system with a tuner is equivalent
resonating the antenna. The only difference
is that there may be some additional losses within the tuner and
transmission line which are generally
minimal at HF frequencies. Also, there is a reduction in bandwidth of
the antenna system.
Yes it does change the currents and voltages in the antenna. The
G5RV was an antenna developed back
when we had tube transmitters with a PI network output that could
accommodate a broad range of
impedances. It also was made to work on 5 bands.
Just about every AM broadcast antenna system has an antenna
matching network. It is a shame that
they don't work.
73
Bill wa4lav
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