[Elecraft] Antenna tuning or matching unit
Ron D'Eau Claire
ron at cobi.biz
Tue Feb 20 22:35:47 EST 2018
Alan your explanation opens the door to erroneous conclusions. Frequently a
feed line does NOT have a low SWR by design. It is still a feed line in that
it carries RF from the transmitter site to the radiator with a minimum of
radiation from itself. A classic example is the Zepp antenna: a 1/2 wave
long radiator fed at one end with a 1/4 wavelength long open wire feed line.
The SWR on the feed line is intentionally very high because its function is
to translate the very high impedance at the end of the radiator to a low
impedance easily handled by the transmitter, nowadays generally using a
matching network commonly called a "tuning unit" since modern transmitters
are designed for a 50 ohm non-reactive load.
Another example is a wire radiator fed at the center with open wire feed
line for operation on a variety of bands. Again the SWR in the feed line
will be very high, depending upon the length of the radiator, the length of
the feed line and the frequency of operation. But, using a feed line with an
impedance of 450 to 600 ohms, the SWR in a real-world H.F. installation
where the radiator is at least 1/2 wavelength long at the lowest frequency
of operation, the SWR on the open wire feed line will not exceed about 20:1
so the losses will be very low. The real advantage to this setup is that the
matching network can be in the shack and within easy reach of the operator
instead of being mounted remotely at the center of the radiator.
I present the use of 50 ohm coaxial line without a matching network as a
"special case" where it possible to design a radiator or system of radiators
that presents an impedance at its feed point that is a close enough match to
50 ohms without the network. However, using a low-impedance like such as the
common coax means we must pay special attention to the SWR on the feed line
to avoid excessive losses. For example, with 50 ohm coax in an HF
installation, it is easy to realize an SWR of greater than 100:1 and very
high losses. In such a case a matching network at the transmitter will not
reduce the feed line losses.
73, Ron AC7AC
On 2/20/2018 8:16 PM, Alan B via Elecraft wrote:
> This all depends on what is meant by antenna tuning.
> When teaching newbies the wrong phrase can cause problems that are not
seen till later.
> Antennas are tuned with wire cutters or a hacksaw so the feed impedance is
the desired value, ideally 50 ohm resistive or at least a match for the
feeder concerned.
> Of course that is not always practical so an antenna matching unit brings
the antenna or antenna plus feeder input impedance to the value wanted by
the transmitter.
> Too often I have seen students believe the ATU alters the current and
voltage distribution on the antenna so it looks exactly like the
distribution on a dipole of the correct length for the frequency concerned.
> Granted the currents and voltages might change as the ATU is adjusted but
that does not make the antenna radiate more efficiently. The reflection at
the feeder/antenna junction is unchanged.
> In training we use the term antenna matching unit, AMU, to avoid best we
can the students starting off with the wrong impression.
> Amongst ourselves we can get away with slack terminology, we all know what
is meant; in front of trainees it is a different story.
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