http://elecraft.85.s1.nabble.com/Antenna-tuning-or-matching-unit-tp7638335p7638341.html
I agree with Ron in that one must understand using an ATU at the transmitter does not change feedline loss. In fact it adds additional system loss due to loss incurred in the tuner. If the transmitter can not deliver rated power into the load that is a different issue to resolve. The ATU may accomplish this, but with added system loss.
> On Feb 20, 2018, at 9:35 PM, Ron D'Eau Claire <
[hidden email]> wrote:
>
> 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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