Then there are those of us who use high ratio end fed half waves for specific frequencies with great luck. I do this on 160 for 1.900 coverage. This allows me to deviate marginally from that frequency with a matching unit. Works great near 3.8 as well. Now to get it further into the trees. Winter has hindered my latest experimental antenna. NVIS is better than nothing :-).
73, Bill K9YEQ -----Original Message----- From: elecraft-boun...@mailman.qth.net <elecraft-boun...@mailman.qth.net> On Behalf Of Ron D'Eau Claire Sent: Tuesday, February 20, 2018 9:36 PM To: elecraft@mailman.qth.net Subject: Re: [Elecraft] Antenna tuning or matching unit 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. ______________________________________________________________ Elecraft mailing list Home: http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/mmfaq.htm Post: mailto:Elecraft@mailman.qth.net This list hosted by: http://www.qsl.net Please help support this email list: http://www.qsl.net/donate.html Message delivered to k9...@live.com ______________________________________________________________ Elecraft mailing list Home: http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/mmfaq.htm Post: mailto:Elecraft@mailman.qth.net This list hosted by: http://www.qsl.net Please help support this email list: http://www.qsl.net/donate.html Message delivered to arch...@mail-archive.com
