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Old 08-08-2008, 05:59   #16
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Originally Posted by btrayfors View Post
Partly, these losses are due to high SWR and partly (or mostly) they may be due to the fact that the line between the tuner and the antenna is a radiating part of the antenna system itself.
Most manual tuners either have a balanced output or coaxial output or both. On a boat, placing the tuner near the radio, we would use coaxial cable to feed the antenna, connecting the center conductor to the backstay lead-in wire and the shield to the ground plane. In this case, the coax does NOT radiate the signal. The signal travels between the center conductor and the inside surface of the shield til it gets to the feed point. Some of the signal can then travel back down the outside of the shield which can be radiated. To prevent this we would use a choke balun in the form of ferrite beads or several turns of the coax onto a cylindrycal form whose diameter is several inches.

Eric
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Old 08-08-2008, 06:17   #17
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Originally Posted by sildene View Post
From what I've seen, the Auto-tuners do not appear to have an SWR display, so how would you know your communication problem is due to a poor impedance match (e.g. it may be higher than 2:1, but you wouldn't know). Do you ever use a separate SWR meter with auto-tuners, or is there no point?
Some tuners such as Icom and SEA provide an indication on the transceiver that the tuner has successfully tuned. The tuner does not change the impedance mismatch at the feedpoint, it resonates the antenna system and provides impedance matching between the tuner and the radio. When using a tuner, reflected power is not necessarily lost power. It is re-reflected by the tuner and the only losses are due to transmission line losses. The greater the SWR, the greater the loss. With a tuner at the antenna feed point, this loss is nearly eliminated.

Eric
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Old 08-08-2008, 07:26   #18
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For further in-depth information on this subject, I highly recommend the book, Reflections 2. You can read some of it online from this site and in particular I would suggest chapter 7 for this subject. It can be quite technical, but the layman can still get a lot out of it.

Eric
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Old 10-08-2008, 21:23   #19
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Thanks for thread replies

Thanks again for the solid info, and pertinent references, Eric and Bill.

Martin
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