Hi Tom, Ok, one more swing at it.
The "tuner" as Don states (and we agree), is transforming the 50 Ohm impedance of the transmitter, into something that mates better with the impedance of the antenna for (hopefully*) better transfer of energy to/from the antenna. There are similar circuits inside every radio to transfer the energy between sections. The proper term would be "impedance matching circuit or device" but we're taught (and stuck with) 'tuner'. Now, using our beloved K3 in this example: Tuner in bypass produces ~50 Ohm impedance at the back of the K3. Tuner in use produces an impedance of somewhere between 5-500 Ohms at the back of the same K3. You don't know what that impedance is. The SWR meter is expecting (because transmitters are usually set to) 50 Ohm impedance. If that impedance is off, the meter reading is probably wrong. Sure, it will read something, but the reading is worthless (even as a power meter) because of the mismatch. The amount of error is dependent on the amount of mismatch (which we don't know). So you're correct, that they are only accurate at 50 Ohm impedance (give or take a small percentage). This is why you put the SWR meter between the output of a known impedance (50 Ohm coming out of the transmitter) and the matching device (50 Ohm input). As the device changes LC values to compensate for (match) the reactance(s) of the antenna, the standing wave at the meter is reduced. This is how you can tell that the matching device is transferring more energy to the antenna (the reactance is 'tuned' out). As Don also accurately stated, it doesn't make the antenna work ANY better, but it does transfer more energy TO the antenna (disregarding tuner and line losses) because of better matching. And note that I most carefully said that the meter makes a reading AT THAT POINT IN THE FEEDLINE. If you add/subtract patch cables to an external meter (or alter feedline length), your reading may very well be different. The 'trap' that many hams fall into is that the feedline is treated as a 'hose' between transmitter and antenna. It isn't; but is PART of the entire circuit (it is not passive). Which leads to your next comment. What you were referring to was that the impedance of the antenna is best read (is duplicated) at half wave intervals on the feedline (ignoring the added feedline reactance). Most hams simply cut to convenient lengths (me too) because we're using multiple band antennas (or at least feeding them that way) and we tend to let the 'tuner' take up any slack (or ignore the losses). This is why the K3 meter, sensing at the transmitter output and before the internal tuner, is the best place to measure SWR. The tuner matches THAT point in the feedline system for best transfer of energy. The bottom line is simple. If you are using a matching device (a tuner) the ONLY place that a SWR meter will accurately read what you expect, is after the amplifier and before the matching device. This should also show you why short patch cables should be used to attach the SWR meter to the transmitter, to minimize error from reading at a random point in the feed (it's more accurate AT the transmitter). Does that help? Rick WA6NHC * I threw in this caveat because I once had a very nice, high power, homebrew 'tuner' that I could feed with the 200 watt transmitter and get a 'perfect match' with *nothing* delivered to the attached end fed random wire. The 'tuner' simply converted it into heat. If I retuned using a different LC combination, I was heard. Don't ask what type etc., because I don't remember and stupidly sold it over 30 years ago. :o( -----Original Message----- From: Tom Azlin N4ZPT Not sure I understand this. I did not think an SWR meter was supposed to work properly only when seeing 50 ohms restive. If the line is not flat it is terminated in to something other than the feedline impedance, i.e. not 50 ohms restive in this discussion. I would expect the SWR meter at some point in the feedline to be accurate under those conditions because it is supposed to measure the standing wave ratio of the feedline with respect to that 50 ohm resistive. If I then have an antenna coupler in the radio that is matching the transmitter to that non-50 ohm impedance why would the SWR meter change to inaccurately measuring the SWR? Putting the K3 into bypass or letting it "tune" should not change the SWR measures at some point in the feedline. The K3 may show 1:1 simply because it properly transformed the line impedance to 50 ohms restive. Perhaps I am missing something here. Sorry for continuing the discussion. 73, tom n4zpt On 1/2/2012 8:06 PM, Rick Bates wrote: > Let me see if I can correctly state, with clarity for all to understand so > this thread can die peacefully and we can move on (please?). > > The K3 puts out power into an internal tuner (if installed) and then to an > SO-239 at the rear of the radio. If the tuner is in bypass, the impedance > is very close to 50 ohm resistive at that connector. In this case, any SWR > meter (bridge) downstream to the antenna will read the SWR (AT THAT POINT IN > THE FEEDLINE which may/not agree with the K3 reading). > > If the tuner is NOT in bypass, the impedance at the same SO-239 will be > +-10:1 of 50 ohms. Since any other SWR meter (bridge) between the K3 and > the antenna is probably NOT seeing the expected 50 Ohm impedance, the > reading of OTHER than the K3 meter will be called into question and more > than likely wrong. > > Repeating, when the internal tuner is used, the K3 meter is the only one > that is accurate. > > Thank you and Happy New Year, > Rick wa6nhc ______________________________________________________________ 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
