Many of the questions about use of extra tuners with an ATU are answered by a review of the fundamental assumptions for the design of simple (cheap) power and swr meters.
Most common tuners assume that the input to the tuner is 50 ohms. The SWR calibration is based on the ratio of reflected to forward power. If the input impedance is not 50 ohms then calculation of power is not accurate because the meters are not smart enough to measure actual power based on the true input impedance. They indicate power based on a voltage or current measurement. The real power will vary if the input impedance changes. When two tuners are used in series, such as the K3 ATU to an MFJ* to an amplifier, the impedance at the output of the ATU and the input to the MFJ is rarely 50 ohms. Only if the amplifier input is 50 ohms resistive and the MFJ is bypassed, then the SWR indicated by the MFJ might be close to 1 to 1 after the ATU is tuned. However, if the amplifier input impedance is perhaps 100 ohms, then the ATU will match the 100 ohms and the MFJ measurements will be referenced to the 100 ohms. The SWR based on forward and reflected power indicated by the MFJ will not be accurate. The forward power indicated by the MFJ may show twice the real input power when the input termination is 100 ohms. Power = (V squared)/R. As you can see the indicated power changes as the resistance changes. The real power is not changed, just the reference has changed and is not accounted for. So, do not waste time reading the power at the output of an ATU unless you know that the load is 50 ohms. Bypass the ATU and use the MFJ and you may get a reasonable indication on the MFJ when the transmitter output impedance is 50 ohms. Or delete the MFJ and use the ATU indicators. Now if you have a true power measuring watt meter that actually calculates current times voltage without using the input impedance as a reference then these measurements might be of some interest. * MFJ is used here to represent the most commonly available SWR meters using forward and reverse power indicators that assume a 50 ohm input impedance including the common Bird and others. I did not get into the additional errors introduced when the reference is a complex impedance and not just a simple resistance. True power is based only of the resistive part of the impedance. So again the error is compounded with the simple P=(I squared) times R approach Aloha, John KH7T W7BRS wrote: > Elecrafters, > > I know this is off-topic, but the range of ham-smarts on this list is > pretty good so I'll throw the question here and hopefully get some of that > ham-smarts too. > > OK -- K3/100 with KAT3 is the rig. > > I have it connected through a tuner and then to the SB-200 and then Dipole > > K3/100/ATU <-> MFJ tuner <-> SB200 <-> Dipole > > If I use Auto mode on the KAT3, put the MFJ tuner in Bypass and hold TUNE > then the KAT3 goes and brings the SWR (according to the display) to a > favorable value (1.0 to 1.2). I use a MFJ tuner between my Rig and Amp > because the Old Bard told me to. Actually my setup is different than > what I just described, but I'm simplifying this for the question at hand. > > [ What I really do is K3 <-> Small MFJ Tuner <-> SB200 <-> Big MFJ Tuner > <-> Dipole ] > > On the watt meter of the (small) MFJ Tuner, the indication is I have a non > favorable SWR, more reflected power than had I put the KAT3 in bypass and > manually tuned the MFJ Tuner to a favorable SWR (1.0 to 1.2 or so). > > So, can someone explain in basic terms what's happening when I use Auto > mode on the KAT3 and see favorable SWR on the KAT3 (via K3 display) but do > not see favorable SWR on the MFJ tuner? > > This is not a round-about way of expressing a problem with the K3. On the > contrary - I couldn't be more pleased with the KAT3 performance and so > forth. > > I'm asking this question as a data-point before I decide to build the KAT2 > for my finished K2. > > Thanks in advance, > > -jeff > > > -- > Jeff Wandling DE W7BRS K3 #2105 http://w7brs.com/k3 > k3u...@w7brs.com > _______________________________________________ > Elecraft mailing list > Post to: Elecraft@mailman.qth.net > You must be a subscriber to post to the list. > Subscriber Info (Addr. Change, sub, unsub etc.): > http://mailman.qth.net/mailman/listinfo/elecraft > > Help: http://mailman.qth.net/subscribers.htm > Elecraft web page: http://www.elecraft.com > > > _______________________________________________ Elecraft mailing list Post to: Elecraft@mailman.qth.net You must be a subscriber to post to the list. Subscriber Info (Addr. Change, sub, unsub etc.): http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/subscribers.htm Elecraft web page: http://www.elecraft.com
