Sorry I need to add some thing I left out. The ATR-10 modification I am using is without the series capacitor sorta speak. The MFJ 989B is fed into a tap on the coil. Cause of the T network on the tuner There is series capacitance on the input to the external components. So the external components is an inductive L network with a cap from the antenna going to ground. 100pF seams to give the MFJ 989B a bit more tuning range than the 250pF cap.
Jim K9TF On Tue, Oct 8, 2013 at 12:56 PM, Jim GM <jim.gmfo...@gmail.com> wrote: > Tom W8JI made some measurements back in 1997 on the ATR-15 when it first > came out. Not sure how this will print out. > > * > * > > *The ATR-15 measures as follows for load R and loss on 1.8 MHz: > 12.5 ohms 21.8% (27.8%) > 25 ohms 16.5% (22.9) > 50 ohms 10.6% (11.0) > 100 ohms 8.9% (10.7) > 200 ohms 8.1% (10.4) > 400 ohms 7.4% (10.1) > 800 ohms 6.9% (8.8%)* > > > Doubling the value of C cuts low impedance losses in half. I put 500 pF > air variable caps in the ATR-15 and the 12.5 ohm loss was 13.1 percent. > > I don't even come close to the MFJ-989 results they published. I > measured the values (they are the ones in brackets above) for the 989 > using both an HP 4191A analyzer and a Harris RF voltmeter, as well as > confirmed on meters. > > > My question is where in the tee match was the extra capacitance installed? > was it going to ground? > > N4XM commented in 1997 on the MFJ 989 review. > > >*>>T network tuners (like the 989, Xmatch, Vectronics, Tucker, * > >*>>Murch, etc) handle the least power on 160 and with capacitive* > >*>>reactance low resistance loads. T network tuners handle MORE * > >*>>power into higher resistance loads or loads with some amount * > >*>>of inductive reactance. * > > > w8JI comments > > >*L network tuners (like the Ten-Tec, Nye Viking, etc) handle more* > >*power into impedances near 50 ohms, but often do a poor job * > >*matching reactive or very low impedance loads on low frequencies. > >(Sometimes* > >*these tuners are called Pi-networks, even though* > >*they do not really function as a pi except perhaps on 15 or ten* > >*meters) * > >** > >*In a T network tuner, maximum efficiency and power handling * > >*generally occurs when maximum and equal amounts of * > >*capacitance are used in the capacitors, and the least amount of* > >*inductance is used. This is true even though many other settings* > >*will produce a low SWR.* > > > > I later read more archives. What came up was the ATR-30 How beefy it was. > What the plate spacing on this tuners capacitors? I would think on a > short antenna like mine it is not enough. > > So back to the ATR-10. The W8JI design with variable taps seams to handle > high power with nor arcing problems. > I just do not know why this is working for me. Can some one please > explain this to me I am confused? All this reading > requires a younger mind. > > Jim K9TF > > On Sat, Oct 5, 2013 at 1:54 PM, Jim GM <jim.gmfo...@gmail.com> wrote: > >> >> My antenna is a 99 foot inverted L for 160M with 95 short radials 2000 >> feet of wire. >> >> What I am trying now is W8JI suggested modification design to improve the >> ATR-10 design by taps on the coil for adjustment. Works well. >> http://www.w8ji.com/antenna_tuners.htm >> >> Had my MFJ-989B at the feed point, I had to add a capacitor to ground >> 250pf so it could work. Inductance was at max farthest away from ground. >> Voltage was so high at 300 watts in, the 0.125 meshed plate spacing arced >> over, it has a T network and I had expected this to happen. >> http://www.mfjenterprises.com/support/MFJ-989B/MFJ-989B.pdf I run 500 >> watts. >> >> What I was looking at is removing the coil and the input would be at the >> top of C1. I made a test with QRP using 2 small spaced caps taken out of an >> MFJ-901A. Works well. Because of the high voltage situation would there be >> an issue with this and I need to go to a 0.25 inch spacing between plates? >> Vacuum variables are out of my price range. >> >> ATR-10 with W8JI modifications is a high pass tuner or is it? Removal of >> the coil would make this a low pass circuit. Besides that on 160M which one >> would be more efficient, and off the top of your head what would be the >> percentages? If you had my make do with what I have setup, what tuning >> network would you use? >> >> Does any one have an email address for MFJ Service department? I have >> the wrong one. I sent my MFJ 998RT to their Service department, the High >> voltage that was developed with 300 watts pretty much done it in. The >> ATR-10 W8JI redesign has greatly reduce the High voltage that I was seeing >> with out the inductor. >> >> -- >> Jim K9TF >> > > > > -- > Jim K9TF > -- Jim K9TF _________________ Topband Reflector