L matching network

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K1JJ:
Well said, Rick -

One potential problem with adjusting an L network... What if the antenna is ~near 50 ohms and the transmitter is ~near fixed 50 ohms but we still want the match tuned to 1:1?

Tapping the cap on one side of the coil or the other may not do the trick. (pi)    IE, a close 50 ohm to 50 ohm match is sometimes hard to tune/match.  I've run across this problem when driving a ~50 ohm linear tube input with another 50 ohm amplifier in cascade.  The solution was to hang the coil from the input to ground (T-match) and put the caps in series on each side. There is always a sweet spot that gives a 1:1.

As for in-field antenna tuning, my favorite method is to get out the MFJ-259B ant analyzer with the coil and capacitor and a pile of clip leads. Play around with positioning and coil taps until 1:1 is found. I would favor the cap going to ground to help reduce harmonics, (pi) but as said, sometimes a T-match is the only one that will work, but has less harmonic suppression since the caps are in series.  In the case of a 50 ohm to 50 ohm  antenna situation, it would equate to a vertical radiator of slightly longer than 1/4 wave to produce 50 ohms. So, it might be wiser to purposely make the antenna longer than 1/4 wave  (>50 ohms) to be able to use a FB pi match using a higher impedance feed.

T

W3RSW:
Your close case sure does happen, especially if the antenna is cut to Natural resonance right in the middle of the band.  If close to 1:1 then simply add a tuner bypass circuit. This usually calls for a double deck or similar switch though.  You don't need the tuner for several KHz perhaps around that node.  ;D.

When adding all that wiring and new switch contacts, still be sure to keep stray capacity as low as possible or 10 meters, etc. may be only close to match with main cap set fully unmeshed.  Switch may have to be at rear of cabinet next to in/out connectors with shaft extension to front of cabinet to achieve this.

I've seen reactance approach infinity / zero on one side of the 1:1 point, then wildly swing to opposite infinity / zero on the other side.  When I first encountered this I added parallel capacitance up the woz as I approached that node before realizing what was happening.

Don't ask what 'infinity / zero' is; you just have to experience it. Convient way to describe a wild unknown reactance that won't behave.  ;D

WA2SQQ:
So to answer the question, yes I have an analyzer. Am I capable of using it to its full potential – no.

I’m not embarrassed to say that smith charts and complex impedances have always been my black hole. I’m sure it can be simple once you find the explanation that flips the switch in your brain and makes the light go on. To get around this Inverted L project I decided to make a simple L network, measure the values that give me the match I need, and substitute fixed values at the feed point. From the info you’ve all shared I just need to experiment to see what configuration I need. I wish I could look at the complex impedance and know exactly what I need. I appreciate the explanations which I have learned from. The only stupid questions are those we don’t ask!

KA2DZT:
Rick

Didn't your teacher ever tell you that you can't divide by zero?? ;D

Don't try to figure out Smith Charts, you'll just be going around in circles. :(

Fred

W3RSW:
Nor can infinity plus, or times, or divided by, or integrated by, or differentiated by anything still equal anything but infinity. That's the beauty of a very powerful concept.

If your divide by zero observation is directed,  the "/" symbol didn't mean divide by, but meant a coupling of concepts, e.g., I'm a Jack of all trades, fisherman/astronomer/amateur radio operator ...etc.  ;D

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