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ATU 101 refresher course

O

OKCRadioGuy

OK... I've forgot some AM theory in my feeble brain. I should add the resistive and reactance component together (on the tower side) when re-designing an ATU, correct? So for 1600kcs on a tower tht measures 31 ohms and +51.6 reactive I could use two caps in series with a value of 1547pf tapped between them with a coil at 6.4uh to ground? Is this correct (90 deg network)?
 
In it's simplest form, take the square-root of the product of the input and output resistance values, then add the antenna reactance value to the calculated leg value going towards the antenna (-39.4+ 51.6=-91.0). With these values, input reactance will be -39.4 ohms (2526.6 pf), shunt leg will be +39.4 ohms (3.92uH) and the final output leg value will be -91 ohms (1093.5 pF). This is +90 degree network if I understood correctly; if not, just toggle the input and shunt-leg reactance values and subtract the +39.4 output leg inductive value from the +51.6 antenna reactance value. You should be good to go! Some call it a "phantom T"....

Merry Christmas!

w/
 
This is kind of a sidebar item. It's interesting how many systems were designed with -90 networks just to save parts. Years ago, one of my "grayer haired" mentors taught me to count the nodes from the output tank out through the ATU. If you keep them properly offset, -, +,-, +, etc, the system will really bark!
 
Indeed fewer parts also in olden days, -90 networks were chosen as they are low-pass filters where +90 are high-pass. Transmitters type accepted prior to January 1, 1960 harmonics weren't required to meet the 43+10log of power requirement, harmonics only needed to be "as low as possible" and low pass networks aided that. Some also used stubs that were open-ended 1/4 wl xmission lines at 3f.

Merry Christmas!

w/
 
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