[TenTec] Model 1225 SWR and Wattmeter Kit

Steve Hunt steve at karinya.net
Tue Jul 22 17:23:30 EDT 2014


Things are seldom as simple as they at first appear!

The Model 1225 - like all the SWR meters I've ever built or owned  - 
does _not_ simply indicate power by measuring line voltage (V) and 
computing V^2/50. Rather, it takes a sample of the line voltage and 
current, from which it derives forward and reflected powers. The 1225 
uses capacitive divider (C2,C3,C1) to generate a sample of the line 
voltage (V), and current transformer L2 to generate a sample of the line 
current (I).

This is how it works:-

The basic line equations are:
Line voltage V = Vfwd + Vref
and
Line current I = Ifwd - Iref

The meter assumes a Zo of 50 Ohms (actually 47 Ohms - the value of R1) 
and generates a voltage proportional to I*Zo by driving the current 
sample through resistor R1.

So we now have two internal voltages proportional to:
Vfwd + Vref
and
50*I = 50*Ifwd - 50*Iref = Vfwd - Vref

Summing the two and rectifying with D1 produces a voltage proportional 
to 2*Vfwd; subtracting them and rectifying with D2 produces a voltage 
proportional to 2*Vref. These voltages are then displayed on the 
twin-needle meter as Forward and Reflected powers, Pfwd and Pref.

If you need to know the power delivered to the load, you must manually 
subtract Pref from Pfwd - the 1225 can't do that for you; but an LP-100A 
can if it's switched to the "Net" power mode.

Steve G3TXQ






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