Modify SWR meter

Question

I am trying to modify a SWR meter (http://www.mondoplast.ro/Reflectometru-powermetru-Performer-DF2469-pg_ft-3597) and add a new scale (now it has 200W/50W and 20W/5W) for 2W/0.5W.

I have made some measurements and it does not work exactly how I expected so I am stuck... The SWR meter is a cross needle Ampere meter (100uA) with 1,8k ohms internal resistor. In the directional coupler is only 1 shunt resistor of 150Ohms.(the schematic for the coupler is similar with the one in the left (http://f1frv.free.fr/main3h_SWR_Bridges_fichiers/DET_bridge_R2.png)) The scale selector adds a capacitor between the FWD/REF line to the ground in the peak position:

FWD:

200W 220uF

20W 100uF

REF:

50W 100uF

5W 47uF

(here I expect this to be for the average not for peak... ) And also adds in the FWD/REF line a series resistor:

FWD:

200W - 145,5K

20W - 33,4K

REF:

50W - 54,5k

5W - 12,5k

(Actually in the schematic there is a fixed resistor an a variable one in series, here is the measured value of the resulted resistor) What values of resistors should I put for 2W/0,5W. (I expected that the resistor to variate linear eg. 200W 145,5K 20W 14,55k) Can you please help me or at least send me to a documentation of how a SWR circuit/bruene bridge works. Thank you, Bogdan

Answer 1

I'm not totally up to speed on SWR bridges, but currently learning to make my own antenna tuner system. This isn't a complete answer, but more of me thinking through this.

The bottom diodes and capacitors are generally called a Peak Detection circuit. Think of it like a half wave rectifier of the signals of forward and reverse power.

C1 and C2 are forming an AC voltage divider. The signal coming from the capacitors is the forward primary signal. This is reinforced on the forward side of the peak detector and subtracted from the reversed side of the peak detector. Reversed power then is accumulated by the reversed peak detector (this is the part I'm still trying to wrap my mind around.)

For calculating the values to use, I would take the expected frequencies and calculate Capacitive reactance. The ratio of the current ranges should predict the smaller ranges. I would expect it to be linear as we are talking about scaling essentially AC power. I think we need a full schematic including the meter shunt. Then we can start figuring out where the reactances are in parallel that make this look nonlinear with the change of one.

What are the values of C2 and R1? I'm also not quite understanding where in the schematic you are talking about for the series resistors. Is that after the FWD and REF connection in the schematic?