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G8MNY  > TECH     22.11.23 10:22z 132 Lines 5727 Bytes #14 (0) @ WW
BID : 60795_GB7CIP
Subj: QRP SWR Bridge
Sent: 231122/1013Z @:GB7CIP.#32.GBR.EURO #:60795 [Caterham Surrey GBR]

By G8MNY                                        (Updated May 09)
(8 Bit ASCII graphics use code page 437 or 850, Terminal Font)

For QRP work a resistive (lossy -6dB) bridge, that you switch out after use is
the simplest type. However I don't think these are made commercially! You
sometimes see the simple circuit with QRP rigs. If you run QRP & can't build
one yourself I suggest you get a local to build it, possibly a novice!

This uses a double pole changeover switch (measure-operate), another single
pole C/O switch for SWR Forward & Reverse & optionally a calibrated power
switch. The 3x 50R rated at a quarter of the RF power (& each can be made
from a 47R or 2x 100R, or 3x 150R for better power rating & must be non
inductive carbon or metal film types). 1 RF diode OA81/schottky for sensitive
work or 1N4148 up to 10W, 2 caps, series meter calibration variable R (220K)
preset & 220K SWR variable & a meter (100uA) (from old CB SWR meter?).

          OPERATE                            OPERATE
TX Socket o컴o컴>o컴컴컴컴컴컴컴컴컴컴컴컴컴o<컴o컴컴o Load Socket
                 o컫컴컴컴컴컴50R컴컴컴컴컫훟           (e.g. aerial/ATU)
          MEASURE                          MEASURE
                  50R   Diode     1n      
                   쳐컴캐>냐컴쩡캐냐컴o컴>o SWR
                             V           o Cal & Power
                      Power o o SWR      
                           /   \         
                         Cal   SWR       
                  50R    Power  Pot       
                        Meter  === 1n    

When in operate mode the RF goes straight through. In measure mode the Tx sees
2x 50R in series & in parallel with a 50R in series with the load. So when the
load is 50R, the Tx sees the correct 50R. If the load is 0 the Tx sees
100//50 = 33R, or if the load is infinity the Tx sees 100.

The diode detector & meter look at the difference in voltage & phase between
half the Tx voltage & the load & 50R. Only when the load is exactly 50R
resistive, the detector see no RF voltage.

If a 2 pole 3 way switch is available then the SWR, CAL SWR, & POWER function
can easily be made on 1 switch with a 2nd 1nF cap.

          OPERATE                        OPERATE
TX Socket o컴o컴>o컴컴컴컴컴컴컴컴컴컴컴o<컴o컴컴o Load Socket
                 o컫컴컴50R컴컴컴컴컴쩡o           (e.g. aerial/ATU)
          MEASURE                      MEASURE
                  50R             1n ===
                      Diode       o컴 SWR read
                   쳐컴캐>냐컫컴컴>o컴 SWR Cal
                            V     o컴 POWER
                     Power o o o     
                         旼 읕    === 1n
                        Cal   SWR    
                  50R   Power  Pot    
                       Meter  === 1n 

It is best to make it from an old CB meter (Band/frequency sensitive) as you
get the Box, Meter with a SWR scale, sockets, switches & pot. Sensitivity is
down to about 0.2V of RF at the germanium/Schottky detector which is about
0.8mW. For powers greater than 5W, a silicon 1N4148 should be used to withstand
the PIV.

In this design there is RF across the power calibration & SWR variable Rs, so
these leads must be very short!

Depending on the layout & components used the meter could be accurate to VHF.

The three 50Rs must be resistive non inductive type! (NO WIRE WOUNDS). They can
each be made from 1/2/3 resistors, 1 can be say a 1W 47 for a 4W bridge rating
or 2x 1W 100R (6 in all) for an 8W rating, or 3x 1W 150R (9 in all) for
a 12W mean rating.

As you will see the scale is far from linear! From this table you can mark up a
new scale if you have not got an old CB SWR meter. These tables do not take
account of diode loss!

 SWR   SCALE         SWR    SCALE         SWR    SCALE
 1.0    0.0%         2.0    33.3%         6.0    71.4%
 1.05   2.4%         2.2    37.5%         7.0    75.0%
 1.1    4.8%         2.4    41.2%         8.0    77.8%
 1.15   7.0%         2.6    44.4%         9.0    80.0%
 1.2    9.1%         2.8    47.4%        10.0    81.8%
 1.3   13.0%         3.0    50.0%        15.0    87.5%
 1.4   16.7%         3.5    55.6%        20.0    90.5%
 1.5   20.0%         4.0    60.0%        40.0    95.1%
 1.7   25.9%         5.0    66.7%        Inf    100.0%

With a fixed preset value it is also a power meter. Unlike pickup line
reflectometer type of SWR bridge (CB) sensitivity is flat across all bands.

 0.00W      .0%         1.5W    38.7%
 0.03W     5.5%         2.0W    44.7%
 0.05W     7.1%         2.5W    50.0%
 0.1 W    10.0%         3.0W    54.8%
 0.15W    12.2%         4.0W    63.2%
 0.2 W    14.1%         5.0W    70.7%
 0.25W    15.8%         6.0W    77.5%
 0.3 W    17.3%         7.0W    83.7%
 0.5 W    22.4%         8.0W    89.4%
 0.8 W    28.3%         9.0W    94.9%
 1.0 W    31.6%        10.0W   100.0%

N.B. 10W fullscale assumes the 3 Rs are 3W each (e.g. 9x 1W 150R carbon) !

In MEASURE mode the Tx only sees 6.25% of the load's reflected power, so this
is always a good SWR match on tuning up. Also the tuning up aerial power is
only 25% of Tx power reducing Band QRM.

See also my Tech buls on "Drake WH7 QRO HF SWR Bridge",
"A Homebrew HF SWR Bridge", "PEP Meter modification",
"HF ATU & SWR Bridge VC300LP/QT-1" & "Meter Damping & Speed Up".

Why don't U send an interesting bul?

73 De John, G8MNY @ GB7CIP

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