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G8MNY > TECH 12.11.25 18:45l 132 Lines 5741 Bytes #5 (0) @ WW
BID : 44879_GB7CIP
Subj: QRP SWR Bridge
Path: JH4XSY<N3HYM<CX2SA<ED1ZAC<GB7CIP
Sent: 251112/0935Z @:GB7CIP.#32.GBR.EURO #:44879 [Caterham Surrey GBR]
From: G8MNY@GB7CIP.#32.GBR.EURO
To : TECH@WW
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!
CIRCUIT.
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 and
220K SWR variable and a meter (100uA) (from old CB SWR meter?).
OPERATE OPERATE
TX Socket oﾄﾄoﾄﾄ>oﾄﾄﾄﾄﾄﾄﾄﾄﾄﾄﾄﾄﾄﾄﾄﾄﾄﾄﾄﾄﾄﾄﾄﾄﾄﾄo<ﾄﾄoﾄﾄﾄﾄo Load Socket
oﾄﾂﾄﾄﾄﾄﾄﾄﾄﾄﾄﾄ50Rﾄﾄﾄﾄﾄﾄﾄﾄﾄﾂﾄo (e.g. aerial/ATU)
MEASURE ｳ ｳ MEASURE
50R Diode 1n ｳ
ﾃﾄﾄﾄﾄｳ>ｳﾄﾄﾄﾂﾄﾄｳｳﾄﾄﾄoﾄﾄ>o SWR
ｳ V o Cal & Power
ｳ Power o o SWR ｳ
ｳ / \ ｳ
ｳ Cal SWR ｳ
50R Power Pot ｳ
ｳ ﾃﾄﾄﾄﾄﾄｴ ｳ
ｳ Meter === 1n ｳ
GndﾄﾄﾄﾄﾄﾄﾄﾄﾁﾄﾄﾄﾄﾄﾄﾄﾁﾄﾄﾄﾄﾄﾁﾄﾄﾄﾄﾄﾄﾄﾄﾁﾄﾄﾄﾄﾄﾄﾄﾄﾄﾄGnd
HOW IT WORKS.
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 and meter look at the difference in voltage and phase
between half the Tx voltage and the load and 50R. Only when the load is exactly
50R resistive, the detector see no RF voltage.
WITH SWITCHING.
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 oﾄﾄoﾄﾄ>oﾄﾄﾄﾄﾄﾄﾄﾄﾄﾄﾄﾄﾄﾄﾄﾄﾄﾄﾄﾄﾄﾄo<ﾄﾄoﾄﾄﾄﾄo Load Socket
oﾄﾂﾄﾄﾄﾄﾄ50Rﾄﾄﾄﾄﾄﾄﾄﾄﾄﾄﾂﾄo (e.g. aerial/ATU)
MEASURE ｳ ｳ MEASURE
50R 1n ===
ｳ Diode oﾄﾄﾙ SWR read
ﾃﾄﾄﾄﾄｳ>ｳﾄﾄﾂﾄﾄﾄﾄ>oﾄﾄｿ SWR Cal
ｳ V oﾄﾄｴ POWER
ｳ Power o o o ｳ
ｳ ﾚﾄﾙ ﾀﾄｴ === 1n
ｳ Cal SWR ｳ
50R Power Pot ｳ
ｳ ﾃﾄﾄﾄﾄﾄｴ ｳ
ｳ Meter === 1n ｳ
GndﾄﾄﾄﾄﾄﾄﾄﾄﾁﾄﾄﾄﾄﾄﾄﾁﾄﾄﾄﾄﾄﾁﾄﾄﾄﾄﾄﾁﾄﾄﾄﾄﾄﾄﾄﾄﾄﾄGnd
CONSTRUCTION.
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 and SWR variable Rs, so
these leads must be very short!
Depending on the layout and 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 47R 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.
SCALES.
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.
POWER SCALE POWER SCALE
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 USE.
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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