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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 50Ω Rs rated at ¼ of the RF power (& each can be made from a 47Ω or 2x 100Ω, or 3x 150Ω 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 (220kΩ) preset & 220kΩ SWR variable & a meter (100uA) (from old CB SWR meter?).

OPERATE OPERATE
TX Socket o──o──>o──────────────────────────o<──o────o Load Socket
o─┬──────────50Ω─────────┬─o (e.g. aerial/ATU)
MEASURE │ │ MEASURE
50Ω Diode 1n │
├────│>│───┬──││───o──>o SWR
│ V o Cal & Power
│ Power o o SWR │
│ / \ │
│ Cal SWR │
50Ω Power Pot │
│ ├─────┤ │
│ Meter === 1n │
Gnd────────┴───────┴─────┴────────┴──────────Gnd

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

The diode detector & meter look at the difference in voltage & phase between half the Tx voltage & the load & 50Ω. Only when the load is exactly 50Ω 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──o──>o──────────────────────o<──o────o Load Socket
o─┬─────50Ω──────────┬─o (e.g. aerial/ATU)
MEASURE │ │ MEASURE
50Ω 1n ===
│ Diode o──┘ SWR read
├────│>│──┬────>o──┐ SWR Cal
│ V o──┤ POWER
│ Power o o o │
│ ┌─┘ └─┤ === 1n
│ Cal SWR │
50Ω Power Pot │
│ ├─────┤ │
│ Meter === 1n │
Gnd────────┴──────┴─────┴─────┴──────────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 & 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 50Ω Rs 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 47Ω for a 4W bridge rating or 2x 1W 100Ω (6 in all) for an 8W rating, or 3x 1W 150Ω (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 Load 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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