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G8MNY > TECH 13.04.24 09:48z 132 Lines 5727 Bytes #18 (0) @ WW
BID : 5358_GB7CIP
Subj: QRP SWR Bridge
Path: SR8BBS<SR1BSZ<IW0QNL<JH4XSY<JE7YGF<LU4ECL<I0OJJ<GB7CIP
Sent: 240413/0935Z @:GB7CIP.#32.GBR.EURO #:5358 [Caterham Surrey GBR] $:5358_GB
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 & 220K SWR variable & a meter (100uA) (from old CB SWR meter?).
OPERATE OPERATE
TX Socket oÄÄoÄÄ>oÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄo<ÄÄoÄÄÄÄo Load Socket
oÄÂÄÄÄÄÄÄÄÄÄÄ50RÄÄÄÄÄÄÄÄÄÂÄo (e.g. aerial/ATU)
MEASURE ³ ³ MEASURE
50R Diode 1n ³
ÃÄÄÄij>³ÄÄÄÂÄij³ÄÄÄoÄÄ>o SWR
³ V o Cal & Power
³ Power o o SWR ³
³ / \ ³
³ Cal SWR ³
50R Power Pot ³
³ ÃÄÄÄÄÄ´ ³
³ Meter === 1n ³
GndÄÄÄÄÄÄÄÄÁÄÄÄÄÄÄÄÁÄÄÄÄÄÁÄÄÄÄÄÄÄÄÁÄÄÄÄÄÄÄÄÄÄGnd
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 0ê the Tx sees
100//50 = 33R, or if the load is infinity the Tx sees 100ê.
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.
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 oÄÄoÄÄ>oÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄo<ÄÄoÄÄÄÄo Load Socket
oÄÂÄÄÄÄÄ50RÄÄÄÄÄÄÄÄÄÄÂÄo (e.g. aerial/ATU)
MEASURE ³ ³ MEASURE
50R 1n ===
³ Diode oÄÄÙ SWR read
ÃÄÄÄij>³ÄÄÂÄÄÄÄ>oÄÄ¿ SWR Cal
³ V oÄÄ´ POWER
³ Power o o o ³
³ ÚÄÙ ÀÄ´ === 1n
³ Cal SWR ³
50R Power Pot ³
³ ÃÄÄÄÄÄ´ ³
³ Meter === 1n ³
GndÄÄÄÄÄÄÄÄÁÄÄÄÄÄÄÁÄÄÄÄÄÁÄÄÄÄÄÁÄÄÄÄÄÄÄÄÄÄGnd
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 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 47ê 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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