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G8MNY > TECH 04.08.05 23:01l 118 Lines 5132 Bytes #999 (0) @ WW
BID : 56930_GB7CIP
Read: DJ4KI GUEST OE7FMI
Subj: QRP SWR Bridge
Path: DB0FHN<DB0RGB<OK0PPL<DB0RES<ON0AR<TG9IKE<HS1LMV<CX2SA<GB7YFS<GB7CIP
Sent: 050804/2107Z @:GB7CIP.#32.GBR.EU #:56930 [Caterham] $:56930_GB7CIP
From: G8MNY@GB7CIP.#32.GBR.EU
To : TECH@WW
By G8MNY (Updated Feb 05)
For QRP work a resistive (lossy -6dB) type 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 50ê Rs rated at ¬ of the RF power (& each can be made from a 47ê
or 2x 100ê, or 3x 150ê 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 (220Kê) preset & 220K SWR
variable & a meter (100uA) (from old CB SWR meter?).
OPERATE OPERATE
TX Socket oÄÄoÄÄ>oÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄo<ÄÄoÄÄÄÄo Load Socket
oÄÂÄÄÄÄÄÄÄÄÄÄ50êÄÄÄÄÄÄÄÄÄÂÄo (eg aerial/ATU)
MEASURE ³ ³ MEASURE
50ê Diode 1n ³
ÃÄÄÄij>³ÄÄÄÂÄij³ÄÄÄoÄÄ>o SWR
³ V o Cal & Power
³ Power o o SWR ³
³ / \ ³
³ Cal SWR ³
50ê Power Pot ³
³ ÃÄÄÄÄÄ´ ³
³ Meter === 1n ³
GndÄÄÄÄÄÄÄÄÁÄÄÄÄÄÄÄÁÄÄÄÄÄÁÄÄÄÄÄÄÄÄÁÄÄÄÄÄÄÄÄÄÄGnd
HOW IT WORKS.
When in operate the RF goes straight through. In measure the Tx sees 2x 50ê in
series in parallel with a 50ê in series with the load, so when the load is 50ê,
the Tx sees the correct 50ê. If the load is 0ê the Tx sees 100//50 = 33ê,
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 & 50ê. Only when the load is exactly 50ê
resistive with the detector see no difference.
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
oÄÂÄÄÄÄÄ50êÄÄÄÄÄÄÄÄÄÄÂÄo (eg aerial/ATU)
MEASURE ³ ³ MEASURE
50ê 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
50ê Power Pot ³
³ ÃÄÄÄÄÄ´ ³
³ Meter === 1n ³
GndÄÄÄÄÄÄÄÄÁÄÄÄÄÄÄÁÄÄÄÄÄÁÄÄÄÄÄÁÄÄÄÄÄÄÄÄÄÄGnd
CONSTRUCTION.
It is best to make it from an old CB meter as you get the Box, Meter with a SWR
scale, sockets, switches & pot. Sensitivity is down to about 0.2V of RF at the
detector which is about 0.8mW.
In this design there is RF across the power calibration & SWR variable Rs, s
these leads must be very short!
Depending on the layout and components used the meter could be accurate to VHF.
The two 50ê Rs must be resistive non inductive type
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%
1.1 2.4% 2.2 37.5% 7.0 75.0%
1.1 4.8% 2.4 41.2% 8.0 77.8%
1.2 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 (eg 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.
Why don't U send an interesting bul?
73 De John, G8MNY @ GB7CIP
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