RF measuring bridge for the standing wave alignment
An even QRP station is dependent on the fact that the RF produced by the TX the antenna is achieved and radiated by this completely. A measure for it, how much energy is not accepted by the antenna system and into the output stage is reflected, is the standing-wave ratio. How it can be measured with simple means, this contribution shows.
It concerns a bridge circuit for RF, whose application for the standing wave alignment decreases/goes back with QRP stations on George Grammer, W1DF and in the " QRP - Notebook " by Doug DeMaw, W1FB (1) is contained.
The application of this bridge circuit for standing wave measurement has the advantage that only small RF voltages are needed for the measurement. Thus the bridge circuit is suitable outstanding for QRP. Even here has one often the problem that with conventional standing wave measuring instruments, which do not operate according to the principle of the Reflektometers - if they are not particularly appropriate for QRP - power output for the full rejection of the display been sufficient. Thus an exact SWR is not possible - for measurement. The established circuit achieves already 40 mW RF of a RF generator full-scale of the display.
Fig. (1) shows the schematic diagram. The measuring bridge is appropriate for 50W. Resistances the R1... R3 to take, i.e. use if possible no wirewound resistors or coiled of layer resistances should be induction-poor.
The 50 W receives one by parallel connection from 2 pieces 100 W resistances. I use so-called metal film resistors with an input of 1 Watt. After I had examined such a resistance more exactly, by removing the dye film with solvent, I had to state that I deviated here from the pure teachings: The layer of metal had a spiral structure! The circuit to 30 MHz operates nevertheless satisfyingly.
Resistances the R1... R3 should be able to take up the double performance, which is supplied to the bridge at RF to total. With 3 resistance à 2 Watts (parallel connection) is laid out the bridge only to 3 Watts of transmitting power (continuous line). Brief tests with performances around 5 Watts of RF are also possible.
Disadvantage of this circuit is that the monitor function is not possible, i.e. often remains the measuring instruments in the antenna line and enables the check of the presence of RF. However actually needs one for the QSO drives no jumping pointer. It is to be gone around by the bypass circuit over switch S1 possible the measuring bridge, better is it however after the measurement the device from the antenna line to be removed.
In order to structure this device fast and inexpensive, it offers itself to convert a SWR measuring instrument from the CB-Radio-Area. There are these devices in the CB - radio requirement for scarce under 20, -- DM. Such a "blank" bring along a solid casing, an instrument movement with a sensitivity of approx. 200 mA, two female connectors, the necessary diode and the potentiometer for further use. Additionally one only one rotary switch (2 levels, 3 contacts) and three 50 W of resistances needs.
First the pcb (printed circuit board) is removed. On it are two diodes, of which one can be recycelt for our purposes. Then the change over switch is forwardbackwards removed. Here the rotary switch must be installed. In addition one makes of a small piece pcb material a mounting plate with 6 mm a drilling, which carries the switch and which is screwed from the rear to the front plate.
Before the switch is finally built into the device, it should be wired.
The remaining circuit is freely wired at the rear wall between the female connectors. As solder supports I have so-called uhf condensers 3 pF used, which rest against the rear wall, but was not glued to. That keeps whole ones between the sockets and the rotary switch mechanical by the rigid connecting leads of the resistances and condensers.
To calibrate the scale, one needs induction-poor laboratory resistors, which one can attach to antenna socket of the device.
If RF is because of the bridge and the antenna link with 50 W a resistance is final, then bridge zero are present, i.e. the pointer of the instrument may not deflect. In this configuration the max. usable frequency of the measuring bridge can be determined. With my copy the pointer deflects half pointer width with 30 MHz, which one can still aktzeptieren even. Thus the short wave area is entered.
In the position CAL with the automatic controller P1 full-scale is stopped.
60 W if a resistance is attached, then a SWR of 1,2 is situated: 1 forwards, the pointer position can be marked.
With further resistances the remainder of the scale can be calibrated.
50 W 1:1
60 W 1,2:1
75 W 1,5:1
100 W 2:1
The relation of the laboratory resistor to the value 50 W is equal to the voltage standing-wave ratio.
To measurement transmitter and antenna are brought attached, the switch into the position to CAL, brought RF created (pressed on the morse key) and the display with the automatic controller P1 on full-scale. Then to SWR is switched and the result is read off. If that decreases/goes back completely pointers on zero, we have a SWR of 1:1.
1 "Blank" CB SWR Meter "CB -Master 420", "SWR-2" or similar. e.g. Conrad stock No.: 26 03 04-99
6 resistors 100 W /1 Watt
2 condenser 10 nF
1 resistor 1 kW
4 condenser (or ceramic soldering points)
1 rotary switch
Fig. (1) schematic diagram
Fig. (2) The "Blank", an usual SWR measuring instrument from the CB-Radio-Area.
Fig. (3) View into the wiring of the measuring bridge for the standing wave alignment .
(1) Doug DeMaw, W1FB: QRP - Notebook, 3. Edition 1990, ARRL Inc., ISBN 0-87259-306-1