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20150207 - SWR Bridge

posted Feb 7, 2015, 1:05 AM by Onno Benschop   [ updated Jun 8, 2015, 4:15 AM ]
SWR Bridge:

Please refer to a suitable "bridge" diagram.


It seems to me that this type of measuring device COULD be ideal for amateurs who use low power, and/or are new to the hobby.

Being relatively inexpensive, it might well be a project that provides interest, as well as encouraging experimentation.

It can indeed be very accurate, which is a big plus.

Build the unit inside a metal box, with the "ground" of the bridge network bonded to the box.

Use BNC or N connectors for better UHF capability.

The standard "bridge" configuration has 4 "arms".

In this case, three are resistors, and the fourth is the antenna under test.

The three 'resistor' arms are each 50 ohms and non-inductive.

Each arm can be made of (up to) three (carbon) resistors in parallel.

For example, 2x 100 ohms or 3x 150 ohms. (carbon 1 watt?)

'Less resistors' is better of course, to minimise stray inductance and capacitance which can affect the readings.

Because of this stray inductance and capacitance, bigger resistors are only practical when using the bridge at HF or below.

For VHF and UHF, making use of SMALLER resistors and CLOSER spacing allows accuracy at frequencies up to at least 1 GHz.

With even greater care in construction, and by using surface mount components, accuracy far above 1 GHz can be realised.

The power handling capacity of the bridge will be 4 times the "wattage" of one 'resistor' leg.

So for 3x 150 ohms 1 watt per leg, each leg would be 3 watts, and the total power capability would be 4x3=12 watts.

Measurement of voltages at the "halfway" points on the bridge allows the SWR to be calculated. (refer to a diagram)

To do this at HF, and, surprisingly, even at VHF, I have found that small signal silicon diodes work well. (1N914 or 1N4148 etc) But shottky diodes might be better?

For UHF and above, shottky diodes are a must. (1N6263 for example)

While it is usual to measure the (halfway) input voltage and the difference between this and the corresponding output voltage, I am lazy.

I simply measure these input and output voltages directly, and work from that.

I think this way is more accurate. It avoids "diode drop" errors, which could occur for the lower voltage 'difference' reading.

This way also simplifies construction (slightly), and favours the lessening of stray capacitance. (I think ^_^)

It also allows you to know if the impedance of the antenna is above or below 50 ohms.

After reading the input and output voltages, and comparing the output to the input voltage, (for example, 10% above or below the input volts) use the formula "100 plus percentage divided by 100 minus percentage".

A ten percent difference is excellent. 110/90 (1.22:1)
A twenty percent difference is good. 120/80 (1.5:1)
Thirty percent, not so good. 133.3/66.6 (2:1)
A fifty percent difference is undesirable. 150/50 (3:1)
Not really hard to work out. ^_^

OR....

If the input & output readings are A & B respectively, and B is the lower reading, the SWR is: (2A-B) divided by (B).

Where B is the higher reading, invert this to: B divided by (2A-B).

OR....

Calculate the difference voltage (A-B or B-A) and use the formula: "input voltage plus difference divided by input voltage minus difference".


Some final thoughts.

The voltages as measured are peak voltages. If you have a LOW SWR reading, AND an accurate input (halfway) VOLTAGE reading, square this voltage reading, and divide by 25 to get the APPROXIMATE power output of the transmitter.

(5 volts peak...5x5=25 divided by 25 = 1 watt output power)

Note that this type of measuring device should be used while checking SWR only, and NOT left in line during normal use of the transceiver.

The resistances in the bridge would consume most of the power before it reached the antenna. Likewise, it would attenuate the received signal.

The above is a generalised view, and I think it is mostly correct.

There might be an error or two however. If so, please let me know..

Ipso fatso, my case rests.

Paul VK6WZ

#project
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