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Syllabus Sections:-

Measurements

9A4   55  Understand that signal generator and similar devices will have a source impedance and the effect on the signal level of attaching different load impedances


Let's simplify this into terms which bring it to the real world. For a signal generator let's use your transmitter. We know that the amateur transmitter is designed to have an output impedance of 50Ω and that you use 50Ω coax to connect to your antenna.
However you know that not all antennas exhibit an impedance at the point of connection of 50Ω, this leads to a mismatch between the transmitter and the antenna which results in some of the signal being reflected back from the antenna to the transmitter, this you can assess on an SWR meter.

Matching Impedance is an important item with regards to an item under test as the signals being reflected caused by the mismatch of the impedances can lead to serious problems, in the case of antenna mismatch with your transmitter, damage will occurr.

There is relationship between the incident wave (transmitted wave) and reflected wave (from the antenna) which can be calculated by value of the source impedance  (ZS) and load impedance (ZL) to give the reflection coefficient in terms of these impedances:

Now this  equation is very similar to that of the SWR on the examination sheet.


Now the syllabus says "Understand that signal generator and similar devices will have a source impedance and the effect on the signal level of attaching different load impedances" and it is hoped that from the above that you now have an understanding that difference in impedance will have an effect on the results of measurement.

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9A4   55 continued Recall that not all measuring equipment will have a 50Ω input impedance.

Here you just need to remember that not all measuring equipment will have a 50Ω input impedance.

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9A4   55 continued Understand that the choice of measuring equipment may have an effect on the the measurement results and on the object under test.


Depending upon what you are measuring you need to exercise some care because moving coil meter (volt and ammeters) draw current from the circuit under test and thus cause an error in the reading.
In the page 97 Fig. 14.5 it shows the measurement of a voltage across a resistor and an explanation in the text as to the type of error.

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9A5  55 Understand that steady RF power may be determined by measuring the RF potential difference across a dummy load and that a steady audio signal e.g. from an audio oscillator, will be required for AM and SSB measurements.

The power from a steady RF signal can be determined by using a dummy load with a capacitor and a diode. The BRATS have a project which members can see on the BRATS FORUM which shows the construction and the two probes which can be connected to a digital meter or oscilloscope to measure voltage of a low wattage signal and then a calculation to determine the power from the voltage.

A steady audio signal is required else the voltage being measured across the dummy load will be varying for both AM signal and SSB signal.


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9A5  55 continued Understand the meaning of PEP (peak envelope power) of an SSB transmission and that it may be determined using a peak reading power meter or an oscilloscope and dummy load.

Definition of PEP

Peak envelope power (PEP), is the maximum average power supplied by the transmitter to the antenna transmission line during one radio frequency cycle taken under normal operating conditions.

Peak Envelope Power (PEP) is the only accurate and meaningful way to compare transmitter power levels between stations

SSB POWER METERS.

RF power meters need a continuous carrier to read continuously, which presents a problem with SSB as the power is never continuous, varying with speech. With the addition of a suitable capacitor or time constant the capacitor is charged on voice peaks, holding the reading as the human voice goes up and down, if there is a pause in the signal i.e. no audio signal present, then the meter reading will drifts slowly down to zero.

The diagram below is of an SWR bridge which is also a Peak Reading Power Meter which will be dealt with more in the next section but is added here as in the text above we mention the Capacitors to store the charge.

PEP Peak Envelope Power.

Another and perhaps more correct way to measure pep is to use an oscilloscope, dummy load and an audio 2 tone generator.

Set the audio tones to say 600hz and 2khz (not harmonically related) and the same amplitude, connected to the audio input of the SSB Tx, set the scope to measure the voltage across a dummy load. The pattern shown on the oscilloscope will be approximately as shown below.

The Peak Envelope Power (PEP) is the maximum average power supplied by the transmitter to the antenna transmission line during one radio frequency cycle taken under normal operating conditions.

On the scope screen you need to read the peak to peak voltage say reads 80 Volts peak to peak then PEP = VRMS2/R

Find the RMS of 80V peak to peak / 0.707 = 56.56

With PEP = VRMS2/R ( R = 50 the dummy load)

then,

PEP = 56.562/50

=3199/50

= 63.98 say 64W

The deflection on the oscilloscope is due to proportion of the voltage causing it.

Not everyone has an oscilloscope or 2 tone generator so the pep reading meter is preferred.

THE DUMMY LOAD.

A dummy load is a resistor or a combination of resistors which is used in place of an antenna for the purpose of testing or tuning. The resistor nominally 50 and is of a non inductive material such as carbon.




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