Miscellaneous 
Test Equipment built by G3NGD for Measuring the PEP 
Power output, linearity, and to check for the presence 
of Harmonics in the output of a Radio Transmitter.
 
The photograph shown above is that of the G3NGD Two-tone Oscillator. This generates two frequencies, not harmonically
related. In this case 700Hz and 2KHz were chosen.
The unit also comprises a Pulser which switches the oscillator
on and off  to prevent damage to the output valves, which should be driven on a reduced duty cycle. 
The Transmitter is connected to a 50 ohm Dummy Load for the
following measurements to be made. In John's case, he uses a
KW107 SUPERMatch, which in addition to the Dummy Load has 
a Power Meter, which indicates the 'mean power'.
 
If a Power Meter isn't available, one could use a Radio Frequency Ammeter
(The one above was used on the T1154 Transmitter), connected in series with
the Dummy Load. The Power can be calculated using the formula W=I² x R.
A Radio Frequency Voltmeter could be connected in parallel with the Dummy
Load and the formula W=V² / R could be used.
To measure the PEP Radio Frequency Power Output of the Transmitter
the output waveform should be displayed on a cathode-ray Oscilloscope.
It only requires a 'sniff' of the signal' to inject into the 'Y' plates of the CRO.
 
The waveform shown on the CRO above is that of  the output from John's KW204 SSB Transmitter being driven by his Two-tone Oscillator.  This shows a pure output without distortion.  If  the waveform 'flat-tops',  then the output stage is being overdriven, and 
damage will occur to the output valves. Just before the waveform 'flat-tops', the power 
should be noted, as that indicated on the Power Meter.
The reading taken should be multiplied by two. This is the maximum value of PEP. In John's case, the Power Meter indicated 50 watts, so the PEP output power was 100 Watts.
When the Two-tone Oscillator is disconnected, the Microphone is connected. 
The speech  peaks when speaking into the microphone must not exceed the upper and lower limits of the waveform as shown above.
 
The photograph above is that of  the Spectrum Analyser built by G3NGD in 1994. It is used in conjunction 
with a Cathode-ray oscilloscope. The Analyser is used to check for harmonics in the output of a Transmitter. Harmonics can be  troublesome as they can cause unnecessary interference. 
 
The Spectrum Analyser has a built in Crystal Calibrator
giving 1MHz and 10MHz calibration points.
The screen display above shows the 10MHz markers
up to a frequency of 100MHz. Other 'blips' are those
of strong nearby signals being picked up and displayed.

The Analyser can be switched to various bandwidths:
10MHz, 5MHz, 1MHz, 0.5MHz, 0.1MHz and 0.05MHz per division. The above display is switched
to cover 10MHz per division.

 
The Photograph above shows the display from John's
Spectrum Analyser, which spans from LF to 110MHz.
A short length of wire was connected to the input and
Medium-wave signals can be seen on the low frequency
side and the VHF f.m. radio signals (88 - 108 MHz) to
the right. Each vertical bar (the x-axis) on the 'Graticule' 
indicates 10MHz on this setting. The Y axis is calibrated
in dBm./cm.
Note: dBm = decibels above 1mW (one milliwatt).
The maximum input signal to the Analyser is -20dBm
otherwise overloading will occur. Built in attenuators are
fitted to allow for a maximum input signal of +10dBm.
 
 
The CRO display above shows the radio frequency
output of John's KW 204 Transmitter on a frequency 
of  2.0MHz. The second harmonic at  4.0 MHz can 
be seen and  it  is -46dBm down.
The manufacturers specification for the KW 204 states
that the "Second Harmonic is 40dB down".
Of interest:
John's absorption wavemeter doesn't indicate any 
harmonics, on any frequency, because it is not as 
sensitive as a Spectrum Analyser. If however, any
harmonics were indicated when using an absorption
wavemeter, then they would have to be suppressed.
The regulations for Radio Amateurs state that it is
only necessary to use an absorption wavemeter so
long as the instrument scale is calibrated to at least
the second harmonic (preferably the third harmonic).

 
The CRO display above shows the Spectrum Output of the Yaesu
FT-101E Amateur Radio Transceiver. The Manufacturers specification
States "Spurious Radiation as -40dB down or more".
 
The display above shows the Spectrum Output of G3NGD's version of
the Chatterbox Transmitter. There were no other harmonics displayed.
In conclusion, it is very handy to have a Spectrum Analyser, however
basic it is, to check the output of a transmitter for harmonics.
It is certainly better than an Absorption Wavemeter as it is more
sensitive. (You wouldn't observe the output of the VHF f.m. radio
stations on an absorption wavemeter.)
G3NGD first used his Spectrum Analyser to check the effect of his
home built low-pass filter, in suppressing  the harmonics from his Top
Band Chatterbox Transmitter. (Field Effect Transistors are prone to
generating harmonics.)

It is well worth the effort involved to build a Spectrum Analyser  - the details are printed below.
The circuit diagram and construction details for the Spectrum Analyser can be found in 'The Radio Amateur's Guide to EMC', published by the Radio Society of Great Britain. Also, Radio Communication Journal, November 1989 -  Pages 44 to 49 inc.

 
The above photograph shows a Voltage and Current Multi-meter made by Wates Brothers Ltd. in the early 1920's. This forms part of  the G3NGD collection of  'Radio Memorabilia'.
The Scales are:  0 - 6V d.c.; 0 - 150V d.c. for
voltage measurement  and  0 - 30mA d.c. for current. The instrument is housed in a black case.

In the early days of  Radio, 2V accumulators
were used to supply the 2Volt heater valves.
For example the - HL2. (John used an HL2 valve
to make his first Single Valve Radio Receiver in the early 'fifties') The high tension voltage used 
was 90V. This type of instrument would have been suitable for checking the voltages. It would not be suitable for reading voltages on electronic circuits as used today because the instrument has a 'low sensitivity', i.e. low ohms/volt.
(The instrument would take more current than the
circuit!)
In the early days, Radio receivers were powered by batteries.
Ever Ready made the 'All Dry' Portable Model N receiver which
used the '"Batrymax" B107 (90V) and the "Alldry" 14  (1.5V)
batteries. Other Battery manufacturers also sold Radio receivers.
There was a large number of  h.t. Dry Batteries available up to 
the 1960's. Transistors took over and these old radios became 
obsolete.
As late as 1950, Vidor made the 'LIDO' Model CN411. This 
used the Vidor L5039 (90V) and Vidor L5048 (7.5V) batteries.
The Vidor Model CN421 line-up was DK92, DF91, DAF91 and DL94. The Battery voltages were 90V and 7.5V.
'Cossor Radio' made the Model 499. The valve line-up was 1R5,
1T4, 1S5 and 3S4. The battery voltages were 1.5V & 90V.
'Ekco Radio' made the (Princess). The valve line-up was DK91, DF91, DAF91 and DL92. The Battery used was the Batrymax B114 (1.5V and 69V) connected via a four pin combined socket.
.......... there were many others.
You make like to visit another page showing 'Radio Memorabilia'
at:   Marconi Coherer detector / V24 Valve
 
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 G3NGD: June 2005