50 ohm 150 watt RF dummy load
Do you use a dummy load when testing your transmitters. Not only is it a good idea, it’s a
requirement of your licence to use one. Plus you don’t want to cause interference on our bands.
Here’s a design that has good VSWR up to 500 MHz. It uses a Bourns™ 150 watt 50 ohm terminator
chip, only 6 mm x 9 mm x 1 mm
thick.
The need for a dummy load
I have a Yaesu FT-301 transceiver
that I’ve owned since new. This
transceiver, when introduced in 1976,
was said to be the first solid state
ham rig with a 100 watt output. My
FT-301, after being in storage for many
years, was put back into service with
a SG-237 Smartuner and long wire. It
has now been in use for around two
years.
As the transceiver is now 33 years
old, I decided that I should check
the alignment to ensure that it was
putting out a clean powerful signal.
First step was to download the user
manual from the internet and print
it. I decided to start with the carrier
frequency adjustments. On reading
the manual it says to inject a 1 kHz
audio signal and to adjust the RF
output to 50 watts. Then to inject a
300 Hz audio signal (same amplitude)
and adjust the carrier frequencies for
12.5 watts output on each sideband.
The manual did not say to adjust the
carrier frequencies to 8.9985 MHz
for USB and 9.0015 MHz for LSB. This
puzzled me so I placed the SSB filter
onto the N2PK VNA (vector network
analyser) and did an insertion loss
trace to determine the filter’s band
pass shape.
My VNA software that drives the N2PK
VNA has a matching tool that allows
you to measure the filter at 50 ohm
impedance, then to display the trace
as if the filter was matched with 500
ohms.
Figure 1 shows that the centre
frequency is not 9 MHz and that the
carrier frequencies need to be set to
suit each individual filter. This is why
a power meter and not a frequency
counter is used to adjust the carrier
frequencies.
Okkk sob thanks By; Wincell Rp; 150,000 / + ongkir
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