should be less than 1.5:1. Less than 2.0:1 is acceptable but some reduction in power may be seen
and the amplifier will work less efficiently and generate more heat. At about 2.5:1 the amplifier
will signal an alarm and enter protection. A power reset should be performed and the amplifier
should not be used again until the VSWR has been reduced.
The antenna should be tuned so that the load is as close to 1.1:1 as possible, see above. This should
also be carried out with the amplifier switched off and the drive radio at low power, 10W should
be sufficient for most tuners to carry out the matching process. If an Automatic ATU is used it
should, once it has found a suitable match, be disabled such that it may not carry out another tuning
cycle until instructed to do so.
Depending on the type of antenna and ATU being used it may be possible to generate very high
and low impedance loads that can look to the amplifier almost like open or short circuits, operating
the amplifier at full output whilst trying to tune a non resonant antenna with a tuner either manual
or Automatic can be catastrophic for the transistors due to the load mismatch. The protection circuit
will in most instances cut in at a VSWR of 2.5:1 but if the amplifier is being used at full power
whilst tuning the antenna it is possible that the transistors may sustain damage due to the load mis-
match. You have been warned!!
Now that the input power has been adjusted correctly, the antenna tuned, (If required), operation
can continue. Input power can be adjusted to give the required output power or the amplifier may
be run at it’s full output.
If using a resonant antenna and having the amplifier set to AUTO (Filter selection), band changes
can be as fast as the time it takes to change the band on the drive radio. However changing band on
a non resonant antenna always requires the amplifier to be switched off whilst the antenna is re-
tuned to an acceptable VSWR.
MODE:
The HLA300 may be used for all of the common narrow band transmission modes such as SSB,
CW,AM,FM, SSTV and data modes etc.
Warning: Transmit Time.
Hi Duty cycle modes such as FM and Data modes etc. operate the amplifier at full power all of the
time unlike modes like SSB and CW that are either intermittent or only reach peak output for very
short times, these high duty cycle modes will run the amplifier much harder and generate more heat
in the same amount of time. It should be noted that the amplifier although capable of being used
with these modes should not be operated continuously. A transmission time of more than a minute
or two should be avoided to avoid excessive transistor junction temperature. The exact time for
transmission in these modes will depend on numerous factors such as is the amplifier fitted with the
cooling fans, How good the ventilation is around the amplifier and is there sufficient space for
freely flowing air to circulate, etc. If the ambient temperature is high this will reduce the total time
in transmission. Common sense should be exercised if the heat sink is becoming too hot to touch
then sufficient time should be allowed to let it cool down before reuse. The amplifier has a thermo-
stat that will block the operation of the amplifier if too high a temperature is reached and automati-
cally returning back operation when the temperature has reduced.
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