[CQ-Contest] Amplifier harmonics and Coax Stubs for SO2R

[ssimons at manitousys.com]

Responding to the "amplifier harmonics and required stubs" thread,

Yes, the exciter (rig) will produce some level of harmonic energy. This is
the typical nature of an exciter. Every exciter design includes a low pass
filter that attenuates the harmonics. 
The performance of the filter determines the exact attenuation value.
Filters are also designed to operate into a 50 ohm resistive load for
advertised performance. 
Each rig is tested to be compliant (to FCC and other set limits) in a
laboratory setting - that is into a high quality 50 ohm dummy load.

Next, each amplifier type (tube or transistor) has its own set personality
traits - tubes by their nature amplifier a signal in a cleaner fashion -
this is a function of the tube type amplifier design and circuitry - the
input and output matching circuits are high "Q" and, by their nature, will
act as low pass filters to keep the amplified harmonic energy low. If an
un-tuned input is used, then there is a chance that the low level harmonics
(from the exciter) will pass to the tube and get amplified. The transistor
amplifier's matching circuits are low "Q" by design and typically do not act
as good low pass filters. So, all well designed solid state amplifiers
include low pass output filters. 

Now, we have real-world conditions where the exciter and the amplifier may
not be perfectly matched causing the built in filters to be a bit less
effective in reducing harmonics. We also have conditions where the amplifier
does not have sufficient output power "headroom" and/or an operator pushes
the amplifier for maximum output. In this case, the amplifier will be driven
into compression and the (output) harmonic energy dramatically rises.
Additionally, use of an amplifier into a reactive (non 50 ohm) antenna load
will result in the amplifier's internal filter to be less effective.

Each station's equipment lineup and operation style will result in
difference's in the resultant harmonic energy. The goal, is to determine IF
the harmonic energy will cause a problem in YOUR station. If there IS a
problem, proper troubleshooting techniques should to be employed to
understand and then correct the individual issues. The fixes may include
amplifier input/output filters, better matching of the antenna load, receive
filters and better exciter to amplifier matching.

Hope this description helps.  
73,
Steve
   
W1SMS
Steven M. Simons
ARRL CT Section Technical Coordinator
w1sms at manitousys.com  
Tel.203.733.2110     Fax.203.746.1459
33 Ball Pond Road  Danbury, CT 06811
Mailing Address: PO Box 8066  New Fairfield, CT 06812 
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