|[AMRadio] RE: GB> Antenna Tuner Wonderings|
k4kyv at hotmail.com
Sun Dec 26 20:26:04 EST 2004
>With BA unit wit Pi networks you don't need a TransMatch (they ARE NOT
>antenna tuners). Most will match anything between 50 and about 500 ohms.
>exception is the Heath DX-60. It is limited to 50 to 75 ohms becase of the
>built in LP filter.
>You need a resonant antenna for each band you want to work. Some could be
>multi band antennas.
>A TransMatch does not correct antenna problems. It hides them from the
>transmitter. The Pi network in the BA rigs is the same thing that was in
>older transmatches to connect link couples transmitters to coax lines.
>It is the solid state riceboxes that really need a transmatch.
Think of it this way. In tube type rigs, the tank circuit always came with
the rig. It could be a pi-network or a link-coupled circuit, but the
purpose was the same, to match the output of the tube(s) to the load
impedance of the feedline. With the tube output, the optimum load was
usually several thousand ohms, so the resonant tank circuit was used to
match the tube impedance to the low feedline impedance.
With modern solid state rigs, the optimum load impedance is close enough to
the nominal feedline impedance that the final can be connected directly to
the feedline without any matching network, or with a simple non-resonant
broadband autotransformer or transformer. In theory, no tank circuit is
needed, but the trade-off is that the rig is very sensitive to load
impedance, and anything very far removed from 50-ohm nonreactive will not
work. Also, the nonresonant transormer provides no harmonic suppression,
and provides no flywheel effect for the final. This means the PA must
operate in linear mode even for CW in order to produce a clean signal.
In the real world, few ham antennas other than a dummy load provide a 50-ohm
nonreactive load to the transmitter at every frequency within a band or over
several bands. A matching network is necessary. This matching network is
often called a "transmatch" and its purpose is to transform the feedline
impedance to 50 ohms nonreactive at the operating frequency. Like the tank
circuit in the tube rig, it must be resonated at the operating frequency;
this may be done manually or automatically with sensing circuitry.
In other words, in the older tube type transmitters, the tank circuit came
as an integral part of the rig. With the newer solid state rigs, the tank
circuit comes as a separate outboard unit that you have to pay extra for,
but with a few rare exceptions, the tank circuit is just as much an
essential part of the transmitter, which will not work without one.
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