[ARC5] Why an UNUN?

[J. Forster]

> A transmitter does not present a "load" as such to the input end of a
> transmission line.  The transmitter delivers power to the transmission
> line, i.e., "drives" the line.
>
> The commonly used term "output Impedance" is misleading, a transmitter
> does not "output" an impedance.  What we are actually referring to
> here is that impedance value which when connected to the output
> terminals of the transmitter  will reflect a value of impedance  back
> to the plate circuit of the PA tube that will, in turn, cause the tube
> to provide a desired power transfer to an external load.

Not really.

A transmitter, like a battery or other power supply, can be modelled as a
Thevinin Equivalenrt, an ideal Voltage Source in series with an Output
Impedance.

There is a circuit theory theorem that, for maximum real power tranfer,
the load impedance should be equal to Complex Conjugate of the source
impedance.

This means equal to, if the source and load are purely resistive.

This means if one is inductive, the other must be capacitive, and the
inductive reactance equals the capacitive reactance.

This means if the source is complex (resistive + inductive), the load must
have a load where the resistive componants are equal and the capacitive
reactance is equal, and opposite, to the source inductive reactance.

ie:  The L & C are resonant at the operating frequency.

> Approximately 12,500 ohms reflected to the plate circuit of the 250TH
> PA would be correct in the case of a BC-610.  (About 5K for a command
> xmtr.) Most transmitters include some form of   impedance translation
> network to allow connection to commonly  encountered load impedance
> values, 50 or 75 ohms for ex.  Variable link coupling to the
> transmitter's tank coil is a simple and common example of such a
> network.

The variable link is essentially like a Variac for RF. It's a pair of
coils, arranged to have variable coupling.

> The tank coil and the link form a transformer circuit and
> varying the mutual coupling between the tank and the link has the
> effect of varying the turns ratio of the transformer windings and
> hence the impedance transformation ratio.

Essentially yes.

> " Loading" is the descriptive term commonly applied to the process
> whereby the impedance reflected back to the plate of the PA is
> adjusted so as to deliver a desired power output value to an antenna
> or other load.

More correctly, is is a variable transformer adjusted to maximize the real
power transfer between the plate circuit and the real part of the
antenna's impedance.

> Note that we normally talk in terms of purely resistive impedance
> values here.  We are not necessarily limited in doing so, however.
> The calculations are just more complex when reactance must also be
> taken into account.

Generally, you want to "match" the real part of the impedances and
resonate "tune out" the reactive part of the antenna.

-John

========
>
> Dennis D.  W7QHO
> Glendale, CA
>