Saandy,
I respectfully disagree, in part. I believe you want to re-think this.
I know it "looks like" you are just paralleling two inductors, but the whole
thing must be considered.

There is such a thing as loaded Q which is what you will see when all is
connected up. When un-connected, you have an "un-loaded Q" situation...and
they are indeed different. The loaded Q is a very important and specific
consideration when designing filters. The loading of the external circuitry
(and resulting loaded Q) has to be taken into account when designing for a
required bandwidth.
Also, there can be a match, and should be for sensitivity, aka maximum
power transfer. Well done, the receiver input tuned circuit will appear
resistive or nearly so. The tuned circuit(s) provide impedance matching
between the antenna input (50 ohms) and the first device (usually higher Z),
as well as filtering -- two functions for the price of one.
Side-bar: When considering noise figure, there can be a
different ball game, but it is a relatively small
difference - impedance wise.
A well designed RF transformer will reflect (look like) 50 ohms at its
output (receiver input) due to the (supposedly) 50 ohm antenna/feed-line
connected to its input (antenna side).

Obviously I am assuming a 50 ohm system, to explain.

A tap is a way of impedance matching and the auto-transformer analogy is
a good way to look at it. I call this a "Mental Model" for better
understanding what is going on - it works to analyze things. Because 50
ohms is a relatively low value, tapping a coil is a way to get to a lower
impedance from the relatively high impedance at the top of the coil/tuned
circuit. Capacitive tap is another common way (they just have differing
secondary [side] effects in far-out attenuation).

Comments?
73, Steve, K,9.D;C'I




"Saandy , 4Z5KS" wrote in message
oups.com...

gojamo wrote:
Can you connect a broadband rf transformer to a receiver front end tank
circuit rather than to a resistive load? The inductance of the side of
the transformer connected to the tank circuit would be several times
the inductance in the tank circuit, and changes to the overall
inductance (parallelled inductances) of the circuit would be taken
into account. Thanks

...sure you can. I think you noticed that sometimes the antenna is
connected to a tap on the inductor, thus creating an autotransformer.
The one thing to beware of is to make sure that the input impedance,
reflected through the transformer is high enough, so as not to load the
tuned circuit. it's NOT a match. If matched, the impedance of the
transformer's secondary would be equal to the tuned circuit's
impedance, thus lowering the Q of the tuned circuit to one half the
original value.
Saandy 4Z5KS