Dave,
A good summary - but can I just pick you up on one minor issue: the
configuration required for the L-network does *not* depend on whether
the load impedance is higher or lower than the required impedance, as is
often quoted. It's not that simple.
Here's a couple of examples which illustrate the point for a LP
L-network providing a match to 50 Ohms:
* A load of 5+j16 and a load of 500+j16 both require the *same*
configuration - shunt C at the load side
* A load of 5+j16 and a load of 5+j13 require *different* configurations
- shunt C at the load side and shunt C at the source side, respectively.
73,
Steve G3TXQ
On 01/12/2010 17:30, David Gilbert wrote:
>
> That's why when I use TLW I initially specify large value capacitors so
> I can see what's going on first. I do the same thing with the W9CF
> online tuner app. I don't think there are many cases for an antenna
> tuner where you'd want to specify a Q since generally speaking you
> simply want it to be low ... you don't want to have to adjust the
> setting as move around the band. Most of the time (not always), an
> L-Network is going to give you the best efficiency, and the best
> impersonation of an L-Network by a T-Network is when either the input
> capacitance or the output capacitance is as high as you can make it.
>
> Generally speaking, there are only three reasons to use a T-Network in
> the first place:
>
> 1. L-Networks only match in one direction, and it makes a difference
> whether the load is less than or greater than the source as far as which
> side of the inductor (assuming a shunt inductor) the capacitor is on. A
> T-network is capable of pretending it is an L-Network in either direction.
>
> 2. The L-Network capacitance and inductance values get almost
> impossible to achieve for loads that are close to the source impedance.
> Try some samples and you'll see what I mean. In those situations a
> T-network essentially uses the extra capacitor to shift either the
> source or the load impedance so that the L-Network portion can
> accomplish a match. An older generally well regarded commercial tuner
> billed itself as a more efficient L-Network tuner, but they included
> provision for switching in a fixed capacitor (or two, if I remember
> correctly) on the other side of the coil in order to match loads near
> the source.
>
> 3. An L-network only has two degrees of freedom ... the coil and the
> capacitor ... and both have to be continuously variable to achieve a
> match. That means you have to use something like a rotary inductor,
> which also requires a turns counter if you want to be able to quickly go
> back to predetermined settings. Both are expensive. A T-Network allows
> you to find a match for an inductance that is merely close to the
> desired value ... i.e., a fixed coil with taps ... even if the resultant
> efficiency might not be optimum.
>
> 73,
> Dave AB7E
>
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