[Amps] *** SPAM *** Re: Antenna traps

[Ian White GM3SEK]

Peter Chadwick wrote:
>Rich said:
>>**  An L-network always has the lowest Q possible.  Is mo' Q mo' betta?<
>Rubbish. Do the math. The Q is the sq.rt of the impedance ratio -1.
>So the bigger the impedance transformation, the higher the working Q. 
>In any two variable system, Qw is an uncontrolled variable. A 3 
>variable network allows Q to be defined. That's the advantage of the T 
>or pi (which is the dual of the T).

I think Rich is more right than Peter on this one. We all agree that the 
network should have the lowest possible working Q, in order to achieve 
the lowest possible internal losses and the highest possible power 
handling capability.

When transforming between resistive impedances, the lowest possible 
working Q is:
> the sq.rt of the impedance ratio -1
Any lower value of Q will not achieve a match. Any network with three 
variables (eg a T) can also achieve a match, but always with higher than 
the minimum possible values of working Q and internal losses.

An L-network only has two variables, so the working Q is automatically 
determined for you. With a resistive load, this is automatically the 
lowest that can be achieved - so that is an advantage. The only case 
where an L-network won't give the lowest possible Q is with certain 
highly reactive loads... in which case, you need to switch to a totally 
different configuration of L-network.

This last point is the major DISadvantage of L-networks: lack of 
flexibility. There are a total of 8 different configurations, and they 
all have a limited matching range. Between them, they can match any 
impedance (except a 1:1 match requires theoretically zero or infinite 
component values) but it's a matter of finding which one out of the 8, 
and most practical L-network tuners can switch between a maximum of 2 
configurations.  However, I'd conjecture that for any pair of 
impedances, there will always exist at least one L-network configuration 
that can match them at the lowest possible working Q.



-- 
73 from Ian GM3SEK         'In Practice' columnist for RadCom (RSGB)
http://www.ifwtech.co.uk/g3sek