On 23 Jun 2005 12:29:51 -0700, "Brian Kelly" wrote:


Oliver Gebele wrote:
Hi,

i am using a Collins-Filter with 2 variable
capacitors and a switched inductor (Pi).
Obviously there are infinitely many possibilities
to match an antenna.Are there any preferred
possibilities? And how would i adjust my box
to do this? (Maybe there are already links on
the internet that i did not find.)

Inductors are lossy. Capacitors are not. Use the tuner settings which
provide a 2:1 VSWR match or better with the least possible amount of
inductance. In other words use the least amount of inductance you can
get away with.

Let's examine this statement.

First all should note that this is a pi-network. Suggestions to use
an MFJ (tee-network) tuning procedure are wrong.

Here is a random example made up on the spur of the moment.

Assume the load is 20 +j50 @ 14 MHz. SWR = 5.2

Also assume that the tuning capacitor(s) Q = 500 and the inductor Q =
200, both typical values.

A nearly optimum solution, from a match and loss standpoint is:

Cin = 140.3 pF, L = .958 uH, Cout = 333.6 pF.

The input z = 49.69 +j0.03 or SWR = 1.006 , Loss = 0.09 dB.

Now, let's force the inductance to a lower value.

Cin = 422 pF, L = 0.67 uH, Cout = 471.3 pF.

The input Z = 49.6 -j0.04 or SWR = 1.008 but the loss = 0.18 dB.

The loss doubled when the inductance was lowered.

All of this is easily calculated using a free tool: XLZIZL.xls.







TIA & 73, Oliver

w3rv