Antenna tuning network

I am planning to buy an antenna that I can fit in the loft, this cannot be longer than 10m so will be OK from 14MHz up, but will need an ATU for lower bands. I have two SDRs (one receiver on 3.5MHz and one transceiver on 7MHz) so I need to design an ATU to match their 50R output to the antenna.

A simple network for matching different impedances is the 'L' network. Which can be on one of two configurations:



These are just the same, as you can swap IN for OUT! The right hand one is the one to chose if your antenna has a higher impedance than the transceiver 50R output. The values of L and C are calculated like this, ignoring the reactive parts and assuming IN & OUT are resistive:



Here's a table of values calculated in a spreadsheet for f = 3.5 to 28MHz, Rin (Rs) = 50R and Rout (RL) from 100 to 1000R: