[AMPS] Review of Antenna Tuner from Dayton

[Ian White, G3SEK]

Jon Ogden wrote:
>
>> Well Jon, it might be advisable to spend time looking at voltages in
>> T network tuners. If you run the numbers, you'll see the largest
>> problems causing arcing are excessive Q on the low bands.
>> Excessive Q is caused by not having enough capacitance.
>
>Well, my problem was that my capacitance on my old tuner was pretty much 
>maxed out.  I couldn't add more.  Believe me, I remember our 
>conversation. But when the cap is fully meshed when it arcs, there's not 
>a lot one can do.
>
Actually, there is. In RadCom about a year ago, G3LNP described a very
simple add-on rotary switch to short the capacitor completely at
maximum. This converts the T-network to an L-network. By shorting either
one C or the other, you can have either a step-up or a step-down L-
network. 

It still won't match everything, of course, but it increases the
impedance range beyond what can be reached with the available maximum C
in the ordinary T-network.

As for network Q, any "three-knob" network such as the CLC-T gives
opportunities for achieving a match at a higher-than-necessary loaded Q
(QL) which leads to unnecessary losses. At least with an L-network or
any other "two-knob" circuit, you have the comfort of knowing that QL is
the optimum value - because it's also the *only* value that will also
give a match!

73 from Ian G3SEK          Editor, 'The VHF/UHF DX Book'
                          'In Practice' columnist for RadCom (RSGB)
                           http://www.ifwtech.com/g3sek

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