[TenTec] "Z" match question

[Dr. Gerald N. Johnson]

On Fri, 2008-06-13 at 17:05 -0400, Marty Hartwell wrote:
> Hi
> 
> I have an MFJ version Antenna tuner. I want to convert it over
> to a Z match.
> First question. If I stack two 2" toroids I think type 41 material
> will this handle 100 watts? Then what size wire would be alright to
> wind. I am thinking I will use solid 14g. with insulation. Right now
> I am looking at regular house wire using the black and white to keep
> the windings separate.

 A single 2" diameter toroid of 61 material tends to handle a KW or more
according to my ARRL Antenna Book.

I'm not a fan of power wire with PVC insulation. Its bulky and and I
suspect some PVC is lossy at RF. And black may be more so if the black
comes from carbon black pigment.

You can check your design. Figure that the inductive reactance of the
winding needs to be at least 5 or 10 times the circuit impedance at the
low frequency end of the operating range, yet the wire needs to be less
than a quarter wave long at the high frequency end of the operating
range.

> Second question. I think the variables in the current tuner are around
> 500pf which should be aright to reuse, how big and what kind would be
> a good cap to put in for the switchable add on?

470 pf, just about the delta C range of a 500 pf variable capacitor.

> Third question. Has anyone else tried this and did it work?
> 
> Marty kd8bj

Haven't tried it. Using a ferrite toroid in a tuned circuit leads to
much tuning confusion because the permeability is changed so much by
signal level which changes the inductance. Ferrite makes good broad band
transformers, while powdered iron makes relatively stable inductances.

73, Jerry, K0CQ