Topband: Re: ON4UN Antenna

[Tom Rauch]

Hi Roy,

> "Toroids are lucky to make a Q of a hundred unless the reactance
> required is low, and they suffer from heating because losses are
> concentrated in a small area that isn't exposed to airflow."
> 
> Hi Tom,
> 
> Well,
> 
> I'm still running my T-400-A core in the PA tank as L1 in the PI-L net on
> 80M and did it on 160M for years at 1300 watts output.  Have felt it after
> hours of CW use rag chewing and it was always at the ambient temp. in
> there like everything else.

Exactly Roy, and that illustrates the point I tried to make.

In a tank circuit, the amount of inductive reactance required is 
typically in the low hundreds of ohms, in the output inductor of a pi-
L much less. The inductors are part of a system with low reactance 
shunting capacitors, so the stray C of a toroid doesn't hurt the 
system very much.  

Toroids fit into some tank applications very well, even though 
properly designed air-core inductors (if you have space) almost 
always have higher unloaded Q's.

In loading systems requiring high amounts of inductance with very 
little shunt capacitance, the distributed capacitance in a toroid (and 
to a lesser extent core losses) make for very poor performance 
compared to conventional inductors.

That's why we never see toroids in Q-critical systems requiring high 
values of inductance when the toroid is operated without low 
shunting capacitances in the external system, but they work OK in 
tank systems, baluns, and other applications.

73, Tom W8JI
w8ji@contesting.com



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