Topband: Core Mix

[Tom Rauch]

> In general, cores for transformers should have relatively 
> high mu
> and low loss.  #43 material by far the lowest loss, while 
> the other
> materials all have about twice as much mu. If I had #43 of 
> suitable
> size, I would use that first.

I wouldn't use 43...not for receiving.

When receiving loss is virtually meaningless until  the loss 
is so high the noise figure of the receiving system sets the 
noise floor. It really isn't important if loss is 1 dB or 20 
dB.

For example most of my receiving antennas run through over 
1/2 mile of F-11 style coax (CATV cable the same diameter as 
RG-11) and each hub has hundreds of feet of F-6 (like RG-6 
size but for CATV) before reaching the house. All the 
amplifiers are in the house, and the system easily limits on 
propagated noise even on the quietest night.

What do I care about the difference between a core that has 
0.1dB loss and 0.5dB loss? Nothing. It is meaningless.

On the other hand if I was running a kW through a 
transformer I'd have to be picky, because .5 dB loss means 
11 percent of 1000 watts, or 110 watts, would be heating the 
core. It would be better, heat-wise, to have only 2% of 1000 
watts (20 watts) making heat.

I use a higher mu core for receiving because then I could 
use less wire and make a better transformer for bandwidth 
and isolation. I'd NEVER use a 43 for receiving unless it 
was a resonant transformer on a low frequency.

Also I'd never use a trifilar winding for a transformer. We 
really don't want the coax shield having a direct path to 
the ground the antenna uses. The ground for the antenna and 
the antenna should never connect to the coax. The 
transformer should always be an isolation type with separate 
primary and secondary windings. We should also do what we 
can to minimize the coupling between those windings (and 
Faraday shields won't help, you need some spacing and 
smaller windings to reduce stray capacitance).

It turns out 73 materials are about ideal for BC band 
through about 20MHz or more. That's what I use, and I never 
use a toroid.

73 Tom