Topband: FT-240-77 Toroid Impedance R + jX Components

[Tom Rauch]

> Help me understand how this translates to a practical
> use at the feedpoint of a receiving antenna.

http://www.w8ji.com/common-mode_noise.htm


> I want to provide a common-mode choke at the feedpoint of
> a Beverage or flag/pennant receiving antenna fed with
> RG-6 or RG-8X feed line for 1.8 and 3.5 MHz. Power is
> not a consideration.

You will probably never get the siolation you need with a
choke of any form in the case of something like a
Pennant/Flag. You need an isolated winding primary-secondary
style transformer, not a choke.

Chokes are only useful if ou have a low shunting impedance
to ground, and they are most useful if the antenna or
feedline common mode impedance is inductive (or the same
reactance sign as the CM choke) at the point of insertion.

The common mode impedance has little connection to the
differential or normal transmission line mode impedance.

> I've been winding 10-15 turns of the feedline through an
> Amidon FT240-77 (2.4-inch #77 material) core. From Mike's
> data, the large number of turns is providing a large -jX
> component, in addition to the large resistive component.

That suprises me with only 10-15 turns if it is on 1.8 or
3.5 MHz. My experience is maybe 7MHz or higher is the
parallel resonant point. I'll measure some similar cores
today.

> For a common-mode choke, would I be better off:
>    1) decreasing the number of turns; or
>    2) Using a toroid of a different material; or
>    3) Eliminating the toroid completely.
>    4) Doing something else altogether.


Number 4, unless it is in a system where you have some
assurance there is low or modest shunt impedance. Small
ground independent loops generally look capacitive in common
mode. The last thing you want is something that looks
inductive in CM between the loop and the feedline. Also the
CM impedance is high, and that means the series impedance
has to be extremely high to have any effect.

73 Tom