Topband: Flag Antenna Feedline Decoupling

[Tom Rauch]

> I am getting ready to build a set of flag transformers and had plan
> to put a string of ferrite beads on the feedline to decouple it from
> the antenna. Can anyone tell me how many beads I should use to
> get adequate decoupling? I will be using Amidon FB-73-2401 beads
> (ui = 2000, thickness 0.2"). I was thinking that 25 would be a good
> number (~300 ohms series impedance at 1.8 MHz).

Use a bead that is mostly resistive at 1.8MHz, but remember the 
bead will only help if you put it between your receiver and a ground 
rod on the feedline near the antenna.

Having modeled the common mode problem, I'm amazed at how 
sensitive these antennas appear to be for even small amounts of 
unbalanced coupling.

It's mostly a case of getting the transformer balance to match the 
antenna balance. A compensation system would be the best 
engineering solution, but difficult to adjust in the field.

Since that isn't practical, it looks like you need to really keep the 
capacitance between primary and secondary to less than maybe 5 
pF or so. That would mean bringing the feedline straight away from 
the antenna for at least some small distance, unless you are lucky.
  
73, Tom W8JI
w8ji@contesting.com


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