Topband: Flag Antenna Feedline Decoupling

[Michael Tope]

The following is a summary of the responses that I received to my 
query on decoupling feedlines from flag antennas. Thanks to all who
replied. 

73 de Mike, W4EF...............................

----- Original Message ----- 
From: "Michael Tope" <W4EF@dellroy.com>
To: <topband@contesting.com>
Sent: Sunday, December 10, 2000 2:14 PM
Subject: Flag Antenna Feedline Decoupling


> 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).

From: "Tom Rauch W8JI" <w8ji@contesting.com>

> 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.

From: "Larry Molitor W7IUV" <w7iuv@earthlink.net>

> I have found that if you use the proper transformer, you don't need any 
> beads on the feedline. I have two flags up now and have done testing with 
> beads and without and can see no difference. However, if you do not have a 
> good isolation type transformer, it may be difficult to get enough beads on 
> there to help.
> 
> If you want to put them on anyway, use enough to fill up at least 12 inches 
> of coax, preferably more. Use type 73/77 material or the type sold for EMI 
> suppression on computer cables.
  
From: "Bert  VE3QAA" <kimba@sprint.ca>

> I would go for more like a hundred.  300 ohms is not very much
> isolation.

From: "Pete Smith" <n4zr@contesting.com>

> I've been told you need at least 50 for that frequency, but you might do
> with less, particularly if the feedline is buried or otherwise closely
> coupled to ground.


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