[TowerTalk] Bead baluns

[Bill Coleman]

On 2/6/02 7:42 PM, Kenneth Hirschberg at info@cal-av.com wrote:

>To answer the question that you asked, regarding beads for RG-303/U:
>
>For 40-10 you will have good results with 43 mat'l. (e.g. 50 beads of Amidon
>FB-43-2401).

43 material has a maximum impedance at around 200 MHz. Perhaps not the 
best choice for HF -- as you will require more beads.

>For 80-10, 50 beads of 73 mat'l. (FB-73-2401) will do fine. (all the way
>down to 160M actually)   

You could use the FB-77-6301. Except you won't need 50 of them. Each is 
twice as long as the 2401, so you'll need half. And the type 77 material 
has a greater impedance per bead than the 43 material at HF. So you won't 
need so many. 20 should be sufficient. Two dozen is $10.00 from Amidon. 
Compare that to $18.00 for 48 of the FB-()-2401, whether you get 43 or 73 
material. (March 1998 price list)

> 40 beads of 73 mat'l. plus an additional 10 beads
>of 43 mat'l. at the dipole end would do slightly better if you expect either
>a significant mismatch, or are going to run high power on the higher
>frequency bands.

Mixing bead type doesn't help. 

>Pay attention to weatherproofing the connections.  The beads themselves
>don't care about moisture, but are frangible, and could use some protection
>from mechanical threats, e.g. shrink tubing.

If you pot them, or otherwise seal them up, they don't disappate heat as 
well. With higher power, as the ferrite heats up, it becomes more lossy. 
That could lead to a thermal runway, at least to the curie temperature.

So, be careful about sealing them up.

I used 10 FB-43-1020 beads on my A3S. I didn't bother to seal them up at 
all. Of course, these beads are huge, and may be a bit more durable as a 
result.


Bill Coleman, AA4LR, PP-ASEL        Mail: aa4lr@arrl.net
Quote: "Not within a thousand years will man ever fly!"
            -- Wilbur Wright, 1901


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