Hi all -
It is true that more #43 material will be required to obtain a given choking
impedance, compared to #73 material. However, it is a better choice for 20
through 6 meters because it will exhibit less heating at higher power
levels.
73, Ken
----- Original Message -----
From: "Bill Coleman" <aa4lr@arrl.net>
To: "Kenneth Hirschberg" <info@cal-av.com>; <abowen@nettally.com>
Cc: <towertalk@contesting.com>
Sent: Thursday, February 07, 2002 9:39 AM
Subject: Re: [TowerTalk] Bead baluns
> 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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