At 9:53 AM +0300 10/9/02, Tõnno Vähk wrote:
>...is it OK to use short toroids instead of beads for balun?
Yes, certainly. Just keep in mind the smaller choking impedance of a
one-turn "winding" on a "toroid," which is thinner than a "bead."
E.g.:
One "turn" through an FT-140-77 toroidal core (actual measurements
with Autek Research RX Vector Analyst Model VA1):
Freq.| |Z| |
(MHz)|(ohms)|
-----+------+
3.5 | 15
14.0 | 11
28.0 | 13 angle 33 deg
One "turn" through Fair-Rite p/n 2631102002 (type 31 ferrite) "bead"
(actual measurements again):
Freq. | |Z| | Angle
(MHz) | (ohms) | (deg)
------+--------+--------
1 | 46 | 58
2 | 66 | 43
5 | 84 |
10 | 112 | 45
20 | 153 | 40
I'm sorry that I do not have measurements for the same ferrite types
or the same frequencies. You can interpolate frequencies, though.
I would prefer not to use either type 31 or type 77 in a choke balun
for HF, especially for high power, because they are rather lossy so
they would get hot. Type 43 would be better, and type 61 (or 63??)
better yet; however the lower-loss materials also have less impedance.
>Does it make the balun any less efficient when threre are 20-30
>toroids instead of 4-5 beads??
If the impedance is the same, then the balun will be just as efficient.
73 de Chuck, W1HIS
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