At 10:38 AM -0400 7/18/03, Joe Reisert W1JR wrote:
>The "W1JR Balun" uses a 2.4" OD ferrite core. The winding consists
>of 12 turns of RG141 or RG303 (the later recommended) teflon coax in
>a "contra-wound" fashion.... The core used is the standard 2.4" OD
>called the T240 by Amidon and others. The material used is Fair-Rite
>type 61 (Indiana General Q-1) which has a initial permeability of
>125.... If you use 12 turns, the impedance will be greater than 500
>Ohms at 80 meters...and typically much greater than 1,000 Ohms from
>7-30 MHz with no resonances.
...to which I replied, "I'll measure one and report back."
I just wound a "W1JR Balun" as specified, using an FT-240-61 toroid
and RG-303; then I measured its common-mode impedance (between the
ends of the coax shield). The core that I used was the one that I
had left on the shelf, out of a lot of ten bought earlier this year,
which had the least permeability of the lot (which is why it was
left). Therefore I expected the impedance of this choke to be
somewhat less than Joe had quoted at 3.5 MHz, and it is; but there's
nothing to get upset about. My measurement results:
Freq.| |Z| | Angle |
(MHz)|(ohms)| (deg) |
-----+------+-------+
3.5 | 408 | 90 |
7.0 | >1k | |
14.0 | >1k | |if the
21.0 | >1k | |
28.0 | >1k | |
30.0 | >1k | |
33.3 | 610 | 29 |
Basically, I confirm what Joe has written about his balun. It _is_
very good. Like Joe, I have never seen a commercial balun so good.
I have only one, _minor_, reservation about this balun, relating to
its ability to handle high power with high SWR. According to the
back-of-the-envelope calculation that I just did (and which I invite
anyone to check), nearly 10 W will be dissipated in this balun
(mostly in the center conductor of the RG-303) if 1500 W is
transmitted through it at a frequency of 28 MHz, and if the SWR in
the coax is equal to one. This (10 W) dissipation should do no harm
to the balun, because the Teflon dielectric of the coax should take
the heat, unless you do something foolish like potting the balun in
foam.
My minor concern is that the dissipation _could_ be dangerous if the
SWR were _high_. If the SWR were equal to, say, 9, then for the same
net (i.e., forward minus reverse) power transmitted, the current in
the coax could be tripled and the power dissipated could be greater
by a factor of nine. Dissipating 90 W in such a small volume could
cause trouble.
All things considered, however, Joe's design has to be considered
_excellent_. Thanks, Joe, for sharing it with us.
73 de Chuck, W1HIS
73 -Chuck, W1HIS
|