[qrp-l.org] Current Balun, 1:1, w/Large Beads
w5usj at 9plus.net
Sat Sep 29 13:51:11 CDT 2007
A couple of years ago, George, W5YR (SK), developed a version of the W2DU
bead balun using large Fair-Rite beads. The beads for RG-58/8X are about
1/2 inch in diameter and 1-1/8 inches long. Material is type 43 and they
are available from Mouser.
Note that Palomar sells a kit using similar beads -- more on this later.
George did a neat demo at one of our QRP club meetings showing how the
currents stay balanced even when the load is unbalanced. I've been able to
do a similar demo using 4ea 200 Ohm 1% resistors as a load connected in
series across the output of the balun. The test instrument, MFJ-259B, is
connected ahead of my z-matcher which is connected to the input of the
balun. The setup is tuned for a match on 20 meters. The center of the load
is grounded back to the signal input side after the tuner. Then the ground
is moved to one side or the other between two 200 Ohm resistors. If all is
well, the SWR doesn't change and it didn't.
The balun tested consisted of 5 of the large beads over a short length of
RG-8X fitted with a connector on one end. The total choking inductance is
about 19uH with each bead contributing about 3.8uH. You could also use
RG-58 or RG-400. RG-400 is the same size as RG-58 but uses teflon
insulation. Choking inductance is about the same. (I'm using 2 beads at
the feed point of my 6 meter quad.)
The Fair-Rite beads are available from Mouser, part number 623-2643540002
for the RG-58/8X size. These are loose on 58 and snug of some 8X or RG-59
depending on insulation thickness. The weight for 7 beads is 6
ounces. Check Mouser shipping cost on this weight. Current catalog price
is $0.56 ea.
The larger size for RG-8/213 etc. is part number is 623-2643102002. Current
catalog price is $1.67 ea. Weight for 5 beads is 13 ounces.
I wound 3 turns of RG-400 through one of the large beads. The choking
inductance was about 23uH. Haven't tried the balance test on this
Palomar has kits of beads, material unknown to me, that are about the same
size as those from Fair-Rite. The cost is about $9 and $18 for the two
size kits. Shipping is $8 per order.
Chuck Carpenter, W5USJ, Point, -|- Rains Co., TX -|- EM22cv -|- 72/73
52 years -|- 19 as K2OFN and 33 as W5USJ -|- Most fun = QRP since 1984.
Website = http://www.w5usj.com hosted by Hamnutz.com
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