At 11:55 AM 7/11/03 -0700, Jim Lux wrote:
>And even better, there is a detailed data sheet with derating and
>impedances on it..
>http://www.cal-av.com/baluns-EB1-ds.html
>I do have a question about the term "reactance"...
>
>Does this mean if I hook an impedance bridge between, say, the shield of
>the SO-239 and the black wire, that I'll measure some combination of L and
>R the magnitude of which is around 1K?
>
>It's also unclear whether the voltage rating is from connector to ground
>(case of device) or from coax shield to coax inner conductor. 500V isn't
>a huge amount of power into a 1.4 K reactance..(a bit more than 100W) If I
>were running a kilowatt to a somewhat unbalanced 10 m dipole that put 10%
>of the power on the outside of the coax (say, by mutual coupling), and the
>balun happened to be at the wrong point, wavelength wise, you could easily
>get 500V..
>
>1:2 SWR is a return loss of 10 dB (i.e. a tenth of the power is reflected
>back)
>1:3 is 6 dB (i.e. a quarter of the power is reflected back)..
Coincidentally, I have recently done some measurements, using an MFJ259B,
on a solenoid choke (18 turns (closewound) of RG-8X on a 4 /2 inch piece of
PVC pipe, versus 50 Type 77 beads on a length of 1/4" teflon coax. I'm not
trying to suggest that these designs are optimum -- in fact, a friend
opines that the solenoid, in particular, has too many turns for the
job. The bead choke numbers don't seem that much different from Cal-Av's
though, at least at the lower frequencies.
The numbers were.... drumroll:
Solenoid
F (mhz) Z (ohms) Phase Angle (deg)
2 384 90
2.8 637 90
3-14 H (> 650)
16 579 90
18 510 90
20 423 90
24 293 90
25.9 210 42 (lowest magnitude measured)
27.4 324 90
Bead choke
F Z Phase
1.69-14 H
14.8 650 60
15 644 60
16 612 62
17 583 64
18 530 62
20 488 67
22 450 66
24 417 66
I suspect that the measurements above 20 MHz or so should probably be
discredited, because my test rig had a pair of 16-inch clipleads in it, but
the lower-frequency readings are interesting. A friend who is much more
knowledgeable about the theory of such things says that the Phase Angle
readings are a clear signal of much larger losses in the beads, hence the
heating talked about in discussions of bead chokes encapsulated in foam
material.
The MFJ is somewhat limited in this role, largely because it will not read
impedances over 650 ohms. Maybe someone out there with a better piece of
test gear could repeat (and correct) these measurements.
Numbers aside, I use the F-12/Cal-Av chokes on my tribanders and 40m beam,
and the homebrew bead chokes on the feedpoints of all my 80-meter array's
elements. So far no failures, despite 1300 watts CW in contest service
(well, maybe 1000 on 10) for several years. My homebrew chokes are just
wrapped in electrical tape, though.
73, Pete N4ZR
The World HF Contest Station Database was updated 17 June 03.
Are you current? www.pvrc.org/wcsd/wcsdsearch.htm
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