At 01:07 PM 7/11/03 -0500, you wrote:
I tend to agree with you, (at least I think your implicit point) that cores
on a coax length are an entirely adequate, economical and wide band solution
to blocking outside the shield reverse RF current, ie act as a balancing
device by prevention of incidental reflections.
Coincidentally, I had 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 73 beads on a length of 1/4" teflon coax.
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 it is clear that
the bead choke is at least somewhat better as a common mode choke than the
solenoid, particularly at low frequencies, but 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.
73, Pete N4ZR
The World HF Contest Station Database was updated 17 June 03.
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