Pete,
Solenoid baluns have their place in our antenna systems. They are
relatively inexpensive and easy to make. The principle problem with
solenoid baluns is that they are "sort-of" narrow band. If they are OK at
1.8-3.5 MHz, they will probably exhibit resonances at frequencies above 10
or 15 MHz.
For example, on my network analyzer I measured the impedance on the outside
of about 50 feet of RG8X close wound (about 39 turns) on about a 4.5" PVC
tube (similar to what you used). At low HF frequencies it was great with an
impedance of about 650 Ohms at 1.8 MHz, 1.3K at 3.5 MHz and 2.4K at 7 MHz
but almost no impedance at about 14 MHz where it apparently had a self
resonance! There were many other resonances as frequency increased at
various frequencies such as 21 and 28 MHz. Hence, for 160 through 30
meters, this was a great balun but not good above these bands.
73,
Joe, W1JR
At 04:41 PM 7/11/2003 -0400, Pete Smith wrote:
>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
>
>
>
>_______________________________________________
>
>See: http://www.mscomputer.com for "Self Supporting Towers", "Wireless
>Weather Stations", and lot's more. Call Toll Free, 1-800-333-9041 with
>any questions and ask for Sherman, W2FLA.
>
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