Hi Pete,
My experience is that PVC works fine as a form for high Q RF coils.
I've measured Qs of up to 450 on loading coils wound on PVC pipe.
I've appended a paper I wrote on measurements of coaxial baluns wound on
PVC forms.
73,
Ed Gilbert, WA2SRQ
eyg@hpnjlc.njd.hp.com
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Having access to a Hewlett-Packard 4193A vector impedance meter at
work, I have made measurements on a number of baluns, coaxial and
otherwise. For my beams I was particularly interested how many turns
and on what diameter are optimum for air core coaxial baluns, and what
the effect of bunching the turns was (formless). Using the remote
programming capability of the HP4193A along with an instrument
controller, I measured the magnitude and phase of each balun's winding
impedance at 1 MHz intervals from 1 to 35 MHz. For comparison, I also
made measurements on a commercial balun which consists of a number of
ferrite beads slipped over a short length of coax. I've appended some
of these measurements so you can draw your own conclusions.
PVC pipe was used for coil forms. The 4-1/4 inch diameter baluns were
wound on thin-walled PVC labeled "4 inch sewer pipe". This material
makes an excellent balun form. It's very light weight and easy to
work with, and I obtained a 10 foot length at the local Home Depot for
about 3 dollars. The 6-5/8 inch diameter forms are 6 inch schedule 40
PVC pipe which is much thicker, heavier, and more expensive.
Each test choke was close-wound on a form as a single-layer solenoid
using RG-213 and taped to hold the turns in place. The lengths of
cable were cut so there was about 2 inches excess at each end. This
allowed just enough wire at the ends for connections to the HP4193A's
probe tip. After data was collected for each single-layer
configuration, the PVC form was removed, the turns were bunched
together and taped formless, and another set of measurements was
taken. I have only included the "bunched" measurements in the table
for one of the baluns, but the trend was the same in each case. When
compared to the single-layer version of the same diameter and number
of turns, the bunched baluns show a large downward shift in parallel
self-resonance frequency and poor choking reactance at the higher
frequencies.
Interpreting the Measurements
-----------------------------
All the baluns start out looking inductive at low frequencies, as
indicated by the positive phase angles. As the frequency is
increased, a point is reached where the capacitance between the
windings forms a parallel resonance with the coil's inductance. Above
this frequency, the winding reactance is reduced by this capacitance.
The interwinding capacitance increases with the number of turns and
the diameter of the turns, so "more is not always better".
The effects of a large increase in interwinding capacitance is evident
in the measurements on the balun with the bunched turns. This is
probably a result of the first and last turns of the coil being much
closer together than the single-layer coil.
An important requirement of these baluns is that the magnitude of the
winding reactance be much greater than the load impedance. In the
case of a 50 ohm balanced antenna, the balun's winding impedance is
effectively shunted across one half the 50 ohm load impedance, or 25
ohms. A reasonable critera for the balun's winding impedance for
negligible common mode current in the shield is that it be at least 20
times this, or 500 ohms. The measurements show, for example, that 6
turns 4-1/4 inches in diameter meet this criteria from 14 to 35 MHz.
The measurement data also reveals the power loss these baluns will
exhibit. Each of the measurement points can be transformed from the
polar format of the table to a parallel equivalent real and reactive
shunt impedance. The power dissipated in the balun is then the square
of the voltage across it divided by the real parallel equivalent shunt
impedance. While this calculation can be made for each measurement
point, an approximate number can be taken directly from the tables at
the parallel resonance points. At 0 degrees phase angle the magnitude
numbers are pure resistive. I didn't record the exact resonance
points, but it can be seen from the tables that the four single-layer
baluns are all above 15K ohms, while the ferrite bead balun read about
1.4K. These baluns see half the load voltage, so at 1500 watts to a
50 ohm load, the power dissipated in the coaxial baluns will be less
than 1.3 watts, and the ferrite bead balun will dissipate about 13.4
watts (neglecting possible core saturation and other non-linear
effects). These losses are certainly negligible. At 200 ohms load
impedance, the losses are under 5 watts for the coaxial baluns and
53.6 watts for the ferrite beads.
Conclusions
-----------
- A 1:1 coaxial balun with excellent choking reactance for 10 through
20 meters can be made by winding 6 turns of RG-213 on inexpensive 4
inch PVC sewer pipe.
- For 40 or 30 meters, use 12 turns of RG-213 on 4 inch PVC sewer
pipe.
- Don't bunch the turns together. Wind them as a single layer on a
form. Bunching the turns kills the choking effect at higher
frequencies.
- Don't use too many turns. For example, the HyGain manuals for my 10
and 15 meter yagis both recommend 12 turns 6 inches in diameter. At
the very least this is about 3 times as much coax as is needed, and
these dimensions actually give less than the desired choking impedance
on 10 and 15 meters.
Measurements
------------
Magnitude in ohms, phase angle in degrees, as a function of frequency
in Hz, for various baluns.
6 Turns 12 Turns 4 Turns 8 Turns 8 Turns Ferrite
4-1/4 in 4-1/4 in 6-5/8 in 6-5/8 in 6-5/8 in beads
sngl layer sngl layer sngl layer sngl layer bunched (Aztec)
---------- ---------- ---------- ---------- ---------- ----------
Frequency Mag Phase Mag Phase Mag Phase Mag Phase Mag Phase Mag Phase
1.00E+06 26 88.1 65 89.2 26 88.3 74 89.2 94 89.3 416 78.1
2.00E+06 51 88.7 131 89.3 52 88.8 150 89.3 202 89.2 795 56.1
3.00E+06 77 88.9 200 89.4 79 89.1 232 89.3 355 88.9 1046 39.8
4.00E+06 103 89.1 273 89.5 106 89.3 324 89.4 620 88.3 1217 26.6
5.00E+06 131 89.1 356 89.4 136 89.2 436 89.3 1300 86.2 1334 14.7
6.00E+06 160 89.3 451 89.5 167 89.3 576 89.1 8530 59.9 1387 3.6
7.00E+06 190 89.4 561 89.5 201 89.4 759 89.1 2120 -81.9 1404 -5.9
8.00E+06 222 89.4 696 89.6 239 89.4 1033 88.8 1019 -85.7 1369 -15.4
9.00E+06 258 89.4 869 89.5 283 89.4 1514 87.3 681 -86.5 1295 -23.7
1.00E+07 298 89.3 1103 89.3 333 89.2 2300 83.1 518 -86.9 1210 -29.8
1.10E+07 340 89.3 1440 89.1 393 89.2 4700 73.1 418 -87.1 1123 -35.2
1.20E+07 390 89.3 1983 88.7 467 88.9 15840 -5.2 350 -87.2 1043 -39.9
1.30E+07 447 89.2 3010 87.7 556 88.3 4470 -62.6 300 -86.9 954 -42.7
1.40E+07 514 89.3 5850 85.6 675 88.3 2830 -71.6 262 -86.9 901 -45.2
1.50E+07 594 88.9 42000 44.0 834 87.5 1910 -79.9 231 -87.0 847 -48.1
1.60E+07 694 88.8 7210 -81.5 1098 86.9 1375 -84.1 203 -87.2 778 -51.8
1.70E+07 830 88.1 3250 -82.0 1651 81.8 991 -82.4 180 -86.9 684 -54.4
1.80E+07 955 86.0 2720 -76.1 1796 70.3 986 -67.2 164 -84.9 623 -45.9
1.90E+07 1203 85.4 1860 -80.1 3260 44.6 742 -71.0 145 -85.1 568 -51.2
2.00E+07 1419 85.2 1738 -83.8 3710 59.0 1123 -67.7 138 -84.5 654 -34.0
2.10E+07 1955 85.7 1368 -87.2 12940 -31.3 859 -84.3 122 -86.1 696 -49.9
2.20E+07 3010 83.9 1133 -87.8 3620 -77.5 708 -86.1 107 -85.9 631 -54.8
2.30E+07 6380 76.8 955 -88.0 2050 -83.0 613 -86.9 94 -85.5 584 -57.4
2.40E+07 15980 -29.6 807 -86.3 1440 -84.6 535 -86.3 82 -85.0 536 -58.8
2.50E+07 5230 -56.7 754 -82.2 1099 -84.1 466 -84.1 70 -84.3 485 -59.2
2.60E+07 3210 -78.9 682 -86.4 967 -83.4 467 -81.6 60 -82.7 481 -56.2
2.70E+07 2000 -84.4 578 -87.3 809 -86.5 419 -85.5 49 -81.7 463 -60.5
2.80E+07 1426 -85.6 483 -86.5 685 -87.1 364 -86.2 38 -79.6 425 -62.5
2.90E+07 1074 -85.1 383 -84.1 590 -87.3 308 -85.6 28 -75.2 387 -63.8
3.00E+07 840 -83.2 287 -75.0 508 -87.0 244 -82.1 18 -66.3 346 -64.4
3.10E+07 661 -81.7 188 -52.3 442 -85.7 174 -69.9 9 -34.3 305 -64.3
3.20E+07 484 -78.2 258 20.4 385 -83.6 155 -18.0 11 37.2 263 -63.2
3.30E+07 335 -41.4 1162 -13.5 326 -78.2 569 -0.3 21 63.6 212 -58.0
3.40E+07 607 -32.2 839 -45.9 316 -63.4 716 -57.6 32 71.4 183 -40.5
3.50E+07 705 -58.2 564 -56.3 379 -69.5 513 -72.5 46 76.0 235 -29.6
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