[Elecraft] Balun for K-1
Tom Hammond NØSS
[email protected]
Sat Jul 20 13:13:02 2002
At 09:57 AM 7/20/02, James R. Duffey wrote:
>I would sugest using an air wound coax choke balun. THe HAndbook and ANtenna
>Book should tell you how to make one. For the 7MHz to 21 MHz range of the 4
>module K-1 I would suggest 10 ft of RG-58 wound into 7 turns or so. Connect
>it like regular coax on the transmitter end, and connect the shield to one
>side of the feed line, and the center conductor to the other side. - Dr>
>Megacycle KK6MC/5
There appears to be a bit more to 'correctly' making coaxial chokes then
merely winding x-turns.
Here's some data I found a number of years ago. Think it might be of
interest to some builders.
73,
Tom Hammond N0SS
__________________________
NOTE: The table included below MUST be viewed using a FIXED-PITCH FONT.
The lines are somewhat long (c. 76 chrs.) and may possibly wrap
the the next line. Sorry about that.
__________________________
From: Ed Gilbert, WA2SRQ
[email protected]
-----------------------------------------------------------------
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
Freq 4-1/4 in 4-1/4 in 6-5/8 in 6-5/8 in 6-5/8 in beads
MHz sngl layer sngl layer sngl layer sngl layer bunched (Aztec)
---- ---------- ---------- ---------- ---------- ---------- ----------
Mag Phase Mag Phase Mag Phase Mag Phase Mag Phase Mag Phase
1.0 26 88.1 65 89.2 26 88.3 74 89.2 94 89.3 416 78.1
2.0 51 88.7 131 89.3 52 88.8 150 89.3 202 89.2 795 56.1
3.0 77 88.9 200 89.4 79 89.1 232 89.3 355 88.9 1046 39.8
4.0 103 89.1 273 89.5 106 89.3 324 89.4 620 88.3 1217 26.6
5.0 131 89.1 356 89.4 136 89.2 436 89.3 1300 86.2 1334 14.7
6.0 160 89.3 451 89.5 167 89.3 576 89.1 8530 59.9 1387 3.6
7.0 190 89.4 561 89.5 201 89.4 759 89.1 2120 -81.9 1404 -5.9
8.0 222 89.4 696 89.6 239 89.4 1033 88.8 1019 -85.7 1369 -15.4
9.0 258 89.4 869 89.5 283 89.4 1514 87.3 681 -86.5 1295 -23.7
10.0 298 89.3 1103 89.3 333 89.2 2300 83.1 518 -86.9 1210 -29.8
11.0 340 89.3 1440 89.1 393 89.2 4700 73.1 418 -87.1 1123 -35.2
12.0 390 89.3 1983 88.7 467 88.9 15840 -5.2 350 -87.2 1043 -39.9
13.0 447 89.2 3010 87.7 556 88.3 4470 -62.6 300 -86.9 954 -42.7
14.0 514 89.3 5850 85.6 675 88.3 2830 -71.6 262 -86.9 901 -45.2
15.0 594 88.9 42000 44.0 834 87.5 1910 -79.9 231 -87.0 847 -48.1
16.0 694 88.8 7210 -81.5 1098 86.9 1375 -84.1 203 -87.2 778 -51.8
17.0 830 88.1 3250 -82.0 1651 81.8 991 -82.4 180 -86.9 684 -54.4
18.0 955 86.0 2720 -76.1 1796 70.3 986 -67.2 164 -84.9 623 -45.9
19.0 1203 85.4 1860 -80.1 3260 44.6 742 -71.0 145 -85.1 568 -51.2
20.0 1419 85.2 1738 -83.8 3710 59.0 1123 -67.7 138 -84.5 654 -34.0
21.0 1955 85.7 1368 -87.2 12940 -31.3 859 -84.3 122 -86.1 696 -49.9
22.0 3010 83.9 1133 -87.8 3620 -77.5 708 -86.1 107 -85.9 631 -54.8
23.0 6380 76.8 955 -88.0 2050 -83.0 613 -86.9 94 -85.5 584 -57.4
24.0 15980 -29.6 807 -86.3 1440 -84.6 535 -86.3 82 -85.0 536 -58.8
25.0 5230 -56.7 754 -82.2 1099 -84.1 466 -84.1 70 -84.3 485 -59.2
26.0 3210 -78.9 682 -86.4 967 -83.4 467 -81.6 60 -82.7 481 -56.2
27.0 2000 -84.4 578 -87.3 809 -86.5 419 -85.5 49 -81.7 463 -60.5
28.0 1426 -85.6 483 -86.5 685 -87.1 364 -86.2 38 -79.6 425 -62.5
29.0 1074 -85.1 383 -84.1 590 -87.3 308 -85.6 28 -75.2 387 -63.8
30.0 840 -83.2 287 -75.0 508 -87.0 244 -82.1 18 -66.3 346 -64.4
31.0 661 -81.7 188 -52.3 442 -85.7 174 -69.9 9 -34.3 305 -64.3
32.0 484 -78.2 258 20.4 385 -83.6 155 -18.0 11 37.2 263 -63.2
33.0 335 -41.4 1162 -13.5 326 -78.2 569 -0.3 21 63.6 212 -58.0
34.0 607 -32.2 839 -45.9 316 -63.4 716 -57.6 32 71.4 183 -40.5
35.0 705 -58.2 564 -56.3 379 -69.5 513 -72.5 46 76.0 235 -29.6