[TowerTalk] The Balun Blues
Kenneth D. Grimm
grimm@lynchburg.net
Tue, 03 Feb 1998 15:01:31 -0500
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K7LXC@aol.com wrote:
> If anyone that has the original thread on coaxial wound RF chokes, it
> might be a good time to re-post the juicy parts.
>
> 73, Steve K7LXC
Well, Steve, since it was the article posted here last October that convinced me
to replace my baluns (as they fail) with coiled coax chokes, I just happen to
have it ready to go. One of the problems with the original posting was in
keeping the formatting. I have edited the table so that it displays correctly (I
hope) and will attach the message in its entirety below. Please let me know if
the formatting is munged. To all who haven't, READ THIS!!! To all those who
have but didn't pay attention, you now have a second chance. :-)
73, Ken K4XL
--
Ken K4XL
grimm@lynchburg.net
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Date: Sat, 11 Oct 1997 15:53:57 -0500
To: Tom Osborne <w7why@mail.coos.or.us>, towertalk@contesting.com
From: Bob Perring <perring@infocom.net>
Subject: Re: [TowerTalk] Coaxial Choke Balun
THE FOLLOWING IS SOME TEXT I SAVED FROM SEVERAL YEARS AGO.
AS FOLLOWS:
=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=
=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=
=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D
Here's something I wrote up a while ago on coaxial balun measurements. I=
t's a little long, but I think there's enough contesters out there buildi=
ng these things that it's worth posting. =
73,
Ed Gilbert, WA2SRQ
eyg@hpnjlc.njd.hp.com
---------------------------------------------------------------------
Having access to a Hewlett-Packard 4193A vector impedance meter at work, =
I have made measurements on a number of baluns, coaxial and otherwise. F=
or my beams I was particularly interested how many turns and on what diam=
eter are optimum for air core coaxial baluns, and what the effect of bunc=
hing the turns was (formless). Using the remote programming capability o=
f the HP4193A along with an instrument controller, I measured the magnitu=
de 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 balu=
n 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 wo=
und 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, a=
nd I obtained a 10 foot length at the local Home Depot for about 3 dollar=
s. The 6-5/8 inch diameter forms are 6 inch schedule 40 PVC pipe which i=
s much thicker, heavier, and more expensive.
Each test choke was close-wound on a form as a single-layer solenoid usin=
g 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 e=
nough 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" measu=
rements 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 diamet=
er and number of turns, the bunched baluns show a large downward shift in=
parallel self-resonance frequency and poor choking reactance at the high=
er frequencies. =
Interpreting the Measurements
-----------------------------
All the baluns start out looking inductive at low frequencies, as indicat=
ed by the positive phase angles. As the frequency is increased, a point =
is reached where the capacitance between the windings forms a parallel re=
sonance with the coil's inductance. Above this frequency, the winding re=
actance is reduced by this capacitance. The interwinding capacitance inc=
reases with the number of turns and the diameter of the turns, so "more i=
s 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 togeth=
er than the single-layer coil. An important requirement of these baluns =
is that the magnitude of the winding reactance be much greater than the l=
oad impedance. In the case of a 50 ohm balanced antenna, the balun's win=
ding impedance is effectively shunted across one half the 50 ohm load imp=
edance, or 25 ohms. A reasonable critera for the balun's winding impedan=
ce for negligible common mode current in the shield is that it be at leas=
t 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 exhibi=
t. Each of the measurement points can be transformed from the polar form=
at of the table to a parallel equivalent real and reactive shunt impedanc=
e. The power dissipated in the balun is then the square of the voltage a=
cross it divided by the real parallel equivalent shunt impedance. While =
this calculation can be made for each measurement point, an approximate n=
umber can be taken directly from the tables at the parallel resonance poi=
nts. 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 th=
e ferrite bead balun read about 1.4K. These baluns see half the load vol=
tage, so at 1500 watts to a 50 ohm load, the power dissipated in the coax=
ial baluns will be less than 1.3 watts, and the ferrite bead balun will d=
issipate 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 an=
d 15 meter yagis both recommend 12 turns 6 inches in diameter. At the ve=
ry least this is about 3 times as much coax as is needed, and these dimen=
sions 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 Fer=
rite
4-1/4 in 4-1/4 in 6-5/8 in 6-5/8 in 6-5/8 in be=
ads
sngl layer sngl layer sngl layer sngl layer bunched (A=
ztec)
---------- ---------- ---------- ---------- ---------- --=
--------
Frequency Mag Phase Mag Phase Mag Phase Mag Phase Mag Phase Ma=
g Phase
1.00E+06 26 88.1 65 89.2 26 88.3 74 89.2 94 89.3 41=
6 78.1
2.00E+06 51 88.7 131 89.3 52 88.8 150 89.3 202 89.2 79=
5 56.1
3.00E+06 77 88.9 200 89.4 79 89.1 232 89.3 355 88.9 10=
46 39.8
4.00E+06 103 89.1 273 89.5 106 89.3 324 89.4 620 88.3 12=
17 26.6
5.00E+06 131 89.1 356 89.4 136 89.2 436 89.3 1300 86.2 13=
34 14.7
6.00E+06 160 89.3 451 89.5 167 89.3 576 89.1 8530 59.9 13=
87 3.6
7.00E+06 190 89.4 561 89.5 201 89.4 759 89.1 2120 -81.9 14=
04 -5.9
8.00E+06 222 89.4 696 89.6 239 89.4 1033 88.8 1019 -85.7 13=
69 -15.4
9.00E+06 258 89.4 869 89.5 283 89.4 1514 87.3 681 -86.5 12=
95 -23.7
1.00E+07 298 89.3 1103 89.3 333 89.2 2300 83.1 518 -86.9 12=
10 -29.8
1.10E+07 340 89.3 1440 89.1 393 89.2 4700 73.1 418 -87.1 11=
23 -35.2
1.20E+07 390 89.3 1983 88.7 467 88.9 15840 -5.2 350 -87.2 10=
43 -39.9
1.30E+07 447 89.2 3010 87.7 556 88.3 4470 -62.6 300 -86.9 95=
4 -42.7
1.40E+07 514 89.3 5850 85.6 675 88.3 2830 -71.6 262 -86.9 90=
1 -45.2
1.50E+07 594 88.9 42000 44.0 834 87.5 1910 -79.9 231 -87.0 84=
7 -48.1
1.60E+07 694 88.8 7210 -81.5 1098 86.9 1375 -84.1 203 -87.2 77=
8 -51.8
1.70E+07 830 88.1 3250 -82.0 1651 81.8 991 -82.4 180 -86.9 68=
4 -54.4
1.80E+07 955 86.0 2720 -76.1 1796 70.3 986 -67.2 164 -84.9 62=
3 -45.9
1.90E+07 1203 85.4 1860 -80.1 3260 44.6 742 -71.0 145 -85.1 56=
8 -51.2
2.00E+07 1419 85.2 1738 -83.8 3710 59.0 1123 -67.7 138 -84.5 65=
4 -34.0
2.10E+07 1955 85.7 1368 -87.2 12940 -31.3 859 -84.3 122 -86.1 69=
6 -49.9
2.20E+07 3010 83.9 1133 -87.8 3620 -77.5 708 -86.1 107 -85.9 63=
1 -54.8
2.30E+07 6380 76.8 955 -88.0 2050 -83.0 613 -86.9 94 -85.5 58=
4 -57.4
2.40E+07 15980 -29.6 807 -86.3 1440 -84.6 535 -86.3 82 -85.0 53=
6 -58.8
2.50E+07 5230 -56.7 754 -82.2 1099 -84.1 466 -84.1 70 -84.3 48=
5 -59.2
2.60E+07 3210 -78.9 682 -86.4 967 -83.4 467 -81.6 60 -82.7 48=
1 -56.2
2.70E+07 2000 -84.4 578 -87.3 809 -86.5 419 -85.5 49 -81.7 46=
3 -60.5
2.80E+07 1426 -85.6 483 -86.5 685 -87.1 364 -86.2 38 -79.6 42=
5 -62.5
2.90E+07 1074 -85.1 383 -84.1 590 -87.3 308 -85.6 28 -75.2 38=
7 -63.8
3.00E+07 840 -83.2 287 -75.0 508 -87.0 244 -82.1 18 -66.3 34=
6 -64.4
3.10E+07 661 -81.7 188 -52.3 442 -85.7 174 -69.9 9 -34.3 30=
5 -64.3
3.20E+07 484 -78.2 258 20.4 385 -83.6 155 -18.0 11 37.2 26=
3 -63.2
3.30E+07 335 -41.4 1162 -13.5 326 -78.2 569 -0.3 21 63.6 21=
2 -58.0
3.40E+07 607 -32.2 839 -45.9 316 -63.4 716 -57.6 32 71.4 18=
3 -40.5
3.50E+07 705 -58.2 564 -56.3 379 -69.5 513 -72.5 46 76.0 23=
5 -29.6
=
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