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Optimum Coax Coiled Turns Choke Baluns for Antennas




 
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Author Topic: Optimum Coax Coiled Turns Choke Baluns for Antennas  (Read 31233 times)
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K1JJ
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"Let's go sailing, Tommy!" - Yaz


« on: May 21, 2006, 03:59:57 PM »

Here's an interesting piece written by WA2SQR. He measured various coax baluns and compared results. It shows that some of the conventional wisdom is incorrect, especially when using these techniques above 7mhz.  Using more turns than necessary degrades the choke's performance. Bunched turns starts to be a no-no above 7 mhz, though acceptable on 160/75M.  Might as well use a coil form for all bands to minimize the tendancy for the end turns to arc to each other. There is increased RF voltage across the coax choke, just like any RF choke. I once arced a bunched turn 40M choke. Since using a 4 1/4" form with a single layer, it has been FB.

Notice the comments about increased power loss using ferrite beads (though not too bad) and see the phase shift from a starting 89 degrees as the techniques change and frequency goes up.

20 bunched turns in a hank or ferrite beads appear to be OK for 160M to give the 500 ohm minimum RF choke parameter without degradation. But, use the techniques shown next for 75M and above to be sure.


I'm making up many coaxial baluns here for all bands. Based on this study, I plan to  wind the RG-213 coaxial choke baluns with these dimensions. The turns are optimized for each band if anyone wants to use them.

All using 4 1/4" diameter ABS plastic sewer pipe (Home Depot):


75M  -  20 turns
40M  -  12 turns
20M  -   7 turns
15M  -   5 turns
10M  -   4 turns


73,
T


----------------------------------
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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