Chameleon CHA F-Loop Antenna
Parameters: 5-30 MHz
Antenna Parameters Measured with a Vector
Network Analyzer
by Dr. Carol F. Milazzo, KP4MD (posted 30 May 2015)
E-mail: [email protected]
INTRODUCTION
The small magnetic loop is a useful compromise antenna for limited
space and portability. In the light of the many subjective anecdotal
reports extolling their performance, comparisons of their measured
operating characteristics against a validated model provide objective
evidence that is needed to assure and understand efficient design and
operation of small magnetic loop antennas.
Some desired goals include:
Non-reactive antenna to transmission line impedance matching for
efficient power transfer. This requires adjustment of the
impedance transformation ratio due to variations in loop
impedance over extended frequency ranges (see figure 8 below);
Low-resistance large-diameter loop material with minimal use of
non-soldered mechanical connections;
Use of a high-Q vacuum or split-stator or butterfly air variable
capacitor to minimize dielectric losses and to eliminate rotor
contact resistance;
Adequate capacitor plate spacing to handle the expected voltage
for the transmitter power level; and,
Narrow bandwidth (high Q) at resonance. For any specific
frequency and magnetic loop antenna, its Q is proportional to its radiation efficiency. Broad bandwidth at the resonant frequency is
not desirable as it indicates that power is radiated as heat (resistive loss) rather than as radio frequency energy.
These characteristics impact the material cost, size, portability, and performance of the antenna and should be available for the radio
operator to make an informed choice in the purchase or construction of a magnetic loop antenna to meet their requirements.
TEST ANTENNA
The test antenna was the Chameleon CHA F-Loop, a 0.74 m (2.44 feet) diameter radiator loop of DX Engineering DXE-400 MAX brand
of LMR-400 coaxial cable mounted on a tripod at 1.52 m (5 feet) height above ground. (Some design features of this antenna have been
revised in later production). The antenna was oriented vertically with the capacitor below and feed point above as shown in the
photograph. A miniVNA Pro Vector Network Analyzer was attached directly to the antenna connector on the tuning box, with no
intervening transmission line other than the supplied two 24 cm (9.5 inch) sections of 50 ohm hard line coaxial cable joined with a PL-258
double female connector that support the fixed 20.3 cm (8 inch) diameter coupling loop of 32 mm (0.125 inch) by 190 mm (0.75 inch)
aluminum bar. The vector network analyzer was linked to the controlling computer via Bluetooth to obtain a pure reading of the antenna
parameters unaffected by any attached cables in the near field.
Frequency adjustment is accomplished through a 6:1 planetary reduction drive and a dual gang 365 pF air variable capacitor across the
open ends of the loop radiator. With the toggle switch in the "B" position, the capacitor gangs are connected in series to achieve an
effective capacitance range from approximately 5 pF to 182.5 pF with twice the voltage rating of a single gang. This yielded a frequency
range of 7.5 through 31.3 MHz on the tested antenna. In the "A" position the SPST toggle switch shorts out one of the capacitor gangs,
rendering the capacitance range from 10-365 pF and the frequency range from 5.3 through 24.4 MHz. The "B" switch position is
preferable for operation above 7.5 MHz, since in the "A" switch position, the rotor contact resistance would introduce some power loss,
the capacitor voltage rating is half that of the "B" position, and the frequency adjustment is coarser than in the "B" position. Payne has
published the relation of frequency, capacitance and resistance loss in this type of dual gang capacitor.5
1. This curve of the measured SWR demonstrates the 18 kHz 2:1 VSWR bandwidth of the CHA F-Loop antenna when the
capacitor is adjusted for 5.3 MHz. The SWR minimum at 5.295 MHz is 1.168:1.
2. This curve of the measured SWR demonstrates the 24 kHz 2:1 VSWR bandwidth of the CHA F-Loop antenna when the
capacitor is adjusted for 7.1 MHz. The SWR minimum at 7.1 MHz is 1.345:1.
3. At 10.1 MHz the SWR minimum is 2.111:1.
4. At 14.15 MHz the SWR minimum is 2.292:1.
5. At 18.1 MHz the SWR minimum is 2.232:1.
6. This curve of the measured SWR demonstrates the 30 kHz 2:1 VSWR bandwidth of the CHA F-Loop antenna when the
capacitor is adjusted for 21.2 MHz. The SWR minimum at 21.2 MHz is 1.953:1.
7. This curve of the measured SWR demonstrates the 113 kHz 2:1 VSWR bandwidth of the CHA F-Loop antenna when the
capacitor is adjusted for 24.9 MHz. The SWR minimum at 24.9 MHz is 1.607:1.
8. This curve of the measured SWR demonstrates the 181 kHz 2:1 VSWR bandwidth of the CHA F-Loop antenna when the
capacitor is adjusted for 28.4 MHz. The SWR minimum at 28.4 MHz is 1.294:1.
9. Plot of measured Magnetic Loop Antenna Impedance at Zero Reactance vs. Frequency.
10. Plot of measured 2:1 VSWR bandwidth at Zero Reactance vs. frequency.
11. Plot of calculated Magnetic Loop Antenna Q vs. Frequency. (Calculator on http://owenduffy.net/calc/VswrBw2AntQ.htm)
12. Plot of calculated Magnetic Loop Antenna Free Space Efficiency vs. Frequency. (Calculator at
http://owenduffy.net/calc/SmallTransmittingLoopBw2Gain.htm)
13. Plot of calculated Magnetic Loop Antenna Free Space Gain vs. Frequency. (Calculator at
http://owenduffy.net/calc/SmallTransmittingLoopBw2Gain.htm)
14. Azimuth Radiation Patterns for CHA F-Loop Antenna
modeled for 14, 21 and 28 MHz at 5 feet above Average
Ground (4nec2 model). These gains are greater than the free
space values due to the additive effect of ground reflection.
15. Elevation Radiation Patterns for CHA F-Loop Antenna
modeled for 14, 21 and 28 MHz at 5 feet above Average Ground
(4nec2 model). These gains are greater than the free space
values due to the additive effect of ground reflection.
DISCUSSION
SPECIFICATIONS AT SWR MINIMA (Z0 = 50 Ω)
The observed parameters correlate with the NEC model
predictions, except for an additional 2-3 dB loss on 5 and 7 MHz
with the antenna in the "A" switch position likely due to increased
ground losses at those frequencies and the insertion of capacitor
rotor contact and switch contact resistance into the radiator loop
circuit. (The revised design of this antenna substitutes a capacitor
of greater range and allows continuous tuning from 7 through 30
MHz through the dual capacitor gangs in series). I had initially
suspected that the ring terminal connections in the radiator loop
circuit were responsible for the CHA F-Loop's lower Q than a
similar comparison antenna with a ceramic insulated capacitor. However, these data reveal that the dual gang capacitor's phenolic
insulation is the more significant factor yielding a capacitor Q
factor that decreases from 1500 at 10 MHz to near 100 at 28 MHz. Soldering the capacitor connections to the SO-239s and
substituting a capacitor with ceramic insulation could theoretically
improve the gain at 28 MHz by 5 dB (nearly one S unit) but at a
prohibitive increase in material cost and weight. With its existing
capacitor, the resistive losses from the ring terminal connections to
the SO-239s are relatively insignificant.
The predicted CHA F-Loop azimuth radiation pattern in Figure 14
appears nearly omni-directional, becoming more directional with
its feed point and the capacitor positions exchanged, placing the
capacitor above and the feed point below as in the comparison
antenna. Such directivity may be desirable when local radio
frequency noise needs to be minimized, but its implementation can
affect other considerations such as physical stability and proximity
effects during antenna tuning. The location of the heavier
capacitor tuning box at the bottom does offer greater physical
stability when resting on a flat surface, but it also brings the high
voltage ends of the radiating loop into proximity to the user during
adjustment and likewise causes a variable change in antenna
frequency after removing the hand from the tuning knob.
The fixed ratio style coupling loop typically exhibits some
impedance mismatch over this wide frequency range. Indeed the
CHA F-Loop non-reactive feed point impedance at resonance
varied from 9.9 ohms at 5.3 MHz to 38.4 ohms at 28 MHz. The
customary 5:1 loop diameter ratio would suggest a 5.9 inch
coupling loop instead of the CHA F-Loop's 8 inch loop. A
different coupling loop diameter may yield a lower minimum
SWR with the overall non-reactive feed point impedance closer to
50 ohms.
The correlation of the measured and the NEC model predicted
parameters validates both the NEC magnetic loop models and
Frequency
MHz
5.3
7.1
Impedance 44+j5 57ohms
j14
SWR
2:1 SWR
bandwidth
kHz
10.1 14.15 18.1
21.2
24.9
43j35
28j13
32-j7 39-j2
32j29
27j21
28.4
1.168 1.345 2.111 2.292 2.232 1.953 1.607 1.294
18
24
N/A
N/A
N/A
30
113
181
Polarization: Vertical at low elevation angles transitioning to
horizontal at high elevation angles.
Power Rating: 10 watts CW or 25 watts SSB per manufacturer.
Frequency range: Switch position A 5.3-24.4 MHz; Switch
position B 7.5-31.3 MHz
ANTENNA PARAMETERS: Measured vs. NEC
Model
Predicted Efficiency and Free Space Gain
2:1
Eff.
Free
Gain
Frequency. Z0 SWR
Eff. %*** Space** dBi***
Q**
MHz
%** per
Gain
per
ohms BW
kHz*
Model
dBi
Model
28.4
38.4 170 118 13.9 14.2
-6.8
-6.8
24.9
30.0 120 146 11.6 11.5
-7.6
-7.7
21.2
18.1
14.15
10.1
22.9
18.0
14.1
11.6
81
57
40
30
186
225
252
222
9.10
6.88
3.58
1.19
8.7
6.53
3.94
1.73
-8.6
-9.9
-12.6
-17.5
-8.9
-10
-12
-16
7.1
5.3
11.6
9.9
26
22
168 0.31
178 0.14
0.61
0.22
-23.3
-26.9
-20
-25
* 2:1 SWR bandwidth for measured Z0 at zero reactance frequency
** Antenna Q, Efficiency and Free Space Gain dBi derived from
measured data with calculators by Owen Duffy1,2.
*** Efficiency and Free Space Gain dBi per Model calculated with
the specified NEC model parameters.
Payne's observed relation of frequency, capacitance, and resistive
losses of 365 pF dual gang air variable capacitors5. This
information will be useful in the design of other NEC antenna
models that employ this ubiquitous type of capacitor.
CONCLUSION
The Chameleon CHA F-Loop is one of several commercially
available compact magnetic loop antennas. A full size antenna is
preferable when the space is available. Small antennas such as
loaded monopoles, loaded dipoles and magnetic loops compromise
gain but offer practical communication solutions when space is
limited and when portability and rapid deployment are essential.
These measurements confirm that in operation the CHA F-Loop,
similar to other compact antennas, is expected to yield signal
strengths between one to a few S units lower than a full size
antenna. The NEC models presented may be useful to optimize
desired operating characteristics when planning a particular
installation.
NEC Model Parameters4
Height above ground
5 feet (1.524 m)
Simulated ground type
Average
Loop diameter
2.44 feet (0.74 m)
Loop circumference
92 inches (2.34 m)
Loop NEC model
segments
18
DXE-400 cable outer
diameter
0.405 inches (10.3
mm)
Outer Jacket material
REFERENCES
1. Calculate Antenna Q from VSWR bandwidth measurement,
Duffy, O, VK2OMD
2. Calculate small transmitting loop gain from bandwidth
measurement, Duffy, O, VK2OMD
3. ZPLOTS Impedance Plots Using Excel Charts, Maguire D,
AC6LA
4. A Universal HF Magnetic Loop NEC Model, Milazzo, C,
KP4MD
5. Measurement of Loss in Air Variable Capacitors, Payne, AJ
6. Capacitor Losses and Wire Resistances, Smith, KJ
4nec2 ZPlots
Model* Data
5.3
MHz
5.3
MHz
PVC
7.1
MHz
7.1
MHz
Outer braid diameter
0.32 inches (8.13
mm)
10.1
MHz
10.1
MHz
DXE-400 braid
conductivity
4500000 mhos/m
14.15
MHz
14.15
MHz
Capacitor Q=Xc/Rc*
118-1800*
18.1
MHz
18.1
MHz
21.2
MHz
21.2
MHz
24.9
MHz
24.9
MHz
28.4
MHz
28.4
MHz
LINKS
1. Chameleon CHA F-Loop (Manufacturer)
2. Chameleon CHA F-Loop Antenna Photo Album
ZPLOTS3
DATA FILES
& 4nec2
MODEL
FILES
*In switch position A: Rc = 0.032 +
5800/(F
×C ²) + 0.0039×√F
) 5,6
MHz
pF
MHz
In switch position B: Rc = 2 ×
(5800/(F
×C ²) + 0.0039×√F
MHz
pF
) 5,6
MHz
* Free Space
Models
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