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's published relation of frequency, capacitance and resistance loss in this type of capacitor⁶ was used in the 4nec2 antenna reference models⁵. Each 4nec2 model yielded an Average Gain Test result of 1.000, the optimal model-adequacy figure of merit. Payne's equation is for a single capacitor gang. The 0.032 ohm contact resistance was omitted and the remaining capacitor loss factors were doubled to calculate the resistance loss for two gangs in series. The 4nec2 model predicted parameters were compared against the measured data for validation.

DISCUSSION

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 antenna⁴ with ceramic stator supports.   However, these data reveal that the dual gang capacitor's phenolic stator supports are 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 stator supports 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 antenna⁴. 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 and greater radiation efficiency when resting on the ground or another reflective 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.

SPECIFICATIONS AT SWR MINIMA (Z₀ = 50 Ω)

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

* 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 Duffy^1,2.
*** Efficiency and Free Space Gain dBi per Model calculated with the specified NEC model parameters.

CONCLUSIONS

The correlation of the measured and the NEC model predicted parameters validates both the NEC magnetic loop models and Payne's observed relation of frequency, capacitance, and resistive losses of 365 pF dual gang air variable capacitors⁶. This information will be useful in the design of other NEC antenna models that employ this ubiquitous type of capacitor.

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.

LINKS

1. Chameleon CHA F-Loop (Manufacturer)
   
2. Chameleon CHA F-Loop Antenna Photo and Test Data Album

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. 14-30 MHz Magnetic Loop Antenna, Milazzo, C, KP4MD
5. A Universal HF Magnetic Loop NEC Model, Milazzo, C, KP4MD
6. Measurement of Loss in Air Variable Capacitors, Payne, AJ
   
7. Capacitor Losses and Wire Resistances, Smith, KJ