Investigating the MicroLOOP
Last month I quickly looked at the antenna I called “Pascal’s Tennis Racquet” devised by Pascal Veeckmans, ON4CFC (An article is scheduled for the July 2001 issue of antenneX). So, this month I put the delayed CFA inspection trip planned for Egypt on the back burner and looked at this antenna in more detail. The first version was as described last month: a 20-inch ring connected to the coax ground and a 45-turn tuning coil connected to a 4-inch diameter plate in the center of the ring (click here for metrics chart). Figure 1 shows this antenna.
A Surprise
At first I ran it directly connected to the MFJ analyzer, and found it had fair match over a broad band, and radiated between -50 and -44 dB signals at the spectrum analyzer that is connected to a small probe antenna 55 feet away. Everything was hot from the MFJ box to the center plate, so tuning it meant creeping up and just touching lightly the knob while remaining far from everything else. A 20-turn 1:1 choke balun quieted things down, but changed the tuning characteristics to two narrower bands. Then I substituted a 4-inch diameter ring for the central plate. Surprise! The tuning curve was identical to the plate, but the radiation averaged 3.8 dB stronger! Evidently only the edge of the central plate is important! It acts like the central area of the plate provides only eddy current losses as the charges attracted toward the ring surge to the edge and back during the RF cycle!
The Field Factor
Seeing this, I wonder if the usual CFA drawing of the E fields is true. This drawing usually shows E lines curving down to the ground plane in quarter circles from all along the cylindrical E plate surface. Could it be that a more accurate depiction would show the E field all originating on the bottom edge of the cylinder?? Is there a similar loss on the E plate?? Could this be why it is so hard to make the CFA work?
Table 1 shows the data for the antenna alone, with no balun. The data points are the frequency of best SWR versus tuning coil turns shorted by a clip lead from the center or top to a particular turn. At each point the frequency and SWR is recorded from the MFJ antenna analyzer, which is used as a source. The amount radiated is measured by a 1-meter long probe antenna hanging from a tree branch outside 55 feet away, which is connected to an HP 8558B spectrum analyzer. Up to 37 turns were shorted by the clip lead. Note the wide bandwidth, moderate SWR, but rather poor radiation compared with the magnetic loop, which radiates just stronger than -40 dBm.
At each point, the tuning coil inductance was calculated with the formula in the 1961 Radio Amateur’s Handbook:
L (uH) = a^2*n^2/(9*a+19*b)
a = coil radius
b = coil length
n = number of turns
The capacitance required to resonate at each point with the calculated inductance is shown. Note that the capacitance is in the ballpark of what I would expect for any structure like this antenna — between 10 and 20 pF. The best frequency is shown in green in Table 1. But, this configuration is very hot, and that makes anything connected to it part of the antenna—so a balun was added.
Table 2 shows the same antenna, connected to a balun made by winding 20 turns of RG-58 cable around a 2-inch PVC form. The balun does two things: its inductance isolates the antenna from the cable and reduces currents on the outside of the cable, and it balances the antenna feed (instead of an unbalanced cable feed). It is clear that the balun influences the tuning — the wide band becomes two narrower bands: a low band between 7.95 and 9.72 MHz and a high band between 11.029 and 19.728 MHz. The low band radiation is greatly improved: -38.7 dB at 7.95 MHz versus -47.4 dB at 8.55 MHz dB. The high band is slightly down: -46 dBm at 14.56 MHz versus -44.8 at 13.486 MHz. Note that the capacitance to resonate at the best frequencies is always in the 10-20 pF range.
Table 3 shows what happens when a ring with the same 4-inch diameter replaces the center plate. The low band’s best radiation point appears to require a larger tuning coil since it is getting better with 0 turns shorted. Even at the lowest frequency reached, it is radiating -43 dBm at 7.985 MHz. The high band radiates -40 dBm—a 6-dB improvement. The big surprise is that the tuning curve of frequency versus 1 turn steps in the tuning coil is unchanged! Table 4 shows the tuning curve and the radiation curve versus coil size. The two configurations tune within an average of 11 kHz of each other over all 37 points of the coil change. The average radiation is increased 3.82 dB!
All the above data shows the best SWR frequency for each number of turns in the tuning coil. Table 5 shows the effect on the bandwidth at a point near the 20-meter ham band. This table shows that even though the plate version without a balun achieves a 1:1 SWR, its radiation barely peaks above the -45 dBm level at that frequency and is -42 dBm at 2 steps away. There is no visible multi-step radiation peak with frequency. The ring version with a balun, on the other hand, radiates -40 dBm at the 1:1 SWR point.
A Conclusion Perhaps
These experiments suggest that in antennas of the E field type should avoid conductors that extend away from the direction to the opposite electrode and should always tune with the coil to resonate with 10-15 pF. The structure that shows a high voltage terminal capacitance in this range seems to have the best chance of achieving 1.0 SWR, and the highest possible voltage on the high voltage element.
The next logical step in understanding this interesting antenna, which radiates better than any other capacitive antenna I have tried, is to use a different size center ring and see if anything changes the dynamics. I think this antenna might make a nice 40 and 20-meter field day antenna with a size near the dimensions of this model.
LAB NOTES: The MicroLOOP - Part 2
During my investigations last month I found that the MicroLOOP (an innovation by Pascal Veeckmans, ON4CFC) antenna worked better when the central electrode was a ring instead of a plate. This month I investigated the effects of changing the size of the center ring, the size of the outer ring and the shape factor of the tuning coil.
The MicroLOOP is essentially a very high L/C ratio resonant circuit. The C element has two parts: the distributed capacitance associated with the coil, and the capacitance between the inside and outside rings. The antenna radiates due to the E field associated with the C outside the coil. During the RF cycle, the energy-per-cycle is stored in the inductance, then the capacitance. The high Q tells us that the energy stored is much more than the energy lost per cycle due to radiation and wire resistance. For a given magnetic energy stored in the coil, the voltage across the capacitor to store the same energy rises as 1/ (square root of C) and the angular frequency (2*pi*frequency) is (omega)^2 = 1/LC.
Something Within
The experiments described below show that the best frequency associated with a distributed capacitance of around 12 pF. It also shows that something else is acting within this antenna since the best match, a 1:1 SWR, only occurs at certain frequencies even though a fair match can be achieved almost anywhere with a suitable inductor. Thus, the MicroLOOP with a 20-inch diameter outer ring and 8-inch inner ring gives a 1:1 SWR resonance at 7.5 MHz and another at 14.5 MHz (each with a different coil) and poorer SWR in between these frequencies. These frequencies are each part of a series of resonances: the 7.5 MHz resonance moves up to 9.3 MHz at the coil size that also gives 1:1.1 SWR at 14.57 MHz. The best frequency can be moved by changing the circumference of the outer ring, which moves both the resonant frequency at a particular L and the lowest SWR frequency. Other data shows that the form factor of the coil has little effect on performance at a fixed frequency, but as the coil becomes long and thin with small turns diameter, it takes more turns to achieve the same frequency. This suggests that the coil distributed C is less for a long, thin coil.
Table 1 shows the data for the 20-inch diameter outer ring, 8-inch diameter inner ring antenna. Two resonance strings are followed as the size of the tuning coil is varied by shorting turns with a clip lead. The position of the shorting tap and the corresponding un-shorted number of turns is shown in the first two columns of Table 1. The next six columns show the resonant frequency, the SWR, and the signal received at a 1-meter probe 55 feet away for taps from 0 to 30 turns. Note that the best SWR, 1.1, at the lowest resonance happens at about 7.5 MHz, and as the coil is reduced in size, the SWR gets worse and the resonant frequency is well below the best SWR for the next higher string of resonances.
The last 3 columns show the computed inductance for each tap position and the capacitance required to resonate at the best frequency. The capacitance at the 1:1 frequency is 16 pF for the low band and 13 pF for the high band.
Table 2 shows similar data for a center element that is a 1-inch copper band bent around the 1-inch diameter PVC center pipe of the antenna. Comparing with the data for the 8-inch ring, the lowest frequency possible has moved from 7.192 MHz to 8.6 MHz. At this frequency, the SWR changed from 2 to 1.7. The tap to achieve the high band best frequency, about 14.5 MHz, moved from 30 turns shorted to 26 turns shorted. The SWR remained about 1.0. The radiation was unchanged.
Size and Shape
Thus, it seems that the size and shape of the center element interacts strongly with the tuned frequency of any particular size of L, but the SWR behavior at any particular frequency is always about the same however the tuning reaches it.
Table 3 shows the passband about the best hi-band frequency, 14.5 MHz. Table 4 shows the same passband for the 1-inch band. The smaller center structure has a slightly higher Q as shown by the narrower bandwidth.
Table 5 shows the tuning range of the low band for the 3 sizes of center structure: 1 inch, 4 inch, and 8 inch, with the data aligned in frequency. All cases use the same coil taps, and the center structure sets the frequency range possible with the coil taps. The best SWR frequency is marked with a colored bar and the implied distributed capacitance is shown. Note that only the 8-inch ring actually included a 1:1 SWR frequency in its tuning range; the other structures approached but didn’t reach the best frequency.
Table 6 shows the same comparison for the high band. Again, the active turns at the best frequency increased from 11 to 19 as the structure size decreased and the implied capacitance changed from 13.87 to 10.43 pF, but the best SWR frequency remained at 14.5 MHz. The tuning range moves around under a fixed envelope of best SWR versus frequency.
For Twenty
To be used for the 20-meter ham band, this antenna must be moved down to say 14.050 MHz. Two increases in the outer ring circumference were tried to move the best SWR frequency down from 14.5 MHz. Table 7 shows the effect of increasing the outer ring slightly. 7.5 inches was too large an increase – the best SWR frequency moved down to 13.7 MHz. The tap to achieve that frequency moved from 22 to 24, a 2 step decrease in the size of the tuning coil and a 3 step increase in the inductance associated with the best SWR. Reducing the circumference increase to 5.25 inches brought the antenna into the 20-meter band. Table 8 shows the passband of the 20-meter antenna as finished.
Re-Coil
A last configuration change tested was the form factor of the coil. All of the above data used the same 45-turn air wound inductor. Two new coils were wound: a short, fat coil with 11.8 turns with a 4.5-inch diameter, and long, thin 47-turn coil with a 2.14-inch diameter. The idea was to change the distributed capacitance of the coil. Table 10 shows some coil calculations adjusted to reach the 12 uH value that tunes the antenna. Table 9 shows the antenna results. The most noticeable thing is the number of turns of the long thin coil that produced 12 uH – 47 versus the 31 computed. The short fat coil computed correctly. Otherwise, the two coils give almost the same performance. This parameter seems to be unimportant.
Conclusion and Next
In conclusion, there appear to be only a few best values for the size of the outer ring to achieve a 1:1 matching at a given frequency, but much greater freedom possible in the choice of the tuning coil and E field structure. The mechanism that produces this behavior is not obvious. It seems best to avoid metal length in the same direction as the E field (like the 4-inch plate last month).
The next step is to try to compare this antenna with a dipole, and see if it radiates enough to achieve some signal reports. An investigation of the inductance or resonance of the outer ring might also reveal some secrets of the MicroLOOP antenna.
Originally posted on the AntennaX Online Magazine by Joel C. Hungerford, KB1EGI
Last Updated : 30th May 2024