CFA FS Measurements Under Controlled Conditions
Recently, short antennas have once again attracted the attention of broadcasters. A short antenna would be useful for low-power applications and especially to be mounted on places like building tops. Of course this kind of antenna is not intended to replace the optimum monopoles or vertical dipole where maximum efficiency, maximum gain and antifading properties were achieved after exhaustive studies and after long experience both theoretically and practically applied.
CFAs, short monopoles, short dipoles and short folded monopoles have been analyzed from the theoretical and practical point of view in order to choose the simplest and more efficient model to fulfill downtown stringent requirements. For a long time now vertically polarized antennas for medium frequencies amplitude modulation (MF AM) broadcasting systems have been studied exhaustively especially for antennas higher than one-eight wavelength These studies were undertaken in order to resolve ways to improve the broadcast coverage by increasing antenna efficiency or an increase in the surface field strength. At the same time efforts were focused on increasing in the night-time broadcasting and determine ways of optimizing the antenna vertical radiation pattern thereby reducing the fading caused by the ionospheric or sky wave
Vertical antennas with height lower than one eight wavelength for broadcast service are still not really very well studied because of poor interest in low efficient radiators At the lower part of the spectrum frequencies only short antennas are used due to mechanical and economical restriction in the antenna size and real estate properties. At medium frequencies the possibility to get a high efficient radiator has been achieved, but at the present time, high cost real estate and height reduction due to ecological, esthetical or aeronautical problems or low power downtown installation make a short antenna or reduced size antenna a necessity.
The equivalent loss resistance of a short radiating systems due to ground plane, conductors and isolator losses is generally higher than the radiation resistance and for this reason high radiation efficiency is difficult to achieve. Efficiency of an antenna as with other systems, is the relation between the radiated power or useful power to the total power injected into the system or the dissipated and radiated power sum. It is well known that the monopole antenna radiation resistance is a function of its height-wavelength relationship and when this relationship is low, low radiation resistance is the result.
Antenna directivity is an antenna property, it increases the radiated power in some space regions and in the short monopole antenna case the maximum radiated power or maximum field strength is obtained along the earth and uniform in azimuth when the ground plane is perfect (infinite in extend and with infinite conductivity).
The reference antenna universally accepted nowadays is a unitary directivity radiator (1 or 0 dBi) or the isotropic source and its radiation is completely uniform into any space direction (173.2 mV/m at 1 Km for 1 kW radiated).
The short vertical monopole radiation pattern depends on the cosine of the elevation angle and this property is achieved for any vertical antenna lower than one-eight wavelength height over a perfect ground plane. Because of this property the directivity is almost the same for a very short monopole (3.0 or 4.77 dBi) and a quarter wave vertical monopole (3.27 or 5.15 dBi). Very short monopole with no losses has a field strength of 300 mV/m at 1 Km for a kW radiated (109.54 dBμVm) and a quarter-wave monopole produces 313.2 mVm at the same distance and radiated power (109.92 dBμV/m). This means an increase of 4.4 % in field strength, or a very small increase taking into account the huge increase in height.
Directivity as a function of a vertical antenna height can be seen in the top of Figure 1 as the first quasi-horizontal line Clearly, almost constant directivity as a function of antenna height can be seen. Very small increase in directivity is achieved increasing the antenna height from practically nothing to a quarter wavelength. For this reason nothing can be done to improve the antenna radiation characteristics because the radiation pattern is almost constant for short antennas (height lower than one eight wavelength). It is a physical antenna condition governed by Mother Nature.
Attention must be concentrated on radiation efficiency because radiation resistance value depends on the square of the antenna height,
Where Ra is the input antenna resistance and RL is the equivalent total loss resistance of the antenna environment. where the tuning system is included. In this particular case, maximum efficiency means maximum antenna gain for the same directivity. It is well known that gain is directivity times efficiency and as efficiency reaches unity, gain equals directivity and an antenna with theoretical behavior is achieved:
In order to increase this efficiency any effort increasing the radiation resistance is welcome. This statement can be seen in Figure 1 where monopole antenna gain is plotted as a function of the equivalent total loss resistance.
It can be seen that gain is lower for the shortest antennas for the same total loss It can be seen clearly from Figure 1, a sharp gain decrease for monopole antennas lower than 0.05lambda., even for lower total loss resistances. At the same time a decrease in antenna reactance can be useful because it reduces the antenna “Q” and an increase in antenna bandwidth is obtained.
“Q” is defined as the quotient between the input antenna reactance module to the input antenna resistance. Unfortunately, a reactance or “Q” decrease is only obtained here with an antenna height increase. In broadcast applications this property is paramount in order to get a transmission as free as possible of distortions due to the transmitter loading variations into the pass band. [1] [2]. Concentration in an efficiency increase for antennas lower than one tenth of wavelength is the task here and for this reason, calculations and measurements are carried out on scaled-down models. Achievements, possibly could be applied on real scale antennas in the near future, but, real scale model investigations are very time consuming and generally very expensive.
Radiation pattern is a cosine function of the elevation angle, and for this reason this antenna is intended for low power applications (power lower than 10 kW) where nighttime broadcasting is reduced and antenna anti-fading properties are not considered an important issue. For higher power applications standard monopole or dipole closer to half wavelength height must be used, in order to get maximum radiation efficiency and the best anti-fading properties. Calculated elevation radiation pattern is not shown here because it is a simple cosine function, well known, with a maximum at ground level and the radiated power decreases to one half at a 45-degree elevation (-3 dB). Radiated power is proportional to a square E or H field, as it is well known, and it must be as low as possible at a high elevation angle for MF AM transmitting applications. Of course this radiation pattern is not desirable for the perfect MF AM system.
CFA Experimental Results
CFA experiments were carried out over a metallic ground plane and in a real ground environment using reduced scale models at frequencies of 10 and 20 MHz. A monopole height was chosen as 1 meter, or 0.0333
and 0.0667 respectively, for both frequencies. Input impedance was measured by means of an impedance meter (HP 4291A) placing the test fixture in the input monopole antenna port under the ground plane in order to obtain the maximum accuracy. Using a piece of coaxial line always causes errors in the impedance translation and the input impedance is not measured precisely. This is a delicate matter due to the low resistance values involved and the instrumentation accuracy in measuring them. Tuning systems are attached directly under the ground plane and the monopole antenna connection is very short. Tuning systems are made up of a “T” type impedance match in a low pass version network. Two types of inductance were used: air core inductors and Q2 ferrite core inductors. In the first case low loss inductors are obtained with Q higher than 200 and in the second case Q obtained is around 30.
The metallic ground plane is made up of aluminum sheets and the reference monopole is installed over the same ground plane at a distance of 5 meters. In this case wave propagation is performed over a very high conductivity medium practically ideal. 5 meters is a distance of 0.1667 for 10 MHz and 0.3333 for 20 MHz. It would be a distance of 50 and 100 meters at a real scale model of 1 MHz, or at an intermediate distance between near and far field. Of course reactive near field is at a lower distance for a monopole antenna. A one meter monopole loaded with a 50 ohms resistance was used as an electric field probe and a calibrated loop (ETS mod. 6509) as an electric and magnetic probe.
Received power is obtained by means of a spectrum analyzer (HP 8563E) where any possible interference is completely avoided and power is measured at the actual frequency with good accuracy. In any measured case, useful power to noise ratio or useful power to interference power is always more than 30 dB so as to assure that the power measured is only the received power from the monopole model. During measurements, received power and input impedance of the transmitting system were always under control. At the central frequency, input tuner impedance was maintained as close as possible to 50 ohms (VSWR lower than 1.1) so as to guarantee the maximum transmitted power in each measured model and always at the same value. This is a very important factor to monitor during the antenna model efficiency or gain comparison. This comparison can be made directly in dB referred to the short monopole without top loading or to the quarter wave monopole used as reference. Absolute gain is more difficult to determine due to the reference probe calibration. Generally, loop calibration is specified with an accuracy of +/- 2 dB, much more than needed for precise measurements and at the same time absolute probe value depends strongly on the place were the probe is installed.
Calculations according to measured values over metallic ground plane, and with probes close to it, give a disagreement with certified values on more than 5 dB. Nevertheless, knowing the inductor losses and the transmitted power and using Friis [6] link equation, quarter wave monopole has given a radiation efficiency close to 97 percent or very close to 5 dBi absolute gain. This is a very good standard reference for any other short antenna placed in the same place as the quarter wave monopole Taking into account the best tuning of any short antenna and with transmitted power and input impedance under strict control, the comparisons can be considered adequate and very close to the actual gain and efficiency values.
From here on there are a lot of missing images, they are missing from the original as well – MD0MDI
Figures 6a and 6b give the measured received power as a function of frequency close to 10 and 20 MHz. At 10 MHz (H/=O .0333) the received power increase for 1 meter diameter top loading was 5.5 dB while for the same loading the received power increase was 6 dB at 20 MHz (H/=0.0667).
The CFA model has been made with the same height as the monopole model. The height is 1 meter, a monopole diameter of 75 millimeters and top load (hat) is 0.45 meters obtaining an H/a=13.333 relationship. Its input impedance was measured as function of frequency or antenna height for the E plate and D plate At the same time input impedance for the E plate was measured both with the D plate disconnected and connected to ground. The same measurements were made for the D plate with E plate disconnected and connected to ground. In this last case on the E plate connected to ground, some interaction can be observed in the resistance values. Very small differences have been observed on the other measurements Figures 7a thru 7f show the input impedance measured for this antenna model.
This antenna was tuned with two “T” matching systems: one for the E plate (monopole) and the other for the D plate and they were tuned one at a time charging the other port with 50 ohms at the tuning system input. After several operations, due to both structure interactions, the best tuning measurements for both inputs was obtained. A 90-degree delay line, precisely calibrated with the impedance meter, was connected to the D plate and checked in order to get a consistent 50 ohms of input. Both inputs were connected to another “T” matching system and 50 ohms input for the entire system was obtained. Power received at 10 MHz and 20 MHz with the previous probe can be seen in Figures 8a thru 8d and compared with the short monopole without (D/H=O) and with loading (D/H=1). (H/=0.0333 or H/=0.0667) are the corresponding heights at the measured frequencies. An increase in receiving power was 2.8 dB compared to a short monopole (H/a=80) without any top loading at 10 MHz.The same measurement was made connecting the D plate to ground and the CFA monopole was retuned. An increase of 3.6 dB over the short monopole was obtained. Nice surprise!! Disconnecting the D plate and retuning the CFA monopole, the receiving power increase was 4.9 dB!! (see Figure 8b)
The D plate was removed and the CFA monopole was retuned obtaining an increase of 5 dB over the short monopole perfectly tuned. This is a standard cylindrical monopole (H/a=13.33). The same measurements were carried out at 20 MHz obtaining an increase of 5.0 dB for the CFA antenna compared to the short monopole of the same height (H/
=0.0667) but 1 dB less than a top loaded monopole (D/H=1) as can be seen in Figure 8c. With the D plate connected to ground and the CFA monopole retuned, the increase was 7.5 dB and with the D plate disconnected, the increase was 7.8 dB. In both cases 10 and 20 MHz (H/=0.0333 and 0.0667) the CFA with both E and D plate excited, gave less received power in the receiving system or less transmitting gain.
By removing the D plate, the increase was 8.4 dB. It can be pointed out further that by connecting the D plate to ground without the tuning systems causes a gain increase according to measurements. Figures 9a and 9b shows the received power for a cylindrical monopole (H/a=13.33) with different top loading. Similar results as in the previous case have been obtained at 10 and 20 MHz. It seems that these increases in receiving power are excessive but they are due to the tuning systems inductance losses. And, this is because the tuning systems have used the ferrite core inductors and any small increase in radiation resistance increases significantly the radiated power when system losses are involved For this reason tuning inductors were replaced with air core inductors in order to get better efficiency in the short antennas and having closer results the theoretical values
New measurements were made at 20 MHz over metallic ground plane. In this case, a quarter wave monopole was used due to shorter physical dimensions and both short monopoles (H/a=80 and H/a=13.33) with different top loading. Nevertheless, the CFA was not measured again because its behavior determined by measurement is not as their inventors claim (gain better than a quarter wavelength monopole or compared to a half wave monopole) and it can be replaced by a simple and more efficient loaded monopole.
Figure 10 shows the short monopole antenna (H/a=80 and H/a=13.33) absolute gain measured over metallic ground plane as a function of the top loading factor (D/H), using a quarter wave monopole as reference. In this case air inductors have been used to perform the measurements, in order to get results as close as possible to the theoretical predictions. Maximum gain improvement measured over metallic ground plane was 4.1 dB for a monopole H/a=80 and D/H=1.
For a monopole H/a=13.33 and DH-1 was 3.1 dB at a frequency of 20 MHz or H=0.0667 in both cases compared to a short non loaded monopole (Ha=80) of the same height/ These results are very close to the calculated predictions as it was seen previously. This example is very important because it shows the small difference in gain between short and quarter wave monopoles when losses are low, especially for top loaded monopoles.
Measurements of the “umbrella loading monopole” has been carried out for the same height as in the previous monopoles (Ha=80 for H/=0.0333 and H/=0.0667). Increase in input resistance with loading is not significant as in the top loading case as it was predicted in the theoretical calculated cases, and this increase has a maximum. This behavior was determined theoretically and measurements indicate exactly the same. For this reason gain increase has a maximum depending on the antenna height. Figure 11a and 11b show the umbrella loading received power measurements at 10 and 20 MHz (H=0.0333 and H/=0.0667).
The resulting gain over the short monopole without any top load is displayed as a function of loading factor value (D’/H) and it can be seen in Figures 12a and 12b. In this case the loading diameter is two times the isolated guy length for 45 degrees sloping angle. In these measurements, tuning systems employed air core coils and for this reason, for 20 MHz, it can be compared with the previous cases measured over the same metallic ground plane and the absolute gain over a quarter wave monopole can be obtained This absolute maximum gain is 3.6 dBi achieved for D’/H=1 2 or D’/=0.08, H/a=80and H/=0.0667.
Top loading monopoles were measured over real ground at 20 MHz. In this case, a quarter wave monopole was installed over a square metallic ground plane placed over grassy terrain. The metallic ground plane radius was 1 meter approximately or 0.0667, quite small compared with the operating wavelength. This small ground plane will produce a reduced radiation efficiency for the quarter-wave monopole, but more reduction is expected for short monopole antennas. In this case, a transmitter (ICOM 726A) with an automatic tuner was used. Output power was adjusted at 10 watts for every measurement and no reflected power was read in the reflectometer as each antenna model was perfectly tuned. Received power was measured with a spectrum analyzer (HP 8563E) and a 50-ohms loaded short monopole (H/=0.0667) was placed over a small aluminum sheet at several distances.
Figure 14 shows the absolute gain determined for these short antennas in such small metallic ground plane conditions where maximum gain for a loading factor D/H=1 of a monopole H/=0.0667 is around -4 dBi and smaller for lower loading This Figure presents very well the imperfect ground plane effect on monopole antennas and especially on the shorter antennas where this effect is even more noticeable. For a quarter wave monopole with an input resistance close to 40 ohms, efficiency was reduced to 70% approximately, but efficiency reduction is more important and close to 13.5% for a top loaded monopole with 1 meter diameter hat (D=0.0667, H=00667, D/H=l) This efficiency decrease is even more noticeable for a non-loaded monopole of the same height were the efficiency would be close to 1.6% (D/H=0 H=0.0667).
The only way found to improve this behavior is making the metallic ground plane as big as possible and using a minimum loss tuner. Several dB can be gained this way as it was shown previously over a metallic surface. Of course this is no possible in reality, but extensive metallic ground plane under the radiating antenna is paramount. From all calculations performed and measurements taken, the top load is a simple and effective way to increase the short antenna gain even if directivity doesn’t increase, but surface wave field strength can be improved significantly approaching the quarter wave monopole gain.
Measuring the VHF Region
Vertical radiation patterns of a quarter monopole and a half wave monopole models over a metallic ground plane in VHF region have been measured. The CFA VHF model vertical radiation pattern has been measured in the same ground plane. CFA patterns are quite similar to a quarter wave monopole or short monopole because the difference between them is insignificant. This comparison can be seen in Figure 15. In these measurements, maximum radiation is not at zero degree elevation due to ground plane edge effects, but this effect will modify the performance of any antenna. The importance here is showing there are no “magical effects” in the CFA radiation behavior because it behaves similar to a short monopole as the measurements indicate.
Measurement Results Summary
Below is a table when condenses a summary of the key measurements made and as described above.
==== RESULTS OF MEASURED ABSOLUTE GAIN ~ f=20 MHz ~ H/=0.0667 ====
OVER METALLIC GROUND PLANE
REFERENCE QUARTERWAVE MONOPOLE
G=5.0 dBi η=97%
Short Monopole H/a=80 D/H=0 G=0.2 dBi η=35%
Short Monopole H/a=80 D/H=1 G=4.3 dBi η=90%
Short Monopole H/a=13.33 D/H=1 G=3.3 dBi η=71%
CFA H/a=13.33 D/H=0.4. G=2.5 dBi η=59%
OVER REAL GROUND
REFERENCE QUARTERWAVE MONOPOLE
G=2.6 dBi η=51.3%
Short Monopole H/a=80 D/H=0 G=-14 dBi η=1.3%
Short Monopole H/a=80 D/H=1 G=5.0 dBi η=10.5%
Short Monopole H/a=13.33 D/H=1 G=6.0 dBi η=8.4%
CFA H/a=13.33 D/H=0.4. G=8.0 dBi η=5.3%
Conclusions
Different short antennas have been analyzed by calculations and in the model cases where the better gain could be achieved, measurements over scaled models were performed. It was determined by calculations and measurements the top loaded and umbrella loaded monopoles exhibit gains not far from a quarter wave monopole when almost perfect ground plane is used. Measurements over a CFA antenna show similar behavior as that of a cylindrical monopole of the same height and top load. For this reason, the cylindrical monopole is preferred due to its simple tuning system. At the same time its measured radiation pattern gives the evidence of its directivity and gain. Like any short antenna, its gain depends strongly on a perfect ground plane.
Thus, it is concluded that all the work performed previously by the myriad technicians, engineers and scientist remains well-founded in that effective radiated energy still depends on very efficient antennas. Short antennas can be improved substantially by means of a perfect ground plane and top loading, but within the scope of the configurations described here, they never can be more efficient or outperform an optimum monopole or dipole close to half a wavelength. For this reason for a high power installation where maximum field strength over the earth and optimum anti-fading properties are needed, traditional tall antenna, monopoles or dipoles, are still the best choice!
Acknowledgements
Special thanks to Mr. Juan Skora for his assistance in model measurements and working on personal computer in order to convert thousand of data into curves, a more simple methods to get a quick view of the antenna impedance and efficiency behavior. To Diego Schweitzer who was supporting the initial measurements and the mechanical model work and to all the CITEFA, Antenna and Propagation Division Personnel their permanent help and support.
References
[1] Doherty W. , “Operation of AM Broadcast Transmitters into Sharp Tuned Antenna Systems” PIRE. N° 37, pp. 729, July 1949.
[2] Westberg J., “The Dependence of AM Stereo Performance on Transmitter Load Phase”. Broadcast Electronics, Quincy, IL.
[3] Smith C. E., Johnson E. M., “Performance of Short Antennas” PIRE N° 35, pp. 1026, October 1947.
[4] Smith C. E., Hall J. R., Weldon J., “Very High Power Long Wave Broadcasting Station”. PIRE Vol. 42. August 1954.
[5] Trainotti V., “MF AM Grounded Dipole for Stereo and Digital Transmissions Trans. on Broadcasting”, Vol. 45, #3 September 1999.
[6] Friis H., “A Note on a Simple Formula” PIRE Vol. 34, pp. 254, May 1946.
[7] Rosseler G., Vogt K.,. “Investigation on Umbrella Aerial”, Wireless Engr. p20, 141, March 1943.
[8] Howe, G .”Calculation of Aenal Capacitance”, Wireless Engr. #20,157, April 1943.
[9] Laport. E..”Radio Antenna Engineering”, McGraw Hill, N.Y. 1952.
[10] Wheeler, H., “Small Antennas”, I. R. E. Trans. on Antennae and Propagation, Vol. AP-23, pp. 462-469, Julv 1975.
[11] Schelkunoff S., Friis H., “Antenna: Theory and Practice”. John Wiley, N.Y., 1952.
Originally posted on the AntennaX Online Magazine by Valentin Trainotti
Last Updated : 21st May 2024