Reflector Antennas Revisited

8/6/2019 Reflector Antennas Revisited

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Reflector Antennas RevisitedJ. Richter

In the process of documenting some of my old data and photographs in the field of RF-studies I

preserved some of my old notes from being lost.

AEG-Telefunken's antenna laboratory and a historical overview of the projects has been

documented in a movie1 describing the years 1955-1990, but some of the technical details may be

added from my notes recorded 1972-1980.

public domain-photogtraph by Martin Hofer.

Original uploader was Rssel0815 at de.wikipedia

1 Fnfzig JahreAntennenlabor (German spoken)

Fig.1: One of the towers at the AEG-Telefunken antenna-lab, southern Germany
http://de.wikipedia.org/wiki/User:R%C3%BCssel0815http://de.wikipedia.org/http://de.wikipedia.org/http://video.google.de/videoplay?docid=-6701798658157733794http://video.google.de/videoplay?docid=-6701798658157733794http://video.google.de/videoplay?docid=-6701798658157733794http://video.google.de/videoplay?docid=-6701798658157733794http://de.wikipedia.org/wiki/User:R%C3%BCssel0815http://de.wikipedia.org/http://video.google.de/videoplay?docid=-6701798658157733794


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Reflector Antennas

Published byRichard Bartz, Munich akaMakro FreakunderCreative Commons-Licence

Namensnennung-Weitergabe unter gleichen Bedingungen 2.5 US-amerikanisch (nicht portiert)

The above Cassegrain antenna (put into service from 1964) clearly illustrates the most important elements in the feedsystem and the secondary reflector with its large support structure of 8 rods. For the Symphony system working with

very low receiver levels (1 pWatts) a parametric amplifier with an amplification factor of 10.000 has been designed2.

2 Radio Bulletin June 1973, page 242

Fig. 2: The biggest facility for satellite communication in Raisting, Bavaria, Germany.
http://commons.wikimedia.org/wiki/User:Richard_Bartzhttp://commons.wikimedia.org/wiki/User:Richard_Bartzhttp://commons.wikimedia.org/wiki/User:Richard_Bartzhttp://commons.wikimedia.org/wiki/User:Makro_Freakhttp://commons.wikimedia.org/wiki/User:Makro_Freakhttp://en.wikipedia.org/wiki/de:Creative_Commonshttp://creativecommons.org/licenses/by-sa/2.5/deed.dehttp://creativecommons.org/licenses/by-sa/2.5/deed.dehttp://en.wikipedia.org/wiki/Parabolic_antennahttp://commons.wikimedia.org/wiki/User:Richard_Bartzhttp://commons.wikimedia.org/wiki/User:Makro_Freakhttp://en.wikipedia.org/wiki/de:Creative_Commonshttp://creativecommons.org/licenses/by-sa/2.5/deed.dehttp://en.wikipedia.org/wiki/Parabolic_antenna


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Parabolic antennas

The reflecting surfaces are shown in red, the feed antenna in green, and typical supporting

structures inblue. A few typical rays are shown, in black. The types are: (1) Axial or front feed. Thisis the most common type, with the feed located in front of the dish, on the beam axis. One drawback

of this type is that the feed and its supports partially block the beam. (2) Off-axis feed or offset-

feed. The primary reflector is asymmetric, with the focus, and the feed, located to one side, outside

the beam area. This avoids the problem of the feed obstructing the beam. It is used in home satellite

dishes. (3) Cassegrain. In this design, the feed is located on or behind the dish, and its beam

illuminates a small hyperboloidal secondary reflector at the focus of the dish, which reflects the

beam back to the parabolic dish. It is used in antennas with bulky or complicated feeds, such as

satellite ground antennas and radio telescopes. (4) Gregorian. This is similar to the Cassegrain,

except the secondary reflector has a concave ellipsoidal shape.

Wikimedia Commons: Chris Burks (Chetvorno), the author of this work,

releases it into the public domain for any use whatever.

Fig. 3: Diagram showing

types of parabolic antenna

feeds.
http://commons.wikimedia.org/wiki/Main_Pagehttp://commons.wikimedia.org/w/index.php?title=User:Chetvorno&action=edit&redlink=1http://commons.wikimedia.org/wiki/Main_Pagehttp://commons.wikimedia.org/w/index.php?title=User:Chetvorno&action=edit&redlink=1


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Example of a Cassegrain antenna patternAn example for a Cassegrain antenna pattern may be found in the following diagram. The 4.5 m antenna has beenmeasured at 6 GHz for circular polarization. Theoretically antenna gain may be estimated by a formula:

G = 10 . log (q . (.D/)2 ) .... (dBi)

in which q represents an efficiency factor, usually between 0.5 and 0.6. D represents the diameter and the

wavelength. The antenna's -3dB-beam-width usually may be estimated between 60 . (/D) and 70 .

(/D). Therefore a 4.5 meter antenna at 6 GHz may reveal a pattern with a 3 dB width of

approximately 0.66 corresponding to 60 . (/D) and a gain of 46 dBi at a q-value of 50%.

q D/ .D/ (.D/)2 Gain (dBi)

100,00% 90 283 80000 49 dBi

50,00% 90 283 80000 46 dBi

Fig. 4: Cassegrain antenna pattern 4.5m (at 6 GHz - circular polarization)


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Generally all Cassegrain antennas applied a shaping procedure in which the secondary reflector's curvature had beencalculated for an optimal tapered antenna aperture. In the classical Cassegrain antenna the secondary reflector will be

supported by a number of rods, which may cause some reflections as sketched from an imaginary offset focus in thefollowing drawing:

Fig. 5: Reflections by support structures in a 4,5m Cassegrain Antenna


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Early measurement technologies

Originally antennas used to be measured for three frequencies,for example at 9, 10.5 and 12 GHz in

a frequency band ranging from 9-12 GHz. The antenna patterns and the antenna gains had to be

documented. We also specified an error range for the antenna gain numbers which must have been

in the 0.3 dB range. A standard paraboloid antenna might have been recorded with 40 dB 0.3 dB

for 11 GHz.

Pulse modulation allowed us to produce a singular pulse, which was very useful for tracing the

sources for reflections. I did find an image in the web for these kind of devices, such as the CF101

Pin Modulator. Most of these devices allowed on/off-modulating the transmitting waveguides with

100 ns pulses, if I remember these technological details.

Some of the generators could be modulated with the use of a internal PIN-diode. I remember to

have used the HP 8690 B Sweeper Mainframe, which had been equipped with different plug-ins for

various frequency bands. This type had been prepared for remote control with external DC-

voltages, but we also used klystrons, Gunn-diodes and later modern HP-IB-controlled equipment.

Measurements were rather cumbersome and required either the aid of an assistant at the transmittterstation or the time consuming travel between transmitter and receiver stations for switching

polarizations, frequencies, calibrating equipment, etc.

In 1972-1977 complex calculations such as an antenna's noise temperature or the antenna-reflectors'

shaping curvatures had to be made manually with the aid of a slide rule. Complex calculations had

to be done on a TR4 orTR440mainframe computer, which might be programmed in Algol 60 /

Algol 68.
http://www.google.de/imgres?hl=de&client=firefox-a&hs=cD8&sa=X&tbo=d&rls=org.mozilla:de:official&biw=1366&bih=638&tbm=isch&tbnid=pSV0PMpJ-GLiMM:&imgrefurl=http://qrmtechnologies.com/pin_modulator.htm&docid=2jPLyXv2GS5yIM&imgurl=http://qrmtechnologies.com/pin_mod.jpg&w=360&h=251&ei=g9mtUN-yEoXItAbRrYDACw&zoom=1&iact=hc&vpx=226&vpy=160&dur=3243&hovh=187&hovw=269&tx=146&ty=115&sig=114402964464676649961&page=1&tbnh=149&tbnw=250&start=0&ndsp=22&ved=1t:429,r:1,s:0,i:88http://www.google.de/imgres?hl=de&client=firefox-a&hs=cD8&sa=X&tbo=d&rls=org.mozilla:de:official&biw=1366&bih=638&tbm=isch&tbnid=pSV0PMpJ-GLiMM:&imgrefurl=http://qrmtechnologies.com/pin_modulator.htm&docid=2jPLyXv2GS5yIM&imgurl=http://qrmtechnologies.com/pin_mod.jpg&w=360&h=251&ei=g9mtUN-yEoXItAbRrYDACw&zoom=1&iact=hc&vpx=226&vpy=160&dur=3243&hovh=187&hovw=269&tx=146&ty=115&sig=114402964464676649961&page=1&tbnh=149&tbnw=250&start=0&ndsp=22&ved=1t:429,r:1,s:0,i:88http://helmut-singer.de/stock/1025.htmlhttp://en.wikipedia.org/wiki/Klystronhttp://en.wikipedia.org/wiki/Slide_rulehttp://en.wikipedia.org/wiki/Slide_rulehttp://de.wikipedia.org/wiki/TR_440http://de.wikipedia.org/wiki/TR_440http://de.wikipedia.org/wiki/Algol_60http://de.wikipedia.org/wiki/Algol_68http://helmut-singer.de/stock/1025.htmlhttp://en.wikipedia.org/wiki/Klystronhttp://www.google.de/imgres?hl=de&client=firefox-a&hs=cD8&sa=X&tbo=d&rls=org.mozilla:de:official&biw=1366&bih=638&tbm=isch&tbnid=pSV0PMpJ-GLiMM:&imgrefurl=http://qrmtechnologies.com/pin_modulator.htm&docid=2jPLyXv2GS5yIM&imgurl=http://qrmtechnologies.com/pin_mod.jpg&w=360&h=251&ei=g9mtUN-yEoXItAbRrYDACw&zoom=1&iact=hc&vpx=226&vpy=160&dur=3243&hovh=187&hovw=269&tx=146&ty=115&sig=114402964464676649961&page=1&tbnh=149&tbnw=250&start=0&ndsp=22&ved=1t:429,r:1,s:0,i:88http://www.google.de/imgres?hl=de&client=firefox-a&hs=cD8&sa=X&tbo=d&rls=org.mozilla:de:official&biw=1366&bih=638&tbm=isch&tbnid=pSV0PMpJ-GLiMM:&imgrefurl=http://qrmtechnologies.com/pin_modulator.htm&docid=2jPLyXv2GS5yIM&imgurl=http://qrmtechnologies.com/pin_mod.jpg&w=360&h=251&ei=g9mtUN-yEoXItAbRrYDACw&zoom=1&iact=hc&vpx=226&vpy=160&dur=3243&hovh=187&hovw=269&tx=146&ty=115&sig=114402964464676649961&page=1&tbnh=149&tbnw=250&start=0&ndsp=22&ved=1t:429,r:1,s:0,i:88http://en.wikipedia.org/wiki/Slide_rulehttp://de.wikipedia.org/wiki/TR_440http://de.wikipedia.org/wiki/Algol_60http://de.wikipedia.org/wiki/Algol_68


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4.5 m Antenna (top view)

The 4.5 m Antenna had been designed for the Symphony satellite program.

Fig. 6: Example: 4.5 m Antenna (top view) (4 & 6 GHz band)
http://en.wikipedia.org/wiki/Symphoniehttp://en.wikipedia.org/wiki/Symphonie


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Hanover Fair 1977

At the Hanover Fair the 4.5 m Antenna has been exhibited.

Fig. 7: 4,5 m Antenna (4/6 GHz) at the Hanover Fair 1977


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Fig. 8: 4,5 m Antenna 4/6 GHz) in Allmersbach im Tal


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Radiation sources in Cassegrain antennas

In 1974 we focused a study for the influence of reflections at the support structure of the secondary

reflector. The radiation sources in these Cassegrain antennas may be categorized in half a dozen

categories, illustrated in the following sketch. At the bottom the main paraboloid reflector may be

identified. Just underneath the focus f2

we may see the secondary reflector which usually will be

sized at 10-12% of the main reflector's diameter. The secondary reflector causes a major

contribution to the antenna's gain losses by blocking a parts of the illumination near the central area.

The secondary reflector (sub-reflector) is to be supported by 3 or 4 (sometimes ranging up to 8

rods as in the cover photograph). The secondary reflector will also produce shadows (I) and

spillover radiation (region VI in the sketch), which may be found in at some wide angles, usually

remote from the main beam.

A major source for losses are the secondary reflector's blocking losses (I). The supporting rods will

produce some reflection losses and shadows, which need to be minimized. Various reflection losses

(IV and II) may be generated from radiation from focus f2.

Of course the feed itself, which is located at focus f1 will also produce some spillover (V), whichhowever in this design largely may be blocked by the support rods in the principal directions

(horizontal and vertical).

Note that some of the reflected waves from the f2- focus are being obstructed on their way to the

main reflector and may only start contribution to the plane wave from (II) at an angle of 20.

Fig. 9: Reflections, shadows and Spillover in Cassegrain Antennas (side view)


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Reflections in support structures

To identify the rods' reflections we increased some of the dimension in the supporting structure until

we found some clear contributions which could be designated to some of the parameters we had

modified.

Some parts of the support structure's parts would contribute according to optical laws, but

sometimes the same elements also obeyed geometrical laws. Obviously these reflections occurred at

some specific angles which could be derived from geometricalconsiderations. The reflected rays

seemed to be focused in an imaginary focus mirrored at the other side of the support structure.

Fig. 10: Shadow structures at the aperture for Cassegrain Antennas (front view)


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Typically these contributions occurred at an offset-angle of 16-25. Varying the dimensions and

angles in the support structure we were allowed to control the reflections in the antenna pattern.

We used a high gain antenna with rather low sidelobe-levels (-20 dB) to identify the disturbing

effects. Due to a very low level of tapering the first sidelobe appeared at 20 dB below main lobe

level. The 115 D/-antenna must have reached a gain of 49-50 dBi.

The feed horn's contribution might be displayed as an imaginary pattern up to 30 dB below mainlobe level. The feed horn's pattern however has been reflected up to the edge of the secondary

reflector and merely starts contributing to the spillover from ca. 12. In this position however the

feed's spillover between 12 and 20may have been blocked by the support rods.

The support rod's contribution reached an average level of 7-8 dBi over isotropic radiation. The

rods' width (22 mm) almost reached a wavelength (26.5 mm at 11.3 GHz) and the rod's radiating

aperture may have been close to 50 2 . Of course the rods had been oversized to demonstrate the

contributions to the antenna pattern.

Somewhere around 90 the spillover of the secondary reflector can be identified. All these

components added up to contribute to the antenna's far field.

Fig. 11: Several contributions to antenna pattern


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Fig. 12: Reflections by support structures in a 3m Cassegrain-Antenna at 11.3 GHz


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Antenna Gain Measurements

After the introduction of computerized equipment the antenna's gain pattern was to be measured

with the help of a HP9825, which also controlled all motors and most of the measurement tools.

Software using cartridges and (in later years) the 8 inches-sized floppy discs.

Directional couplerWe started by registering a file for the directional coupler and stored these data to cartridges. These

data were to be used for later measurements by loading the data from cartridges or discs.

Initially all power meters were to be calibrated. Frequency settings and power levels were allowed

to be settled before saving the data. Frequency increments may have been set to 20 MHz.

Fig..13: Example: directional coupler R120


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Antenna Gain

To initially measure the antenna's gain we used two identical antennas (or alternatively the 3

antennas measurement system), e.g. for a standard antenna in the 9-12 GHz band.

Theoretically antenna gain may be estimated by a formula:

G = 10 . log (q . (.D/)2 ) .... (dBi)

in which q represents an efficiency factor, usually between 0.5 and 0.6. D represents the diameter

and the wavelength. The following table may illustrate some typical reflector antenna gain

characteristics:

q D/ .D/ (.D/)2 Gain (dBi)

100,00% 115 361 130525 51.1 dBi60,00% 115 361 130525 48.9 dBi

50,00% 115 361 130525 48.1 dBi

Smaller antennas reaching D/- ratios of 40 achieve gains around 40 dBi. This will be illustrated in

one of the following tables.

3dB-Beam-width

The antenna's -3dB-beam-width usually may be estimated between 60 . (/D) and 70 . (/D).

Therefore a 1.2 meter antenna at 11 GHz may reveal a pattern with a 3 dB-beamwidth ofapproximately 1.6.

Frequenz G Q D/ 3 dB-width pattern

70 . (/D)

9 GHz 38 dBi 0,5 33 mm 36.36 1.9

10 GHz 39 dBi 0,5 30 mm 40.0 1.75

11 GHz 40 dBi 0,52 27 mm 44.44 1.58

12 GHz 41 dBi 0,55 25 mm 48.0 1.46

Table 1: Gain and 3 dB-width for 1,2m antenna


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Various types of reflections

In order to meet the far field condition the measurement setup required a minimal distance R > R0of:

R0 = 2 D2 /

For identical antennas gain would now be calculated from:

A = G1 + G2 - 20 log ( 4.R/ )

The antenna range (approximately 390 meters with ca. 25-30 m high towers) revealed some ground

reflections at an antenna angle ofarcsin (25/195) = 7.4 respectively arcsin(30/195) = 8.8 . In the

measured pattern these reflections may be identified between 9 and 10.5 GHz.

The antenna pattern suppresses these ground reflections at a level of approximately 2 x 30 dB

(standard sidelobe level -30dB for an 1.2m antenna at 9 GHz for u = D/ . sin () = 14.5).

Fig. 15: Antenna Gain 1,2m Antenna

Fig. 14: Antenna range (approximately 390m with 2 towers of 25-30 m)


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In the pattern we may observe the antenna gain raising from 38 dBi (at 9 GHz) towards 41 dBi (at

12 GHz). We now may observe a superimposed overlay pattern modulating the curve. These

superpositions are caused by various reflections, including some reflections in the antenna' structure

and ground reflections. Especially Gregory and Cassegrain-antennasmay be suffering from internal

reflections.

In the 1.2m antenna some of the reflections reveal a superposed period of 500 MHz between 9.5

and 11.5 GHz, other superposed periods may be around 50 MHz.

Internal reflections

The 500 MHz-reflections are the main contributions. The amplitude of 0.3 dB may represent a

reflection at -30 dB levels. These major contributions represent a detour length of approximately 20

wavelengths or 0.5 meters. These distances may occur inside the antenna construction, between the

feed horn, the secondary reflector and the main reflector.

Ground reflections

The minor 50 MHz superpositions with 0.05 dB represent a reflection at approximately -40 dB

levels, which may be caused by ground reflections at a detour of 4.6 meters. At the test range the

ground reflections result in a detour path Delta of 4.6 meters, equivalent to approximately 160

wavelengths.

Frequency Detour (Delta/ = 4.6 m / )

by ground reflections

9 GHz 33 mm 139

10 GHz 30 mm 153

11 GHz 27 mm 170

12 GHz 25 mm 184

Table 2: 4.6m-detour ground Reflections for a test range of 390 meters

Measurement range reflections

A third category of reflections occurs between both the receiving and the transmitting antennas. The

detour length now is 2 x 390 = 720 meters, resulting in very short intervals of under 0,5 MHz,

which may only be observed in sweeping mode display at an oscilloscope. On insertion of an

isolator into the setup these reflections could be eliminated. These reflections only occurred at theshort antenna measurement range.
http://en.wikipedia.org/wiki/Parabolic_antennahttp://en.wikipedia.org/wiki/Parabolic_antennahttp://en.wikipedia.org/wiki/Parabolic_antenna


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Beam-width Measurements

Beam-width has been measured by position the antenna in 3 rather freely chosen positions near the

main axis and sweeping the frequency at intervals of small increments. Antenna positions were

always approached from one direction to minimize mechanical hysteresis in the gear systems and

angle measurement.

Assuming a true parabolic beam shape we were allowed to interpolate for the maximum value and

the -3dB-width. The formulas for these calculations will be given below. The formula based on a

parabolic shape of the beam's logarithm-function. Three levels (a1, a2, a3) were to be registered in

order to calculate the maximum value (a0), the maximum's position (0), both -3dB-levels and their

angles.

Fig. 16: Basic formulas for the 3dB-Beam Width Calculations


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Fig. 17: Basic formulas for the 3dB-Beam Width Calculations


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Fig.. 18: Basic formulas for the 3dB-Beam Width Calculations


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The typical, beam-width pattern of a 3.3 m-paraboloid antenna, calculated from 3 subsequent

measurements series may be sketched as follows:

Abb. 19: 3 dB-Bean Width of a 3.3 m Antenna


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Cross-polarization

The system had been programmed to measure cross-polarization patterns as well. The antennas

needed to be scanned layer after layer for one singular frequency. The curves had to be interpolated

to sketch the cross-polarization diagram. It took more than one hour to scan these patterns for one

frequency.

Fig.. 20: Cross-polarization 1,2 m antenna

Fig. 21: Cross-polarization 1,2 m antenna


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Software

HPL-Programming code for the HP9825 (introduced 1976) was rather easy, but programs needed to

be compact without too much comments... The system provided us with 31,420 total bytes of RAM-

memory.

Fig. 22: Basic Control Program HP9825 (Sample source code HPL)
http://www.hpmuseum.org/hp9825.htmhttp://www.hpmuseum.org/hp9825.htm


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Hardware

In the years 1976-1980 the system had been equipped with a lot of controlling interfaces, output

devices and a HP 9885M flexible disk drive (~105K). The calculator also included a tape drive

capable of holding 250K bytes with a 2.7KBps transfer rate and an average access time of 6

seconds.

The remote transmitter equipment had to be controlled by a remote HPIB- connection which had to

be extended to cover the 390 m test range by a underground telephone wire.

There were receivers power meters, programmable attenuators, some precision A/D-converters (4.5

digits 50 msec ADC17I), etc...

Fig. 23: Measurement Setup for Antenna Measurements (1976-1980)


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Safety

Of course we were informed not to view into an open waveguide or feed. As RF-generators we

applied power klystrons (probably up to a few Watts), low power sweeping RF-frequency

generators and some Gunndiodes. We were rather careful in avoiding exposure to RF-fields and

today I feel uncomfortable inside a closed room or a car compartment with someone who is

working with a cellphone. I know the modern cellphones are working at power levels we never

would have admitted to be exposed to so close at our heads...

Mobile communication

For communication in the free field we occasionally did use a set of ancient walky talky's, using

vacuum valves and high voltage converters. These rather clumsy and already antiquated portable

sets may have been the Telefunken type Teleport II-equipment (or similar), which applied 0.4 Watt

RF-power in the frequency band 156..168MHz.

The radiation Standards published at July 1984According to valid laws published in the standard number 57 848 Part 2, dated July 1984, the limits

for personal damage in the far field of antenna radiation allow maximal levels averaged over a

period of time of six minutes:

Frequency 30 MHz up to 3 GHz: 2,5 mW / cm2

Frequency 3 GHz up to 12 GHz: 2,5 f (MHz) /3000 mW / cm2

Frequency > 12 GHz: 10 mW / cm2

Internationally it is suggested to reduce these levels to 10% of the specified values.

Of course these were free field radiation levels. No values had been given to specify the near field

areas.
http://www.radiomuseum.org/r/telefunken_teleport_ii.htmlhttp://www.radiomuseum.org/r/telefunken_teleport_ii.html


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Revisiting the antenna-lab in August 2011

At the 17th of August I revisited the antenna-lab site and did take a few photographs. A new building

had been installed at the site where I had been working 1972-1980.

The transmitter tower is still intact, although the horn's aperture cover of the 1.2 m Cassegrain

antenna is broken, allowing the beetles to build their nests... This probably is the same antenna Iused for antenna measurements.

Fig. 25: 1.2 m Cassegrain antenna

Fig. 24: Main building antenna lab


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Fig. 26: The transmitter tower


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Fig. 27: Transmitter antennas


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Contents

Reflector Antennas...............................................................................................................................2

Parabolic antennas................................................................................................................................3

Example of a Cassegrain antenna pattern ............................................................................................4

Early measurement technologies..........................................................................................................6

4.5 m Antenna (top view)................................................................................................................7

Hanover Fair 1977................................................................................................................................8

Radiation sources in Cassegrain antennas..........................................................................................10

Reflections in support structures........................................................................................................11

Antenna Gain Measurements.............................................................................................................14

Directional coupler........................................................................................................................14

Antenna Gain.................................................................................................................................15

3dB-Beam-width............................................................................................................................15

Various types of reflections............................................................................................................16

Internal reflections....................................................................................................................17

Ground reflections.....................................................................................................................17Measurement range reflections.................................................................................................17

Beam-width Measurements................................................................................................................18

Cross-polarization...............................................................................................................................22

Software..............................................................................................................................................23

Hardware............................................................................................................................................24

Safety..................................................................................................................................................25

Mobile communication .................................................................................................................25

The radiation Standards published at July 1984............................................................................25

Revisiting the antenna-lab in August 2011.........................................................................................26

Reflector Antennas Revisited

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