Attachment

3. Antenna measurement design considerations

When designing an antenna measurement system, there are many parameters that must be considered in order to select the optimum equipment. Begin by considering the components for the transmit site, then move to the receive site. Designing a complete antenna system often requires you to configure the transmit site, then the receive site, and then make adjustments to the transmit site and recalculate the values for optimum performance.

Transmit site configuration

Optional amplifier

G amp

L

2

Transmit antenna

E

RP

L

1

PSG synthesized source or internal PNA source

Figure 8. Transmit site configuration

Select the transmit source

In selecting the transmit source, consider the frequency range of the antenna under test, the distance to the transmit antenna, the available power of the source, and the speed requirements for the measurements. For compact ranges and near-field ranges, the internal PNA source will typically be the best source to meet your measurement needs.

The internal source is faster than an external source and may lower the cost of the complete system by eliminating a source. Large outdoor ranges may require an external source that can be placed at a remote transmit site.

Will a transmit amplifier be used?

Begin by making your power calculations without an amplifier. If after doing the power calculations the transmit power is not high enough, then add an amplifier and run the calculations again.

12

Calculate the effective radiated power

The effective radiated power (E

RP

) is the power level at the output of the transmit antenna.

E

RP

= P source

– (L

1

+ L

2

) + G amp

+ G t

Where E

RP

= Effective radiated power (dBm)

P source

= Power out of the source (dBm)

L

G

1

& L

2 amp

= Loss from cable(s) between source and antenna (dB)

= Gain of the amplifier (if used) (dBi)

G t

= Gain of transmit antenna (dBi)

Note

A calculator which will derive this number for you can be found at:

http://na.tm.agilent.com/pna/antenna

Calculate the free-space loss

The free-space loss (or power dissipation, P

D

) of an antenna range determines the difference in power levels between the output of the transmit antenna and the output of an isotropic (0dBi) antenna located at the receive site. This free-space loss is due to the dispersive nature of a transmitting antenna. A transmitting antenna radiates a spherical wavefront; only a portion of this spherical wavefront is captured by the receiving antenna.

For a free-space, far-field range, this range transfer function is easily determined as follows:

P

D

= 32.45 + 20*log (R) + 20*log (F) where P

D

= Free-space loss (power dissipation) (dB)

R = Range length (meters)

F = Test frequency (GHz)

This equation does not account for atmospheric attenuation, which can be a significant factor in certain millimeter-wave frequency ranges.

Compact antenna test ranges (CATRs) achieve greater transfer efficiency by collimating, or focusing the transmitted power using one or more shaped reflectors. Transfer functions for most CATRs are available from the manufacturer's data sheet or on request. If the transfer function is unavailable, use the free-space loss as a worst-case estimate.

Calculate your range transfer function for the minimum and maximum test frequencies.

Calculate the maximum power level at the output of the AUT

The test channel received power level must be calculated to determine the approximate maximum power level present at the output of the antenna-under-test (AUT). The required measurement sensitivity is determined from the test channel received power level, the required dynamic range, and the required measurement accuracy. The maximum test channel received power level will occur when the AUT is boresighted relative to the transmit antenna.

Note

P

AUT must not exceed the specified compression input levels of the next components (typically either the PNA or in more complex systems, a mixer). See the individual component specifications for detailed information.

P

AUT

= E

RP

– P

D

+ G

AUT where P

AUT

E

RP

= Test channel received power level at output of AUT (dBm)

= Effective radiated power (dBm)

P

D

= Free-space loss (dB, at the maximum test frequency)

G

AUT

= Expected maximum gain of AUT (dBi)

13

14

Dynamic range

The dynamic range required to test the AUT is the difference, in decibels, between maximum boresite level and minimum AUT level that must be measured. Examples of these include side-lobe level, null depth, and cross-polarization levels.

Measurement accuracy/signal-to-noise ratio

Measurement accuracy is affected by the measurement sensitivity of the system. The signal-to-noise ratio will directly impact the measurement accuracy of the system for both amplitude and phase measurements. Figure 9 illustrates the relationship between signal-to-noise ratio and magnitude and phase errors.

Figure 9. Measurement accuracy as a function of signal-to-noise ratio

Determine your signal-to-noise ratio based on the magnitude and phase errors you can accept.

Note

This equation assumes the simplest antenna system with no remote mixing. See Figure 10.

Sensitivity

The PNA should be located as closely as possible to the test antenna to minimize the

RF cable lengths. The measurement sensitivity of the PNA must be degraded by the insertion loss of the RF cable(s) to determine the system measurement sensitivity needed.

Now, determine the sensitivity required of the PNA

Sensitivity = P

AUT

– DR – S/N – L where P

AUT

= Power at the output of the AUT (dBm)

DR = Required dynamic range (dB)

S/N = Signal-to-noise ratio determined above (dB)

L = Cable Loss (dB) from AUT to PNA input

Reference

Test

P

AUT

L

Receiver #1 Receiver #2

Figure 10. Receive site configuration without external mixing

15

Choosing a network analyzer

The frequency and sensitivity requirements of your antenna system will determine the network analyzer specifications. Agilent offers three families of network analyzers: the

PNA series, the PNA-L series and the ENA series. Agilent has developed options for the

PNA series specifically for antenna measurements. Because of these options, the

PNA series is often the preferred analyzer for antenna solutions. However, there are applications which do not require these options and the lower cost PNA-L series or ENA series analyzers may be the right solution. For secure environments, a PNA or PNA-L series analyzer must be used. Select an analyzer from the following table that meets your frequency and sensitivity requirements.

Table 1. Agilent network analyzer typical values

Family

ENA

PNA-L

Model/ option (std./ configurable test set)

E5070B

E5071B

N5230A

Opt. 020/025

N5230A

Opt. 120/125

N5230A

Opt. 220/225

N5230A

Opt. 420/425

N5230A

Opt. 520/525

Frequency range

300 kHz to 3 GHz

300 kHz to 8.5 GHz

300 kHz to 6 GHz

300 kHz to 13.5 GHz

10 MHz to 20 GHz

10 MHz to 40 GHz

10 MHz to 50 GHz

160us

160 us

160 us

160 us

*

*

Frequency stepping speed

(10 MHz/pt at max IF BW with

Sensitivity at test port with 1 kHz no band crossings) IF BW @ Fmax

160 us

< –92 dBm

< –80 dBm

< –99 dBm

< –94 dBm

< –85 dBm

< –75 dBm

< –70 dBm

PNA

E8362B

E8363B

E8364B

E8361A

10 MHz to 20 GHz

10 MHz to 40 GHz

10 MHz to 50 GHz

10 MHz to 67 GHz

Note: Option H11 sensitivity is typically –127 dBm

* Data not available

** Option not available

278 us

278 us

278 us

278 us

< –100 dBm

< –94 dBm

< –94 dBm

< –79 dBm

Sensitivity at direct receiver input with

1 kHz IF BW

(w/Opt. 014 for

PNA) @ Fmax

**

**

< –108 dBm

< –108 dBm

< –97 dBm

< –86 dBm

< –78 dBm

< –114 dBm

< –105 dBm

< –103 dBm

< –88 dBm

Power out

@ Fmax

+10 dBm

+5 dBm

+10 dBm

+2 dBm

+10 dBm

–5 dBm

–9 dBm

+3 dBm

–4 dBm

–10 dBm

–5 dBm

Refer to the ENA data sheet, literature number 5988-3780EN or the PNA and PNA-L data sheets, literature numbers 5988-7988EN and 5989-0514EN for more detailed information.

What to do if the sensitivity requirement cannot be met

If the AUT is located far from the analyzer, requiring long cables, then the loss caused by the cables could be significant, reducing accuracy and dynamic range. You may also be unable to find an analyzer that meets your sensitivity requirements. In this situation, downconverting the signal to an IF signal by using the 85309 LO/IF distribution unit with

85320A/B remote mixers brings the measurement closer to the AUT. This reduces RF cable loss and maximizes accuracy and dynamic range. Options H11 and 014 on the PNA network analyzers both support a remote mixing configuration. Refer to “Receive site

configuration with external mixing” to configure your system.

16