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Estimating Antenna Factor for a MobileOne M40 helical whip

This article explores the expected performance of a MobileOne M40 short mobile helical whip for the purpose of measuring ambient noise at 7.1MHz using FSM and a communications receiver.

ITU-R P.372-8 suggests that the lowest ambient noise that would be expected under reasonable propagation conditions would be the galactic noise with Noise Figure 32.4dB. This equates to a field strength of -46.3dBμV/m in 1Hz measurement bandwidth.

The Antenna Factor is the ratio of field strength to receiver terminal voltage for an antenna.

MobileOne M40

The following pictures show the antenna and roof mounting for least directivity.

MobileOne M40 whip mounted on heavy duty roof bar.
Detail of base and spring. The antenna has 3m of RG58C/U to the step attenuator, and a further 1m of RG58C/U to the measurement receiver.
The roof bars are mounting using a reinforced plastic adapter which requires an RF bonding conductor at both ends of the roof rack bar.

Measurement and modelling

Using a MFJ-259B analyser to measure the impedance looking into 4m of RG58C/U, the whip was tuned for minimum VSWR at 7.1MHz. At this operating point, the base impedance of the antenna should be very close to purely resistive.

The measured input impedance to the transmission line was 48+j32. Using the Transmission Line Loss Calculator, the base impedance (Zload), load end VSWR and line loss were calculated. The results are shown in Table 1.

Table 1: Transmission Line Loss Calculator results
Parameters  
Transmission Line Belden 8262 (RG-58C/U)
Code B8262
Data source Belden
Frequency 7.100 MHz
Length 4.000 metres
Zin 48.00+j32.00 Ω
Yin 0.014423-j0.009615 Ω
Results  
Zo 50.01-j0.83 Ω
Velocity Factor 0.660
Length 51.64 °, 0.143 λ
Line Loss (matched) 0.147 dB
Line Loss 0.231 dB
Efficiency 94.82%
Zload 25.15-j0.14 Ω
Yload 0.039762+j0.000223 Ω
VSWR(source end) 1.87
VSWR(load end) 1.99
γ 4.24e-3+j2.25e-1
k1, k2 1.30e-5, 2.95e-10
Correlation coefficient (r) 0.999884

 The base impedance is 25.15-j0.14 Ω (almost purely resistive as expected) and load end VSWR is 1.99.

The Antenna Factor is estimated for a centre loaded whip with similar current distribution, using the online calculator.

 The termination loss is calculated from the load end VSWR above to be 3.5dB plus line loss of 0.23dB.

The results are shown in Table 2.

Table 2: Calculated Antenna Factor
Height base to coil centre 0.850 m
Height coil centre to top 0.750 m
Overall height 0.0379 λ
Frequency 7.10 MHz
Termination loss 3.50 dB
Antenna Factor 1.77 dB/m

The antenna with an Antenna Factor of 1.8dB/m, which will have receive performance similar to a terminated efficient antenna at this frequency of -14.0dBi gain.

The field strength equivalent to a noise floor of -135dBm in a 2.250kHz wide receiver is calculated using the Field strength / receive power converter, the results are in Table 3. 

Table 3: System noise floor calculations
Frequency 7.1000 MHz
Field Strength Bandwidth 2250.0 Hz
Field Strength distance to source 30.00 m
Field Strength 0.04591 μV/m
-26.8 dBμV/m
0.0001218 μA/m
-78.3 dBμA/m
Normalised Field Strength (1Hz) -60.3 dBμV/m
-112 dBμA/m
Antenna Factor (matched to 50Ω) 1.25 dB/m
Receiver input resistance 50.0 Ω
Receiver Bandwidth 2250.0 Hz
Receiver distance to source 30.00 m
Receiver Voltage 0.03976 μV
-28.0 dBμV
Receiver Power -135 dBm

The system has a noise floor equivalent to a normalised field strength of -60.3dBμV/m. This is well below the expected galactic noise at -46.3dBμV/m and should not limit measurement of ambient noise.


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