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Field strength measurement - Buddipole and FSM

Introduction

A challenge in assessing the impact of BPL emissions on an amateur radio station is to use a portable antenna to make field strength measurements that are extensible to a typical amateur antenna. A portable antenna is usually:

The objective of BPL emission studies is typically to:

Small loops lack sensitivity, and even amplified small loops have a noise floor that is well above expected ambient noise on the HF bands.

 

 

Fig 1: Buddipole on mast and tripod

The Buddipole is a portable HF dipole system which uses loading coils below 20MHz. A Buddipole was obtained on the prospect that it might have a similar pattern to a typical dipole installation and have sufficient sensitivity to be able to measure ambient noise. The system includes the Buddipole Mast and Buddipole Tripod which supports the dipole at a height of 2.85m.

The Buddipole uses a tapped loading inductor in each leg. The inductance and RF resistance were estimated using Serge Stroobandt's Inductance Calculator. The dipole arms are telescopic and brought to resonance by adjusting the coil tap and length of the arm. The system also included a balun at the feed point, and the balun could be set to impedance ratios of 1, 2 and 4 for 50:50, 50:25 and 50:12 Ω ratios.

Fig 1a: Buddipole "Low Band" coil assembly.
 

This article refers to a Buddipole purchased in 2007 and using the  "Low band coil" (See Fig 1a), "Long telescopic whips", and "TRSB". The models and resultant Antenna Factor apply only to that antenna configuration. The antenna length overall was a measured maximum of 5.9m. (Current advertisements suggest that the antenna is longer, but these models and measurements were performed on whips that permitted 5.9m overall maximum.)

Nothing in this article is to comment on the performance of the Buddipole in absolute or relative terms for typical amateur QSO operations.

NEC studies

A series of NEC studies at 7.1MHz were performed on the Buddipole and a typical InvertedV in free space, and over average ground. The ground parameters used were σ=0.005 and ε=13.

Table 1:
  Freespace gain (dBi) Over ground gain (dBi)
Avg Max Avg Max 20°
InvertedV -0.02 2.11 -0.46 6.66 0.63
Buddipole -1.43 0.36

-8.64

-0.92 -8.68
Difference -1.41 -1.75 -8.18 -7.58 -9.31

Table 1 is a summary of the NEC model gain results.

The freespace average gain of both antennas discloses the extent of internal loss. The internal loss in the InvertedV is caused by loss in the copper conductor, and is very small at 0.02dB. The internal loss in the Buddipole is mainly loss in the loading coil, and totals 1.43dB.

The loss over ground includes an additional component, loss in waves reflected by the ground, and which is the main cause of the increase in loss from the free space model. The effect of the nearby ground results in higher total loss for the Buddipole, around 9dB worse than the InvertedV..

Fig 2:

Fig 2 shows the pattern of both antennas. The patterns are very similar, but the Buddipole has less gain at lower elevation angles, up to 1.8dB less gain at 15°.

It is easy to compare the maximum gain to obtain a rating for the performance of one antenna against the other. The maximum gain occurs at the zenith, and since most noise and longer distance signals arrive at much lower angles, the maximum gain is not a good indicator of relative performance for ambient noise measurements.

Another metric is the average gain which would be a good indicator if signals arrived equally from all directions in the hemisphere.

Fig 3:

Fig 3 shows the average gain of the antennas. Average gain is related to efficiency, AverageGain=10*log(ΣPfarfield/Pin)dBi, so efficiency of the two antenna systems is 90% and 14% respectively.

For ambient noise measurement, the average gain is probably the best figure for comparing the antennas, but keeping in mind that the Buddipole will be up to 1.8dB less sensitive to low angle sources compared to the higher InvertedV.

FSM

FSM uses Antenna Factor to relate the field strength of signals or emissions with the receiver terminal voltage that they will produce.

Fig 4: 

Fig 4 is a view of a worksheet to develop a system Antenna Factor of the Buddipole at resonance in each of the bands, taking into account the average gain, feed point impedance at resonance, balun transformation, and coaxial line loss. Additionally, it explores the ratio of expected ambient to receiver internal noise to ensure that there is sufficient sensitivity to make reasonably accurate measurements of ambient noise.

Fig 5: 

Fig 5 shows the system Antenna Factor for a lossless antenna, the Buddipole, an amplified loop with AF=20dB/m, and a 600mm passive untuned loop for each of the amateur bands. Lower Antenna Factor means better sensitivity. Whilst not as sensitive as a lossless or nearly lossless antenna such as a high half wave dipole, the Buddipole is much more sensitive than the amplified loop commonly used for EMC measurements. (The instrument noise floor or noise floor of the loop amplifier are also important in determining the system noise floor.)

Verification

A test was conducted to compare FSM results for ambient noise using each antenna using Antenna Factor derived from NEC model average gain over average ground.

Fig 6: 
0

Fig 6 compares ten observations made with each of the antennas. The median indicated field strength values of the Buddipole are about 1.2dB lower which might be explained by its lower gain at low elevation angles.

Comparison with a small loop

Small loops are often used for field strength measurement.

Fig 7: 

Fig 7 compares the modelled gain of a:

Measurements made with the Buddipole (based on its average gain) will be lower than expected from a loop, for example by around 6.1dB at 10° elevation.

Conclusions

Nothing in this article is to comment on the performance of the Buddipole in absolute or relative terms for typical amateur QSO operations.

The pattern of the Buddipole is fairly similar to the chosen typical InvertedV dipole.

Field strength measurements on the 7MHz band using the Buddipole and a system Antenna Factor derived from NEC model average gain over average ground should be a sound basis to: 

The model for performance on the 7MHz band was validated in a test. The model is likely to be a good predictor of performance on the other bands.

Table 2:
Frequency (MHz) Balun ratio System Antenna Factor (dB/m)
3.6 4 -0.8
7.1 4 -3.0
10.1 4 -3.5
14.2 2 -2.6
18.1 2 -1.3
21.2 1 0.0
24.9 1 1.3
28.5 1 2.4

Note: System AF includes estimated loss in balun and 16m of RG58C/U feed line.

Table 2 is a summary of the antenna settings and AF for each band. The figures assume that the antenna has been adjusted to resonance, and the system AF depends on that assumption. It is very important that the balun setting is per Table 2 and that the antenna is resonant (adequately indicated by VSWR minimum) at the measurement frequency. Whilst and antenna analyser might be helpful in adjusting the antenna to resonance, due to the risk of the broadband detector in many analysers being disrupted by received signals, the most reliable test of resonance is a dip in VSWR measured using a transmitter.

Links

 

Glossary

Term Meaning
AF Antenna Factor
dB decibel - power ratio
dBm decibels - power wrt 1 mW
   

Changes

Version Date Description
1.01 02/07/2007 Initial.
1.02 29/07/2007 Revised coil loss estimates and dependent models.
1.03 04/06/2008 Added detail of  "Low band coil" and "Long telescopic whips".
     

V1.01 12/03/09 13:48:28 -0700 .

Use at your own risk, not warranted for any purpose. Do not depend on any results without independent verification.


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