Chris Scott & Associates -

LP-3 Standard H-Field Antenna  - user's manual

(some illustrations not shown)

LP-3 series Specifications:

Type: Shielded Loop.

Application: Measurement and monitoring of AM broadcast Response: H-field. (E-field attenuated typically 30 dB)

Frequency range: .50 - 5.0 MHz.

Impedance: 50 ohms nominal.

Antenna factor: Varies with frequency, calibrated and documented within +/- 1.5 dB.Termination: Coaxial, BNC

Mounting: Integral bench-top base with 1/4 - 20 female threads for tripod mounting

Polarization: Vertical

Pattern: Figure eight with > 20 dB minima (30 db typical).

Power input rating: Receive only

Height: 21"

Weight: 5 lbs 

1. Introduction

The LP3-series standard H-field antenna is designed for laboratory and field measurements in the range of .5 - 5 MHz. This antenna allows broadband reception over this range for monitoring and measurement purposes. The electrostatically shielded design provides superior rejection of man-made E- field noise. Tektronix, in their excellent 26W-7062-1 Technical Brief, discusses the advantages of using the shielded loop antenna in conjunction with their 2710 spectrum analyzer for "NRSC Measurements". Excellent symmetry achieves typically a 30 dB null at 90 degrees. Each unit is individually calibrated with the final test data included with this manual. A optional tripod is highly recommended for all but the most casual application. This antenna is not designed to be permanently mounted on a structure or vehicle outdoors. It is not designed for transmitting.

2. Basic Operation

For measurement of AM broadcast stations, position the antenna approximately one meter above ground using either a tripod or other suitable support. The fine azimuthal adjustment permitted by the tripod mounting allows greater flexibility and adjustability when direction finding and nulling interference sources. Connect the antenna to the receiver input using well-shielded (greater than 99% braid) 50 ohm coaxial cable terminated with a male BNC connector. An eight-foot length of high quality RG-58/AU cable is used during calibration, and is recommended for best absolute level accuracy. The receive instrument's input should be 50 ohms, non-reactive, to maintain proper loop balance and achieve deep nulls. To obtain the actual RF field value in volts / millivolts / microvolts per meter, simply multiply the antenna output voltage value by the calibration factor (multiplier); for example if the output voltage is 10 millivolts and the calibration factor at the affected frequency is 200, the actual H-Field value (expressed as the plane-wave equivalent E-field value) is 200 times 10 = 2000 millivolts = 2 volts per meter. Although amperes per meter is technically correct, E-Field equivalent values are generally used; this conversion is accurate when used in the normal far-field (plane-wave) environment over homogeneous ground, standard with AM field intensity measurement methods.

3. Directional Pattern

This antenna is vertically polarized, and offers high rejection of electrical noise due to it's inherent H-field only pickup. Like most shielded air-core loop antennas, it exhibits a figure-eight pattern with nulls (or minima) perpendicular to the plane of the loop, in the direction and opposite direction that the coaxial connector points. This minima should exceed 20 dB when the antenna is properly terminated. Depending upon the quality of the receiver RF input termination, null depth may reach 40 dB at some frequencies. Normally, the antenna is used with the maxima (plane of loop antenna) oriented toward the signal source. The maxima 3 dB beamwidth is approximately 70 degrees wide, while the minima is only a few degrees making the null useful for direction-finding should the azimuthal bearing of a signal be sought. Naturally there is a 180 degree ambiguity common to all loop-type antennas without external sense antennas for phase reference. The established calibration factor holds only for measurement within + or - 5 degrees of maxima azimuthal orientation. One very useful feature is the ability to deeply null an interference source, greatly improving accuracy of the subject measurements; while ideally the selected measurement location would provide a 90 degree angle between subject and interference, in practice this is not always possible. If the subject pointing angle error is 50 degrees or less, accurate measurements are still achievable by augmenting the calibration factor with secondary angle factor, which is simply the cosine of the angle. For example, assume that the normal maxima factor is 100, and that nulling the interference resulted in a pointing error to the subject of 30 degrees. The cosine of 30 degrees is .866, so the received voltage will be 86.6 % of the value produced when perfectly pointed.

The corrected factor would be: 1 multiplied by the reciprocal of .866: 115. 

The table below may be used for approximate values.

Pointing Error COS Factor Multiplier

10 degrees .98 1.02

20 degrees .94 1.06

30 degrees .87 1.15

40 degrees .77 1.30

50 degrees .64 1.56

60 degrees .50 2.0 (not recommended)

4. AM Bandwidth measurement

The LP3 series antenna is ideally suited for the AM Bandwidth measurements currently required by the Federal Communications Commission. . Maximum signal-to-noise is desired. Normal field intensity measurement considerations apply, such as avoiding locations in close proximity to electrical distribution systems and other interference sources. In some marginal signal-to-noise locations, proximity to operating automobile ignition systems and / or AC power inverters may contaminate the noise floor and should be avoided. Note also that although the rules specify "no video filter" when using the spectrum analyzer, in practice, a video filter of equal or greater than the selected resolution bandwidth will not slow the video risetime, but can improve the spectral display.

Figure A and B plot the bandwidth of a common solid-state AM transmitter that fails the tests; B shows the same environment with the subject nulled 20 dB. This technique can be useful for verification that the measured energy is actually being radiated by the subject. 

5. Harmonic and Spurious Emission measurements

Since the LP-3 antenna is carefully calibrated from .5 through 5 MHz, relational measurements of out-of-band emissions can be made with only slightly less accuracy than with a field-intensity meter. This is done by dividing the spurious or harmonic field value (in millivolts or microvolts per Meter) by the fundamental carrier field value; each measurement must be made under identical conditions, each value corrected by the appropriate calibration factor.

For example, if we obtain 7 millivolts antenna output at 1.0 Mhz, (having converted from dBm) and 5 microvolts antenna output at 2.0 Mhz under identical conditions of antenna alignment etc., and the calibration factor is 350 at 1.0 Mhz and 230 at 2.0 Mhz, the field values are 350 times .007 = 2.45 volts per meter fundamental, and 230 times .000005 = .0011 volts per meter second harmonic. Using the formula 20 times the LOG (.0004 divided by 1.75), we get -66.5 dB suppression. This obviously is inadequate suppression unless the station is operating at very low power!

NOTE: If a spectrum analyzer or other broadband receiver is used for this type measurement, the limited dynamic range of the instrument may handicap this function. Intermodulation products created in the instrument's RF front-end often create the illusion of high harmonic level. A medium- frequency notch filter is often required to reduce the fundamental level to prevent this error. Figure C shows the 1340 khz carrier nulled with this filter; when the spectrum analyzer is then tuned to sweep the harmonic frequencies, harmonic levels will suffer less than 1 dB insertion loss and will be unaffected by the fundamental. This accessory equipment is available from Chris Scott & Associates.

Although the insertion loss of the MF-NOTCH filter is considered by some to be negligible at second and third harmonic frequencies, we recommend using .8 dB and 1.2 dB, respectively. Spectrum analyzer accuracy is often degraded to + / - 4 dB when the subject carrier is being displayed in the lower 10 dB (-70 to -80 dB ref) range. Empirical data has shown that increasing gain by 10 dB after notching the fundamental signal often reduces this spectrum analyzer error significantly.

In addition, note that shorter vertical structures will become an appreciable portion of harmonic wavelengths prior to the same action at fundamental frequencies; re-radiation from these structures can easily be detected by ensuring that harmonics are properly nulled when the LP-3 is oriented 90 degrees to the subject's radiator.

6. RF Sampling

The LP-3 antenna is also very useful as a close-proximity RF sampling device; connection directly to a scope or frequency counter (beware of potentially high levels) permits low noise coupling while maintaining isolation. In general, proximity to an AM radiator should be kept to at least five feet for low-power stations, and ten feet for stations greater than a kilowatt. When conducting tests in close proximity to the base of an AM broadcast radiator (tower), bear in mind that the RF radiation intensity may be unsafe. A complete treatment of this subject is beyond the scope of this manual, but be aware of the safety requirements unique to the affected site.

6. Re-Calibration

This antenna has been individually calibrated using standard techniques comparable to NIST field-strength meter calibration procedures. Our facilities include a standard medium-frequency magnetic field maintained in close agreement with, and traceable to National Institute of Standards and Technology. The following page details the standard field method and the reduction method. The inherently stable nature of this antenna design, makes re-calibration at yearly intervals unnecessary. However, if the antenna is subjected to abuse in the field, repair and re-calibration is recommended. For organizations which require continuous NIST traceability, the standard re- calibration interval is (24) months. A nominal fee is charged for this service. For reference, the typical LP-3 impedance is plotted below. It maintains a moderate match over the AM broadcast band. 

6. Plain Language Warranty

We warrant this instrument to be free of defects in materials and workmanship for a period of one year from date of shipment. We guarantee that the instrument meets the specifications published in this manual. Chris Scott & Associates will repair or replace any defective item or material when notified within the warranty period.

Prepayment of shipping to our facility and arrangement for a Return Authorization is required.

Specifically Excluded are:

1. Damage during shipping.

2. Damage due to improper use.

3. Instruments which have been modified.

4. Normal wear

5. Re-calibration

This warranty covers only repair or replacement of the defective instrument or affected parts. Chris Scott & Associates assumes NO liability for any damages, losses, or expense resulting directly or indirectly from product use, or any inability to use them separately or in conjunction with other instruments or devices. 

Specifications and pricing subject to change