Generally, the most reliable modulation indication is obtained with a modulation monitor located at the transmitter, fed with an RF sample from thc output circuit of the transmitter.
When the monitor location is remote from the transmitter site, and it is impractical to send the required modulation indication to the operating point (studio) via remote control circuits or sub-audible telemetering, it is necessary to make the modulation measurement from an off-air signal. This measurement is made by using an RF amplifier (Belar RFA-2), which selectively amplifies the desired signal (picked up by a receiving antenna) to a level sufficient to drive a modulation monitor, such as the Belar AMM-2B, AMM-3, or AMMA-1 "The Wizard".
AM stations radiate a signal in which the electric component (E Field) of the wave is perpendicular to the ground; therefore it is classified as vertically polarized. The two receiving antennas appropriate for the reception of this signal are the vertical whip and the loop.
(Horizontal wire antennas greatly attenuate the signal over a vertical antenna, but not the ambient electrical noise, and have undesirable directional characteristics)
Vertical whips have a non directional receiving characteristic responding to the impinging E Field wave, while loop antennas respond to the perpendicular magnetic component (H Field) of radio waves and exhibit a figure-eight directional characteristic when viewed from above.
For proper orientation of a loop, the imaginary plane in which the loop rests should be vertical to the ground and should, if extended, pass through the transmitting antenna. The loop characteristic sensitivity pattern results in a null 90° to each side of the receiving plane described.
The null in the sensitivity pattern of the loop antenna may be used to minimize pickup of off-axis interfering stations. The loop may be rotated so that the null occurs in the direction of the interfering source. This is not recommended, however, if the angle between the desired station and the interfering station is less than 45°.
SHIELDED LOOP ANTENNA:
Two commonly encountered situations dictate the use of a shielded loop antenna: Excessive man made electrical interference and the presence of an interfering station off the axis between the transmitter and receiving antenna.
Symptoms of excessive noise are a crackling sound from the aural output of the modulation monitor, or occasional, seemingly inappropriate pinning of the modulation meter while reading positive modulation.
A shielded loop antenna acquires noise immunity from the electrostatic shielding of the loop afforded by the grounded metallic conduit enclosing the wire coils.
Loop antennas, however, have a much smaller output voltage compared to the actual measured field strength of the desired signal. Belar can provide active loops (LP-1A) with an amplifier mounted in the loop frame to compensate for the loss.
Power for this amplifier is derived from a modification to the Belar RFA-2 RF amplifier (kit shipped with each LP-lA loop antenna if we know you have an RFA-2) which sends the required 15 VDC through the coaxial lead-in. (It is important that the input is not inadvertently grounded, otherwise damage to the RF amplifier will result).
The signal level delivered to the RF amplifier by the amplified loop antenna will be approximately equal to the measured field strength at the antenna. The amplified loop is tuned to the station's operating frequency only to provide overload protection for the loop amplifier, and to slightly improve the selectivity of the overall system.
If the lead-in is properly terminated, the amplified loop will drive several hundred feet of coax.
In cases when the field strength at the receiving location exceeds 30 or 40 mV at the lowest transmitter power employed, an unamplified loop (LP-1) may be employed. This loop is essentially untuned, but the number of turns is selected according to frequency. The output voltage of the LP-1 is approximately l/10 field strength.
Since no isolation exists between the loop and lead-in, it is recommended that the lead-in be limited to less than 100 feet to minimize tuning effects introduced bv the cable.
Both the anplified and unamplified Belar Shielded loop antennas may be ordered for use with other than Belar RF amplifiers. An Optional Outboard power supply is available for this purpose. (Option 01)
OFF-AIR MONITORING PROBLEMS:
In a few situations, off-air modulation monitoring may be impossible. The RF amplifier cannot eliminate co-channel interference (generally occurring at night) and adjacent channel interference. If the interfering signal is sufficiently far off-axis, it may be minimized by a loop antenna, as perviously discussed.
Some RF amplifiers have been constructed with narrow IF band widths to suppress adjacent and second adjacent channel pickup. This is not generally recommended, because the modulation indication then becomes inaccurate due to the lack of high frequency components in the detected signal.
When an RF amplifier is operated in close proximity to a transmitter on another AM frequency, input overload may become a problem. If you suspect an overload problem, watch the RF level indicator when your carrier is turned off. The indicator should drop to the same point on the scale as with the antenna disconnected. If it does not, try reducing the input level, and if necessary, insert a pad.
Directional antennas generally introduce inaccuracies in the field-measured modulation indications. When the monitoring system is operating in the major lobes of a directional pattern, these errors are generally acceptable.
If one wishes to operate an off-air monitor near the null of a directional array, a theoretical investigation of inaccuracies introduced in modulation may be warranted for the particular array.
Belar does not recommend introducing filters between the antenna and the RF amplifier to attenuate adjacent frequency interference. To obtain accurate modulation indications, any external filter must have symmetric amplitude and asymmetric phase response about the carrier frequency, while providing the proper measurement bandwidth.
If you have any additional questions about Belar Shielded Loop antennas (or any Belar product), please contact us.
© Copyright 2000 Belar Electronics Laboratory, Inc. All rights reserved.
Specifications subject to change without prior notice. Return to the LP-1/LP-1A Data sheet What's
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Return to the LP-1/LP-1A Data sheet
What's New | About Us | AM Equipment | FM Equipment | TV Equipment | Service & Support