am nrsc testing with the loop antenna (nrsc bandwidth)

8/9/2019 AM NRSC Testing With the Loop Antenna (NRSC Bandwidth)

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Technical Note:

NRSC AM bandwidth measurements with the loop antenna

Copyright 1994 Chris Scott

This note is a reprint of the article "Looking at Bandwidth with the Loop Device", published September 7,

1994 Radio World. The original photographs have been replaced.

With the first year of AM bandwidth measurements behind us, my associate Roger Hall and I have learned

important lessons about the best way to approach these new tests.

We tested a number of stations and gained some practical knowledge in this area and identified some potential

pitfalls. About thirty percent of the stations we tested initially failed, with an even split between bandwidth

and harmonic related problems. In all cases these problems were curable-the testing led us to transmitter

defects, usually modulator problems or harmonic trap misadjustment. Aside from simply meeting the spectral

purity rules, these tests had beneficial side-effects; true defects were identified and repaired.

According to Bernie Stuecker, Chief, Equipment and standards branch at the commission, low signal-to-noise,

interference from near-frequency stations, and the sometimes misunderstood effects of antenna factor are

probably the most common culprits affecting accuracy of these measurements.

Acceptable procedures

Stuecker sets testing standards and procedures to be used by field operations personnel when determining

station compliance. While at his office discussing AM bandwidth testing methodology, I asked him about the

once common practice of using a communications receiver to check harmonics. Specifically I asked him if it

was acceptable practice. "We would see [harmonic measurement] reports on file at the station indicating that

the second harmonic was barely audible or so many S units below carrier. If it is anything other than inaudible,

how do you know if it meets the attenuation specifications?" Stuecker asked. He indicated that they use a field

strength meter to check harmonic levels.

We found that the receive antenna used with the spectrum analyzer is critical; our experience comparing

various antenna types used with the Tektronix 2712 showed that a broadband shielded loop is arguably the

best choice. Tek published a related technical brief on the subject available in pdf format here.

Signal to noise important

June was our busiest month conducting these measurements, and nearby thunderstorms contaminated the

noise floor on more than one occasion. Locations away from power distribution systems and industrial areas

were naturally the best. Achieving proper signal-to-noise and confirming that the client station was actually

NRSC testing with the Loop Antenna (NRSC bandwidth) http://www.scott-inc.com/htm

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generating the recorded energy were probably the most important tasks.

In difficult cases we found it necessary to shut off the station to verify the emission source. Normal

considerations for taking AM field strength measurements apply, and we found the best results locating at the

specified one-kilometer or closer; when measuring the bandwidth of omnidirectional stations we found that

proximity as close as one or two wavelengths showed the same results but with fewer noise problems.

The rules specify "approximately one kilometer" for operating stations and in my opinion this is a repeatable

compromise between high signal-to-noise ratio and being truly representative of the signal that is radiated intothe far field. It is easy to mistake ambient or electrical noise for the station's emission; if the measurement is

erratic, it pays to corroborate it at a lower noise location or with a transmitter tap sample.

The level that a sample port provides should be checked before connecting a spectrum analyzer direct because

many instruments will be permanently damaged by more than a hundred milliwatts.

When chasing harmonic problems with a transmitter tap the engineer should note what typed of pickup is

used; a reflectometer-type normally exhibits a six decibel per octave increase in sensitivity, while resistive

dividers should be flat. Our equipment power came from a fairly large UPS that produced a sinewave output

and is well shielded. Some units radiate and should be tested prior to beginning a measurement series. We have

since constructed a customized well filtered and shielded inverter, supplied by vehicle dc.

Adjacent Frequency

If the major noise source is

another local station, the

twenty-five db null

obtained with the shielded

loop will not be enough to

reduce it to near the level

of the ambient noise.

Obviously, arranging for

the interfering station to be

off air during the

measurements will

eliminate this problem, but

if it happens to be the

competition, it may be

difficult to convince them

to do this in the middle of

the day, particularly if

measurements need to be

repeated for any reason. In

this case, with the client

station off-air it's best to

record a spectral plot of the ambient RF environment and inclued it in the final data, demonstrating what

cannot be blamed on the staiton. We usually recorded a plot for report inclusion showing the station nulled at

least twenty decibels to identify which signals followed the null. One way to couple energy into a spectrum

analyzer is to use a simple whip antenna connected directly to the fifty ohm input of the instrument. While this

may be useful for quick-and-dirty checks, attempting to get meaningful harmonic data this way will almost

certainly be misleading, often mistaken by ten or even twenty decibels. At least three things must be known

about the test antenna; the antenna factor, or relationship between its output and the field that its placed into,


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the impedance match which affects performance with various lengths of coax, and what mismatch loss needs

to be considered. This performance can be measured over the frequency range of interest (in our case 500

kilohertz to five megahertz) and combined into a calibration factor. Commercially available antennas for this

purpose are rare, so we developed and calibrated our own.

Although the FCC rules granted a grace period for measuring close-in bandwidth, yearly harmonic

measurements were still required. Now that the grace period has passed, both close-in bandwidth

measurements and harmonic measurements are required. Our experience showed that if the stations are in

compliance at the second and third harmonics, higher order products were the same level or weaker.

Favorable locations

The most favorable locations for close-in bandwidth tests are less than ideal for harmonic measurements

because groundwave attenuation increases with frequency and is particularly noticeable at the third harmonic.

In one case while corroborating data recorded at one kilometer, measurement at three showed several decibels

improvement. Whether or not separate measurement locations are acceptable to the commission remains a

question; this practice should probably be the exception, used only when compelling reasons exist.

Transmitter power levels of five kilowatts and above must meet the full eighty decibel specification. Theseoften have harmonic traps and, once properly adjusted, usually have little trouble with compliance.

Transmitters without traps may be more of a challenge. If all else fails, adding a trap to the atu should cure the

problem. Although we used a Potomac FIM-41 field intensity meter for harmonic data, accurate

measurements are available from a spectrum analyzer with a proper antenna. One caveat here: We saw some

artificially high harmonic indications which were created inside the front end of the spectrum analyzer. As

good as modern spectrum analyzers are, accurate eighty decibel on-screen dynamic range may be a stretch

under certain conditions. We initially increased the input attenuation, which changed the ratio more than the

input level; this is the tipoff. Selecting a lower first mixer level helped some, but to get consistently accurate

harmonic data we needed an external tunable notch filter.

Filter insertion losses should be measured in the lab and tabulated for field reference. One final point is the

effect of various program material. Many stations are trending toward talk. This restricted bandwidth audio

often paints a rosier picture than music.

More repeatable results can be obtained using the USASI noise as recommended in NRSC-2. However, the

rules again are mute as to whether this program source is acceptable.

Building and using loop antennas - part II

We now examine design and construction of two loop antennas: a shielded, untuned receiving loop to be used

in the field with the spectrum analyzer, and an unshielded transmitting loop used to generate a frequency-

independent standard field, to calibrate the former. The technique described here is very similar to the way

NIST calibrates AM field intensity meters.

Shielded loop antennas respond primarily to the magnetic component of the RF field, and provide good

directionality in the form of a figure-eight with nulls at right angles to the plane of the loop. Compared to an

amplified (E-field) whip. Empirical tests prove the shielded loop to reject substantially more electrical noise.

The key benefits of the loop stand up well for AM emission testing. Minimal electrical noise pickup, with


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proper site selection an interfering signal can be nulled at least twenty decibels, and antenna factor is easily

generated using the method described below.

The loop is also an excellent general purpose pickup; positioned near an

AM tower, it provides a nice clean sample to a scope, spectrum analyzer,

or frequency counter- all of which are useful to have available when

"proofing." Normally the sensitivity of the loop increases directly with

the speed that the lines of flux cut across the windings; this translates into

a six decibel per octave increase, which is useful for harmonicmeasurements.

More sensitivity makes the lower level harmonic energy easier to resolve.

Actual measurements proved this to hold well until parasitic reactance

becomes a significant portion of the loop impedance. Our initial shielded

loop was constructed of 1.25 inch square aluminum tubing with a gap at

the top to prevent the shorted turn effect. Two turns of number eighteen

insulated wire were

wound inside, using

wooden collars to

maintain wallspacing. More turns

can be used, but the increased inductance and stray

capacitance will reduce the frequency at which antenna

factor anomalies begin. Often, for communications

purposes, the loop winding is resonated with a parallel

capacitor, greatly increasing the Q and the output.

Although this can be useful for improving the antenna

sensitivity and selectivity for harmonic relationships, it

can't be used for bandwidth testing because it's easy to

bandpass out the sideband energy you're searching for.

Although theoretically, balance will be adequate by using

shielding, we grounded the winding midpoint and used a broadband 1:1 transmission-line transformer for

enhancement. A good measure of balance is null depth; this loop averaged more than twenty decibels. The

standard field antenna consisted of a single turn supported by steatite bushings installed at the top of a

shielded meter and matching box. The loop is fed through resistors and a balun. The RF current must be

carefully metered as this must be held constant.

A reliable antenna factor can

be accurately generated using

the near-zone magnetic field of

a small constant current loop.NIST calibrates field-intensity

meters this way. Because the

loop size is very small

compared with the wavelength,

the antenna current remains

essentially constant throughout

the conductor, resulting in the

radiated near-zone magnetic

field being constant over the

decade .5 to five megahertz.


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This field does not follow the

inverse distance rule, falling off

rapidly beyond a couple of

meters. We spaced the loops

two meters apart center-

to-center, which proved to be a

good compromise between

field strength and sensitivity to

placement repeatability. The reference radiator was mounted one meter above ground on a wooden platform,and the shielded loop antenna on a tripod at the same height. Although this mini-antenna range setup is

considerably more tolerant to the presence of adjacent structures that VHF antenna testing, reflective objects

should be kept to a distance of at least three times the spacing between antennas.

We excited the reference radiator with one-hundred milliwatts of harmonic-free RF. At this distance our

FIM-41 meter indicated six millivolts per meter with co-planar (normal) alignment, and twenty-one when

aligned co-axially, the way NIST does it. According to the FIM, this field varied less than three percent over

the decade in question. With this standard field established, calibration of small aperture magnetic field

antennas properly positioned becomes pretty straightforward. If constructed exactly like our shielded loop, the

six decibel per octave rule will hold well, until slightly above two Megahertz, where the slope starts to taper.

When combined with both a modern spectrum analyzer and a notch filter, an accurately calibrated antennacompletes the package necessary to begin "proofing".

A tripod supported the loop antenna in the field, and the entire apparatus is guaranteed to generate interest

from passers-by who must surely believe that you're DF'ing for Nazi spies.

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am nrsc testing with the loop antenna (nrsc bandwidth)

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