The AM band changes dramatically between daylight and dark.
Do field strength measurements turn out differently depending on the time of day?
The AM band changes dramatically between daylight and dark.
Do field strength measurements turn out differently depending on the time of day?
Field Strength for AM BCB
When measuring the base impedance of an AM antenna system, tests can only be made during the relative absence of sky wave energy which could be additive to the detected signal. Sky-wave can be stronger than the output of the signal generator on the design frequency, especially at night. In short, it screws up the accuracy of the measurement done within the signal detector.
The same would be true of field strength measurements done at night. When setting up the pattern parameters of a directional antenna system, measured field strength is used to determine whether the system is set correctly for the proper pattern shape and magnitude. The settings involve phasing of the active and passive antennas and the current level fed to each antenna in the system (2 or more). Nighttime sky-wave does not allow for accurate readings unless the sky-wave energy can be adequately attenuated and filtered out. Even adjacent channel interference can screw up the measurements.
Field strength measurements, to be the most accurate, should be done during sky-wave minimus (plus or minus midday). I have always been somewhat doubtful as to field strength measurements accuracy done on the AM band in the presence of a strong sky-wave signal. Just something to think about. (editors note: The FCC inspectors are aware of this, so should the operator.)
As a broadcast engineer, I have done all of these measurements throughout the years. They are not done, accurately, without some challenges even with the best of equipment. Field strength measurement is not a “slam dunk”.
Isn’t this the reason why FS
Isn’t this the reason why FS measurements are made at a fixed (close in) distance from the transmitter?
“Close In” Field Measurements
The answer to your question is a qualified yes. The qualification being that at 10 kilometers or more from the antenna system, sky wave becomes a significant issue. During AM broadcast directional antenna proofs, measurements at greater distances than 10 kilometers are often required on a variety of cardinal bearings. Plus during program testing, prior to licensing, these stations are required to operate at a significant reduction in power to ensure no interference with other licensed stations. When sky wave is too strong, directional loop receiving antennas, bandwidth filters, notch filters and signal attenuators can come into play in order to get an accurate measurement. In short, the weaker the fundamental signal, the tougher the measurement. Even though the rules call for measurements at specific distances, most fixed locations are an estimate (in reality) of the distance from the radiator. A difference of a couple hundred feet at 5 kilometers, however, shouldn’t change the field strength measurements very much.
The reason, for engineering purposes, for several measurements at specific distances on a specific compass bearings is to determine the shape of the vertical wave off of the antenna. Ground systems and ground conductivity have a great deal to do with the “angle of departure” of the transmitted signal at 90-degrees from the vertical antenna. This is sometimes described by the term “Pseudo-Barlow Effect”. Most of this information is available in the “ARRL Antenna Book”, published periodically.
As a side note: using a good shortwave receiver, tune across the broadcast band in late afternoon and try to pull out a single signal on busy Class “C” and “D” AM frequencies. Now imagine trying to measure field strength of just one signal in the middle of the mayhem. And when you are measuring the signal of a Part 15 station using a certified transmitter and antenna system, how close do you have to be to assure that the measurements don’t contain some sky wave component? Several “background signals” can add significant error to the outcome of the measurements.
Optics and RF
This is sometimes described by the term “Pseudo-Barlow Effect”
Is this similar to the pseudo-Brewster effect?
Neil
Duh….
Right on Neil. Yes. Memory is the 2nd thing to go. LOL. Thanks for the reminder.
The Barlow Effect is when the
The Barlow Effect is when the local enforcement chief Ralph Barlow busts pirates. 🙂