Frequently Asked Questions
For our AM Broadcasting Customers
- How are these antennas typically used?
- What technical documentation is provided with an antenna?
- What options are available for the antennas?
- What type of tuning unit is required?
- Do the antennas require lightning protection?
- What type of ground screen is required?
- What is a Valcosphere?
- What is the life expectancy of these antennas?
- What is the polarization of the antennas?
- What is an azimuth plane?
- What shape is the radiation pattern?
- What is the purpose of 'coil-loading'?
- What is the efficiency of the antennas?
- What is the expected coverage range?
- What is the maximum power handling capability?
- What types of signals can these antennas accommodate?
- What is FSK?
- Can Valcom antennas be used in a phased array?
- What if a fence is needed?
- How would wind and ice affect the antenna?
How are these antennas typically used?
Standard non-loaded Valcom whip antennas are used for both transmitting and receiving applications. For receiving, the antenna may be connected to the receiver directly or through a broadband matching transformer for improved performance over extremely wide bandwidth. For transmitting, the antenna must be used with some type of tuning unit to allow matching of the antenna's input impedance to that of the transmitter at the frequency of operation. Although intended for operation over the frequency ranges indicated in the respective brochures, the actual range of frequencies over which any particular antenna may be tuned depends on the capabilities of the tuning unit used. Certain Valcom antennas are custom manufactured with an integral loading coil. The purpose of the loading coil is to produce an antenna with a self-resonant frequency which is a few kilohertz above the intended operating frequency. An antenna tuning unit which contains a series inductance is connected to the antenna feed point to reduce the antenna resonant frequency to the operating frequency. The tuning unit also transforms the resulting antenna input impedance to the nominal transmitter impedance. Although each coil loaded antenna is manufactured substantially for operation at one frequency only, some frequency adjustment is usually possible depending on the capabilities of the tuning unit. Coil loaded antennas are usually used for transmitting applications but may also be used for receiving.
What technical documentation is provided with an antenna?
All Valcom antennas are provided with a technical manual; which includes the technical specifications, assembly instructions, installation and maintenance procedures for the antenna as well as instructions for any optional accessories ordered.
What options are available for the antennas?
Most antennas are available with either a side or feed-through connection point. Other available options include hinged mounting plates, hinged and non-hinged mounting towers in one foot increments up to eight feet high, gin poles for erecting antennas using a hinged mounting structure, ground screens, lightning arresters, standard and custom impedance matching transformers and top hats (for special wider bandwidth applications).
What type of tuning unit is required?
Valcom can recommend and supply appropriate tuning units for use with our antennas. Complete information on the nature of the application is required such as operating frequency, transmission power, signal type and modulation, etc.
Do the antennas require lighting protection?
It is not advisable to use an external lightning rod system for lightning protection of a Valcom antenna. External structures may adversely affect the operation of the antenna by upsetting the input impedance and absorbing transmitted power which will alter the omnidirectional radiation pattern.
If lightning and high static voltage protection is desired, lightning arresters are available as options for most Valcom antennas. An arrester consists of an adjustable metal mount and electrode which form a precision air gap between the antenna feed point connection and ground. Properly adjusted, it affords lightning and high voltage protection for both the antenna and any connected equipment. The high conductivity conductors used in the antenna itself provide a low resistance path down the antenna to the lightning arrester and reduce the chance of damage to the antenna if it does encounter a direct hit. The ground path is through one of the antenna mounting bolts to the tower or hinge plate to the ground system. The use of a spherically shaped Valcosphere at the top of some antenna models also helps reduce the likelihood of large field build-ups around the tip.
What type of ground screen is required?
For all but very specialized applications, Valcom recommends the VGS-3 6100 Ground Screen. It consists of 36 radials of #10 copper wire each 100 feet long and terminated with a 5 foot grounding rod. The radials are joined to a central ring made up of four quadrant sections which are assembled on site to encircle the base mounting structure of the antenna. The connection to the tuning unit or transmission line is made via four heavy copper wires. The radials are normally buried approximately 12 inches deep. A good quality ground screen is strongly recommended for efficient operation of the antenna. It reduces power losses in the earth near the antenna where current concentrations are the highest. It also provides a convenient connection means for the tuning unit which is usually located at the base of the antenna.
What is a Valcosphere?
A Valcosphere is a spherical terminating top structure used with certain Valcom antennas. Although it provides a modest improvement in radiation efficiency, it is generally employed to extend the effective bandwidth of the antenna around the central operating frequency. In addition, the Valcosphere helps to alleviate the high static charge build-up which can occur around the tip of an antenna during adverse weather conditions.
What is the life expectancy of these antennas?
Valcom antennas, if properly installed and maintained are expected to provide a useful service life of 20 years. They are fully warrantied against defects in materials and workmanship for a period of 10 years from the date of manufacture.
What is the polarization of the antennas?
Radio waves are made up of two components: an electric field and a magnetic field. The polarization of the antenna depends solely on the direction in which the electric field (E-lines) propagates. When an antenna is mounted vertically, the E-lines will be strongest perpendicular to the axis, and will propagate out parallel to the antenna and perpendicular to the earth's surface. This is called vertical polarization. When an antenna is mounted horizontally, the opposite is true; E-lines will propagate out perpendicular to the antenna and parallel to the earth's surface. This is called horizontal polarization. All Valcom whip antennas are mounted vertically and thus exhibit vertical polarization in the azimuth plane.
What is an azimuth plane?
Azimuth is defined as the horizontal pointing of the antenna. The azimuth angle is usually measured in degrees clockwise from a reference point, commonly taken as true north. A plane is created from rotating the azimuth angle a complete 360~ revolution about its reference point. The azimuth angle can be seen in Figure 1 as the angle 4. The azimuth plane in this case coincides with the x-y plane.
What shape is the radiation pattern?
Free standing whip antennas exhibit an omnidirectional radiation pattern in the azimuth plane which is typical of vertical monopole antennas. The field is strongest at low elevation angles and diminishes with higher angles to become zero straight up. Any resulting disturbances in the uniformity of the field are usually due to factors outside the antenna system such as the proximity and construction materials of buildings and other structures, ground connections to other site equipment and geographic land features.
In general, the exact shape of the radiation pattern profile in the elevation plane is complex and will depend on the particular antenna, the operating frequency and the surface condition over which the antenna is operated. These factors will determine the antenna electrical height in wavelengths and the ground plane reflection coefficient upon which the shape of the radiation pattern depends. Antenna and radio textbooks and handbooks provide general graphs of radiation pattern profiles for vertical monopole antennas of various heights and frequencies, usually over a selection of surface conditions (i.e. perfectly conducting ground, water, typical earth, etc.).
One characteristic which is often of interest is the elevation angle associated with the antenna radiation pattern. The elevation angle is the angle at which maximum radiation occurs with respect to the ground plane and can be seen in Figure 1 as the angle c~. For coil loaded antennas, this angle is generally low, between 5 and 30 degrees from the ground and is dependant on the factors mentioned above. Contact Valcom for calculations of the theoretical radiation pattern for any particular application.
What is the purpose of 'coil-loading'?
When looking at the wavelengths for low frequencies (540 kHz to 1700 kHz), the 1/4 λ lengths would be 492 feet and 144 feet m respectively. These are not very practical lengths for an antenna so we can add a loading coil to the antenna to change its electrical height and make it appear longer than it actually is. For example the 492 foot antenna for 540 kHz mentioned above can be made to be only 49 feet high with the proper loading coil. The coil effectively counteracts the capacitive part of the antenna's input impedance which makes it look like a resonant circuit and allows it to absorb 100% of the incident power.
What is the efficiency of the antenna?
The overall antenna system efficiency and gain are related and depend on the antenna's radiation resistance as well as on the other losses that are present within the entire antenna system. Antenna radiation resistance is derived from the height of the antenna in wavelengths which depends on the particular antenna and operating frequency. For coil loaded antennas and antennas operating at frequencies which make them longer than 1/8X, the determination of the radiation resistance for each particular antenna can be quite complicated. Antenna design handbooks may be consulted for the necessary background theory.
When operated between 500 and 1700 kHz, coil loaded antennas exhibit efficiencies ranging from approximately 18% to 65% for the V33070-CL2 and 5% to 50% for the V147-CL2. Coil loaded antennas are preferred at lower frequencies as they help to avoid the use of excessively high voltage production at the output of the tuning unit and at the antenna feed point.
The magnitude of the other losses in the antenna system depends on the ground system employed, the conductivity characteristics of the earth beneath the antenna, the frequency of operation and the tuning unit used. Combined power losses in the tuning unit and ground in the vicinity of the antenna can range from less than 1 dB to over 3 dB (half of the input power!) depending on the specific installation site. Any losses attributable to these factors must be considered along with the antenna efficiency to calculate the actual overall system efficiency.
The use of a ground screen offers a great improvement in efficiency by reducing the losses in the earth resulting from the high concentration of currents flowing in the vicinity of the base of the antenna. This is particularly important at low frequencies. At the frequencies generally used with Valcom whip antennas a ground screen will not have a dramatic effect on the radiation pattern's shape. However, it will significantly increase the antenna system efficiency over a ground system which simply uses a ground rod connection and no ground radials. The use of a Valcosphere will also offer a modest improvement in efficiency but its main use is to increase the antenna's effective bandwidth. Antenna and radio textbooks and handbooks provide graphs of radiation pattern profiles for vertical monopole antennas of various heights over a range of frequencies, usually over a selection of surface conditions (i.e. perfectly conducting ground, water, typical earth, etc.).
What is the expected coverage range?
The coverage range attainable with any particular antenna depends on several factors. The output power of the transmitter is a fundamental starting point in determining coverage along with the tuning unit efficiency and losses in the transmission line between the transmitter and tuning unit. The particular antenna being used is also very important since the antenna's height and degree of coil loading (for coil loaded antennas) determines the antenna system efficiency and gain. The presence of a proper ground screen and ground conductivity characteristics also have a significant bearing on the coverage range. For coil loaded antennas operating at low frequencies (100 kHz to 2 MHz) where they are considered electrically short, the main interest is usually in the strength of the radiation along the surface of the earth in the form of the ground wave. This field strength depends on the height of the particular antenna, the frequency, the transmitting power and the overall efficiency of the transmitting system including the transmission line. Once these parameters are known the expected coverage prediction calculations for a specific application can be performed.
What is the maximum power handling capability?
The maximum power rating for each antenna is indicated in its respective brochure. The values given are for continuous average power application. Unusual modulation or special applications such as simultaneous multi-frequency operation may also be accommodated.
What types of signals can these antennas accommodate?
All Valcom whip antennas accommodate continuous CW (continuous or carrier wave) signals. The compatibility of Valcom antennas with various signal and modulation types is mostly restricted to the bandwidth capabilities imposed by the tuning unit design. For coil loaded antennas the bandwidth is frequency dependant because the operating frequency determines the degree of coil loading necessary in the antenna. Coil loaded antennas can readily accommodate CW, narrow band AM such as non-directional beacons and FSK for data transmission. The integral loading coil greatly reduces the voltage that appears at antenna feed point at low operating frequencies. For wider band applications such as broadcast AM a non-coil loaded antenna may be used for compatibility with the AM modulation signal.
What is FSK?
FSK stands for Frequency Shift Keying which is a simple form of digital modulation for transmitting data. For example, binary FSK is modulating a series of binary ones and zeroes to a set carrier frequency. The amplitude of the data signal remains constant but the frequency and phase shift will vary with the input. That is, for a binary one, the output frequency will be a certain value added to the carrier frequency and for a binary zero, the output frequency will be a certain value subtracted from the carrier and of opposite phase.
Can Valcom antennas be used in a phased array?
Since all Valcom whip antennas exhibit an omnidirectional radiation pattern, they are well suited for a phased array application. A phased array consists of multiple antennas connected together to act as a single antenna which allows the beam width and the directivity of the array to be controlled using electronic methods. That is, the antennas are not physically moved around, their directions are changed by controlling the various phase shifts introduced in the array by the other antennas or by phase shift circuits (see Figure 2). Because of this, an ideal antenna would have an omnidirectional characteristic so that it would have full signal strength no matter which direction it was 'facing'.
What if a fence is needed?
If a fence is required in the vicinity of the antenna, a wooden fence is recommended over a steel wire fence.
How would wind and ice affect the antenna?
Valcom antennas have been tested for ice and wind loading due to adverse weather conditions. Refer to the individual brochures for the capabilities of each antenna.