One of the key components of a directional antenna system, involving a receiving or transmitting aerial device that converts electric power into radio waves and vice versa, is the phasing equipment (aka the phasor cabinet). This system is highly used in directional AM Stations and in the medium wave (AM) frequency band, as it allows the strengthening of the signal in a particular direction and reduces the interference to stations located in other directions that are not desired. It’s possible, in fact, to set the degree of directivity of the antenna’s radiation pattern, which is controlled by the amplitude and phase weights of each individual element.
To assure maximum accuracy and stability, the phasing equipment is used. The phasor manages the power that is sent to each antenna tower in a system. The phasor cabinet holds tuning coils, vacuum/cast mica capacitors, inductors, RF contactors and variable junctions of circuitry connected to the other components; it is all housed in the cabinet where someone can do most of the adjustments (on the front panel’s control knobs) to increase the coverage area or achieve the best antenna gain (beamwidth) and path, or nullify interference.
A phasor can even help fix small transmission problems. It is not uncommon to find a slight change in directivity or performance of antennas. A simple fix, however, can be applied by controlling the cabinet’s knobs. Through this radio component, the phase and power going to each antenna can be adjusted.
On the face of the phasor cabinet, it is possible to check base current levels without the need to open the cabinet doors or remove its panels, as there are indicators showing the mode of operation. However, a tech may need to use the cabinets’ rear doors for easy parts access, to have access to all housed components. At times, an engineer may need check the equipment inside of the cabinet, when needing to inspect the system after encountering a system fault. From the inside, the person is able to insert a measuring instrument, for example, to troubleshoot and fix what may not be working properly.
A good example of the phasor not working properly is when the owner’s antenna monitor readings changes dramatically or there is absence of the shorting bar (J-plug) which may have fallen out of its socket/jack. Every now and then the cabinet requires a radio engineer to discover the problem(s) and ensure the phasor is being fed by the transmitter(s).
Aside from shifting the phase or increasing the amplitude of the signal, for instance, in order to get the needed direction from the antenna system, the phasor unit is crucial to functioning of other devices too, such as the antenna tuning unit (ATU), line-terminating unit (LTU) and antenna coupling unit (ACU), whose feed lines are also correlated to each tower. To get enough signal strength and the right phasing and matching of the array parameters, it is paramount to have each unit working aside the other to gain the desired directional pattern and achieve wanted measurements.
In sum, the phasor, its matching and power dividing equipment for directional AM operation, housed in the cabinet, is purposely used as a phase unit controller which ensures power is being applied to each antenna. It is commonly used as a monitoring system to check pattern changes, of which can be customized to meet the needs of the user.
Common features of a good cabinet include:
• Easy access from front and rear panel via hinged doors
• The ability to check current levels without even opening the front doors
• Digital front controls that include cranks or aluminum knobs for precise adjustments
• Dual AC receptacle in each bay
• Clear identification for each components through engraved labels
• J-plugs positioned for ease of access, including adjacent bridge grounding posts
• Interior panels that divide bays, so that RF isolation can be ensured to day and night networks
• A variety of fixed and variable capacitors
• An input current meter that allows for remote output
Fabrication and placement: Directional antenna phasing units are typically constructed in non-corrosive aluminum weatherproof housings (when placed outside) or placed in metal cabinets (when used indoor) or on support stands; they can also have an open panel and shelf ATU type for wall mounting.
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