Digital Modes Meanderings
The Antenna System Components
The antenna is called a “vertical doublet” as opposed to a “vertical dipole”. The difference being that the word “dipole” usually conjures up the thought that the antenna is resonant at one frequency that usually exhibits a near 50 ohm feed point impedance. That operating parameter is not descriptive of the “vertical doublet”. Like its counterpart the “horizontal doublet”, it will be operating at just about anything but a 50 ohm feed point impedance. The impedance may be smaller or larger than 50 ohms but most likely never exactly 50 ohms except by sheer chance. Luckily we don’t need it to be 50 ohms because the feed line also will not be 50 ohms. Consequently, due to the antenna never being self-resonate at the operating frequency nor impedance matched with the feed line, there will always be some value of reactance that the system will need to counteract or negate.
The feed line used in the antenna system is of the parallel wire type which includes in its family such names as “ladder line”, “window line”, “twin lead”, and “open wire line (OWL)”. These types are all similar in general operating parameters and principles. The vertical doublet is fed with open wire line (OWL) which distinguishes itself from window line and ladder line by having air as the primary dielectric between the wire conductors. This is the lowest loss type of feed line that most amateurs can readily acquire. Unfortunately it is not usually found ready-made. Fortunately it is easily constructed and costs only tens of cents per foot. The exact cost depending on the materials used.
As was pointed out above, there is going to always be some value of reactance in the antenna system that will need to be tuned out. The device used to accomplish this is an antenna tuner or matchbox. In this type of antenna system a matching device is used to literally tune the antenna system to resonance, thereby tuning out the reactance in the entire system and matching the system impedance to the 50 ohms required by the transceiver. This is traditionally called a tuned feeder system. The antenna tuner is a required component of the antenna system and should therefore be included in the design of the system.
The feed line used in the antenna system is of the parallel wire type which includes in its family such names as “ladder line”, “window line”, “twin lead”, and “open wire line (OWL)”. These types are all similar in general operating parameters and principles. The vertical doublet is fed with open wire line (OWL) which distinguishes itself from window line and ladder line by having air as the primary dielectric between the wire conductors. This is the lowest loss type of feed line that most amateurs can readily acquire. Unfortunately it is not usually found ready-made. Fortunately it is easily constructed and costs only tens of cents per foot. The exact cost depending on the materials used.
As was pointed out above, there is going to always be some value of reactance in the antenna system that will need to be tuned out. The device used to accomplish this is an antenna tuner or matchbox. In this type of antenna system a matching device is used to literally tune the antenna system to resonance, thereby tuning out the reactance in the entire system and matching the system impedance to the 50 ohms required by the transceiver. This is traditionally called a tuned feeder system. The antenna tuner is a required component of the antenna system and should therefore be included in the design of the system.