I'm reading lots about center loaded, or top loaded antennas (or base). I've always assumed that the load was a coil of wire. I have not read "yet" what the load is, though. Does it only help to reduce the overall length of the antenna, or is there some other reason.
Thanks for your time.
The loading coil actually allows for a shorter antenna length to be used.
Think of the rope tied to a tree simily again, mentioned just a few threads earlier. If you have it so you have just one high, or one low (as is it was before you started shaking it up and down, or in other words a single point of amplitude)- this represents one half of a cycle, or the point of resonance with a half wave antenna. With a half wave antenna at resonance, the nodes at the ends have zero potential (voltage), with the maximum amplitude at midpoint of that half cycle.
If you now double your frequency of shaking the rope, you now have one high, and one low point, which now represents one full cycle, or in other words- a full wave antenna.
Now, if you want maximum eficiency for transmission, or reception, the full wave antenna is best- but not always able to be achieved; and some designs using quarter and half wave resonance can be as ultimately effective; but not always practical if you are trying to broadcast on 80 meters, 160 meters, or 400 meters (the lowest range of the AM broadcasting band.). That loading coil makes a short antenna look longer (more load), and can allow one single antenna to be tuned to any resonance within certain limitations. Half and quarter wave antenna designs are also very effective, and can be made shorter with the use of a loading coil. The loading coil changes the resonance of the short antenna that may not even be resonant at the frequency you want to use it. CB for example, is defined as "11 meters"- which is a bit over 34 feet; which is not practical for mobile service. At 1/4 wavelength, the antenna is still efectively longer than 8 feet, and some were manufactured that way.
But to get the same performance in a much smaller antenna, the loading coil allows that 40 or so inch antenna to look like the 8 plus foot antenna to the radio.
The point of resonance in a circuit, be it RC, LC, or a crystal; is the point where there is maximum reactance of the circuit (maximum potential[voltage] across the circuit), or highest measurable potential at 1/4 and 3/4 wavelength of the target frequency when tuning an antenna. The antenna forms an RC circuit in relation to the earth.
When the antenna is out of resonance, you create standing waves which add, and cancel to the primary broadcast signal, and the standing waves actually start radiating power (signal) into the coaxial cable, and also the final amplifier stage of the radio.
In ideal circumstances (theoretical), the length of the coax is a full wavelength, or half wavelength in total length, and the antenna is a half wavelength, of full wavelength (quarter wavelength will also work, but the theoretical ideal for best performance in general terms leaves any end points, or terminations at zero potential). Something that can be done, but not always easily.
There is also the issue of balanced, or unbalanced loads and antenna designs, which require a balun as a transition (a form of RF impedance matching transformer), or other "balancing" technique. In a nutshell, almost anythng using coax for transmission, or reception; is using an unbalanced system. (one side is ground referenced and acts as a shield. A balanced antenna system is one that uses a differential between two signal feeds, such as a dipole (TV rabbit ears are one example), and these use a ladder line, or a 300 ohm (sometimes 600 ohm, or even 120 ohm)twin lead. A center fed longwire antenna is an example of a dipole, and with loading coils, can have multiple resonances within some significant limitations.
Unless you are seriously into shortwave listening, or are trying to optimize an antenna design, or are planning on investing in Ham equipment, and do not want to burn it out right away, the ARRL (www.arrl.org) publishes an antenna design handbook, as well as a general manual with a good and thorough description suitable for a vocational school text of electronic theory, and application, that covers even in greater depth than what I have typed here. With a good coverage of basic theory of antennas, and antenna design.
: Man there's a LOT of threads!
: I'm reading lots about center loaded, or top loaded antennas (or base). I've always assumed that the load was a coil of wire. I have not read "yet" what the load is, though. Does it only help to reduce the overall length of the antenna, or is there some other reason.
: Thanks for your time.