Amateur Radio Stack Exchange is a question and answer site for amateur radio enthusiasts. It only takes a minute to sign up.
Sign up to join this community
Suppose I have a coax-fed half-wave dipole antenna. The coax shield goes to one side of the antenna, the center conductor to the other side. The transceiver's chassis is connected (via the coax connector) to both one side of the dipole and to Earth ground via the chassis to Earth ground wire.
I'm confused by having one-half of the dipole tied to the RF transmitter output and to Earth ground. Can anyone explain the electronics behind what is going on?
Welcome to the world of RF (radio frequency) and AC power, where the obvious rules of DC power don't work.
In RF electronics, just because two wires are DC shorted doesn't mean they are RF shorted. In RF, a transmission line (like coax) can be shorted at one end and measure as RF open at the other end if it is an odd multiple of quarter wavelengths between the shorted end and the end you are measuring. Even more confusing, change the shorted end to open and you can measure a short at the other end.
So just because you have the shield connected to "ground" in one or more places doesn't mean the antenna is shorted. The ground can be used to absorb (or induce!) common mode current. If the coax is not symmetrical with respect to the dipole near the feedpoint, it will couple with the dipole and encourage common mode currents. A balun or unun in the right place(s) can reduce this. The simplest version of a choke balun would be a coil of a few loops (typically at least 5) of coax with the loops tightly bound to each other.
In transmission line like coax, currents are suppose to oppose, so that their magnetic fields cancel and the wire doesn't leak RF. Common mode current (where some portion of the currents flow the same direction in both sides of the coax) cause the coax to leak and radiate.
RE (from the OP): I'm confused by having one-half of the dipole tied to the RF transmitter output and to Earth ground. Can anyone explain the electronics behind what is going on?
Just to note that all antenna systems are two-terminal devices — no r-f energy will flow on one "leg" of a dipole without an equal amount of r-f energy flowing on the other leg.
A coaxial cable carries the same amount of r-f energy on the OD of its inner conductor as it carries on the ID of its outer conductor. The r-f current on the outer conductor is confined to its inner diameter due to "skin effect," that isolates the two surfaces of the outer conductor along its length.
At each end of the coax, a path exists between the inner and outer surfaces of the coax outer conductor. So a portion of the r-f energy on the ID of the outer conductor can appear on, and propagate along the outer surface of its outer conductor (or "shield").
This phenomenon occurs whether or not the coax shield has either a direct or an indirect conducting path to a true r-f ground reference such as the earth. IOW, the coax shield does not have the same characteristics for the flow of a-c/r-f current that it has for direct current.
R-F current flowing along a conductor exposed to free space will produce e-m radiation, just as it does when it flows along the two legs of a dipole.
The part of the dipole that is connected to the shield is NOT grounded at RF.
There are common-mode RF signals on the outside of the coax caused by the junction of the unbalanced coax and the balanced dipole. This is why we use choke baluns at the feedpoint: to force all the power present on the center conductor and the inside of the shield onto the dipole.
The same is true for received signals. Without a proper balun, the outside of the coax can pick up unwanted noise.
As others have pointed out in their answers there's a distinction between RF and DC which makes this use of coax still an effective feed line for a dipole antenna. There's going to be plenty of people that consider this bad form as it is feeding a balanced antenna with an unbalanced line. Even so I checked my ARRL Antenna Book and they show a dipole being fed directly with 50 and 75 ohm coax, but they do point out that using a 1:1 balun might be a good idea.
Why this works can be explained by many different theories. One theory I'd use is to point out that the coax is a waveguide and grounding the coax at the source isn't going to have an effect on the RF being transmitted down what is the path of least resistance. A copper strap to ground is not in fact the path of least resistance because the matched load of the dipole is going to have the lower SWR for the frequencies of concern, assuming the dipole is cut correctly.
In a coax-fed dipole, the coax core is connected to one element of the dipole and the braid to the other.
At the rig, the coax core is connected to the hot end of its output and the braid to the chassis, which is the common or cold end.
Incidentally, the chassis is connected to shack earth which is bonded to mains safety earth. This serves 2 purposes - as safety earthing for all equipment in the shack and as a means to discharge static charges on the antenna (more on this later).
The dipole antenna is a balanced load connected to the unbalanced output of the rig through unbalanced coax cable. The result of this imbalance is common mode current flowing on the outer surface of the coax braid, making it a radiating part of the antenna system, which is undesirable. The solution is a 1:1 voltage balun, at the feed point of the dipole, which eliminates common mode current.
Without the balun, the coax braid would have provided a DC path to ground only for the dipole element to which it was connected. With the balun, both the dipole elements have a DC path to ground and static charges are thus effectively dissipated.
