The outside surface of the coax braid forms a conductor which is completely independent of the inner conductor and inside surface of the braid that are delivering differential-mode power to the dipole feedpoint.
The moment you feed the dipole with coax you have "inadvertently" connected a wire from one leg of the dipole back to the shack radio/tuner. If the length of the coax, its routing, and the way it is grounded at the shack results in it being a low impedance path, you can find most of the dipole current flowing that route rather than into the dipole leg. The chances of that happening increase if you use the dipole on several bands. The consequences can be increased RFI and increased noise pick-up on receive; it can also cause pattern distortion, but that's not likely to worry you too much on a dipole.
Just because some folk "get away" without a choke doesn't mean that you will! The conservative engineering approach is to isolate that third wire from the feedpoint by inserting an effective common-mode choke; the cost is probably peanuts compared to your investment in the rest of your equipment.
Balance conversion and impedance converting equally important!DE bd3ct
This might be a silly question but...
Do you need a balun to feed a vertical dipole with coax? By that I mean a 1/4 wave up and a 1/4 wave down from the center assuming you have enough height to get the bottom off the ground. Obviously more likely at VHF than at HF.
It intuitively feels right to connect the center conductor to the up wire and braid to the down because it looks like a ground plane with one radial. I'm guessing that you still need a balun just as much as if it's horizontal but I'm interested in hearing what others think. A real ground plane has multiple radials so is therefore unbalanced and doesn't need a balun whereas a vertical dipole is still balanced. Maybe it depends on how far off the ground it is.
It might help answer this sort of question if you keep separate in your mind the two modes of operation of the coax as I outlined earlier: the delivery of the differential-mode signal to the feedpoint (inner conductor/inside surface of the shield), and acting as an inadvertent radiating element (outer surface of the braid).
Originally Posted by KT1F
So ask yourself if you want to connect a radiating piece of wire between one leg of the vertical dipole and your radio: if you decide it's not a good idea, place a common-mode choke on the coax close to the feedpoint to stop conducted currents, and route the coax away from the dipole at right-angles to minimise induced currents; if you think an extra piece of radiating wire running from one dipole leg back to your radio is a good idea, do nothing!
Not that I know , learning also , for me trying to answer a question with what I think I've learned helps me to learn or correct what I think I know , so here we go .
I have trying to find simple ways to think of and discrib Baluns , also Ununs [ there are are voltage & current Baluns also ] they seem to break down to weather or not you need to transform impedance , or choke off unwanted RF [ it seems to me there are is 1 or 2 more ways to categorize , but this is what I have ? ] .
These all tools that are finite , or each having specific use , and there are many antenna designs some requiring one of the above or not .
As an example , in a horizontal dipole it has a specific impedance at a specific frequency , then you add the feed-line , it also has a specific impedance , then coax at least most of what hams use has a fixed impedance of 50 Ohms .
So depending on the individual antenna design [ what bands your wanting to work ] there may be mismatches between the different parts of the antenna system - dipole & feed-line , if the difference is large enough you need some type of matching between each component , back to the dipole lets say it has 450 Ohms and the feed-line coax 50 Ohms , so you need a 4:1 balun to make the transformation from 450 to 50 .
Or same antenna with 450 Ohm twin-lead to intrance of shack but you may want coax into shack - you need the same transformation to modern gear , 50 Ohms .
Then there are 1:1 choke baluns , because of the design of the antenna there is enough reflected power , so now you want to stop the reflected power from getting into the shack , so the need for a choke balun .
To me it seems that if you can , design the antenna to need the least amount of extra components - such as having resonant antenna for each band , so only the dipole and feed-line .
Once we end up having to make compromises , in comes the need for all the different types of baluns .
But my main point is knowing what each part of the antenna system measures and only adding what is needed .
Now for all the corrections from those that know better , so I can learn more
One problem with a vertical dipole is routing of the feedline. It needs to be at a 90° angle AWAY from the antenna, for at least 1/4 wavelength. As you say, that's relatively easy to accomplish at VHF and above, but can be a major issue at HF frequencies.
Originally Posted by KT1F
Some thing that occurred me a a few yrs. ago is , that to think of antennas like magnets , there is always two poles to a magnet , same with antennas [ or the energy that we are trying to manipulate with antennas ] there is always two poles weather we build two sides or not , the two poles are still there .
As an example , a mag-mount 2m or 440 , the element on top of the mag-mount is one pole , the car is the other side of the antenna , same with a ground mounted vertical , requiring ground radials or a counter-poise .
As for needing a balun or not , it depends on the design of the antenna system weather there is a imbalance - choke balun or a mismatch between antenna & feed-line requiring a impedance match - ratio-balun .
Originally Posted by KT1F
It is good engineering practice to install a BALUN at every BALanced to UNbalanced transition point in the antenna system. Given that, what flavor of BALUN depends upon the conditions at the BALanced to UNbalanced point.
Originally Posted by KD0CAC
There are two possible functions performed by a BALUN. First and foremost is the BALanced to UNbalanced transformation which satisfies the definition of a BALUN. Second is the impedance transforming function. Note that an impedance transforming function may be required at a junction where no BALanced to UNbalanced condition exists.
For instance, many hams ignore the impedance mismatch at the feedpoint of a ladder-line-fed dipole because both the dipole and ladder-line are balanced and therefore do not require a balun. Often, the losses due to the mismatch (high SWR) in that system are tolerable and sometimes lower than matched line coax. The BALUN function needed for such a system should be installed at the ladder-line/coax junction, wherever that may be. Conditions at that point will dictate what flavor of BALUN will best fit the application. A husky 1:1 current-choke-balun seems to be slightly preferable for those kind of multi-band antennas.
73, Cecil, www.w5dxp.com
The maximum power transfer theorem works just as well for a non-resonant antenna as it does for a resonant antenna.