Driving a Balanced Antenna with an Unbalanced Coax - Why A Balun Is A Good Idea

In a recent conversation about the need for a balun on a dipole fed with coax, the comment was made that no balun is needed because the dipole radiates in a balanced manner.

Well, maybe...

If a dipole is positioned above uniform ground and there are no trees, hills, fences, buildings, or powerline poles, then the statement above might be.

However, the real world has trees, has hills, fences, wet and uneven ground, and powerline poles. It would be rare to have one side of a dipole driving the same free-space radiation impedance as the other side. If there is an unbalance, then the outer shield of the coax will radiate RF to make up the difference. An easy way to think of it is to imagine that one side of the dipole radiates 10 amps of RF and the other side radiates 9 amps due to the imbalance. The difference of 1 amp has to get made up. It gets radiated from the outside of the shield of the coax. Stated differently, the coax center conductor carries 10 amps. The inner shield carries 9 amps and the outer shield carries 1 amp.

An extreme case of this condition would be the feeding of an off-center fed dipole. Clearly, such a design is very much out of balance. A good example of this antenna as well as the method of feeding it is found at [7]. I have used this type of antenna many times in the field at campgrounds. The advantage is that the "long end" can be shot up over a tall pine tree and the short end tied off near the ground. The feed point is, then, relatively close to the operating position.

RF on the outside of the coax will radiate and may radiate RF inside the shack.

My favored method for driving a dipole is to feed it with coax and have a balun at the antenna feed point.[1][2]

Properly constructed, the balun will force all of the current coming out of the center conductor to equal the current returning on the inside of the shield. Common mode current is reduced to nearly zero.

This post prompts an idea for a project - a longitudinal RF ammeter. Feedline imbalance can be detected by measuring the current in both legs of the feedline. If there is a balance, the currents will be equal. An easier way is to construct an RF ammeter to directly measure the imbalanced current. Such a device is depicted in [7, 8].


REFERENCES:

1. "Baluns: What They Do and How They Do It,", Roy W Lewallen, W7EL, http://www.eznec.com/Amateur/Articles/Baluns.pdf
2. "Feeding a dipole using a balun", https://en.wikipedia.org/wiki/Dipole_antenna#Feeding_a_dipole_using_a_balun
3. "A Cost Effective Current-mode 1:1 Balun", http://www.arising.com.au/people/Holland/Ralph/CMBalun.htm
4. RF Current Meter http://vk6ysf.com/RF_Current_Meter.htm
5. "A Transmission Line Balance Test Meter, "Lloyd Butler VK5BR", http://users.tpg.com.au/users/ldbutler/Line_Bal_Test_Meter.htm
6. "Measurement of relativity between the Longitudinal and the Differential currents in the Transmission Line", Lloyd Butler VK5BR, http://users.tpg.com.au/users/ldbutler/Line_Diff_Long_Meter.htm
7. "Array Solutions OCF Dipoles", "", https://www.arraysolutions.com/Products/ocf_dipole.htm
8. "Study of the Off Centre Fed Dipole Antenna", "G8ODE", https://rsars.files.wordpress.com/2013/01/study-of-the-ocf-dipole-antenna-g8ode-iss-1-31.pdf