Balancing, local QRM and closed loop antennas
When having local QRM, it is very important to avoid any coupling between the antenna and the feeding coax or open line feeder. You donít want feeders to be a part of your antenna system. On their way to the shack and in the shack, they can pick up all kind of noise sources. So your receiving antenna needs to be perfectly balanced.
Closed loop antennas, like the quad, folded dipole and delta loop, are said to be quieter in a noisy environment than the dipole antenna.
If this is true, is balancing making the difference? How much noise power picked up by the feeder is converted into differential mode power and becomes a signal at the receiver input for the same level of unbalance?
Let me give you my conclusions
1) There is no reason to believe that closed loop antennas are quieter than a half wave dipole when the antennas are well balanced.
Well balanced means appropriate baluns at appropriate places on the feeder and/or appropriate feeder length with appropriate termination. The common mode impedance of the antenna defines what is appropriate.
See also: Balancing antennas
2) When antennas are not well balanced, then closed loop antennas can be (up to 13-18dB) quieter because of the self-balancing property.
The half wave dipole shows almost no self-balancing compared to the closed loop antennas.
See also: Self balancing examples
Closed loop antennas with a coaxial feeder can function well without a balun, but extra balancing still can improve reception.
Comparing antennas in real life is very difficult, because it is not easy to know the measure of balancing. The length and termination of the feeder for instance has also to be taken into account. When using the same feeder, it can be worst case for one antenna and best case for the other. And of course the radiation pattern, polarization, etc., etc., can be different.
A ground-plane antenna with elevated radials is not well balanced. It also needs a balun and/or hf-grounding the coax shield at the feed point.
Balancing the antenna tuner, when using an open line feeder, is equally important!
Better balancing does not necessarily reduce the unintended electromagnetic coupling between the antenna and the feed line.
The feed line also behaves as an antenna and can (re-)radiate the noise picked up in the shack as a common mode current.
The best way in reducing this coupling is by keeping the feed line away from the antenna and/or burying the feed line in the ground.
Last update: April 28, 2012