Extra Class Question of the Day: Feedpoint impedance, antenna efficiency, frequency range

One of the most basic antenna parameters is the feedpoint impedance. Why would one need to know the feed point impedance of an antenna? To match impedances in order to minimize standing wave ratio on the transmission line. (E9A04) The reason that it’s important to minimize the standing wave ratio, or SWR, is that if you’re using coaxial cables, minimizing the SWR will also help you minimize losses. If you minimize losses, you’ll radiate more signal.

Many factors may affect the feed point impedance of an antenna, including antenna height, conductor length/diameter ratio and location of nearby conductive objects. E9A05 For example, we say that the feedpoint impedance of a half-wavelength, dipole antenna is 72 ?, but that’s only really true if the antenna is in free space. When it’s closer to the ground than a quarter wavelength, then the impedance will be different. That’s why you have to tune the antenna when you install it.

Another antenna parameter that’s frequently discussed is radiation resistance. The radiation resistance of an antenna is the value of a resistance that would dissipate the same amount of power as that radiated from an antenna. (E9A15) Radiation resistance plus ohmic resistance is included in the total resistance of an antenna system. (E9A06)

If you know the radiation resistance and the ohmic resistance of an antenna, you can calculate its efficiency. You calculate antenna efficiency with the formula (radiation resistance / total resistance) x 100%. (E9A10)

Vertical antennas are sometimes criticized as being inefficient antennas. Soil conductivity is one factor that determines ground losses for a ground-mounted vertical antenna operating in the 3-30 MHz range. (E9A12) If soil conductivity is poor, ohmic resistance will be high. One way to improve the efficiency of a ground-mounted quarter-wave vertical antenna is to install a good radial system. (E9A11)

The frequency range over which an antenna satisfies a performance requirement is called antenna bandwidth. (E9A09) Normally, the performance requirement is an SWR of 2:1 or less. In fact, you’ll sometimes hear this parameter referred to as the 2:1 SWR bandwidth.

Finally, this section has a question that really doesn’t fit in here about folded dipoles. A folded dipole antenna is a dipole constructed from one wavelength of wire forming a very thin loop. (E9A07)

Comments

  1. Dan KB6NU says:

    Here’s something interesting about the loop antenna and folded dipole antenna. If you read my blog, you’ll remember that I recently put up a 10m loop antenna.

    10m loop antenna

    Well, this loop has a full wavelength of wire and is twice as high as it is wide. In this configuration, the antenna has a feedpoint impedance of 50 ohms.

    If you take that same wavelength of wire and configure it so that the height is very small with respect to the width, you have a half-wavelength, folded dipole. In this configuration, the feedpoint impedance is about 300 ohms.

    Back in the day, hams used this to make folded dipoles out of 300-ohm twinlead, the same kind of wire used to connect TVs to external antennas. Not only did the twinlead serve as the antenna wire, but it also served as the feedline, and because this twinlead was mass-produced, it was relatively inexpensive. To match the 300-ohm impedance to a transmitter, you did need some kind of transmatch or a balun, but overall, this type of antenna works very well and was cheap to build.

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