«-- Letter Re: Roger Mills County, Oklahoma as a Retreat Locale | Main | Note from JWR: --»

Letter Re: Some Transceiver Antenna Questions

Jim;
By purchasing a 60-inch collapsible antenna, I was able to get a lot more range out of my hand-held transceivers, but that's all I know. Can you give a short tutorial on antennas? What is a ground plane, when is it necessary? Would full wavelength be better than 1/4 wavelength? For a base system, would you recommend Yagi or something else? Thanks, - SF in Hawaii

JWR Replies (Updated): To begin, one-half wave antennas are theoretically the most efficient. Shorter fractional wavelength antennas (quarter-wave, 1/8th-wave, et cetera) are used primarily for compactness and lower cost. I was told by our correspondent David in Israel (an experienced ham operator) that a full wave antenna actually cancels out signals on its resonant frequency--the peak and trough energy is 1+(-1) = 0. To illustrate some practical aspects of wavelength: CB radio frequencies have a wavelength of around 10 meters (about 33 feet). It is possible to use a 1/2-wavelength CB antenna at a home or at a retreat, but not mounted on a vehicle. (On a vehicle, even a 1/2 wavelength antenna is often too tall.) The MURS Band (my favorite for short range communications) has a wavelength of around 2 meters, so using a half-wavelength antenna is much more practical. See this index page from the ARRL for a good basic understanding of how both transmitting and receiving antennas work.

A ground plane is a reflective flat surface that limits the downward radiation of an antenna. When operating a transceiver with an antenna mounted on a vehicle with typical steel body panels, the vehicle itself forms a ground plane. This is why the most efficient antenna mounting location is at the top-center of a vehicle. But, unfortunately, this also places an antenna at the greatest risk of impact damage. This explains why bumper-mounted antennas are more popular, despite their distorted transmission characteristics and inefficiency.

A log periodic antenna (LPA) or Yagi-type antenna can be very effective, but keep in mind that like other antennas, they need to be properly polarized. Most mobile two-way radios use vertical polarization. Hence, your LPA or Yagi will not have the traditional horizontal "TV antenna" appearance--rather, it will be flipped on its side, for vertical polarization.

Permalink -