A High Efficiency Extended Length Mobile Antenna
Author: Ralph M. Hartwell II (W5JGV).
Accessed on 19 February 2018, 2013 UTC, Post #1408.
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Operating a mobile amateur radio station can be lots of fun mixed with antenna, grounding, and installation problems common to vehicles. In most cases, mobile amateur radio antennas tend to be small and inefficient.
In this article, Ralph (W5JGV) tries to overcome these restrictions by designing, buidling, and using a mobile antenna that is sturdier and longer than the usual mobile antenna. Ralph's multi-band mobile antenna is 3.42 meters/11 ft, 2 in long from the top of the mounting spring to the top of the antenna itself. A shorting strap is used to operate on the 15, 12, and 10 meter bands. Center-loaded coils are used to operate and 40 and 160 meters.
Ralph's antenna is a well-crafted gem which can be used in mobile, home, and portable operations.
Ralph's construction project covers the following areas:
Evolution of the Antenna Design.
Comuted Radiated Efficiency of a cent-loaded mobile antenna vs a 1/2 wavelength dipole antenna.
Construction details--most materials can be found at your nearby hardware store or building supply outlet.
Four-position magnetic mount needed for roof installation.
Making the 40 and 160 meter coils.
Additional Loading Coil Data.
"A center loaded vertical antenna will not present a pure resistive load at the base of the antenna. Usually, a matching network is added at the bottom of the antenna to cancel the reactance and transform the lower than 50 Ohm feedpoint resistance to something close to 50 Ohms. Since this antenna was designed to operate over several HF bands, a single matching network is impractical. Instead, I chose to connect the antenna through a length of low-loss coaxial cable to an automatic antenna tuner (ATU) inside the vehicle within reach of the operator and out of the weather.
To make the best use of the antenna with this set-up, the loading coils for the antenna should be tuned to resonance at the high end of each band. The antenna will then look electrically "short" to the tuner, which will then be able to tune the antenna to the desired operating frequency. If the loading coil in the antenna is tuned to a frequency below the top of the band, then operation above that critical frequency will cause the antenna to look electrically "long" to the RF. The loading coil will begin acting as a choke and effectively reduce the length of the antenna, causing a severe loss of gain.
The top whip represents a (measured) capacity of 17.5 pF. This value changes by about 0.75 pF as the mast is moved +/- 60 degrees from vertical in any direction.
The tuning of the antenna stays fairly constant as the whip sways, so compensation for bending of the antenna while driving is not necessary, at least not on this vehicle."
Overall, this is an excellent tutorial on how to design, build, and use a high efficiency mobile antenna. If you make this antenna, be sure to avoid low hanging branches and low highway overpasses.
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Aloha es 73 de Russ (KH6JRM).