[Antennas] W3EDP and mobile antennas

2002年5月19日 10:13:01 -0400

You may be laboring under the impression that resonance = radiation! The
tank circuit in your final or amp can be tuned to resonance, but it does
not radiate (at least not very well). Making the antenna shorter and
shorter lowers the radiation resistance of the antenna. An antenna with
a very low radiation resistance leads to higher losses in the feed
arrangement, unless extreme measures are taken to reduce the resistance
of those feed arrangements. For example: an antenna only 8 feet tall
operated on 80 meters will have a radiation resistance of only 1 Ohm! If
we can get the feed line and other losses down to 5 Ohms or less (which
is a reasonable assumption), the overall efficiency will be in the
neighborhood of about 16%. (100 watts in = 16 watts out)! Whereas the
radiation resistance of a full quarter-wave vertical is about 35 Ohms
and with the same 5 Ohm feed loss will yield an efficiency of about 87%.
So, adding that few inches to the Hamstick, represented by the whip, may
very well increase the radiation resistance enough to double the
efficiency of the antenna.
It is to be noted, however, that making the antenna very short in no way
interferes with its ability to radiate. It is only that the radiation
resistance becomes so low that it is nearly impossible to feed it
efficiently.
Harvey/W4TG