[Milsurplus] Car Whip Antenna "gain"

[Jeff Kruth]

Hi Kurt, Ray, et. al.I am always hesitant to comment on this stuff. Its been my profession and hobby for over 50 years. Most folks get bored with science & engineering. However, I feel it is a dis-service to not try to help people to understand what is actually going on.Kurt, do not mix dBm (absolute power) with dB (relative gain). I think you meant your amp example would have an output of +20 dBm (100 mW), while contributing 10 dB of relative gain (taking +10 dBm up to +20 dBm). I usually had to adjust my students thinking every year for dBs versus dBm (or dBw, whatever).
Ray is correct. I had a saying "Antennas are not amplifiers". Sounds, dumb, I know, but it drives home the idea that an amplifier converts DC power to (in this case) RF power. It provides additional energy in the circuit.An antenna ONLY redirects power from where you dont want it to where you do. A truly isotropic radiator does not exist on earth and the closest example is the sun, which, certainly to a first approximation, puts power in ALL directions equally.Now, if you could (magically) cause the sun to put all its power in a 1/2 sphere, that situation would have a power density 2X that of the originally. Or 3 dB of "gain". But the other side is dark, no light. Now if you could put all the energy in half of a half of the sphere (1/2 hemisphere, or a 1/4 of the sun) would would have 4X the original power density or 6 dB over isotropic (6 dBi). And so on. 24 dB of gain is 1/250 of a sphere.ALL antenna gain is comparative. The easiest standard antenna that ANYONE (like NIST, or the ham down the street) can build is a dipole. HOWEVER, we all know a dipole does NOT put energy equally everywhere, but show a 3D pattern, resembling a donut. In an isotropic comparison, the isotropic antenna has 0 dB gain, while a dipole (in a direction broadside to the antenna) will show a gain of 1.6 X greater than isotropic or 2 dB better.For an antenna to have 24 dB gain, it concentrates the power 250X. That means its angular coverage is only 165 square degrees. If you consider 360 degree azimuth coverage (like a 1/4 wave vertical) the the elevation is about 1/2 degree wide (from horizon to sky). Pretty tough. Gain in an antenna equates to angular coverage against 41263 square degrees in a sphere. Efficiency is NOT addressed here, for another day.As a comparison, a yagi BW is roughly 30 degrees by 30 degrees, or 900 square degrees, for an average yagi with ~ 16 dBi of gain.Your 24 dB gain,if compared to a yagi, is a tiny elevation coverage.BUT if you say that the antenna factor of a electrically short whip, ~1m a 1 MHz is -60 dB compared to a resonant dipole (pretty typical), then you (maybe) could be 24 dB greater and ONLY -36 dB down from a dipole. Still pretty bad.I'll stop here, too long. Hope this is of some use.  73, Jeff
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://mailman.qth.net/pipermail/milsurplus/attachments/20250815/b714825b/attachment.html>