--- In firstname.lastname@example.org
, "Ya`akov N. Miles" <ve7alq@...> wrote:
> --- In email@example.com, Jim Dunstan <jdunstan@> wrote:
> > At 07:55 AM 7/8/2009 +0000, you wrote:
> > Very true. I often hear people spousing "reciprocity" between
> > receiving and transmitting with the same antenna. Designing an
> > antenna for receiving is different to designing an antenna for
> > transmission. One just has to
> > look at the AM reception with the automobile radio. I would hate
> > to estimate the capture area of the 3' whip ... let alone modern
> > 10 inch antennas common these days. However they demonstrate
> > excellent MW broadcast reception. If I locate in quiet area ...
> > like a park .... I can receive weak daytime MW stations with the
> > same efficiency as my portable with the assistance of a small
> > tuned loop (sitting on the park bench).
> > Of course when you move into town ... local RFI makes the small
> > tuned loop superior by virtue of its ability to null the noise
> > while the 10 inch whip on the car (parked in the drive) becomes
> > useless.
> [-- CUT --]
> > Jim, VE3CI
> Please allow me to point out that there is a more important
> virtue of a loop MAGNETIC-sensing antenna over a whip ELECTRO-
> STATIC-sensing antenna. For a distance of about 1/3 of a
> wavelength (from my University Honours degree in Physics) the
> STATIC fields (+ the INDUCTION fields) are WAY stronger than
> the RADIATION (= RADIO WAVE) field. Thus at distances shorter
> than 1/3 of a wavelength (about 200 feet at the upper end of MW)
> MAGNETIC-sensing LOOP antennas do not HEAR electrostatic noises
> (such as from TVs, Computers, Electric Motors, etc.) very well
> if at all. This is quite INDEPENDANT of the fact that a
> magnetic LOOP antenna can null out noise if oriented correctly.
> Thus my "PLAY-MATE" AM Radio (600 - 200 Meters, 0.5 - 1.5 mHz)
> does not HEAR most domestic (and industrial) noises closer than
> 200 feet at the Upper end (1.5 mHz) and 600 feet at the Lower
> (0.5 mHz) end of the MW AM Radio Broadcast Band with its 1/2 inch
> LOOP(-stick) diameter loop wound on a Ferrite (=Magnetic!) rod.
> You have to drive your car with its ELECTRO-STATIC "whip"
> antenna into the Park well away from man-made ELECTRO-STATIC
> noises in order to acquire the same "EARS" as my tiny portable
> "PLAY-MATE" AM-only 6 Transistor Radio with its Ferrite
> LOOP(-stick) antenna can do right beside me in the ground floor
> of my wood frame apartment building any time of the day or night.
> 73 de Yakov, VE7ALQ - Fists #11117
I have a problem with the use of 'electro-static' when I feel you may mean electric field. As the E/M wave propagates, of course, it's E & H fields are never constant (Re: Poynting vector - basic E/M propagation.). The short E-field aerial is more of a 'voltage probe' - as is the short wiring to/from fluorescent lights, motors, etc, although it is a 'source'. So, the whip picks up the E-field, while the ferrite cored loop, etc, respond to H-field (magnetic).
Back in the eighties, I and an SWL friend went on a mini-DXpedition to prepare for the real thing - and get away from my local EMI source - coronal discharge on power leads in front of my home. We went into a state park - and set up at the vacant primitive campgrounds in the back of my truck using a freshly charged 105 A-Hr marine battery and his FRG-7 and my SPR-4. We ran short Beverage antennas in three directions. We were greeted with the same horrendous noise level we thought we had left at my home. The fourth direction had a pole with a pole-pig mounted - only 20 ft from our listening spot - and no loads connected. As night fell, you could see the tensioned insulators with corona - on the pole - with no loads - in the middle of no where. On the way home, the battery broke it's bungee cord securement and turned over, getting under the truck bed's pad. It was a horrible experience.
Oh yeah, my greatest honor was that I actually graduated - in physics - Auburn U. '75. I've held a First Class Radiotelephone License since '68.
PS The AL-4 loop on the SPR-4 didn't help - what a noise level!