I have to agree with Jim on this one. Perhaps the biggest concept drilled into me by the RF engineers I worked with at Motorola (I was a digital/software/comm engineer) is that there is no such thing as RF ground. RF can, and is, conducted on any path that it wants. This is especially true for the so-called "ground" and "power" paths, which while appearing to be well bypassed, still will carry RF currents. The problem was really drilled home when I had to track down a problem with a 450 MHz handheld data transceiver being desensed. The cause was the 250th harmonic of the microcontroller main clock, which placed a 14 uV signal on the receiver input. The signal was being conducted on the system ground, including shielding, and into the receiver front-end. How do you solve it? Shift the crystal frequency when on problematic channels.
So, while the concept of a common ground which carries no signals may be an interesting one, in practice it simply does not exist. - Jack Brindle, W6FB. -----Original Message----- >From: Ron D'Eau Claire <r...@cobi.biz> >Sent: Apr 1, 2009 2:00 PM >To: elecraft@mailman.qth.net >Subject: Re: [Elecraft] BL-2 Connection To An Unbalanced Wire Antenna > >Jim, IMX it's a mistake to equate "RF ground" with an Earth connection. > >An RF "ground" is just a low-impedance, low-reactance current sink for RF. >Of course it is an integral part of the antenna circuit. > >An RF "ground" would not be expected to radiate, and most "counterpoise" or >"radial" setups don't radiate a significant amount of energy*: > >1) Counterpoises near the Earth and on-ground "radials" tend to couple all >their energy into the lossy dielectric of the Earth, never to be seen again. >This is how BCB stations achieve a good RF ground generally using 120 0.2 >wavelength radials around their towers to couple the RF into the Earth. > >2) Elevated radials will radiate a lot unless they are carefully balanced >and symmetrical so "legs" produce RF fields that cancel each other outside >of the immediate area of the antenna. Such radials, like any RF ground, >*are* part of the antenna circuit but, when properly designed, they are a >non-radiating "current sink". In the common "ground plane" designs, they >also decouple the radiating element from the feed line, providing an RF >"ground" not only for the radiator but also grounding the feed line at the >antenna so RF currents don't flow down the outside of the coax shield. > >Ron AC7AC > >* Students have asked me what happens if they use only one radial with a 1/4 >wave antenna. I reply that if they make it 1/4 wave long, then elevate them >both into the air and arrange them to run in opposite directions for maximum >efficiency, one 'radial' works just fine. If they draw out the antenna I >described on paper they'll recognize the common center fed dipole antenna. >In that case, there's no problem with the "radial" radiating. > > >-----Original Message----- > >On Tue, 31 Mar 2009 18:43:39 -0700, Ron D'Eau Claire wrote: > >>But it *IS* a ground for RF purposes > >Nope! It has NO relationship with the earth, nor is one needed. This >use of the word "ground" is an ongoing source of confusion and >misunderstandings. Hams to go extremes to install ground rods, >thinking that it will improve the performance of their antennas or >make their radios quieter or fix RFI. A connection to earth does NOT >do any of those things. It IS critical for lightning protection. >That's all. > >Radials are not GROUND in any sense. They are part of the antenna! >Indeed, their purpose is to intercept the fields produced by the >antenna and by providing a low resistance path for return currents, >prevent those fields from producing current in the lossy earth. >There is an excellent discussion of this by Rudy Severns, N6LF, both >in the ARRL Antenna Book and on his website. > >73, > >Jim Brown K9YC > > >______________________________________________________________ >Elecraft mailing list >Home: http://mailman.qth.net/mailman/listinfo/elecraft >Help: http://mailman.qth.net/mmfaq.htm >Post: mailto:Elecraft@mailman.qth.net > >This list hosted by: http://www.qsl.net >Please help support this email list: http://www.qsl.net/donate.html ______________________________________________________________ Elecraft mailing list Home: http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/mmfaq.htm Post: mailto:Elecraft@mailman.qth.net This list hosted by: http://www.qsl.net Please help support this email list: http://www.qsl.net/donate.html
