[Elecraft] BL-2 Connection To An Unbalanced Wire Antenna

[Jack Brindle]

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 <ron at cobi.biz>
>Sent: Apr 1, 2009 2:00 PM
>To: elecraft at 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
>
>
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