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

[Matt Palmer]

Ahh now you are no longer talking about RF ground but the black art of
EMI. Its not to difficult to determine RF ground, you have to have
very good spatial visualization, the ability to hold the 3D model of
the board, chassis and housing in your head, and you need to think
like an electron. It takes practice, but being relatively
inexperienced i've gotten the hang of it after being involved in the
successful design of 2 radios, but then again I have fantastic mentors
and elmers. In the end everything is explained by Maxwell's equations
and if 1/10th of the ham population took the time to understand how
these fantastic formula work and what they mean, 99% of this confusion
and most of the half truths you see passed around as 'common
knowledge' would disappear.


Matt
W8ESE
Former KD8DAO
http://blog.MattIsKichigai.com



On Wed, Apr 1, 2009 at 6:04 PM, Jack Brindle <jackbrindle at earthlink.net> wrote:
> 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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