[Elecraft] K3/100 -Second floor- no RF ground ??

Wed Feb 24 23:44:04 EST 2010

Speaking of lightning grounds. MFJ says use soild wire or flat copper for 
ground wire and never use stranded or braided wire. They claim the latter 
has high impedance to lightning.
Ok, fine....so if my antenna is stranded wire, does it have high impedance 
to lighting? Would it be less safe to make ones antenna from solid 
conductors? Yes...a weird question but it's been bugging me.
Steve
N4LQ
----- Original Message ----- 
From: "Don Wilhelm" <w3fpr at embarqmail.com>
To: <n1al at cds1.net>
Cc: <elecraft at mailman.qth.net>
Sent: Wednesday, February 24, 2010 11:11 PM
Subject: Re: [Elecraft] K3/100 -Second floor- no RF ground ??


> Thanks for that Alan,
>
> I have been saying the same thing for years.  Earth grounds are for AC
> safety and Lightning safety.  They have nothing to do with RF Ground
> reference.  Yes, RF "Ground" is nothing but a reference point for the RF
> voltage (or current).
>
> For our purposes, think of it only as a low impedance (ideally zero, but
> normally higher) for RF at the frequency of operation.  A truly balanced
> antenna will have that point halfway between the feedline conductors at
> the feedpoint.  While that may be 'stretching science' a small amount,
> it is the closest I can get with a visual example.
> When the currents (and voltages) are equal and opposite, RF "Ground"
> will be the value midway between those voltages (or currents).
> Since a balanced feedline should have equal and opposite (differential)
> currents on the two conductors (that is the definition of balanced), if
> the antenna is balanced and the feedline does not pick up radiation from
> the antenna, that RF Gound reference point will be conducted into the
> shack by the feedline.
>
> The problem is that the feedline does pick up radiation from the antenna
> - and that is carried by the feedline into the shack as well - that is
> called common mode current. If that common mode current exists, it will
> bring RF into the shack and must be dealt with by either bleeding it off
> into a low impedance for RF (the counterpoise wires mentioned earlier of
> an artificial RF Ground), or eliminating it at the source by proper
> feedline placement if the feedline is balanced.
>
> Coax fed antennas are a bit different than those fed with balanced
> feedlines.  Due to skin effects, a coax feedline has 3 effective
> conductors - the center conductor, the inside of the shield and the
> outside of the shield.  The currents inside the coax are actually
> balanced - they must be equal and opposite.  When the end of a coax
> feedline is connected to two wires of equal length, the center conductor
> sees only the antenna half for the current to flow onto, but the side
> connected to the shield sees two conductors - one is the antenna half
> and the other is the outside of the coax shield.  If current does flow
> onto the outside of the coax shield, it is common mode current when it
> arrives at the shack end.  The cure is to use a current balun at the
> antenna feedpoint, and run the feedline at right angles away from the
> radiating elements for as great a distance as possible so the coax does
> not pick up radiation from the antenna.
>
> A choke balun is nothing more than an RF Choke - it can be formed with
> ferrite beads over the coax, or by coiling the coax into an inductor.
> The reactance of the inductor thus formed should be greater than 5 times
> the antenna feedpoint impedance (note that this may or may not be not be
> the characteristic impedance of the coax feedline).  The required
> impedance is determined by the antenna, and not by the feedline.  I know
> there have been a lot of things published about "forcing balance" by
> using a balun - I just do not buy that concept.  My education in
> antennas is much more simplistic and clear (to me), and it is based on
> one principle - the current at the end of an antenna *must* be zero.
> Think about that for a while and you may change your thinking about
> anteennas - one does not "force" an antenna to be balanced, that is
> determined by the relationship between the feedpoint and the ends of the
> antenna.  The radiating portion of the antenna sets the "rules" - any
> differences are dissipated as heat.
>
> 73,
> Don W3FPR
>
> 73,
> Don W3FPR
>
> Alan Bloom wrote:
>> On Wed, 2010-02-24 at 18:16 -0800, Jim Brown wrote:
>>
>>> On Wed, 24 Feb 2010 14:18:50 -0800 (PST), callen1155 wrote:
>>>
>>>
>>>> I'm planning on purchasing a K3/100 but am concerned about my second 
>>>> floor
>>>> shack and RF problems.
>>>>
>>> The most important thing to understand is that a connection to EARTH has 
>>> NOTHING
>>> to do with RFI, or RF feedback, or any of the problems you are 
>>> describing.
>>>
>>
>> It's amazing how widely the concept of ground is misunderstood.  For
>> years the ARRL Handbook repeated the myth that you need a good earth
>> ground to prevent TVI.  I think we finally got that straightened out.
>>
>> People say that your low-pass filter won't work without a ground because
>> the shunt capacitors need to be "grounded" but then they are at a loss
>> to explain how low-pass filters work in satellites.
>>
>> I think the fundamental problem is that people don't understand (or
>> forget) that "voltage" means "potential difference", the difference in
>> electrical potential between two points.  It makes no sense to speak of
>> the voltage "at" a particular circuit point unless you define the
>> reference point -- the place the other lead of your voltmeter or
>> oscilloscope is connected to.
>>
>> There can be 1000V difference in potential between earth and the chassis
>> of your radio and the radio will continue to work perfectly normally as
>> long as it is completely isolated from earth.  What matters is the
>> potential difference between the circuit nodes inside the radio, not the
>> voltage with respect to earth.
>>
>> Al N1AL
>>
>>
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