With a truly balanced antenna there should be no RF current from the station
to ground. If you are on the second or third floor and a long way from the
house ground--which is probably a very poor RF ground even if you are close
to it--you better hope things are well balanced. If you are using a
balanced antenna and find you need an artificial RF ground, something is
badly wrong--and as has been said, a long wire to the ground point will
radiate even if it is tuned.
If you have an unbalanced antenna which is fed with coax, you should take
steps such as using a common mode choke to remove ground currents on the
sheath before entering the station. If you have an unbalance antenna fed
with ladder line, you need to do something to balance up the currents before
entering the shack or you will have RF looking for somewhere to go, which is
not good if you do not have a good RF ground. Best to clean things up
before entering the shack.
I used to manage the Lightning Protection Group at Bell Labs. We were
concerned with preventing damage to plant, people, and equipment. Although
lightning strokes have a wide spectral content, the bulk of the energy is at
pretty low frequencies, and if the pulse has propagated in from a strike to
the power plant, it will be even further smeared out. Typical lightning
testing is done with an exponential pulse with a 10 usec rise time and 1000
usec tail. In some cases a narrower pulse is used. Here is some
interesting information from
http://www.vlf.it/naturalnoisefloor/naturalnoisefloor.htm:
"Lightning has enough variants to warrant a series of articles all to
itself. Briefly, a lightning discharge consists of three main parts. A
series of leaders, each about a m sec in duration with peak current about
300 amps and each leader being separated from the next by 25 to 100 m secs
occur over a period of about a sec. The leaders ionize a channel for the
return stroke or main discharge which lasts about 100 m sec and has peak
current of the order 30 kilo amps. Last is the slow tail, an exponentially
decaying current initially of several hundred amps that persists for about
0.5 sec. Each of the three parts produces radiating energy in different
parts of the spectrum. The leaders, being the shortest, produce energy in
the low LF range, generally above 30 kHz. The main discharge produces a
large radiating pulse with energy centered broadly around 10 kHz. The slow
tail, of course, being long, is one of the principal sources of ELF noise."
I would quibble with parts of the description but it is a good summary.
>From this information you can see that something which looks like a good rf
ground at 20 meters may not look like a good ground for lightning and vice
versa.
Bob W2WG
-----Original Message-----
From: tentec-bounces@contesting.com [mailto:tentec-bounces@contesting.com]
On Behalf Of Robert & Linda McGraw K4TAX
Sent: Wednesday, November 30, 2005 8:53 AM
To: Discussion of Ten-Tec Equipment
Subject: Re: [TenTec] Station and AC Ground
If you have a "tuned" ground system, consider that it is part of the
radiating system. Otherwise, your ground is radiating RF. If current
flows on a conductor, it radiates. Usually this is an invitation to RFI
issues on phones, TV's, and such due to the placement of the conductor.
73
Bob, K4TAX
----- Original Message -----
From: "Martin AA6E" <martin.ewing@gmail.com>
To: "Discussion of Ten-Tec Equipment" <tentec@contesting.com>
Sent: Tuesday, November 29, 2005 4:25 PM
Subject: Re: [TenTec] Station and AC Ground
> You can have a tuned "ground" but it only works at discrete
> frequencies. Besides, any current-carrying wire of appreciable
> length, tuned or not, becomes part of your antenna system. If, as it
> should, the RF ground carries no current, an open circuit will work
> just as well. ;-)
>
> For lightning (a broadband pulse), what matters is impedance of your
> ground connection. Resistive (considering skin effect) and reactive
> (inductance). Wide copper stap as short as possible minimizes
> effective resistance and inductance. A half wave wire (cut for some
> wavelength) is unlikely to be good for lightning.
>
> 73 Martin AA6E
>
> On 11/29/05, Gary Hoffman <ghoffman@spacetech.com> wrote:
>> >
>> > The first (RF) is somewhat hopeless, as was pointed out, because a
>> > real (broadband) RF ground has to be very short, say not more than
>> > 1/10 wavelength, to avoid reactive and resonant effects. That means
>> > shorter than 1 meter for 10 meter operations, etc. That's not
>> > practical in most installations. You don't really need a "true" RF
>> > ground, if your antenna system is balanced properly - no common mode
>> > RF on the feed lines, etc.
>> >
>>
>> Or....a half wave ground wire, to repeat the low impedance at earth right
>> at
>> the radio.
>>
>> And, since lightning is RF, I feel that a real RF ground is important,
>> even
>> for lightning.
>>
>> 73 de Gary, AA2IZ
>>
>>
>>
>> _______________________________________________
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>>
>
>
> --
> martin.ewing@gmail.com
> http://blog.aa6e.net
> _______________________________________________
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>
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