Hi all,
Well, I'm enjoying this thread more all the time. Many interesting
technical points being brought forward. And still no flames ! :)
I'd like to add another comment about ground though.
I feel that it is very feasible to have a low impedance path to earth
ground without breaking the piggy bank. However, that said, it is
true that there are several technical hurdles to be overcome. A short
list - not inclusive - follows...
1. You must establish a really good connection to earth... you know...
to the dirt ! One 4 foot cheapo rod from radio shack doesn't cut the
mustard.
You must make an adequate connection to a suitably moist and conductive
layer. Here where I live, that is doable as there is an underground spring
down
about 6 feet that I can drive my 10 foot rods into. And I used several.
2. Then you need the connection to the shack. A piece of no. 22 hookup
wire doesn't do it ! :):)
Large copper pipe, heavy braid, copper strap... those are the ticket. And
good
connections, corrosion resistant !
3. Then there is the distance. If you can't make it in MUCH less than 1/4
wavelegth of the highest frequency of interest, then you will have to use a
longer than 1/4 wire as mentioned many times before. Avoiding the odd
number quarter wave mutliple of course.
4. Single point ground in the shack.
Ok... thats enough. But it's all doable... and none of it is particularly
hard of
expensive.
73 de Gary, AA2IZ
----- Original Message -----
From: "George, W5YR" <w5yr@att.net>
To: "Alderman, Chester" <CAlderma@ora.fda.gov>
Cc: <tentec@contesting.com>
Sent: Monday, October 21, 2002 4:24 PM
Subject: Re: [TenTec] Open Wire
> Let me add another nit or two for the picking:
>
> "Alderman, Chester" wrote:
> >
> > Ouch!! Here's some nit picking:
> >
> > If the feeders are truly balanced, and narrow spaced for the frequency
> > > in use, such as window line 450 ohm nominal and 300 ohm nominal at HF
> > > bands, there should be minimal field outside the feeders due to the
> > > equal and opposite currents principle.
> >
> > What is 'truly balanced'? The only way you get the phase in open
> > wire feeders exactly 180 degrees out of phase is by making absolutely
sure
> > the physical length of each side of your dipole are identical. Feeders
are
> > NOT 'truly balanced', but if you apply common sense, your antenna system
can
> > be balanced.
>
> It requires more than just equal dipole halves to make a "truly balanced"
> system. The environment of each half of the dipole must be the same such
> that each sees effectively the same impedance to ground, in the manner of
> displacement currents, etc. Equal wire lengths with one end a few feet
away
> from a metal building will not provide a balanced antenna system.
> Similarly, each line of the feeder must be in the same electromagnetic
> environment: distributed capacity to ground, other objects, etc.
>
> The merit of the current balun is that it forces equal currents into the
> two conductors of the ladderline or whatever and into the dipole halves.
> But, the contribution to the radiation pattern of each half depends not
> only upon equal wire lengths and currents but upon impedance to ground,
> etc. This in turn influences the magnitude and phase of current returning
> from each dipole half and thus a common-mode current can be created.
> >
> > >
> > > A well designed rig, installed properly, will not have RF in the
> > > shack. This means use of a well balanced tuner, and earthing measures
> > > that avoid having the distance to earth as a quarter wave of the band
> > > in use or odd multiples. This is where many have trouble, especially
> > > with unbalanced antennas.
> >
> > 'RF in the shack' has absolutely nothing to do with a 'well
designed
> > rig, installed properly'!! If the length of one side of your dipole plus
the
> > total length of your feedline is an odd multiple of quarter wave
lengths,
> > you will have high voltages (commonly referred to as 'RF in the shack')
at
> > your operating position. If you happen to hang an end (or both ends) of
your
> > dipole wire directly over the top of your house, chances are great that
you
> > will also suffer with the 'RF in the shack', simple because of the very
high
> > voltage fields always produced at the ends of dipole wires.
>
> Again, a small difference of opinion. There is nothing inherent about
> having a transmission line with a voltage loop at the shack causing "r-f
in
> the shack." True, there is a larger "potential" field - that is a double
> pun, BTW! - at the voltage loop but it is a differential affair and no
more
> radiation or field strength "leakage" can be expected there than anywhere
> else along the balanced line, provided that it is indeed balanced.
>
> Provided that an effective 1:1 current balun is used, a balanced tuner is
> not required. As to "earthing measures" after 57 years of this foolishness
> and a few decades in the profession, I still have serious doubts about any
> measures taken to "r-f ground" anything in the shack. With the shack
> equipment properly interconnected, everything within the shack should be
at
> the same potential, and it that happens to differ from the potential of
> something called "ground" there is usually little or no harm done. To the
> contrary, I have seen many more problems created by seeking that "good r-f
> ground" than ommission of such a conductor corrected. I have never used an
> r-f ground in any shack that I have had and I have never had "r-f in the
> shack."
>
> No brag - just fact! <:}
> >
> > >
> > > Use of true balanced antennas minimizes problems of RF in the shack,
> > > from both coax and balanced feeders. (The coax feed using a balun at
> > > antenna.)
> >
> > True. And a true balanced ham antenna is a dipole with exactly
equal
> > physical length for each side of the dipole AND a loop (such as a quad)
> > antenna. And a uniform environment (impedance to ground) for all
elements of the antenna system.
> >
> > >
> > > That being said, if a particular rig has sensitive digital logic,
> > > modern EMC methods of beads and choke cores on cables, improved
bypassing,
> >
> > > twisting power supply conductors and shielding them, all will help
> > > remove the RFI effects.
> >
> > In my totally humble opinion, all of the above is tried AFTER
you
> > make sure your antenna is not inducing a high voltage field into your
room.
>
> Correct! If you and your equipment are within the induction field of the
> antenna, there will be an r-f potential present on anything conducting and
> current will flow according to the impedance of the conductor and the
> differential field strength in which it is located.
>
> An "r-f ground" for the shack merely provides another current path which
> may or may not improves matters. The "ground" is not an infinite sink for
> all things that we do not want. It is merely another physical object with
> an inherent impedance, etc. and almost always the connection between the
> shack equipment and the "ground" is questionable at best.
> > >
> > > Many hams persist in using off balance, off center fed antennas
> > > without balancing and thus will more easily introduce RF into the
> > > shack.
> > >
> > > Another method little known by hams, to mitigate RF in the shack
> > > involves running the coax feeder through a conduit, (the waveguide
> > > beyond cutoff type of filter). This affords certain off center fed
> > verticals little to
> > > no RF on the coax outer shield. It is a method easily implemented for
> > > a single problem band, although ferrite cable chokes may be easier to
> > > implement for multiple problems.
> >
> > Hmmmm, a 80 meter waveguide...should be an interesting thing to
> > construct.
>
> I have never been able to find any tradeoff in using an unbalanced antenna
> such as the infamous "Windom" (which isn't a Windom at all) in preference
> to a centerfed and hence balanced antenna feed.
>
> As to the waveguide approach, the lumped constant approach of the current
> balun applied to the coax shield is simple and effective - and a lot
> smaller than that 80-meter waveguide!
>
> 73/72, George
> Amateur Radio W5YR - the Yellow Rose of Texas
> In the 57th year and it just keeps getting better!
> Fairview, TX 30 mi NE of Dallas in Collin county EM13qe
> K2 #489 Icom IC-765 #2349 Icom IC-756 PRO #2121
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