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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