Topband: Silent Feedline for RX antennas

[Doug Waller]

> >David K3KY wrote:
> >there might be problems of an electrical/
> >mechanical nature. Unlike single insulated wires, which are
> >easily  twisted together in pairs, trios, etc., I think you would
> >be doing very well to get even 6 to 8 twists per foot, i.e. less than
> >one twist per inch using RG-58/59 coax. Would that be enough?

> Don K4KYV wrote:
> I have seen rolls of "double barrel" coax at hamfests.  It has two
separate
> parallel inner conductors instead of one. Sort of like shielded twin lead.
> I forget the RG number, but it might be a better bet than twisting two
> coax's together.  Does anyone know if they still make the stuff, and any
> available sources?

Don & David & others,

The single coax shield with twin center conductors would be excellent, if
available.  A multitude of problems now faced with the darn unbalanced
feedlines would be solved.  Conversion from balanced to unbalanced,
impedance matching & further isolation, would all be accomplished right at
the receiver with a single toroid transformer.  This would reduce the length
of unbalanced line in the system to a few feet, from transformer to
receiver, and eliminate the inherent receive problems that come with it.

Other advantages would include the gradual stepping down of impedance in two
stages, rather than doing it all at the Flag (or any high-impedance  receive
antenna) feedpoint, and the total elimination of the need for RF Chokes,
even at the feedpoint!

Balanced, shielded lines are covered in ARRL Antenna Book, 18th edition,
pages 24-18 to 24-20.  Using two 50 ohm coaxes in parallel, yields a 100 ohm
feed impedance - a doubling of impedance (2-75 ohm = 150 ohm, etc.).  The
article shows grounding the two shields only at the receiver chassis, but I
believe grounding them to a separate isolated ground would be the best
method, just before the balanced-to-unbalanced receiver transformer.
Twisting of the two coaxes is not mentioned, but appears a good idea to me,
to assure equal exposure to RF by both cables.  A single "double-barrel"
coax would eliminate any need for twisting, and would be easier to handle &
install.

Yes, this may be overkill for a Beverage, but for the family of small RX
antennas with -30 dBi gain on down, directional characteristics are easily
upset by the smallest ingress of signals received by the outside of the coax
shield.  I have an experimental -56 dBi gain phased loop antenna on a 30 ft
boom, with the EZ pattern of an 800 ft Beverage.  The only way I will ever
realize that pattern may be with a near-perfect shielded balanced feedline.
The statement that not gain, but only directional characteristics apply to
RX antennas, is true down to -15/20 dBi, but beyond that, noisy feedlines
(also surrounding re-radiators & added preamp noise) are the limiting
factor.

Although having already ordered FT-150A-F choke toroids, mini-coax to wind
them with, various snap-on ferrites, BNC connectors, coax hoods, metal and
plastic housings, waterproofing materials & several ground rods, to treat
the single UNBALANCED RG-58 lead-in, I will begin a search next week for the
single  BALANCED line coax with two inside (center) conductors, which needs
none of the above.  The single, unbalanced leadin may still need help with
noise induction, even with all the efforts to stop it, since it naturally
does not want to work.  Why continue to add all this effort to a single
center conductor inferior coax leadin if a balanced line coax leadin solves
all these problems?  Where are my parts catalogs..................

73, Doug / NX4D