Topband: Silent Feedline for RX antennas

[k3ky@erols.com]

On 24 Aug 2002 at 0:38, Doug Waller wrote:

> Gentlemen:
> 
> It is generally accepted that low gain receive antenna coax lead-ins, from
> Beverages to Flags, must be well shielded, balanced and choked, to prevent
> unwanted signals and noise from entering the system & degrading performance.
> To this end, I have ordered several FT-150A-F toroids and mini-coax to make
> 3-K-ohm chokes.  I also plan to ground the coax between the new rotary Flag
> and receiver, to drain the shield-received RF.  A dozen long, snap-on split
> ferrites will be installed along the way.
> 
> Suddenly a light:  Why not use two RG-58's or 59's in balanced feedline
> configuration, twisted 2-3 times per inch, to eliminate all the above?  The
> shields at the feedpoint would be bonded, but left floating.  The unchoked
> RF received equally by the outside of the shields (due to twist) would
> induce equal currents onto the two center conductors, which would then
> cancel.  Transformers on both ends of the dual feedlines would match the
> doubled feedline impedance, while providing more common mode protection.
> One coax ground near the receiver should suffice.
> 
> I wish this feedline to be as silent & balanced as possible, as an
> experimental -56dbi gain RX antenna will be tried on down the road.
> 
> My run is only 100 feet, so I could see a longer run of dual coaxes
> eventually surpassing the costs of choking and grounding.  But I could buy
> alot of RG-58 for the price of the three FT-150A-F's & spool of mini-coax.
> If you have already done this, or have comments, please let me hear.
> 
> 73/DX, Doug / NX4D
> 
Sounds like it might be worth investigating. My initial reaction,
however, is that 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?
73, David K3KY