Topband: Pennants/Flag report and K9AY xfmr question
k9ay at k9ay.com
Wed Sep 7 11:23:10 EDT 2005
My comments are included below with snips of your questions.
> after the 4-square mini-array (phased verticals), built in 1994, several
> Pennants and a rotable Flag, now I am going to try also the K9AY loop.
> Recently I got a further lot available where I can put a K9AY loop; it's
> over 100 meters from the Tx antenna and I can reach it with a buried
> feedline and a 5 pole control cable. I have an ICE preamplifier left and
> I am intended to put it down there.
> Now this is my question:
> - in the original '97 QST article K9AY uses a trifilar transformer, but
> it was suggested by many to go to separated windings and to not ground the
> coax cable at the antenna (well detailed in the last ON4UN book)
> - but if I use a preamplifier in the switching box I must ground it, and
> also the feedline and than connect on the same ground the low impedance
> side of the transformer, is it right?
It is my experience that common-mode problems are rare with buried
feedlines, since the earth is a lossy dielectric medium.
For simplicity, I build the commercial K9AY Loops with a single ground for
coax, control ground and antenna ground. I have had one customer ask how to
make a separate antenna ground, but never heard whether he did it or how it
> I saw the pictures of the new commercial version of the K9AY loop (AYL-4)
> on http://www.aytechnologies.com/Products/products.htm through the WX0B
> http://www.arraysolutions.com where the SO239 connector looks grounded,
> so I guess that the low impedance side of the transformer is still
> to ground; but I wonder if the original old trifilar style or a separate
> winding type is used.
> I have both the FT140-43 and FT114-77 cores: which do you suggest ?
Toroid transformers work well, but the windings require a lot of care to get
maximum performance. The original K9AY Loop (1996) is the last time I used a
toroid for an RX antenna -- winding a conventional primary/secondary
transformer on a binocular core is much simpler, and has equal performance.
My K9AY Loop and 9:1 Beverage transformers are "binocular" using two large
77 material beads (Amidon FB77-6301) with separate windings of #22 AWG
wire -- 3 turns for the 50 ohm side and 9 turns for the 450 ohm side. The
measured loss is around 0.2 dB from below 160M to at least 20M. This
material and winding scheme works well at lower frequencies, too, since many
loop users are Broadcast and low frequency DXers.
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