Topband: Noise and reception (K6STI loop)

[k3ky@erols.com]

Hi, Donald-

On 13 Feb 2002 at 4:29, Donald Chester wrote:

> Re the K6STI loop, I built mine directly from the articles. 
     (snip)
> The open wire line is made of #12 copper, the same wire I used for the
> antenna itself, separated by EF Johnson ceramic 1 1/2" feedline
> spreaders.

According to the graph in my ancient ARRL handbook, about 430
ohm characteristic impedance. Probably close enough, but I 
recall K6STI said you want 450 ohms or better or it will not work
well. 

The only modification I made on the design was that the
> exact centre point of the diagonal open wire line is fed with
> identical open wire line extending down to ground level, where the
> tuned transformer is attached, instead of locating the tuning unit
> directly up at the diagonal feeder as the article showed.

This sounds 'dangerous'. I remember the article fussed a good
bit over how critical it was to keep short leads of identical length
within the tuner box- to maintain good balance. Whether or not the
added length of 450 ohm line down to a ground level tuner box
would be a problem, I cannot say. I would not try it personally.

I found a nice, compact, weathertight, plastic box at Radio shack
that was just right for this project. It may now be a catalog only
item. I don't see tham any more when I visit the stores. K6STI
says that the diagonal ladder line may be permitted to sag a lot
without any problems whatever. That is what I let mine do. I can
open the box and tune it from the ground. My antenna is about 8-10
feet up. The box sits at about 5 feet.

> The tuned transformer hit resonance first try.  I find it pretty sharp
> tuning; without a pre-amp, I get a useable signal only over a maximum
> bandwidth of maybe 30 kHz without retuning.

Mine has DC control voltages superimposed on the coax to the
tuner box. It has 3 states- no voltage sets it for 75m. One DC
polarity kicks in some padding capacitance for 80m. The other
polarity switches a relay for 160m to adjust turns ratio and kick
in a 3rd, separate padding capacitance. I can hear 80m signals
clearly when set to 75m and vice versa, although noticeably
weaker. I am guessing that my bandwidth, although narrow,
must be way more than 30KHz.


> I noticed that even minor
> changes in the length of the open wire line that runs down to the
> tuning box, affects resonance of the tuned circuit.

Hmmm... As I said, I would not dare attempt this mod. K6STI
made it sound just a bit too tricky here to obtain good balance.
I suspect your added open wire line may have been most of your
problem.

> The antenna was somewhat a disappointment, but it is quieter than the
> transmit vertical or 5'X 5' shielded loop.

Mine is 'quieter', too, but this is a low output antenna. The important
thing is whether or not the S/N improves. Here I get a markedly
better S/N, night and day different. All this without preamp. I will
eventually get around to building up one of the 2N5109 CATV
transistor designs. I have all the parts...  :o)

 The output is too low for
> good s/n ratio in my 75A-4 receiver.  I tried a 20 dB preamp, but it
> amplifies the background noise just as much as the signal, with no net
> improvement in s/n ratio.  I'm using an old Ameco tube type preamp
> that has two nuvistor triodes in a cascode circuit.  Maybe a modern
> FET preamp would have a better noise figure and make a difference. 
> Certain types of local electrical noise come in over this loop just as
> strongly as on the transmit vertical, but it shows improvement with
> some noise sources.

Same here. Sometimes it picks up some noise sources about as
bad as other antennas here. Other times it is the quietest by far, 
while picking up at least some of the weak DX signal.
> 
> According to the articles, this antenna is supposed to respond poorly
> to vertically polarised groundwave signals.  However, when I listen to
> a local AM broadcast station (approx 8 miles away, on 1550 kHz) I get
> it very strongly on this antenna day or night.  When I compare the
> nighttime strength of the local BC station against that of the skywave
> from distant adjacent channel BC stations, the relative signal
> strengths the local and distant stations is about the same, whether I
> use the loop, my 160M transmit vertical, or my 10M ground plane.  It
> would seem to me that if the loop really discriminates against
> groundwave signals, the local BC station should be way down compared
> to when I listen on the vertical.  If I find a nearby skywave signal
> that has about the same s-meter reading as the local station on the
> vertical, when I switch to the K6STI loop, they still have about the
> same s-meter readings.

There are *so* many variables here. My definite impression is that
my loop does knock down the *high* angle QRM from close by- a
significant amount. Still, my competitors in a pileup are loud.  :o)
Skywave signals from a few states away should not be attenuated
at all. Probably only the really close-in stuff.
I listened to 1500 (local) and again my *impression* is that it is
knocking that signal down somewhat. How much I could not say.
Quite a bit, I suspect.

> Don, K4KYV
> 
                                      73, David K3KY