Topband: Noise and reception (K6STI loop)

[Donald Chester]

Re the K6STI loop, I built mine directly from the articles.  It is 12' above 
the ground and 21' on a side.  I was very careful to set the poles so that 
all sides are equal and in a perfect square.  I believe I got all 4 sides 
and both diagonal measurements each within 1/2" of the other.  The wires 
were measured very precisely.  I used a water level at each support point to 
make sure the loop was perfectly horizontal.

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.  
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.  The purpose of this modification was to make it 
possible to adjust the tuning capacitor while standing on the ground, 
without having to use a stepladder.

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.  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.

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

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.

Don, K4KYV

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