Topband: Noise and reception (K6STI loop)

[Tom Rauch]

One problem with models is we can sometimes design antennas 
that are impossible to build, because the sensitivity is low and the 
common-mode rejection we see using a "perfect" ground 
independent source is simply not available in the model.

This is more of a problem than we often imagine with any antenna, 
and would be a huge problem with an antenna like the K6STI loop! 
It not only takes being able to enter the data in a program and 
understand the results, we also MUST have some basic intuitive 
idea how the systems work. Otherwise we might model the 
unbuildable or unreproducable.

Very few models are verified these days before they hit the press, 
we have to keep that in mind. Unfortunately once something 
appears in an article, it is taken as reliable data for years to come 
no matter how absurd...like the silly notion four truncated coil-
loaded radials (that recently reared its head again, this time in CQ 
Magazine) are somehow superior to a real ground system.

> 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.

You did an excellent job, but you skipped the important part of 
replacing all the soil below your antenna with a homogeneous 
media in a clear empty field. 

When a system has almost no differential-mode sensitivity, like 
this system has, it becomes far too easy to mess things up by 
putting it in the real world and feeding it with real feedlines.

> According to the articles, this antenna is supposed to respond poorly
> to vertically polarised groundwave signals.  
<snip>
> 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.

All you need is a perfect system with perfect balance over perfect 
homogeneous earth, and it provide you with the modelled results 
and not have much groundwave response. The problem is not with 
the antenna design or the modeling, but rather because we live in a 
less-than-perfect world and install our antennas there.


73, Tom W8JI
W8JI@contesting.com