Re: Topband: Skimmer calibration

["Tom W8JI" <w8ji@w8ji.com>]

This is why some time and multiple skimmers must be involved in the 
statistics................otherwise data doesn't mean much.
Without skimmer I never settled on antennas until many dozens of blind AB 
test reports. I think skimmer is a more accurate way, because the human at 
the RX end is out of the picture.


> There's a lot of scatter in the dB measurements from skimmers. If I see
> dozens of spots graphed on the reversebeacon "spots comparison tool" then 
> I
> can believe systemic differences like 3-5dB. But I could never draw that
> conclusion over a single pair of spots.
>
> Any given skimmer will spot a given station on a given frequency at most
> once every ten minutes. But when the geographic density of skimmers is
> large enough (e.g. East coast US or Western Europe) just raw quantities or
> breadth of spots starts being more interesting than exact dB level. Even
> with the paucity of skimmers on west coast of US, I can still see who has 
> a
> 4-square for transmit and how they steer it during the contest.
>
> Tim N3QE
>
>
>
> On Mon, Aug 18, 2014 at 7:53 AM, Tom W8JI <w8ji@w8ji.com> wrote:
>
> > I am not a Skimmer expert, and am just asking. Question:  Are all the
> > Skimmers individually(and collectively) calibrated in concert? Can one 
> > rely
> > on them for comparing scientific data and conclusion to prove or 
> > ascertain
> > a point?Val>>>>
> >
> > Val,
> >
> > A live comparison of S/N ratio or relative level over time is with very
> > few exceptions an excellent comparative test. It is much more accurate 
> > than
> > S meters or absolute levels without a comparison reference. As such, the
> > RBN is a great tool for evaluating systems.
> >
> > The problems are:
> >
> > 1.)  For determining small differences, less than around 5 dB, you have 
> > to
> > know the performance level of the reference antenna or station. (For that
> > reason, I use a simple dipole reference.)
> >
> > 2.) The reference and AUT (antenna under test) have to be reasonably 
> > close
> > together to eliminate propagation variances, but not so close as to
> > interact, and they have to be in the clear. For example, it would be
> > foolish for me to plant a dipole in the middle of a bunch of Yagi 
> > antennas
> > and call it a reference, or put the antenna being evaluated in an
> > obstructed area.
> >
> > 3.) On skywave, there has to be some time involved with readings averaged
> > over time. This is somewhat true if there is more than a few wavelengths
> > distance between antennas, and especially true (almost critical) when
> > comparing different polarization antennas.
> >
> > 4.) Ideally the reference and AUT should be the same polarization, unless
> > we simply want to know which is louder overall.
> >
> > 5.) Antennas have sweet and sour heights for a given set of conditions. 
> > We
> > have to be very careful of this. This is especially true when antennas 
> > are
> > more than a half wavelength high above ground, because the antenna 
> > pattern
> > will be a series of deep nulls that selectively "notch out" a given wave
> > angle.
> >
> > The RBN is an excellent tool. It does not need to be calibrated in
> > absolute level, only in dB, and dB to noise is just fine provided the 
> > noise
> > level of the receive site is steady.
> >
> > One thing I hope we all can do is stop acting so "American" (we are now
> > what, 30th or 40th in math and science?) and get back to constructive
> > exchanges of information. If we stop learning and just pick a position 
> > and
> > fight, which is our trend today, this becomes a useless hobby and there 
> > is
> > no reason to communicate.
> >
> > 73 Tom
> > _________________
> > Topband Reflector Archives - http://www.contesting.com/_topband
>
>
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