Topband: Skimmer calibration

[Tim Shoppa]

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 at 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
> _________________
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>