Topband: Skimmer calibration

[Tom W8JI]

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 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
>> _________________
>> Topband Reflector Archives - http://www.contesting.com/_topband
>>
>
>
>
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