As far as I know, skimmers are not signal-level coordinated, so strictly
speaking, it only makes sense to compare measurements made by the same
skimmer.
Although not an expert in this field, I have performed some tests with
skimmers, both under lab conditions as well as over the air.
First, skimmers will not provide a signal power measurement, but a signal to
noise ratio measurement. If the noise power does not vary with time (which
need not be necessarily true), you could take the provided SNR as a measure
of signal power relative to some constant noise power.
But over HF channels (where signal levels follow a Rayleigh distribution),
there is the additional measurement problem of fading. According to my
observations, skimmers estimate SNR only needing a very short transmission,
even shorter than a second. They will report SNR observed during that
particular interval. But that level may have occured during a signal
enhancement, or during a deep fade or anywhere in the middle. So
measurements from the same station at different times, even only differing
by seconds, may report SNRs differing several or many dBs. Let alone if you
are trying to compare signals from two different stations, whose fading
patterns are almost guaranteed to not coincide in time.
All that said, skimmers provide a valuable tool for estimating signal
levels, provided you are not trying to discern differences exceeding this
"statistical noise". In my experience, you cannot rely on them for that
purpose if you are looking for sub 5-10dB differences in ionospheric paths
with 2 or more hops.
73,
Juan EA5RS
-----Mensaje original-----
De: Topband [mailto:topband-bounces@contesting.com] En nombre de Hugh
Valentine
Enviado el: sábado, 16 de agosto de 2014 22:37
Para: w8ji@w8ji.com
CC: Topband@Contesting.com
Asunto: Re: Topband: Modeling the proverbial "vertical on a beach"
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
---------- Original Message ----------
From: "Tom W8JI" <w8ji@w8ji.com>
To: "Michael Tope" <W4EF@dellroy.com>, <topband@contesting.com>
Subject: Re: Topband: Modeling the proverbial "vertical on a beach"
Date: Thu, 14 Aug 2014 19:06:44 -0400
I looked at comparative data from four or five contests.
I'm too far out of the path and in distance (approaching 1000 miles
different) to compare, but I was still interested in how others in the NE
compared.
If it was really 10-20 dB, shore locations would stand out like a sore thumb
compared to inland locations. Everyone from around New England is about the
same. Heck, K3LR is on the Ohio/PA border and does just as well or better
than coastal stations in signal levels.
10 dB is completely undoable with antenna systems, once someone is at the 10
dB threshold of gain over a dipole at optimal height (except on 160 where
polarization is a player, and we have to compare vertical to vertical). 20
dB would be beyond the world of magic.
Understand I'm not saying there isn't a difference. I'm just saying it isn't
an exaggerated difference that jumps out in the ways we use our systems.
Watch skimmer yourself. It is entertaining to watch.
73 Tom
----- Original Message -----
From: "Michael Tope" <W4EF@dellroy.com>
To: <topband@contesting.com>
Sent: Thursday, August 14, 2014 5:03 PM
Subject: Re: Topband: Modeling the proverbial "vertical on a beach"
> On 8/13/2014 6:28 PM, Tom W8JI wrote:
>>>
>>
>> But skimmer, which displays a relative level, does not show the level
>> difference.
>>
>> Skimmer shows about the same peak levels, but the stations closer or over
>> salt water paths (not localized salt water) have longer openings but no
>> more level for peak level. Anyone can look at that.
>>
>> K3LR is about as strong into Europe, when I look at skimmer levels, as
>> someone on the coast.
>>
>> The exceptions are people right next door to Europe (like VY1).
>>
>>
>> 73 Tom
>
> Tom,
>
> How much skimmer data did you mine before establishing a firm conclusion
> that the advantages of saltwater proximity are exaggerated?
>
> Myself, I think of how well AA7JV and HA7RY have done at various locations
> using antennas that were very close to or in some cases literally in the
> saltwater. The consistency of their topband signals compared to
> Dxpeditions who were confined to inland locations seems to point to a big
> advantage. I'll admit, however, that this hypotheses comes about from
> anecdotal observations filtered through a mental lens that is biased
> towards believing saltwater is a huge advantage.
>
> I think using skimmer is an excellent approach to this question provided
> of course that you have mined enough data to filter out the statistical
> noise.
>
> 73, Mike W4EF...................
>
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