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Date: Fri, 29 Jan 2010 00:46:56 +0000 (GMT)
From: ALAN MELIA <alan.melia@btinternet.com>
To: rsgb_lf_group@blacksheep.org
In-Reply-To: <4B621864.3030501@toya.net.pl>
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Subject: Re: LF: Signal strengths, WSPR reports , propagation
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Hi Piotr, Is it possible that Andy is radiating a much stronger ground-wav=
e at that distance. I know the paths should be reciprocal, but if the weak=
er station ground wave was weaker, swamped in the noise??  Possibly the an=
tennas for the others are radiating much more upward and the ground wave=
 is being attenuated. The addition.subtraction of a ground wave at the sam=
e strength as the skywave could add or subtract 6dB (about the difference=
 in the figures)....I know that the Lowfers in the States see a dead zone=
 at their low power.....but that as with QRSS not a sophisticated system=
 like WSPR.=20

A slant range calculation formula is given on my web site. The apparent re=
flection height at 136kHz at night was lower of course 90-100km. At extrem=
e range the apogee was calculated by raytracing at about 86km (from memory=
) At short range and high angles the signal will penerate further before=
 being returned. The path calculated this was is in error because the refr=
action "shortens" the path over a "mirror reflection" model.
http://www.alan.melia.btinternet.co.uk/simple.htm.

http://www.alan.melia.btinternet.co.uk/fading.htm   Dont take too much not=
ice of the text it is quite old now but the equations work and can be solv=
ed at fading nulls. You need two receivers at different distances to resol=
ve the ambiguities.

An interesting point is that if you can calculate the ground-wave strength=
, you can estimate the skywave strength by the depth of the null(which asu=
mes they are out of phase of course).

I used the thumbnail figure that a watt ERP at 1km generates 300uV/m, for=
 my calculations. These kind of simple models can be very useful in trying=
 to understand what is happening and calculating whether any particular pa=
th is possible...... and its good fun !!

Best Wishes
Alan G3NYK


--- On Thu, 28/1/10, Piotr M=C5=82ynarski <sq7mpj@toya.net.pl> wrote:

> From: Piotr M=C5=82ynarski <sq7mpj@toya.net.pl>
> Subject: LF: Signal strengths, WSPR reports , propagation
> To: rsgb_lf_group@blacksheep.org
> Date: Thursday, 28 January, 2010, 23:06
> Dear Rik, James,LF group
>=20
> it is going to be long.... sorry...
>=20
> Some time ago Jim, G7NKS began a new topic on this
> reflector. it was about a link budget
> for wspr at  given distance. Recently, Rik, ON7YD has
> made a nice thing i.e. comparing reception
> levels of three stations using a 'small statistics' - that
> is exactly what wspr stands for.
> investigations of propagation paths, limits etc...
> Being 'triggered' by these two mails and , additionaly, by
> a phrase 'mill' used by Rik
> which translated into polish sounds very much like my
> surname ( Mlynarski) Mlyn=3Dmill  :)
> I started to think  about the estimate of an effective
> electric field 'felt' by RX station.
> Later, I will use Rik's collected data  as numerical
> example.  Due to the enormous
> and variable  noise at my qth  my reception data
> is not appropriate and lacks some stations.
> I like to do such 'guesstimates' almost from the scratch,
> so let me start
> from the 'basics' i.e.  classical electrodynamics
> which states that in a free space
> the power P radiated by an isotropic antenna in a distance
> R has its density:
> D=3DP/(4*pi*R*R). On the other hand, the mean magnitude of a
> Poynting vector (D) can be expressed
> by an electric field: D =3D E*E/(2*120*pi) ( 120pi =3D
> 377ohm=3Dimpedance of a free space)
> Combining these two one gets that the amplitude of an
> electric field E equals:
> E =3D sqrt(60*P)/R. if R is in [km] then E is expressed in
> miliVolts/metr, P is in Watts.
> I assume that we are dealing with a short vertical monopole
> so following Rik's explanations
> ( veeery useful post to the reflector! ) its directivity
> gain is 3 but as the power in wspr reports
> is given as ERP power so it must be multiplied by a factor
> of 1.64 in order to replace
> power P (isotropic) in the last formula by ERP*1.64 , where
> "ERP" is the reported erp power. Also,
> we can multiply  E  by  0.707 to get the
> effective signal strength E(eff):
> E(eff) =3D 0.707*sqrt(60*ERP*1.64)/R
> I focus ONLY on the ionospheric propagation and  1
> hop.
> At the night, the absorption of incoming wave (500kHz) to
> the ionosphere is really small so ,
> at first approximation, can be simply ommited and we are
> left now with relatively
> simple geometric considearations with further assumption
> that the distance between
> station a (TX) and station b (RX) is not an earth arc but a
> straight line, segment.
> {sure, it can be done more precisely taking into account
> mean earth radius etc...)
> The wave is propagating from  TX antenna with an
> elevation(take-off) angle alfa, the
> reflection takes place in  E layer, say, 120 km above
> earth (H) and the distance
> between stations a and b is "d". At the RX place we are
> interested in the vertical
> component of incoming reflected wave being doubled by
> reflection from the earth
> and assumed  antenna characteristic is described by a
> certain function A(theta)
> {theta=3D usual notation of one of the angles in spherical
> coordinate system)
> In the case of a short vertical monopole:  A(theta) =3D
> sine(theta) =3D cosine(alfa)
> so we have:
> E(eff) =3D E(eff)*2*cos(alfa)*cos(alfa)
> R in formula for E(eff) becomes: sqrt(d*d + 4*H*H)
> and cos(alfa) =3D d/sqrt(d*d + 4*H*H)
>=20
> (in the course of programming it is convenient to multiply
> E(eff) by 1000 thus getting
> the values in microVolts/m)
>=20
> The above final formula gives real values of an effective
> signal strength. It is a 'product'
> of  fundamental physics and simple geometric
> considerations. Obviously, there is an ample room for
> improvements but as i am not a radio-comm specialist i just
> do not know  how to make it -
> some reflection losses, absorption attenuation which both
> seem to be frequency
> and (probably) time dependent etc..
> Anyway, here comes  'numerical'side..
> I took Rik's qth as  RX station and the values of erp
> power as reported in wspr database ( or e-mails)
> by the TX stations:
> G4JNT: 420 km and 200 miliWatts of ERP pwr
> G3ZJO: 411 km     0.2 mW
> G3XBM: 337 km     1 mW
> H =3D 120 km ( the height of reflection point) (have found in
> literature it is 110-120 km on average)
> calculated values of E(eff) are:
> G4JNT: 9.777 microVolt/m  =20
>    G3ZJO: 0.311=20
>    G3XBM: 0.711
> calculated relative differences of signal strengths:
> (first, 'left', stations are favored)
> G4JNT-G3XBM: 22.77 dB
> G4JNT-G3ZJO: 29.95 dB
> G3XBM-G3ZJO:  7.18 dB
> Now, the same differences made by Rik (night receptions,
> wspr readouts, averages etc..)
> G4JNT-G3XBM:  15 dB
> G4JNT-G3ZJO:  23 dB
> G3XBM-G3ZJO:   8 dB
> In the case of Roger,"Water Pistol", G3XBM and Eddie,
> "Dripping Tap", G3ZJO
> theory and experiment are fairly consistent
> if Andy, "Big Gun", G4JNT had run only 40 mW ERP then his
> E(eff) would be 4.372 microVolt/m
> so then the calculated diferences:
> G4JNT-G3XBM: 15.78 dB
> G4JNT-G3ZJO: 22.96 dB
> (pretty nice matching..)
> Final conclusion: "Big Gun" shrinks !=20
>    :)
>=20
>               =20
>               =20
>               =20
>         Yours, Piotr, sq7mpj
> qth: Lodz /jo91rs/
>=20
>=20
>=20