I'm not as familiar with the geometry of meteor scatter propagation as others, so this may be a rookie question. But is there actually a situation where propagation can be "one way"? On several occasions while attempting to work another station, they'll report getting several good decodes from me, while I'm hearing nothing from them. In the past, I've chalked this up to differences in station construction, local noise, etc. Since my antennas are inside the attic, and very small at 6 elements, it's not uncommon for me to have receiving issues. My usually it works the other way for me. For instance, I can often hear other stations on SSB or CW, who cannot seem to hear me. So, "one way rocks"--real phenomenon or should I rank it alongside ghosts, UFO's, and my ex-wife's story about her and that construction worker just being really good friends? 73, Les Rayburn, N1LF EM63nf
Rank it alongside ghosts, UFO's and "good friends" Les. It is not geometrically possible to have one way rocks. Cheers, Barry KS7DX
There is a lot of material available if one wishes to really understand meteor scatter, or, as it is more commonly known, as forward scatter. If you understand the geometry of forward scatter you'll realize that the expression "one way rocks" doesn't really compute. In order to cause a forward scatter reflection, the meteor trail must lie within a plane (called the tangent plane) which is tangent to an ellipsoid having the transmitter and receiver as its foci. The entire reflection path will also lie within a plane (called the plane of propagation), which contains the transmitter, reflection point, and receiver. The plane of propagation will be normal to (at right angles to) the meteor tangent plane. The meteor itself can be at any orientation within the tangent plane - it need not be normal itself to the propagation path. There is, however, greater signal loss when the meteor trail is perpendicular to the propagation plane than when it is parallel to the propagation plane. A third constraint is that most meteor reflections will occur within the narrow altitude band of about 85 to 105 km altitude. Thus, the sphere formed by the 95 km altitude band, the meteor tangent plane, and the ellipsoid having the transmitter and receiver as foci must all meet (or be tangential) at the reflection point. The "classical" equations for forward-scatter from a meteor trail, which have been derived from theory and validated empirically during the heyday of radiometeor astronomy (1945- 1970) , are as follows: Underdense trails (electron line density, Q < 1E14 electrons / meter): Underdense Echo Power The echo power received at the receiving station in a forward scatter underdense echo is given as the product of two fractions: P_r = ((P_t * g_t * g_r * lambda^3 * sigma_e) / (64pi^3)) * ((Q^2 * sin^2(gamma)) / ((r1 * r2) * (r1 + r2) * (1 - sin^2(phi) * cos^2(beta)))), where: P_r = power seen by receiver (Watts), P_t = power produced by transmitter (Watts), g_t = gain of transmitting antenna (dB), g_r = gain of receiving antenna (dB), lambda = RF wavelength (m), sigma_e = scattering cross section of the free electron (m^2), Q = electrons per meter of path, r1 = distance between meteor trail and transmitter (m), r2 = distance between meteor trail and receiver (m), phi = angle between r1 line and normal to meteor path tangent plane, or phi = 1/2 angle between the r1 and r2 lines, beta = angle between meteor trail and the intersection line of the tangent plane and plane of propagation, gamma = angle between the electric vector of the incident wave and the line of sight to the receiver (polarization coupling factor). A useful substitute for sigma_e is: sigma_e = 1.0E-28 * sin^2(gamma) m^2, which reduces in the back-scattter case to simply: sigma_e = 1.0E-28 m^2. Underdense Echo power decay A second useful expression for the exponential decay over time of the underdense echo power is given as an exponential (e^x) raised to a fraction): P_r(t)/P_r(0) = exp(- (((32pi^2 * D * t) + (8pi^2 * r0^2)) / (lambda^2 * sec^2(phi)))), where: P_r(t)/P_r(0) = normalized echo power as a function of time (t), t = time in seconds (sec), D = electron diffusion coefficient (m^2/sec), r0 = initial meteor trail radius (m). The diffusion coefficient, D, will increase roughly exponentially with height in the meteor region. An empirical derivation from Greenhow & Nuefeld (1955) is given for meteor altitudes of h = 80 km to h = 100 km: log10(D) = (0.067 * h) - 5.6, for D in m^2/sec. The initial meteor trail radius is another empirically derived value, given in two studies as: log10(r0) = (0.075 * h) - 7.2, h = meteor altitude (75-120 km) r0 = trail radius (m) and log10(r0) = (0.075 * h) - 7.9. Underdense echo duration An approximate expression for the duration of an underdense trail is given by: t_uv = (lambda^2 * sec^2(phi)) / (16pi^2 * D) Overdense trails (electron line density, Q > 1E14 electrons / meter): The classical expressions for the overdense trails contain many more assumptions and estimations than for the underdense trails. Their full theory is still under development today. However, the classical equations can still be used to glean some of the basic characteristics of these events. I am showing these here in their final form, skipping some intermediate steps and approximations. Overdense echo power P_r = 3.2E-11 * ((P_t * g_t * g_r * lambda^3 * Q^(1/2) * sin^2(gamma)) / ((r1*r2) * (r1+r2) * (1 -sin^2(phi) * cos^2(beta)))). Overdense Echo Duration An approximate expression for overdense echo duration is given by: t_ov = 7E-17 * ((Q * lambda^2 * sec^2(phi)) / D). If you wish to delve more into forward scatter then a good book to read is "Meteor Science and Engineering," D.W.R. McKinley, (McGraw-Hill, 1961) As I mentioned at the beginning there is a lot more to forward scatter than just pointing in the general direction of the meteors and hoping for the best. I'm not suggesting that you sit and do the calculations all the time but they do explain a lot of the varibles in play when you are running with someone. Bottom line have fun and don't get discouraged if you don't always get the result you expected. 73, Barry KS7DX
Hi Les,
-7 dB is fairly typical for a suburban area with a modest-sized yagi. I'm
pointed north right now (one of my "quiet" directions), and I get -7 dB
also
when I remove the antenna. It's worse in other directions.
Yeah, this makes m/s work more challenging, but not impossible, that's for
sure. My best DX is over 1,350 miles using FSK441 on 6m, and I've worked
all
the states I can easily reach that have any WSJT activity in them. But in
order to do this, I have to work pretty good stations on paths over 1,000
miles or so -- guys who are running some suds and some decent aluminum.
The
guys 1,200-1,300 miles away that are running 70 watts to a dipole, I never
hear. If my noise floor was 5 or 6 dB lower, I probably would.
The one question that must be asked about your ambient noise level, though --
where's it coming from? Is it natural or man-made? Natural noise comes
from
local periodic static charge build-up and discharge (not just lightning), the
Sun, and the galactic center. Man-made noise comes from all kinds of other
things, some of which can be limited or even eliminated. So, yeah, try to
determine what noises you have coming from which directions, and try to
figure
out what is causing them.
The first thing to check are your computers. Turn them off (including monitors)
and see how much your noise level drops. This can be different in various
directions, becuase computer noise is often radiated and picked up by the
antenna, especially if it is wide-band hash coming from a bad switching
power
supply. This is typically common-mode noise that uses the power cord and
your
house wiring for an antenna. :-) I've found that most people can lower
their
noise level by a good 1 or 2 dB (sometimes much more) by elminating all
computer-generated noise from their shack.
Bill W5WVO
----- Original Message -----
From: Les Rayburn
To: Bill W5WVO ; Bruce Brackin, N5SIX
Cc: WSJT Group
Sent: Monday, July 14, 2008 12:55 PM
Subject: Re: [wsjtgroup] Newcomer with lots of questions....
I'm starting on mapping the noise floor today. Preliminary results don't
look good. As suggested, I pointed the antenna in a typical direction and
set WSJT to "0" db. Then substituted a dummy load for the antenna. The
difference was about 7db! Ouch!
So, I'm open to suggestions about how to reduce the noise floor. I currently
have a 6el beam, and have a custom made 10 element beam coming, which should
provide a sharper pattern.
73,
Les Rayburn, director
High Noon Film
100 Centerview Drive Suite 111
Birmingham, AL 35216-3748
205.824.8930
205.824.8960 fax
205.253.4867 cell
--------------------------------------------------
From: "Bill W5WVO" <w5wvo@cybermesa.net>
Sent: Monday, July 07, 2008 6:02 PM
To: "Les Rayburn" <les@highnoonfilm.com>; "Bruce Brackin, N5SIX"
<n5six@bellsouth.net>
Cc: "WSJT Group" <wsjtgroup@yahoogroups.com>
Subject: Re: [wsjtgroup] Newcomer with lots of questions....
> Les Rayburn innocently queried:
>
>> 1.) During one QSO, the other station reported
>> hearing me on nearly every sequence, while I
>> heard them much less often. This could indicate
>> several things. Such as I'm deaf on 2 Meters. Or
>> that MS propagation is sometimes better for one
>> station than the other?
>
> Bruce Brackin sagely replied:
>
>>>> Generally indicates one may have receive or noise problem. I'll
> also send you a suggestion off list shortly.
>
> You will hear a lot of talk on PJ about "one-way rocks". I think most
> knowledgeable m/s folks recognize this as silliness.
>
> M/s is a weak-signal mode. The difference between not seeing a ping at all
> and seeing an easily decodeable ping can be as little as 3 dB. This tiny
> received-signal detection differential is easily made up by a difference
> in
> ambient noise floor between two stations.
>
> Here's a simple, non-scientific, but revealing test. On a typical day with
> your antenna pointed in a typical direction, adjust WSJT so that your
> total
> noise floor reads 0 dB, as per usual practice. Now disconnect the antenna
> and replace it with a resistive 50 ohm load. How many dB does the noise
> floor drop? Depending on your ambient environment (urban, suburban, rural,
> wilderness) and the height of your receive antenna (very low up to very,
> very high), you can see a drop of 2 or 3 dB (excellent) all the way up to
> 10
> to 15 dB (horrible), or worse. This determines, in the main, how
> successful
> you are going to be using the WSJT weak-signal modes.
>
> There are some things you can do to decrease your ambient noise floor, and
> I'm sure Bruce will give you some ideas. (In a nutshell: locate specific
> sources of external noise and eliminate them; make your antenna pattern
> sharper so it picks up noise from fewer directions; raise your antenna
> higher to get above the noise.) But at the end of the day, the only way to
> really solve the noise problem completely is to move to a much quieter
> location. Occasionally, this is a actually a possibility in a ham's life
> plan; mostly, it isn't. So you do the best you can.
>
> The other reason for a difference in perceived signal strengths, of
> course,
> is simply transmitter power. If you're running 500 watts, and the other
> guy
> is running 35 watts, you're looking at around 11 dB difference in your
> signals, all other things being equal (including your noise floors). By
> running higher power, you can overcome the other guy's high noise floor --
> but not your own! :-)
>
> 73 and welcome to the Dark Side,
> Bill W5WVO
>
Hi Les,
-7 dB is fairly typical for a suburban area with a modest-sized yagi. I'm
pointed north right now (one of my "quiet" directions), and I get -7 dB
also
when I remove the antenna. It's worse in other directions.
Yeah, this makes m/s work more challenging, but not impossible, that's for
sure. My best DX is over 1,350 miles using FSK441 on 6m, and I've worked
all
the states I can easily reach that have any WSJT activity in them. But in
order to do this, I have to work pretty good stations on paths over 1,000
miles or so -- guys who are running some suds and some decent aluminum.
The
guys 1,200-1,300 miles away that are running 70 watts to a dipole, I never
hear. If my noise floor was 5 or 6 dB lower, I probably would.
The one question that must be asked about your ambient noise level, though --
where's it coming from? Is it natural or man-made? Natural noise comes
from
local periodic static charge build-up and discharge (not just lightning), the
Sun, and the galactic center. Man-made noise comes from all kinds of other
things, some of which can be limited or even eliminated. So, yeah, try to
determine what noises you have coming from which directions, and try to
figure
out what is causing them.
The first thing to check are your computers. Turn them off (including monitors)
and see how much your noise level drops. This can be different in various
directions, becuase computer noise is often radiated and picked up by the
antenna, especially if it is wide-band hash coming from a bad switching
power
supply. This is typically common-mode noise that uses the power cord and
your
house wiring for an antenna. :-) I've found that most people can lower
their
noise level by a good 1 or 2 dB (sometimes much more) by elminating all
computer-generated noise from their shack.
Bill W5WVO