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Multi hopping radio waves
With multi-hops through the ionosphere there will be reflections from
the ground between adjacent hops. If there are N hops there will be N-1 ground reflections. At each reflection a signal loss is incurred. But the ground is a variable quantity. What rule of thumb relating to reflection loss is used by radio professionals when calculating path loss? At the back of my mind I have a figure of 5 dB per reflection. Is this in the right ball park? Thank you. ---- Reg. |
Multi hopping radio waves
"Reg Edwards" wrote With multi-hops through the ionosphere there will be reflections from the ground between adjacent hops. If there are N hops there will be N-1 ground reflections. At each reflection a signal loss is incurred. But the ground is a variable quantity. What rule of thumb relating to reflection loss is used by radio professionals when calculating path loss? At the back of my mind I have a figure of 5 dB per reflection. Is this in the right ball park? Thank you. ---- Reg. ========================================= It has just occurred to me that the loss due to ground reflections may be related to the angle at which waves strike the earth. It is the same angle as the transmit elevation angle and can be quite small. ---- Reg. |
Multi hopping radio waves
Reg Edwards wrote:
========================================= It has just occurred to me that the loss due to ground reflections may be related to the angle at which waves strike the earth. It is the same angle as the transmit elevation angle and can be quite small. ---- Reg. The amplitude and phase of a field after ground reflection depends on the polarization, and is quite different for horizontal and vertical. It of course also depends on ground conductivity and permittivity, frequency, and angle. The equations are simple, and can be found in Kraus and other references. Those equations are used by NEC for determination of the far field pattern. Roy Lewallen, W7EL |
Multi hopping radio waves
The amplitude and phase of a field after ground reflection depends
on the polarization, and is quite different for horizontal and vertical. It of course also depends on ground conductivity and permittivity, frequency, and angle. The equations are simple, and can be found in Kraus and other references. Those equations are used by NEC for determination of the far field pattern. Roy Lewallen, W7EL =========================================== Yes! But what's the ball park, rule-of-thumb value in dB ? ---- Reg |
Multi hopping radio waves
Reg Edwards wrote:
The amplitude and phase of a field after ground reflection depends on the polarization, and is quite different for horizontal and vertical. It of course also depends on ground conductivity and permittivity, frequency, and angle. The equations are simple, and can be found in Kraus and other references. Those equations are used by NEC for determination of the far field pattern. Roy Lewallen, W7EL =========================================== Yes! But what's the ball park, rule-of-thumb value in dB ? ---- Reg I dunno. What's the ball park, rule-of-thumb value of a resistor in ohms? Roy Lewallen, W7EL |
Multi hopping radio waves
Uh, uh,
Wait until Richard Clark sees this and descends on you!!! :-) Reg, you can't just look at HF waves propagation through simple "reflective" glasses. There is considerable amount of propagating going on by refraction, ducting, polarization gets all tumbled around and ground conditions vary so much that even those orderly, same current in a loading coil believers get drowned. There are some propagation prediction programs that will do some predicting +- 59%, but that's about it. Otherwise as W7EL says, if you are looking at the antenna pattern forming properties within few wavelengths you need to consider polarization and ground conditions within the zone, but once you get beyond "first hop" you are on the mercy of propagation Gods. You can look at my oooold article http://members.aol.com/ve3bmv/bmvpropagation.htm which at the time was judged "ridiculous" and "nothing new" at the same time. Now a days, especially low band crowds are coming around and admitting that there is perhaps more refracting/ducting than reflecting going on and trying to figure out when, why, how. W8JI fought another losing battle claiming that there is no high angle propagation on extreme DX signals on 160/80, or skewed path. Now he is "guru" on the subject. Soooo, depends.... what you are after? For "regular" conditions you can apply some ballparks, but for extreme DXing and weak signal comms, there is whole different world outside of formulas. Yuri K3BU, VE3BMV "Reg Edwards" wrote in message ... The amplitude and phase of a field after ground reflection depends on the polarization, and is quite different for horizontal and vertical. It of course also depends on ground conductivity and permittivity, frequency, and angle. The equations are simple, and can be found in Kraus and other references. Those equations are used by NEC for determination of the far field pattern. Roy Lewallen, W7EL =========================================== Yes! But what's the ball park, rule-of-thumb value in dB ? ---- Reg |
Multi hopping radio waves
Roy wrote -
I dunno. What's the ball park, rule-of-thumb value of a resistor in ohms? ========================================= Lord Kelvin was correct. |
Multi hopping radio waves
"Reg Edwards" wrote in message
... With multi-hops through the ionosphere there will be reflections from the ground between adjacent hops. If there are N hops there will be N-1 ground reflections. At each reflection a signal loss is incurred. But the ground is a variable quantity. What rule of thumb relating to reflection loss is used by radio professionals when calculating path loss? At the back of my mind I have a figure of 5 dB per reflection. Is this in the right ball park? Thank you. ---- Reg. Reflection loss can vary from zero to infinity; depending on the material and angle of incidence. For example; take the oblique incidence of a horizontally polarized EM wave on an air/rocky ground interface of:: conductivity 2 mS/m, and relative permittivity 15. The reflection loss at zero degrees is 4.4 dB, increasing to 21.4 dB at the pseudo Brewster angle of 76 degrees, and 0 dB at 90 degrees. Ref. Advanced Engineering Electromagnetics, C. A. Balanis, pp 206 - 214. I have copies of the Mathcad calculations if anybody is interested. Frank |
Multi hopping radio waves
Reg, G4FGQ wrote:
"At the back of my mind I have a figure of 5 dB per reflection. Is this the right ball park?" It could be right for some reflections. E.A. Laport was Chief Engineer of RCA International when he wrote "Radio Antenna Engineering". On page 236 Fig. 3.17 shows the effect of ground conductivity on maximum field strength from a horizontal dipole antenna versus its height in vavelengths. Optimum height would be about 0,50 wavelength to most concentrate energy at a certain vertical angle, 30-degrees according to the RAF Signal Manual quoted by Laport. 30-degrees might hop 1000 kilometers. At the antenna earth reflection point, frequencies between 2 and 16 MHz are reduced to 95% of their prereflection field strengths by ordinary soil from a dipole at 1/2-wavelength height. A reduction to 70.7% of prereflection strength would represent a 3 dB power loss. So no harm done yet by the reflection from an antenna over good soil. Lower antenna height and poorer soil would attenuate more. The angle at which rhe signal strikes the earth in subsequent reflections should be the same as the first reflection from the antenna. Conductivity and dielectric constant at subsequent earth reflection points are what they are. Shortwave broadcasters use vertically stacked horizontal elements to concentrate the vertical beam to avoid multipath interference. They also prefer targets reached on the first reflection from the ionosphere.. Best regards, Richard Harrison, KB5WZI |
Multi hopping radio waves
On Thu, 27 Apr 2006 08:22:17 -0400, "Yuri Blanarovich"
wrote: Uh, uh, Wait until Richard Clark sees this and descends on you!!! :-) Hi Yuri, You mean he might stand a chance of obtaining data instead of griping: W8JI fought another losing battle Reggie, Rule of hand with 5 thumbs.... Land Loss deg 10 20 30 40 band 80 3 4+ 5+ 6 40 3+ 5+ 6 7 30 3+ 6 7 8 20 3+ 6+ 7+ 8+ 10 3 6+ 7+ 8+ Sea Loss deg 10 20 30 40 band 80 .2 .14 .12 .1 40 .3 .18 .15 .13 30 .4 .2 .17 .15 20 .45 .27 .22 .18 10 .52 .38 .28 .27 Washington D.C. - London Autumn Ionospheric 1.35MHz Loss UT dB 00 0 02 0 04 0 06 0 08 0 10 2 12 1.5 14 1 16 1.5 18 1 20 .5 22 0 Earth/Ionospheric Waveguide Loss/10000kM band 80 20+ 40 8 30 3 20 2 10 2 73's Richard Clark, KB7QHC |
Multi hopping radio waves
Reflection loss can vary from zero to infinity; depending on the
material and angle of incidence. ==================================== Yes I know. That's obvious. But what I would like to know is a typical or average value, or a range of most likely expected values found in practice. Some experienced person must know! Needed to crudely ESTIMATE ionospheric multi-hop path loss. Ground reflection losses are a small proportion of the total path loss but not small enough to be neglected. Just a whole number of decibels per reflection will do please. --- Reg. |
Multi hopping radio waves
"Reg Edwards" wrote in message ... Reflection loss can vary from zero to infinity; depending on the material and angle of incidence. ==================================== Yes I know. That's obvious. But what I would like to know is a typical or average value, or a range of most likely expected values found in practice. Some experienced person must know! Needed to crudely ESTIMATE ionospheric multi-hop path loss. Ground reflection losses are a small proportion of the total path loss but not small enough to be neglected. Just a whole number of decibels per reflection will do please. --- Reg. Some approximate examples at 7 MHz with angles of incidence from 0 - 40 degrees: Sea Water 0.2 dB Rocky Ground 5 dB Average Gnd. 2 dB Frank |
Multi hopping radio waves
Thank you Richard, and the others, for digging out and presenting the
information from which I can deduce a ball-park figure. My original figure of 5 dB per hop for a ground reflection was slightly too high. It's nearer to 3.5 dB. I'm still thinking about it. But 4 dB is near enough. Not that it matters very much. I deduced 5 dB from my experience of signal strengths received from such places as the antipodes and the Western US states. It's not surprising hopping across just the North Atlantic results in zero dB loss. The excess path loss, over and above spreading loss, must be due to ground reflection losses. Just calculate backwards from received signal strength, path distance and the probable number of hops. It's easy to do this with dedicated computer programs. For very long distance paths assume that half the ground reflections are due to sea/ocean water and the other half is due to land masses and you won't go far wrong. Ground reflection losses at the transmitting and receiving sites can be ignored. Or such losses can be lumped together to constitute another complete hop. I needed the information to include in a simple program which predicts (approximately) path loss for multi-hop propagation. Program users cannot be expected to know incidence angles, ground conductivity and permittivity, Brewster angles etc, for every hop along a route. Some guesswork is needed on the part of the programmer to make a program useful. Despite your continuing abuse of the English language I know you are quite capable of understanding the foregoing waffle. I'm on South Eastern Australian Merlot tonight. Hic! I think the Chinese will be coming along shortly with their own high quality stuff. They are not just becoming adept with their high technology. Californian wine growers should look to their laurels. But no doubt you are all more immediately concerned with the price of oil, the gas in your tanks, and from where it may be obtained. Iraq and Iran for example. ---- Reg, G4FGQ |
Multi hopping radio waves
Roy Lewallen wrote:
Reg Edwards wrote: The amplitude and phase of a field after ground reflection depends on the polarization, and is quite different for horizontal and vertical. It of course also depends on ground conductivity and permittivity, frequency, and angle. The equations are simple, and can be found in Kraus and other references. Those equations are used by NEC for determination of the far field pattern. Roy Lewallen, W7EL =========================================== Yes! But what's the ball park, rule-of-thumb value in dB ? ---- Reg I dunno. What's the ball park, rule-of-thumb value of a resistor in ohms? Sumptin tween nuttin and everthin Roy Lewallen, W7EL |
Multi hopping radio waves
Frank wrote -
Some approximate examples at 7 MHz with angles of incidence from 0 - 40 degrees: Sea Water 0.2 dB Rocky Ground 5 dB Average Gnd. 2 dB ========================================= Thanks Frank, Your figures for 7 MHz are representative of HF. Nobody is particularly interested in MF, LF or VLF. They confirm my latest estimate of an average, between sea water and dry land, of roughly 3.5 dB per ground reflection. ---- Reg. |
Multi hopping radio waves
"Reg Edwards" wrote in message ... Frank wrote - Some approximate examples at 7 MHz with angles of incidence from 0 - 40 degrees: Sea Water 0.2 dB Rocky Ground 5 dB Average Gnd. 2 dB ========================================= Thanks Frank, Your figures for 7 MHz are representative of HF. Nobody is particularly interested in MF, LF or VLF. They confirm my latest estimate of an average, between sea water and dry land, of roughly 3.5 dB per ground reflection. ---- Reg. No problem Reg. I did the computations recently trying to estimate the path loss, on 7 MHz, from the west coast to Europe via the long path. The various ground losses from all kinds of ground, including polar ice, and fresh water, are all fairly low. The real problem was to estimate the losses due to the incidence with the diffuse, conductive, ionospheric plasmas, and also to consider the possibility that some inonospheric ducting occurs. So far I have not attempted to estimate these losses, but they must be very high, since only considering ground incidence produces unrealistically low path losses. Frank |
Multi hopping radio waves
Dear Group:
The use of one number is crude and suitable only if one has imbibed enough grain alcohol or one does not care about Lord K's admonition or both. The preferred scheme is to model the expected virtual heights at each ionospheric reflection (note that the apparent TOA (angles) involved are not likely to be the same at each of these reflections) and then to use the angles involved to note the expected incidence angle of the intermediate "ground" reflection. The coordinates of the expected "ground" reflection areas are calculated and note is taken as to whether those areas are predominately salt-water, ice, or neither. A heuristic algorithm is applied that depends on type of "ground" and that weakly depends on the expected incidence angle. The result is a statistically significant reflection loss. [Some iteration is needed, which computers are very good at doing.] It is important to note that the apparent TOA of the major mode between two points is unlikely to be the same at both ends of the path because the virtual height is unlikely to be the same near both ends. For HF paths greater than something like 12 or 14 Mm predictions become more complex. In all cases of multiple hops (real DX), the TOA at which the gain of the antennas is effective varies between about 12 degrees and 2 degrees. If one wishes a simple predictor, one could find it in a paper published in the UK in the transactions on a conference on HF propagation in the 80s (as I recall) by a BBC engineer (IEE was the publisher). The entire computer code is given in the paper. It uses a simple heuristic model that was found by the BBC to give satisfactory results over the paths that their World Service used. Note that the BBC used paths that did not include much of the polar regions. To digress: The worst path to a DXCC entity from the Upper Midwest is over the magnetic pole to VU4. 100 watt transmitters, ice reflections, heavy absorption, and roughly 10 db more noise at the far end make the need for power and high antennas almost necessary. 73 Mac N8TT -- J. Mc Laughlin; Michigan U.S.A. Home: "Richard Harrison" wrote in message ... snip It could be right for some reflections. E.A. Laport was Chief Engineer of RCA International when he wrote "Radio Antenna Engineering". On page 236 Fig. 3.17 shows the effect of ground conductivity on maximum field strength from a horizontal dipole antenna versus its height in vavelengths. Optimum height would be about 0,50 wavelength to most concentrate energy at a certain vertical angle, 30-degrees according to the RAF Signal Manual quoted by Laport. 30-degrees might hop 1000 kilometers. At the antenna earth reflection point, frequencies between 2 and 16 MHz are reduced to 95% of their prereflection field strengths by ordinary soil from a dipole at 1/2-wavelength height. A reduction to 70.7% of prereflection strength would represent a 3 dB power loss. So no harm done yet by the reflection from an antenna over good soil. Lower antenna height and poorer soil would attenuate more. The angle at which rhe signal strikes the earth in subsequent reflections should be the same as the first reflection from the antenna. Conductivity and dielectric constant at subsequent earth reflection points are what they are. Shortwave broadcasters use vertically stacked horizontal elements to concentrate the vertical beam to avoid multipath interference. They also prefer targets reached on the first reflection from the ionosphere.. Best regards, Richard Harrison, KB5WZI |
Multi hopping radio waves
Mac, N8TT wrote:
"Note that the BBC used paths that did not include much of the polar regions." That`s an indication that they knew something about broadcasting. There must be a better way somewhere that doesn`t involve a path near a polar region. A broadcaster must advertise his programs well in advance. He can`t find a band that`s open and tailor his emissions to it. He must fire up on schedule with the right program at the designated time and direction regardless of conditions. All is planned long ahead of time. One of the most usless toys we had was a backscatter ionospheric sounder. It worked well but we had no immediate need for the information it provided. We were locked in our schedules for months in advance and today`s ionospheric conditions were short-term indeed. Best regards, Richard Harrison, KB5WZI |
Multi hopping radio waves
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