Signal strenght vs distance

[Danielvi]

Got into this: https://www.pasternack.com/t-calculator-fspl.aspx

On 27 MHz if I calculate the loss after 100m I get 35.57 dB lost and after 500 m 49.55 dB lost.

There are 14 dB (more than 2 S-points) lost in 400 m ???

What is the reference ? 0 m from antenna ?

I'm not the genius of mathematik but that looks strange...

Does anybody knows hot to use it ?

thanks,
Daniel

 

[The DB]

I can see why this would seem surprising at first, but that is no mistake, and in fact how antennas have always worked. The energy radiated from an antenna is dispersed very quickly. It isn't like a laser beam. See yourself as in the center of a sphere, or ball. How many directions are there between you and any given point of the ball? The power radiated from an antenna has to be divided among all (or if you are on an earth half) of these directions. Imagine, every possible angle gets some power, how much of that power do you think actually goes in any one direction? The larger the area (or the larger the ball) the more dispersed the power is on the area's surface.

Because of the ever expanding surface area that shares the same power, power does indeed drop off very quickly. We actually measure transmitted power up into the kilowatts (thats thousands of watts), yet a receiving s-meter measures in micro-watts (actually micro-volts, which is 0.000001 volts, and actually, the lower s-meter readings measure even smaller amounts than that).

This is partly why adding more power doesn't effect a transmitted signal as much as a lot of people want to believe it does.


The DB

 

[Danielvi]

I think I got the point, thanks.
Daniel

 

[Jay Mojave]

“Got into this: https://www.pasternack.com/t-calculator-fspl.aspx

On 27 MHz if I calculate the loss after 100m I get 35.57 dB lost and after 500 m 49.55 dB lost.

There are 14 dB (more than 2 S-points) lost in 400 m ???

What is the reference ? 0 m from antenna ?

I'm not the genius of mathematik but that looks strange...

Does anybody knows hot to use it ?

thanks,
Daniel”

Hello Daniel: Yeah that's right I believe approx. 14 dB. I get different dB numbers but they are 14 dB difference from 100m as compared to 500m

Its 12 dB in free space loss, for 4 times the distance. I get confused at times and throw the #@$! Calculator out the room, I know the feeling….

A basic fact to know is that when the distance is doubled you will have a decrease of -6dB free space loss. I believe that is why S-Meters are supposed to be calibrated 6dB for every S-Unit. Or if the distance is reduced by half you will get a increase of +6dB increase.

The reference for measuring antenna field strength will be in the far field, this allows the radiation wave to be straight and not curved possible causing measurement errors. Another ball of worms.

If you plug in 27 MHz at 1 nautical mile, the free space loss will be 66.4 dB.

Freq in MHZ X Distance Logx20 + or - from distance units = dB in free space loss

Nautical Miles +37.8

KM +32.45

Meters -27.55

Yards -28.33

Feet -37.87

So at 27MHz, 1 NM its 66.4 dB. At double the distance its 66.4 + 6 = 70.4 dB free space loss. Or at 27MHz at ½ NM will be 66.4 -6 = 60.4 dB free space less.

This basic double range -6dB loss, or half range + 6dB increase is a handy fact to know allowing quick and accurate antenna field strength measurements with simple equipment.

The antenna field strength for longer distances you will see that the 6dB rule will cover many miles and is a logarithmic function, meaning it ain’t all bad up close as compared to longer distances.

If a rock is thrown into a pool of water you will see the waves decrease with distance traveled, same physics thing only in plan view.

Hope this helps. Field Strength Measurements will set you free....

Jay in the Great Mojave Desert

See:
http://www.navair.navy.mil/nawcwd/ewssa/downloads/NAWCWD TP 8347.pdf

Pages 4-3.1 thru 4-3.9

 

[StrangeBrew]

Just to make sure I got this. In a nutshell the farther from the antenna you are the farther you have to go to double the distance, as such the farther away you get the less of an effect a certain increase in distance will have. That about right?

If so then for DX purposes this isn't as much of a problem as it would seem to be at first glance, the initial drop may look scary but things would sort of "stabilize" to some degree out in the distance.

 

[The DB]

Just to make sure I got this. In a nutshell the farther from the antenna you are the farther you have to go to double the distance, as such the farther away you get the less of an effect a certain increase in distance will have. That about right?

That is correct, however, it makes certain assumptions, such as the ability to travel in a straight line from the transmitted antenna to the receiving antenna with nothing in between. Unfortunately this assumption neither applies to most local or any DX communications. It is, however, a good starting point.


The DB

 

[StrangeBrew]

I know this mostly applies to line of sight, it's just good to know it's not really working against us much. Thanks for the reply.

 

[Robb]

Better coax = minimizes loss.
Loss changes with freq; under ~24mhz loss becomes less. Above ~24mhz, losses increase dramatically as you go up in freq. But a long run of good coax gets pretty pricey; best to reduce the coax run to a minimum for the reasons of cost, performance, and efficiency.

 

[The DB]

So Robb, what you said is true but, just curious, what does coax loss have to do with what is being discussed in this thread? We are talking about the effects of signal strength with distance after it has been transmitted from the antenna, not as it travels down a feed line...

The posts where they talk about "loss" above, that is really a poor word for what is actually going on, signal isn't actually being "lost", it is simply spreads out more and more as it moves further away from the transmitting antenna.


The DB

 

[Danielvi]

“Got into this: https://www.pasternack.com/t-calculator-fspl.aspx

On 27 MHz if I calculate the loss after 100m I get 35.57 dB lost and after 500 m 49.55 dB lost. View attachment 19979

There are 14 dB (more than 2 S-points) lost in 400 m ???

What is the reference ? 0 m from antenna ?

I'm not the genius of mathematik but that looks strange...

Does anybody knows hot to use it ?

thanks,
Daniel”

Hello Daniel: Yeah that's right I believe approx. 14 dB. I get different dB numbers but they are 14 dB difference from 100m as compared to 500m

Its 12 dB in free space loss, for 4 times the distance. I get confused at times and throw the #@$! Calculator out the room, I know the feeling….

A basic fact to know is that when the distance is doubled you will have a decrease of -6dB free space loss. I believe that is why S-Meters are supposed to be calibrated 6dB for every S-Unit. Or if the distance is reduced by half you will get a increase of +6dB increase.

The reference for measuring antenna field strength will be in the far field, this allows the radiation wave to be straight and not curved possible causing measurement errors. Another ball of worms.

If you plug in 27 MHz at 1 nautical mile, the free space loss will be 66.4 dB.

Freq in MHZ X Distance Logx20 + or - from distance units = dB in free space loss

Nautical Miles +37.8

KM +32.45

Meters -27.55

Yards -28.33

Feet -37.87

So at 27MHz, 1 NM its 66.4 dB. At double the distance its 66.4 + 6 = 70.4 dB free space loss. Or at 27MHz at ½ NM will be 66.4 -6 = 60.4 dB free space less.

This basic double range -6dB loss, or half range + 6dB increase is a handy fact to know allowing quick and accurate antenna field strength measurements with simple equipment.

The antenna field strength for longer distances you will see that the 6dB rule will cover many miles and is a logarithmic function, meaning it ain’t all bad up close as compared to longer distances.

If a rock is thrown into a pool of water you will see the waves decrease with distance traveled, same physics thing only in plan view.

Hope this helps. Field Strength Measurements will set you free....

Jay in the Great Mojave Desert

See:
http://www.navair.navy.mil/nawcwd/ewssa/downloads/NAWCWD TP 8347.pdf

Pages 4-3.1 thru 4-3.9

"Field Strength Measurements will set you free...."

That was a good one

Thanks for explanation.

 

[Robb]

So Robb, what you said is true but, just curious, what does coax loss have to do with what is being discussed in this thread? We are talking about the effects of signal strength with distance after it has been transmitted from the antenna, not as it travels down a feed line...

The posts where they talk about "loss" above, that is really a poor word for what is actually going on, signal isn't actually being "lost", it is simply spreads out more and more as it moves further away from the transmitting antenna.


The DB

My mistake.
Sorry.