Transmitter range

Hi all,
This is a really interesting discussion. The "quick and simple" answer is reference to the formula for a free-space path loss:

PathLoss-km (dBd) = 28.1 + 20Log(d) + 20Log(f)

Look at the frequency bit:

@ 500kHz (0.5MHz) 20Log(f) =-3dB
@ 100MHz 20Log(f) =+20dB

There is a 23dB difference (200x) between the path losses for the two frequencies. So if you were in outer-space (no planets or black holes) then you would get 14x more distance with 0.5MHz than with 100MHz. But you would need bigger antennas for the lower frequencies (dipole length = 1.5m or 300m).

HOWEVER
On earth, 100MHz will be refracted but NOT reflected by ionised layers, so once more your range would be limited to distance-to-horizon (unless you have a piss-poor receiver).
0.5MHz will come back to earth after reflection, so you could expect

1 - Groundwave to horizon, then nothing until ...
2 - 120km(ish) with the first skip
3 - 350km(ish) with the second skip (assuming you have enough TX power)

This is exactly what Glenn wrote, but I believe in fewer words.

Just had to stuff in my two-penneth :-)

BR Harry - EA/SM0VPO
(QRV tomorrow 12 noon European clock time - 14MHz and/or 18MHz from IM86bs)

Jalex2 wrote:Hi Harry
I have a simple question  for you.
Lets say  we have a transmitter at 500 mw and 100 Mhz 
and a transmitter running 500 mw at 500Khz  
What would the difference in range be assuming they both have properly tuned simple dipole antennas on them?

Please read these articles:

Radio Waves and Communications Distance - ARRL:
http://www.arrl.org/files/file/Technology/tis/info/pdf/8501031.pdf

How far can I talk - Radio Range information:
https://www.freewaycom.ca/pages/how-far-can-i-talk-radio-range-information

AM Radio Propagation (800kHz):
http://www.wirelesscommunication.nl/reference/chaptr03/pel/pel_am.htm

https://en.wikipedia.org/wiki/Radio_propagation
Quote: "...
At lower frequencies in the MF [includes 500kHz], LF, and VLF bands, due to diffraction radio waves can bend over obstacles like hills, and travel beyond the horizon as surface waves which follow the contour of the Earth. These are called ground waves. AM broadcasting stations use ground waves to cover their listening areas.
..."

https://en.wikipedia.org/wiki/Category:Radio_frequency_propagation

https://en.wikipedia.org/wiki/Line-of-sight_propagation
Quote: "...
However, at frequencies above 30 MHz (VHF and higher) and in lower levels of the atmosphere, neither of these effects are significant. Thus, any obstruction between the transmitting antenna (transmitter) and the receiving antenna (receiver) will block the signal, just like the light that the eye may sense. Therefore, since the ability to visually see a transmitting antenna (disregarding the limitations of the eye's resolution) roughly corresponds to the ability to receive a radio signal from it, the propagation characteristic at these frequencies is called "line-of-sight". The farthest possible point of propagation is referred to as the "radio horizon".

In practice, the propagation characteristics of these radio waves vary substantially depending on the exact frequency and the strength of the transmitted signal (a function of both the transmitter and the antenna characteristics). Broadcast FM radio, at comparatively low frequencies of around 100 MHz, are less affected by the presence of buildings and forests.
..."

https://en.wikipedia.org/wiki/Non-line-of-sight_propagation

https://en.wikipedia.org/wiki/Radio_propagation_model

-

Please note that a half-wavelength dipole antenna for 500kHz would be 300 meter high or wide.

Inspiration for a 500kHz antenna (radioamateurs can use 472 - 479 kHz):

Antennas for 136kHz:
http://www.strobbe.eu/on7yd/136ant/

http://www.strobbe.eu/on7yd/136ant/#Meander
Quote: "...
When scaled to 136kHz this 0.029 wavelength antenna becomes 64m high [17,4 meter high at 500kHz] and for the 21 elements over 1.3km (!) [354 meter at 500kHz] of wire would be needed. But, assuming 60 groundloss, this antenna would perform almost 18dB better than a vertical monopole of the same height.
A meander antenna can be built rather compact arround a grounded tower.
..."

Harry did make a multiband meander antenna for shortwave:
https://web.archive.org/web/20190803180809/http://213.114.137.174/antennas/balcant2.htm

He used a few "special" nylon food preparation boards from Ikea to isolate the center antenna rod (old CB antenna) from the metal handrail.

Hi Harry,
perhaps I am wrong, but I believe the Earth diameter is 12742 km.

---------------------------------
Money cannot buy happiness, but it is hard to be happy without it.

VBR Ivan

Hi Ruud and Jalex,

If you look at my homepages you will see a complete article dedicated to just this question. Search for "path losses":

http://213.114.137.174/data/pathlos.htm

Basically, a transmitter does not have a "range".

Take your transmitter power in dBm (plenty of on-line calculators).
Take your receiver sensitivity in dBm.

The difference between the two levels is the path loss (in dB) that you can have (link loss budget). If you want a good "fade margin" then take another 20dB from this. If you have a gain in the antenna(s) then add this to the link budget. If there are obstructions, then these create a loss, which should be removed from the budget. See the article.

The pathloss is APPROX (simple figures so I can use my fingers, and without taking off my socks)

30dB + 20*Log(distance in km) + 20*Log(frequency in Hz).

----------------------------

If you have a 1 Watt transmitter and you increase the power to 100 Watts, then you will have a 20dB increase in signal strength. This is 3 S-points on a hamradio receiver, or 10x the voltage at the receiver. It can bring a signal out of the noise.

An antenna with a 20dB gain will give exactly the same effect, but the 20db gain will be applied on BOTH uplink and downlink.

If you have a 20db preamp in the receiver then this will also have the same effect (assuming the received signals are well above the RF noise level left behind by "the big bang").

Finally, another "rule of thumb":
if you have an obstruction then the signal reduces. The Earth is rounded, so at VHF the horizon can be very close. The earth is about 8000km in diameter, but since VHF rF will also bend a little, then RF believes the Earth is about 11000km Diameter.

If the world were a completely smooth ball, then the distance to horizon is the square-root of the antenna height multiplied by 4.125km.

If you antenna is only 1m above ground, then every dustbin, postbox, car, gatepost and shovel-leaning-against-the-wall will affect the signal. But if you get you antenna 100m above ground then the distance to horizon can be 41km (if there are no hills).

If your antenna is 100m above ground then only 10mW (+10dBm) will get you 41km, if the receiver sensitivity is good (-107dBm) (link budget = 117dB). At 1m height, 10-Watts (1000x) would be doing well if you got 3km.

Have I explained this well?

As frequency increases, the signal is absorbed more by physical objects (atmospheric moisture, trees, buildings, etc). Hence you need more power to make up for the signal loss. For constant power, the range decreases because the signal losses increase with increasing frequency. 

Another factor affecting terestrial communication range is the tendency of low frequencies to follow the curvature of the earth's surface by reflecting off of atmospheric layers or refracting through atmospheric layer boundaries. This allows for very long distance communication at low frequency. As frequencies get higher, the tendency is for the signal to pass through the layer boundaries rather than reflect, and refraction angles tend to lessen. The result is shorter range, and increasing power won't help - all you're doing is blasting a signal into space.

Maybe Harry can shine more light over this!

Hi Harry
I have a simple question  for you.
Lets say  we have a transmitter at 500 mw and 100 Mhz 
and a transmitter running 500 mw at 500Khz  
What would the difference in range be assuming they both have properly tuned simple dipole antennas on them?