2.4 GHz vs 72 MHz - Free space loss
 

2.4 GHz vs 72 MHz - Free space loss


The free space loss between transmitter and receiver increases with frequency. The increase is 6 db for every doubling of frequency, or the same as for doubling of distance.

2400 MHz (2.4 GHz) is much higher frequency than 72 MHz. We have to double 72 MHz five times to reach approximately 2400 MHz (72-144-288-576-1152-2304). The free space loss is therefore 5 times 6 or 30 db more on 2400 MHz than 72 MHz. Thirty decibels is quite a lot. Same as the difference between a 1 milliwatt transmitter and a 1000 milliwatt transmitter.

The free space loss can be calculated for any frequency and distance by the formula:

L=32,45 + 20log f + 20log d where L= loss in db, f= frequency in MHz og d= distance in km.

See also a good description here:
Path Loss at VHF, UHF and Microwave frequencies
A free Space graphical representation
http://www.qsl.net/vk3jeg/pathloss.html

I used Excel to calculate the free space loss for various distances using the RC frequencies 35 MHz, 72 MHz and 2.4 GHz. The result is quite interesting. See the comments on the bottom of the table.



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What does this mean?
On 72 MHz (and 35 MHz) we have much more reserve signal from the receiving antenna to the receiver. We can use a simple wire antenna without problems. The attitude of the model usually does not affect reception.

On 2.4 Ghz, where the strenght of the received signal is much lower, we have to be very careful when installing the antenna. The attitude of the model can have adverse effect on the signal strength, so we may have to use two antennas spaced 90 degrees apart. Both Spektrum and Futaba use this approach.

On 2.4 GHz we can also have problems with Fresnel zone, especially if the model is flying low far away. See Planning a Microwave Radio Link http://www.terabeam.com/solutions/wh...micro_link.php

We can have multipath interference both on 72 MHz and 2.4 GHz, but multipath interference can affect us more on 2.4 GHz because the received signal strength is so much lower if the model is far away.

How shall we install the 2.4 GHz antennas in a model with a carbon fiber reinforced fuselage? The attention can be considerable, and the signal strength margin may not be enough.

I think it is very important for us to understand the differences in radio propagation on 2.4 GHz and 72 MHz (or 35 MHz) . Microwave propagation is different from VHF propagation. When installing fixed 2.4 GHz radio links we can use directional gain antennas (parabolic reflector) to compensate for the increased free space loss, but of course not in our case.
http://www.terabeam.com/solutions/wh...micro_link.php is a good introduction to microwave links.


Best regards

Agust

RE: 2.4 GHz vs 72 MHz - Free space loss
 

OK- -so is the sky falling in ? by shifting from 2.4 vs 72 ?
absolutely not.
The practical application for use in radio controlled models is always -for all practical uses- based on having a visual link ( you have to see the model well enough to effectively control it
For the lion's share of the audience -- that is 1/2 mile. 1/2 of 5280 feet or 2640 ft. a quarter of a mile is 1320 ft and any old drag racers know full well that even seeing what car is doing at that distance , is pretty iffy.

Going the other direction -- the thermal hunters --will be further away but always higher in the air not along the earth's surface.
When testing the DX6 radio - we used a small model a Diablotin about 48" wingspan (but a huge chord ) and stuffed an altimeter in it .
at 1900 ft you could still see the model and sorta decide which way it was travelling but for precise control , the model was clearly way out of range .
the radio tho, was working nicely .
would I shift back to 72? for any reason ? unless it had the same capabilities as my 2,4? - --not on yer life

RE: 2.4 GHz vs 72 MHz - Free space loss
 

And when comparing the two you cant ignore the processing gain from spread spectrum, which is at least 17-18db and can be as high as 30. I do agree though, with full CF birds it may be hit or miss. Not all carbon layups will see considerable attenuation but then some may see the signal attenuated to the point of not being usable.

RE: 2.4 GHz vs 72 MHz - Free space loss
 

Yes, path loss is a biggie. SS manufacturers have gone to a lot of trouble to overcome it... polarization diversity, error correction, fail safe, etc. It is unfortunate that 2.4 GHz is the only band available for RC SS. We would certainly benefit from a lower frequency allocation, say a few hundred MHz. I wish the AMA would champion this cause with the FCC, although maybe such an allocation is just not possible. The bottom line, however, is that 2.4 GHz is working well enough.

There are some inherent losses at 72 MHz that we tend to overlook. For example, 72 MHz uses 1/4 wave monopoles without a proper ground plane or "conterpoise". Accordingly there are impedance mismatch losses at the receiver and transmitter, which I expect amount to at least a few dB at both. 72 MHz is not a polarization diversity system, so there will always be nulls, and these may be many dB. Also, 72 MHz has to contend with greater atmopheric noise than 2.4 GHz.

RE: 2.4 GHz vs 72 MHz - Free space loss
 

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RE: 2.4 GHz vs 72 MHz - Free space loss
 

I think this is one thing that we are going to have to trust the manufacturers to solve for us.
BTW most of the interplanitary communications use 3GHZ, the so called "S" band

RE: 2.4 GHz vs 72 MHz - Free space loss
 

Path loss is of concern for RC because the transmitter and receiver must both use omni-directional antennas. In applications such as space communications very high gain antennas are needed. These must be precisely aimed.

Path loss and antenna gain tend to compensate one another. For an antenna of a given physical size, the gain increases with frequency in the same proportion that path loss increases with frequency.

RE: 2.4 GHz vs 72 MHz - Free space loss
 

The theory stuff is nice
do any of you who are posting "theory ", have these systems and can you give some data -even if it is simple observed flight occurances?
The reason I ask is that tho I don't doubt much of the theory I see - I have not seen ANY results in using the 6000/6100/7000rx and the DX6 and 7 --which shows that there is anything to be concerned about -design wise -- using the systems as intended.
I spent this afternoon again flying two different electric models -trying to create a 'situation - range -lower voltage - hard throttle application -which may confound the rx.
No luck today -will try again - these were models with a 480 EFlite motor using the 25 amp Eflight ESC and one with a Hacker 20/20 and the Phoenix 25 amp ESC and TP gold stripe 1300 and 2100 ma packs models wee both using the 6100rx
-high performance aerobatic stuff.

RE: 2.4 GHz vs 72 MHz - Free space loss
 

You know that higher propagation loss for 2.4 GHz vrs. 72 MHz is a two edge sword. Yes, we might need the processing gain to have hihge receiver sensitivity, but ther eis another very good side! Namely. isolation from interference generated remotely from our flying site! This is the basic principal of frequency reuse that is applied in AM, FM and TV broadcast as well as cellular telephony. Becasue of these extra losses we are flying around in a nice local bubble relatively safe from same frequency transmission from the next county or next park.
Loss can be a good thing!

RE: 2.4 GHz vs 72 MHz - Free space loss
 

Blah...blah...blah...


Just go fly. 8-)

RE: 2.4 GHz vs 72 MHz - Free space loss
 

Some of the spacecraft use omni-directional antennas.
40 years ago on the Lunar Orbiter program we were able to maintain communication with 2 watts of transmitter power on the up link and 1 watt on the down link. The spacecraft had an omni-directional antenna.
The station at Goldstone had an 85' parabolic antenna and a cryogenic cooled MASER preamplifier. The downlink telemetry threshold was -154 dbm with the antenna at Zenith. With the antenna pointed near the horizon the threshold went up to -135 dbm.
Whats that distance? 186,000 miles?

RE: 2.4 GHz vs 72 MHz - Free space loss
 

My interest in 2.4 GHz is possibly because I have been dealing with radio for decades, both at home and at work. I was once a radio amateur (ham, callsign TF3OM) an built and operated many transmitters from 1 to 1500 watts. In fact my first RC transmitter was homebuilt. At work I have designed several point-to-point data links, mainly on 450 MHz. My experience on 2.4 GHz is however limited.

I really hope that we will not have many problems using 2.4 GHz for remotely controlling our models. Futaba has just announced that they will soon sell equipment based on their industrial experience, so maybe I will give it a try some day (I am using Futaba equipment). However I think it is very important to understand the differences in propagation on 72 MHz and 2.4 GHz. Then it is easier to avoid any pitfalls we may encounter. It is clear that we must be careful when we install the antenna in our models, especially the expensive ones with carbon fuselage, etc.

By the way, my first leagal home built ham transmitter was only 2 watts on 14 MHz. This was enough communicate over the Atlantic using morse code. A friend of mine built a really low power transmitter, about 100 mW, but was able to communicate over thousands of miles.


This was many years ago, in the sixties. Now I am more interested in model airpales than radio :-)

Regards, Agust

RE: 2.4 GHz vs 72 MHz - Free space loss
 

From what I have read, they do not have many link problems in spite of the very low power transmitters and omni-directional antennas. From what I have seen, this is the way of the future

RE: 2.4 GHz vs 72 MHz - Free space loss
 

To my knowledge, all communications satellites still use omnidirectional antennas. Omnis are used during launch for telemetry and tracking. The high gain antennas are not deployed from their stowed position on the spacecraft until it arrives on station in geosynchronous orbit. The satellite tracking stations on earth use high gain tracking antennas to receive and transmit to the omni. The frequency is usually Ku band - 12 GHz or so. Typical transmit power is a few watts.

I think the 2.4 GHz / 72 MHz path loss subject is very interesting. I don't understand why other posters are upset. Nobody here has said that 2.4 GHz doesn't work, I have only said that a lower frequency would be a better choice. I have heard of no cases, even anecdotal cases where there has been a problem with 2.4 GHz for RC use. If someone finds this thread uninteresting or annoying they should simply stop reading it.

RE: 2.4 GHz vs 72 MHz - Free space loss
 

OK- for this application-- Visually directed control input (under a mile and at elevated points of reception). - why would a lower freq help?
-I should think that higher would be better based on the problems we typically see in our models -specifically, intrnally generated noise such as the turbine engines -metal bumping/sliding on metal - Carbon fibre (the signal on high freq can peek between the fibres -can't they?)

RE: 2.4 GHz vs 72 MHz - Free space loss
 

This is an interesting suject to me.
I used to be a electro-magnetic compatibility test technician for automotive components. And as a general observation, the higher the freq the lower power was needed to generate the same RF field (volts per Meter). But the signal strength decreases more over a distance than lower frequencies (Free Space Loss)

The Fresnel Zone is independent of frequency and is determined by distance ( i.e. Curvature of the earth and line of site. So there should be no difference between the 72MHz and 2.4GHz frequencies.

The main thing to remember is the noise floor at any given time for the two frequencies. The rule of thumb is the higher the frequency the lower the noise floor. When we did free range testing (open air - - outside of the Anechoic Chamber) the noise floor was greatly reduced at frequencies above 1.6GHz. The natural harmonics of lower fundamental frequencies lose lots of power the further they are away from the fundamental freqs.

A good example of the noise floor is we have two rooms Room A and Room B

Room A is 72MHz and Room B is 2.4GHz

Two people are trying to have a conversation with each other accross in Room A and Two people are trying to have a conversation with each other accross in Room B.

In room A there are 450 other people having idle chit chat, like at a wedding after the drinks have been flowing for a while. The two people have to yell and may not even hear each other.

In Room B ther are 10 other people whispering and the two original people can talk with out yelling.

The free space loss in 2.4GHz is there but you don't need the excess power that you do in 72MHz to overcome the noise floor.

Not to mention the data transmission bandwidth increases by over 30 times (faster servo response times, possibly increased sensitivity in future servos).

But all in all I think this will be good for the hobby.

RE: 2.4 GHz vs 72 MHz - Free space loss
 

Thats some good info Apmech.

RE: 2.4 GHz vs 72 MHz - Free space loss
 

Fresnel zone takes frequency into consideration!

http://en.wikipedia.org/wiki/Fresnel_zone

(Frequency won't post)

Did several reams of paper worth of these calculations back before software was available for point to point calculations (back in the 6gHZ days) :)

RE: 2.4 GHz vs 72 MHz - Free space loss
 

That increased bandwidth point is an interesting consideration. Hopefully someone will recognise that and design a new system that take advantage of it. For 50 years our system had to contend with a less than 10khz bandwidth. We have developed a surprising system that fits in that. With microprocessors and digital filters a frequency division multiplex system could be designed that would eliminate latency in all channels and provide more accurate servos. The future will be exciting.

RE: 2.4 GHz vs 72 MHz - Free space loss
 

As far as bandwith goes, we are moving from 1 MHz of spectrum at 72 MHz to 80 MHz of spectrum at 2.4 GHz.
Using spread spectrum techniques is a proven way to make good use of this wide spectrum as the new systems are doing.

As far as path loss goes:
I looked at the nomogram someone sent earlier and interpreted that
At 1 mile, on 2.4GHz, the path loss is 104db
Starting with a 1 watt (+30dbm) transmitted signal minus the 104db path loss leaves a respectable -74dbm signal at the receiver.

That still is a fairly strong signal at 1 mile which increases 12 db when the distance is reduced to 1/4 mile.

RE: 2.4 GHz vs 72 MHz - Free space loss
 

As it stands -- using non digital servos -but of current (last 2 years ) technology-- the difference in hand output to airplane response is much better than before -
Tho it(actual time differences) is all measured in very small increments - -if you have both types of model equipment, in a performance model -- you can easily detect the difference
the human ability to feel infintisimally small differences . is apparantly not appreciated .
Those who have a background in playing classical musical instruments ,are well aware of this.
The present lack of optimal use of bandwith may exist -but the improvement -with the present switch from 72 -to 2,4 -even on top of the line radios is instantly apparant.
We are at a point where it takes a healthy dose of "expo" to smooth the improved response.
The servo tracking is better and I would suspect -the servos actually respond to smaller command error-tho I don't have any data -other than "feel" to back this up.
If I were to put in my order for servo "improvement" --it would be to up the typical servo voltage
All of my servos are now operating at at least an unloaded 6.6 volts --except for my little foam models which use BEC circuits and run servos at under 6 volts.
My batteries for the others put out 6.6 volts even under max servo loads -these are the A123 cells now making entrance into the model airplane scene. No regulators- just batt to servo. Some servos will now fail using these cells - If you stall the servo-- the gears strip - some others will likely burn motors but I watch the loading . The response time of the servos is better in all cases on the 6.6 and also the BEC supplied servos -which are being controlled thu the 2.4 DX7 and 6100 type rx.
How come no one has posted response time differences ? The heli guys had some good info recently showing signal input to out put times --which was interesting .

RE: 2.4 GHz vs 72 MHz - Free space loss
 

I sit corrected ;), Thanks for that cool Info!!! When doing chamber testing in emc everything is done pretty much at 1M so the fresnel zone never really came into play. Maybe that's why all the standards call for almost everything at 1M?? But I did the calculations in excel and if I calculated them correctly (understood them correctly) the radius of the fresnel zone actually decreases (which is a good thing) with higher frequency

Please correct me if I did this wrong.
r = radius in feet
D = total distance in miles (1 mile in this case)
f = frequency transmitted in gigahertz. (2.4 for 2.4Ghz and .072 for 72MHZ)

for 2.4 GHz ---- r=72.05*(1/(4*2.4))^.5 = 23ft

for 72MHz --- r=72.05*(1/(4*.072))^.5 = 134ft

here is the link to another interesting website
http://www.zytrax.com/tech/wireless/calc.htm

I checked my calculations above against this website and they came out good. Means 60% of the Radius of the 1st Fresnel zone must (Should) be unobstructed. The line of site plus a radius of 14 ft around the LoS at the 0.5 mile distance must be clear with 2.4GHz for a 1 mile distance
The line of site plus a radius of 81 ft around the LoS must be clear with 72MHzfor 1 mile distance

This should ease some apprehensions