Replies to This Discussion

Permalink Reply by Jake Stew on July 30, 2012 at 9:38am

Just read some of the 5G posts here.  Terrible range and extreme interference problems.

The 1.2g video TX/RX I have has quite a range of frequency output.  I use something like this...

http://www.ebay.com/itm/1-2G-Hz-Wireless-Audio-Video-Transmitter-Re...

It lists the frequencies as...

0.9G/1.2G/1.3G With the larger channel number TXes you get quite a large frequency range.  IMHO this is by far best type of setup since you get the best range using a better frequency band and still have lots of channels you can use to minimize interference.

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ModeratorPermalink Reply by Sgt Ric on July 30, 2012 at 10:20am

Jake, do you find any GPS reception issues from the relatively close frequencies?

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Permalink Reply by Jake Stew on July 30, 2012 at 10:34am

I haven't noticed any.  I'm only running 700mw and haven't really done much testing to see if certain channels will cause problems.

I do wonder what kind of PA filtering is in these things.  It can't be very tight with such a large range unless they're using something beyond my understanding.  Being cheap Chinese products I can't imagine there's anything fancy in there.

I should also point out that I use a fairly large plane for an airframe and separate the antennas as best I can.

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Permalink Reply by Rick Stewart on July 30, 2012 at 10:41am

I am currently using a 1.2 1W video setup.  I would suggest a low pass filter if you are using 2.4G radio gear.  With out the filter I find range on the radio is shortened.

Rick

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ModeratorPermalink Reply by Carl Campbell on August 1, 2012 at 1:04pm

Thanks for the input guys.  I have decided on a 5.8Ghz system as I am only operating a quad/heli in rural missouri where there is no internet let alone 802.11n AP's  I am shooting for a range of 500m.  Ill be adding decent antennas to the TX and RX. 

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DeveloperPermalink Reply by R_Lefebvre on August 1, 2012 at 1:10pm

I've got a 200mW 5.8GHz system I've been playing with.  So far the reception with the rubber ducky antenna is so poor I haven't even bothered mounting it on an aircraft yet.  I've got some BlueBeam circular polarizing antennas in the mail that I'm going to try.

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Permalink Reply by Vernon Barry on August 1, 2012 at 1:48pm

5.8GHz is the problem. Radios 101 tells you the higher the frequency, the more directional the signal is, the more sensitive it is to exactly tuned and matched antennas, and the least amount of range for given power.

100mW at 5.8GHz is probably got less range than 10mW at 900mHz

All a directional antenna does  is trade power for limited acceptance angle. In other words, a directional antenna doesn't make your receiver get 10x range. It does make it 10x more sensitive that the two antennas must be pointed right at each other. In most cases, this is worse since you cannot garantee that fact at both ends.

 

To prove why 5GHz sucks, who uses 802.11 A still? The answer is only those who want short range or are stuck with interference because B, G,N are all 2.4 GHz which goes further under the same TX power.

 

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Permalink Reply by Jake Stew on August 1, 2012 at 2:05pm

But the phone companies tell us that more G's = better!

They wouldn't lie to us would they?

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Permalink Reply by Vernon Barry on August 1, 2012 at 2:11pm

http://www.afar.net/tutorials/how-far/

Free-Space Loss

As the radio signal travels through space, it deteriorates for two reasons:

• The signal spreads out in space, proportional to the square of the distance.
• Some of the signal is absorbed by the atmosphere (especially on a rainy day; the microwaves will heat up the raindrops, and that energy comes right out of your signal!) The higher the frequency, the greater the attenuation.

The free space loss can be calculated according to the formula

-L = C + 20 * log(D) + 20 * log(F)

where D is the distance, and F is the frequency in MHz. The constant C is 36.6 if D is measured in miles, and 32.5 if D is in kilometers. The following are some examples of free space losses:

Distance		Loss at F=
in miles	in km	900 MHz	2.4GHz	5.8GHz
1.6 mi	2.5 km	99 dB	108 dB	116 dB
3.1 mi	5 km	106 dB	114 dB	122 dB
5 mi	8 km	110 dB	118 dB	126 dB
6.2 mi	10 km	112 dB	120 dB	128 dB
10 mi	16 km	116 dB	124 dB	132 dB

These figures do not take into account deterioration due to weather. Typically, we recommend allowing 15 dB of "fade margin" to accommodate for weather, antenna alignment, and other miscellaneous losses.

 

The "G" in telephone speak is Generation, AKA 3rd Gen, and 4th Gen. They are using multiple technologies (encoding, bandwidth, and software) to take the same spectrum and shove more data than plain old single channel communications like pure FM.

But, yes, the phone companies are full of crap. Like SMS uses the unused data space in the overhead channels of the network, but they still charge $0.10 per text to use something that was being wasted.

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Permalink Reply by Vernon Barry on August 1, 2012 at 2:49pm

Also, for those not in the know and missed the math behind those numbers, keep in mind dB is LOG. So a 10dB loss isn't like 100mW- 10 mW = 90mW , it's more like 100mW= 20dBm minus the 10dB= 10dBm or only 10mW

This is why I said that the difference in frequency is huge!

The table says at 1.6miles 900Mhz has a loss of 99dB but 5.8GHz has a loss of 116dB or a 17 dB higher loss for 5.8GHz.

Thus at 100mW TX power for both systems (AKA 20dBm) the negative 17dB loss of choosing a 5.8GHz TX means that system has the equivalent of 3dB or 2mW TX power!!!!

This chart here makes it easy to see dBm and mW equivalents. http://www.rapidtables.com/convert/power/mW_to_dBm.htm

mW to dBm conversion table

Power (mW)	Power (dBm)
0+ mW	-∞ dBm
0.01 mW	-20.0000 dBm
0.1 mW	-10.0000 dBm
1 mW	0.0000 dBm
2 mW	3.0103 dBm
3 mW	4.7712 dBm
4 mW	6.0206 dBm
5 mW	6.9897 dBm
6 mW	7.7815 dBm
7 mW	8.4510 dBm
8 mW	9.0309 dBm
9 mW	9.5424 dBm
10 mW	10.0000 dBm
100 mW	20.0000 dBm
1000 mW	30.0000 dBm
10000 mW	40.0000 dBm
100000 mW	50.0000 dBm
1000000 mW	60.0000 dBm
10000000 mW	70.0000 dBm
100000000 mW	80.0000 dBm
1000000000 mW	90.0000 dBm
10000000000 mW	100.0000 dBm

 

I hope the one thing people take away is that you REALLY want to use 900MHz even if it's only 100-200mW (20-23dBm), it's so much more power than even 1 watt (30dBm minus 17dB= 13dBm of 5.8GHz !

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Permalink Reply by Rick Eis on August 11, 2012 at 5:09pm

Barry,

I have the 3dr 915 Mhz telemetry system, which works very well.  In addition I have a cheap video system  http://www.hobbyking.com/hobbyking/store/uh_viewItem.asp?idProduct=....  Somehow,  I got the idea they could work together ??  Was that way out to lunch?  Actually, they do work very fine together,..... except when I turn on the motors !...ha.  So, is it possible I could get them to work or do I need to get a new system for one or the other?

Thanks for you work,

Rick

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Permalink Reply by Vernon Barry on August 11, 2012 at 6:33pm

Actually, they do work very fine together,..... except when I turn on the motors

So, you pretty much answered the question by yourself. By that, you stated the only time you have interference is when the motors spin up. Without wiring diagrams of your setup, I have to make some asumptions. Let's say that both the camera, OSD, and video TX also run off of the main battery? I don't know where you tied them in or if you inadvertently caused a ground loop. You probably also don't have a specific power filter from the main battery to the video TX, OSD or Camera. The ESCs are going to send some serious noise to the battery, and while you could try ferrite beads on the ESC battery side wires, it may not be enough and also, might hurt performance. You can go full strength filtering (meaning something more than just a ferrite bead) on the supply to your video setup though. I would place an inductor (say 100mH) in series on the positive lead and a cap in parallel to the ~12 V feed you are sending to the video setup (camera, OSD, and video TX).

Here's a link to a circuit http://williamson-labs.com/480_dec.htm

Next, assumption. I think you mean the video gets interference but the telemetry stays up? Or if they both drop, then the answer above still should help. The why behind this is that the video TX is an amplifier and probably is picking up common mode noise riding the power wires and amplifying the crap out of it. This noise would drop all RF and you could be losing the RC link too and just don't know it yet (it would hurt the range pretty badly). This broadband noise will be eliminated by either filtering it before it hits the video equipment, or alternatively, get another 3 cell battery and just power them with  a clean 11.1 volts. You could go further and supply a clean 12 volts via a regulator and a 4 cell, but just a guess, I think you have more than 1 amp draw between the camera, OSD, and video TX, making a 7812 linear regulator out of the question.

Point being, the interference is likely not coming from the telemetry because it's power source is through the APM, which is (hopefully) filtered by the BEC or ESC).

Again, knowing how they make these cheap video TXs, they are using a broad band amplifier and it's likely not got a great common mode noise rejection thus anything on the input wires is likely to just get broadcast and cause serious issues. The fact that itworks with no motors shows the basical signal is fine, somehow you are getting interference from the motors. If we solve that, you should be fine.

Searching also came up with this http://fpvlab.com/forums/showthread.php?5010-rcmodelreviews-method-...

You're not the first person with this problem. It can be solved cheaply.

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