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Home✈️Aircraft🚁HelicopterHelicopter Main Discussion › Transmitter Power Output
05-29-2016 08:27 PM  3 years ago
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Four Stroker

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Transmitter Power Output
I used my cheap spectrum analyzer to check all of the transmitters at the field yesterday. This is a relative measurement.

-28 dBm - JR and one Spectrum DX7
-35 dBm - Futaba TM 7, two DX 8
-42 dBm - one DX 9

The transmitters were placed at the same distance from the spectrum analyzer and the antennas were parallel.

7 dB is a factor of about 5 so the top line transmitters were transmitting 5 times the power of the second line.

The bottom transmitter was transmitting about 1/5 as much power as the middle line AND the bottom transmitter was transmitting about 1/25 of the power of the top line transmitters.

Now assuming the top line is 100 mW the middle line are at most 20 mW and the bottom transmitter is a mere 5 mW. Now 5 mW was considered a long range WiFi a few years ago BUT something ain't right here.
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05-29-2016 08:38 PM  3 years ago
Climax

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Crikey, that's a big disparity!Electronics, Physics, Helicopters, Fixing Things...
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05-29-2016 11:11 PM  3 years ago
Flyin for Jesus

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Help a dBm idiot out here...
Is the JR and DX7 the best or the worst?
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05-29-2016 11:13 PM  3 years ago
Climax

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They were putting out the most power.Electronics, Physics, Helicopters, Fixing Things...
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05-30-2016 01:03 AM  3 years ago
Flyin for Jesus

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Thanks for the clarification... Figured they were in best-worst order...but the numbers threw me.
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05-30-2016 07:15 AM  3 years ago
Climax

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I wonder if "identical" transmitters would show similar variations, that would be an interesting experiment.Electronics, Physics, Helicopters, Fixing Things...
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05-30-2016 10:32 AM  3 years ago
Eury

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What I wonder is if it actually means anything or makes a difference beyond that DX9 having better battery life. People have been flying the DX9 for years in everything from tiny stuff to giant scale and haven't had issues. Or maybe that particular DX9 wasn't working right and another would have different results.Nick Crego

Citizen #0168
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05-30-2016 11:14 AM  3 years ago
Climax

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I suspect that there is variation (although that's a shocking amount). Personally I'd want the full 100mW or so.Electronics, Physics, Helicopters, Fixing Things...
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05-30-2016 12:17 PM  3 years ago
rpat

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OK I am flying a DX9 black edition and a 14SG Futaba. Now you have me worried. Was the DX9 an older version or the DMSX version?trex 700fbl cal30,minititan,, trx600fbl,trex250,logo 500,Velocity N2
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05-30-2016 07:45 PM  3 years ago
Eury

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Dx9 has only been available in DSMX. You are perfectly illustrating my point. What does it matter what the power output of a transmitter is? The only thing that matters is if it is reliable and has enough range. If it's putting out 1mw and still has enough range that you can't see far enough to out fly it, then how is that worse than one that puts out 1000 times as much and has range to the moon when they both have the same reliability and the limiting factor on both is your eyes? Wouldn't the lower power be a good thing since your battery is going to last longer?

It used to be that some countries had lower power (France and Japan if I remember right), and it was much lower than allowed here in the US. Those radios still had more than enough range and worked perfectly.
Nick Crego

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05-30-2016 07:56 PM  3 years ago
Four Stroker

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Atlanta

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Tested some more transmitters today.

New DX8 at -27 dBm.

Two TM14 at -45 dBm.

One 10CG at -52 dBm.

Now the antenna stop on the 10CG was broken and the antenna would go round and round and round winding up the coax. Moving the antenna around some brought it up to -35 dBm same as the TM 7. I told him to buy a new antenna !

I recall that there was a TM14 recall because of losing power. I told them to send them in for a checkup !

Now I am starting to guess that -27 is about 100 mW. This makes -35 about 20 mW which some of the FCC tests have gotten. -42 is about 5 mW which is probably still OK for most pilots. BUT the transmitters that are reading below this need servicing.

Now my original 12Z/TM14 would get 600 feet in range test mode which was 23 dB down from full power.

Twisting antennas with a crimped coax U.FL connector at one end is probably a bad idea. There was a bad batch of DX8's one of which crashed two hell's at my field. They replaced the RF section and antenna and battery and gave him some receivers ! Not even close to the actual damage.
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05-30-2016 08:07 PM  3 years ago
Four Stroker

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There were some spectacular crashes in Germany using 10 mW with DSM2 and that other company with one channel - XPS ? But this is probably an indictment of NOT HOPPING and not 10 mW.

Japan could not get reliable DSM2 with 10 mW and invented DSMJ hopping which allowed 100 mW. This morphed into DSMX for the USA.

So how much is enough ? With hopping I would guess 5~10 mW is plenty for flying 1000 feet away. But some of these broken antenna transmitters are at the edge.
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05-30-2016 08:36 PM  3 years ago
Andy from Sandy

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With the Futaba hopping all over the place are sure you are able to measure the output properly?
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05-30-2016 08:39 PM  3 years ago
Four Stroker

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Not a problem. Takes several minutes to hit all of the channels but they are all of the same magnitude in dBm anyway.
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05-30-2016 08:57 PM  3 years ago
Andy from Sandy

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Not an expert here but it is the effective power that has to be measured and I wonder if what you are measuring is the spot power at just one point in time?

Does any of this make any sense?

http://www.rowetel.com/blog/?p=136
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05-31-2016 01:43 AM  3 years ago
Four Stroker

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At a minimum, all of the Futaba were FASST. So the relative difference holds. I never claimed it was a power measurement but a relative measurement between transmitters. You don't like the results ? Your brand is offended ?
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05-31-2016 07:21 AM  3 years ago
Andy from Sandy

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You mentioned power in your first post. I was just curious if the way you have made the measurements is correct for the different types of spread spectrum but I do now see what you say about the various Futaba transmitters.

There is nothing to dislike about your results and I don't think my brand is offended.

I like this kind of stuff, I wish I had access to the equipment and the knowledge to use it.
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05-31-2016 03:43 PM  3 years ago
Four Stroker

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Andy;

In many ways you are correct to object. Power is complicated. The power is spread out over the say 99% power bandwidth OR the 6 dB OR 20 dB down from peak bandwidth which differs depending on the type of transmitter protocol. For example Spectrum is about 1 MHz bandwidth per channel, Futaba is 1.6 MHz, and FHSS is very small. JR claims to be 3 MHz. The Spectrum analyzer is giving the power in a single resolution bandwidth bucket.

So really a careful analysis should compensate. But for the same protocol like FASST, the relative measurements should be roughly OK. The difference between Futaba FASST and Spectrum DSMX is a couple dB.

I am seeing tremendous differences within the same protocol.

I could think about this and look at the FCC documents for the different protocols.

PS I am thinking about the antenna position and what the box is made of. Folding an antenna sideways against a metal case has to kill the efficiency. All test should have been done with the antenna sticking straight out.
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06-01-2016 02:40 PM  3 years ago
Four Stroker

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Atlanta

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Summary:

-27 dBm - new DX8 (DSMX)
-28 dBm - JR (DMSS) and one Spectrum DX7 (DSM2)
-35 dBm - Futaba TM7 (FASST), two DX8 (DSMX)
-42 dBm - one DX9 (DSMX)
-45 dBm - two Futaba TM14 (FASST)
-52 dBm - one 10CG (FASST)

The differences in protocols is small. All of these are DSSS. The 6 dB bandwidth is the important thing for DSSS. JR quotes the 20 dB bandwidth which is bogus. The 6 dB bandwidths are:

~1.6 MHz - FASST, DMSS
~0.9 MHz - DSM2, DSMX

~250 KHz - FHSS not tested.

The resolution bandwidth of the cheap spectrum analyzer is 500 KHz. All of the above measurements are valid +/- 3 dB.

Conclusions:

These 2.4 GHz transmitters either age or twisting the antenna is degrading the U.FL crimped coax connector.

I am sure the RF module is farmed out in most brands. Getting the power out from the U.FL connector on the board to the aether is the job of the antenna. I am sure some of these are hand made using a cheap crimper since Hirose charges a bunch for theirs.

The range check mode of FASST is 23 dB down. 35 dB down for DMSS. I have not measured DSM2/X. Some of these transmitters are putting out a little bit more power than range test mode.

I am bored with this now.
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06-01-2016 10:17 PM  3 years ago
AirWolfRC

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2.4 SS is a pulse transmission.
Don't know what the scanner is measuring.
Is it really capable of measuring peak pulse power ?
At that price . . . I wonder.
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