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Typical transmitter RF output power?

Old 01-09-2003, 03:27 AM
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downeym
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Default Typical transmitter RF output power?

I bought a lightly used JR652 transmitter from someone and changed the crystal to channel 40. Everything seems to work ok, but I noticed when I do a range check, the range is significantly shorter than the airtronics transmitter I was using before. I know you are supposed to send the transmitter back to the manufacturer for retuning, but I actually have a nice spectrum analyzer at work I was able to check it with.

My airtronics transmitter outputs about 150mW of RF power. When i compare it to my new JR , it only outputs about 85mW! If you open the back of the transmitter there is a sub board that looks like it has the RF amp on it and has 2 adjustments. These adjustments seem to shift the transmitted frequency, but do not affect the output RF power at all. It seems that all I can do is recenter the frequency and call it a day.

Does anyone have any experience with tuning these transmitters? It seems that my new computer transmitter is not nearly as powerful as the 10 year old airtronics transmitter I was using before!

I am sure there are some HAM type people out there that have some experience tuning their own equipment. Any advice? Are my readings typical?
Old 01-09-2003, 03:42 AM
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Phil Cole
 
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Default Typical transmitter RF output power?

I think the 150 mW is in the typical range. 85 mW would seem a bit low, but it's probably adequate for the intended purpose (transmitting to a receiver thats within visual range).

Antenna-down range checks should only be used as a qualitative check to see that nothing has changed during day-to-day operation.

Transmitters with similar output power and range with fully extended antennas may may wildly different results with retracted antennas.

Did you put the original crystal back into the 652 and measure the power?

With the 150 mW to 85 mW difference you would expect the ranges to be different by a factor of around 0.7 (square root of 2).
Old 01-10-2003, 05:53 PM
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JPMacG
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Default Typical transmitter RF output power?

How are you measuring power output? By disconnecting the antenna and soldering in a coaxial connector with ground and using a power meter and load?

The transmitter may not be tuned to 50 ohms at the factory. I expect at the factory they tune the transmitter to the input to the whip antenna, which is probably low resistance and high reactance. Therefore power measurements made with a 50 ohm meter would be misleading.
Old 01-10-2003, 10:33 PM
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downeym
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Default Typical transmitter RF output power?

My input to the SA is definately not matched... I was just using a test clip to clip onto the antenna iteslf.... The thing that worried me more than anything is that the output measures half the RF power as my other transmitter. The antenna down range check reflects this also, which is why I wanted to measure it in the first place...
Old 01-10-2003, 10:52 PM
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Phil Cole
 
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Default Typical transmitter RF output power?

Different transmitter designed will react differently to mismatched loads (retracted antenna or 50 ohm SA input).

All an antenna-down range comparison tells you is the range with the antenna down.

In fact you could argue that a lower range with the antenna down is better since the transmitter has better shielding, and only emits RF via the intended path.
Old 01-11-2003, 10:08 AM
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easymoney
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Default Typical transmitter RF output power?

The maximium RF output power limit set forth by FCC for radio control is 750mW and new JR Txs normally transmitts between 500 to 600mW.

Its better if you send the radio to JR service centre for optimum output because if you change the inductor settings then you might be able to get more power but with lots of side band transmissions (harmonics) which can effect other channels. Also you might not have any idea of the input impedence of the antenna for JR and I am sure its not 50 ohms.
Old 01-11-2003, 10:08 AM
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Default Typical transmitter RF output power?

The maximium RF output power limit set forth by FCC for radio control is 750mW and new JR Txs normally transmitts between 500 to 600mW.

Its better if you send the radio to JR service centre for optimum output because if you change the inductor settings then you might be able to get more power but with lots of side band transmissions (harmonics) which can effect other channels. Also you might not have any idea of the input impedence of the antenna for JR and I am sure its not 50 ohms.
Old 01-11-2003, 04:05 PM
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downeym
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Default Typical transmitter RF output power?

that is why I wanted to put the transmitter on the spectrum analyzer. I was able to fine tune the center frequency and I done have any out of band issues.... If the JR guys cant give me any more power I dont think I will send it in.

Unless there is another adjustment I dont know about, there is nothing else they can do....
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Old 01-11-2003, 04:11 PM
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Forgues Research
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Default Typical transmitter RF output power?

The maximum output allowed is only reached by Multiplex TX and that is 750mw.
Old 01-11-2003, 06:53 PM
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Default Typical transmitter RF output power?

after thinking about it for a little while I realized that the signal looked too clean on the Spectrum analyzer. The frequency spike was too sharp. In order to have a FM Modulated signal, you need some frequency shift, right? So i zoomed way in and took a snapshot of the airtronics transmitter I have on the same frequency.
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Old 01-11-2003, 06:55 PM
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Default Typical transmitter RF output power?

I then adjsuted the JR transmitter to try to qualitatively obtain the same signal pattern. Enough frequency shift to give it the same look to the signal with the same basic center point and frequency shift pattern.
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Old 01-11-2003, 06:56 PM
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Default Typical transmitter RF output power?

I then wanted to try to make an objective measurement of the radiated output power of both transmitters. I cut a piece of bare wire about the same length as the antenna on the receiver, and clipped my probe onto it.. I then took my airtranics transmitter and turned it on across the room.
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Old 01-11-2003, 06:57 PM
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Default Typical transmitter RF output power?

then i did the same with the JR transmitter.
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Old 01-11-2003, 06:59 PM
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Default Typical transmitter RF output power?

It looks like the radiated output power with the antenna fully extended is about the same for both transmitters. The issue I must have had was with the impedance mismatch with the test equipment an the probing method. I need to go to the field and try a real range test like before and see if my newly tuned transmitter is doing better now....
Old 01-11-2003, 08:54 PM
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Default Typical transmitter RF output power?

that seems like it is much better now... I think adjusting the frequency shift potentiometer seemed to make the difference. I still have one receiver that seems to be very short ranged, but that might be the receiver itself, not the transmitter.

Using my sailplane (which ahs an airtronics receiver) I can probably walk at least 100 feet from the plane before I start getting some noise with the antenna down. Can't ask for much more thatn that I guess...

Has anyone ever tuned a 555 receiver? I assume that there are some adjsutments in there that can be adjusted for frequency shift as well, right?
Old 01-14-2003, 05:29 AM
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MO_Radio_Tech
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Default Typical transmitter RF output power?

Tuning your own transmitter is usually a bad idea. It is against FCC regultations and AMA rules. The adjustments you made to tune frequency (with the variable cap and pot) may be correct, but the more important tuning is in the coils. You can tune for max output with these coils and your Spec Analyzer, but you can not tune for proper current drain. You may overdrive the regultator or that output transistor. This would lead to radio failure. Horizon can tune the radio for about $20 with shipping. If you swap crystals (in violation of FCC and AMA), have them tune the radio to the center of the band. With the deck in the 652, it will be nearly perfect thoughout the whole band when center tuned. If you only fly one crystal, have them tune to it, and get max range with lower current drain.

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