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50 MHz and Simjet

Old 08-21-2004, 02:11 AM
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Default 50 MHz and Simjet

Are there any known issues with using 50 MHz radios with the Simjet 1200 SP?

I am seeing a large reduction in range. Antenna down checks with the ECU off are over 200' with the model sitting on the ground. With the ECU on, I get 20 or 30'.

Two different receivers on ch. 7 50.940 MHz.

I'll be trying another 50 MHz channel just in case it's a spot frequency problem, as well as the usual moving of things around.

It's in a Facet 1200, with the Rx battery on top of the ECU. The ECU, pump, solenoids, ECU battery are roughly in the positions shown on the plans.

The battery and ECU switches are mounted in the positions provided on the first full-height former.

The receiver is mounted to the side of the rear compartment, next to the rudder servo. The antenna is strung to the top of the fin.
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Old 08-21-2004, 08:24 AM
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Default RE: 50 MHz and Simjet

Phil, your installation looks alright. It is almost identical to every Facet I've done with the exception of the Rx battery, so I don't think it's that. Kevin or Ed should know about the 50 MHz, but I have not heard anything about it.

Trying another channel is a good idea and you may also want to try the test with 72 MHz, (may be a pain to borrow another radio, but may be worth it from an informational stand point). It wouldn't need to be anything fancy either as it is only for reference.

George
Old 08-22-2004, 01:46 AM
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Default RE: 50 MHz and Simjet

After spending some time walking back and forth with transmitter in hand I found that disconnecting the throttle lead from the receiver got the range out to something acceptable (10% reduction of the ECU-off range), provided I was on channel 9 (50.980 MHz) instead of channel 7.

I then tuned my scanner across the 50 MHz RC band. There was a general low level noise right across the band which went away with the ECU off. For the scanner to hear this it had to be within inches of the ECU. However, there was a peak at around 50.942 MHz which easily be detected by the scanner when it was placed near the receiver.

I had a look through the 72 Mhz band as well. There was low level noise similar to what I found at 50 MHz. There were one or two peaks as well, and a really strong one at 72.000 MHz.

If I listen to the noise detected by the scanner it does reduce somewhat when I disconnect the battery lead extension going to receiver. Disconnecting the throttle from the its extention significantly reduces the noise at the scanner.

I'll try a ferrite bead on the throttle lead, and see if I can arrange the Rx battery to be further away from the ECU. The ideal spot for the battery for balance purposes is an inch or back from where I have it, anyway. I did try moving it back by the receiver, but that didn't do anything for the range.

I also have a 72 MHz PPM receiver that I can use to see if the range reduction is less pronounced on that band.
Old 08-22-2004, 03:41 PM
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Default RE: 50 MHz and Simjet

Hi Phil,

We've not yet been made aware of any issues on 50MHz, but you're also the first person I know of to report back that they were doing so. You beat me to the punch on the ferrite bead as a solution, let us know how it works out.

As far as problems in the 72MHz band go, we have had one issue with the IRDT Tx board and channel 19, but that's it. The peak at 72MHz doesn't surprise me, 3rd harmonic of the 24 MHz crystal. (If you check with your scanner, there should be a bigger peak at 48MHz, but nobody that I know of uses that band for RC flying.)
Old 08-22-2004, 11:04 PM
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Default RE: 50 MHz and Simjet

Hi Kevin,

The ferrite didn't have any effect.

I moved all the extra cable length to a bunch just in front of the ECU and put the receiver battery beside the top fuel tank. That way I could get acceptable range provided I didn't connect the throttle lead from the ECU. An optical isolator might work here, except that it would be a bit of a pain to have another battery just to power it. If I could get power from the ECU, or knew what the ECU input circuit was like I could figure something out.

I did do a bit better at 72 MHz. I'll get a PCM receiver and see if I can make that workable. I'm not to thrilled by the prospect of using 72 MHz, but I guess most people manage.
Old 09-07-2004, 12:13 AM
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Default RE: 50 MHz and Simjet

I have managed to get what seems to be acceptable range test results.

The main change I've made to to move to 72 MHz from 50 MHz. Before I chose the 72 MHz crystal, I scanned the ECU, looking for the frequencies with the least signal strength. I found quiet spots near the channels that end in '5'. E.g. channels 15, 25, 35.

Compared to the signal levels my scanner sees on 50 MHz, the 72 MHz band is much quieter, except for a few spot frequencies, like channel 1 which is only 10 kHz away from the third harmonic of the ECU's main clock. The second harmonic at 48 MHz is also there, but not as strong as the third. This is expected since pure square waves consist only of odd numbered harmonics.

Right now, I think the only way I'd get satisfactory results on 50 MHz would be to improve the RF suppression in the ECU. I'll stick with 72 Mhz for the moment.

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