Radio Transmissions affecting Altitude

[mmarien]

This is an old problem but time to fix it. When I transmit on my radio the altitude rolls up and the fuel level goes down. It's not a problem normally but when on AP and altitude hold, the plane takes a dive trying to maintain altitude when I transmit.

I've upgraded from an iCOM A210 to a GTR200 which didn't change the behavior. I also replaced the cable with RG400 with no noticeable difference. The antenna is a bend back type on the bottom just behind the seats and the ground plane is a 3' x 4' (approx) tin foil on the inside of the belly plate of my Glasair II. I've never had any complaints about either radio so I assume the transmissions are good. Receiving is crystal clear except at 120.00.

The problem is worse at low frequencies. At 120.00 and only 120.00 there is background noise on the radio. Sounds like computer noise. At high frequencies (130+) the altitude changes little or none at all. Tower here is 118.30 so it's a real problem. I usually disconnect the AP before I transmit or I have to fight the pitch servo.

My EMS has been RMA'd for the fix so those gauges work fine. The fuel level changing doesn't bother me although once in a while a passenger will point out the disappearing fuel while I'm transmitting.

I'm open to suggestions.

 

[dpbarnes]

I have the same problem with fuel level and the radios.  Whenever I transmit the fuel level goes to zero.  I have lived with it since the beginning but I to would like to dig into the problem and solve it.  I had the cht and egt problems and sent the engine module in for Dynon to update.  That solved that problem.  I fly a Lancair ES and it has very long capacitance probes.  All the comm antennas go down the right side of the plane and the right level seems to be affected more than the left.  I am using the Dynon converter located right at the end of the probes.  I figure the interference is either coming straight into the probe itself or the converter module or the  wire between the converter module and the engine module.  If no one has a idea where to start I thought I might experiment a little.  Maybe disconnect the probe and put a fixed capacitor across the module and test.  Maybe shielded cable between the two modules. Anyone have any ideas?

 

[jakej]

Murray
I have sent you a pm.

Jake J

 

[Dynon]

mmarien: We've had a few (literally a few over thousands) reports of EMI from the radio getting back into SkyView ADAHRS and affecting the sensors. It depends on signal paths and other RF voodoo magic, and I won't claim to be an expert on these topics. In the cases where radio tx is influencing the flight instruments, this filter attached to the ADAHRS has sometimes helped. That won't solve the fuel level issue though, and I'm not sure if we're fundmentally chasing an EMI or a grounding issue here, frankly. I'll ask our engineering team to weigh in here.

Dan - same goes to you: I don't have immediate advice, but I'll see what our engineering team can offer.

 

[mmarien]

Thanks. I'll order one and give it a try.

 

[mmarien]

The problem is worse at low frequencies. At 120.00 and only 120.00 there is background noise on the radio. Sounds like computer noise.

I solved this problem. It's the ZAON. Turn it off and the noise on the radio goes away. :-?

I did get the capacitive filter suggested for the altitude problem, but haven't had the chance to install it so can't comment yet. I do have a work around. I was using the AP disconnect button to when I wanted to transmit but had to reset the AP again after the communication. Pressing AP after disconnecting with the disconnect button assumes current climb rate and wings level. I discovered (probably common knowledge to most) that if I use the AP button on the SV menu to disconnect and then reconnect, it will resume with the previous settings. So both NAV and altitude settings are preserved and the AP will get me back on course if I wandered off during communication. Makes life simpler.

 

[mmarien]

In the cases where radio tx is influencing the flight instruments, this filter attached to the ADAHRS has sometimes helped.

Case closed. I installed the filter and tried transmitting on a few frequencies on the ground. The VSI jumps a bit but the altimeter seems to be unaffected by the TX.

The fuel gauge also remained unchanged during TX. Go figure? I can't see that it's connected to the ADAHRS in any way. I'll give it a go in the air in a couple of days to see how it works with everything running.

 

[mmarien]

Quick update. I had a chance to check it out in the air and the altimeter no longer is affected by radio transmissions. The fuel gauge is still affected but it's not a real problem. I'm a happy flyer.