Rf Radio Noise

[Steve McGowan]

If your having problems with the Sky Dat guage system on the
Sparrow Hawk. (when you transmitt with the radio/push to talk)

The problem is your radio coax is too close to the wire bundle in the instrument pod.

To remedy this cut a 3/4 hole in the forward area directly below the radio coax attach point of the radio. Run the coax straight down from the radio. Re-attach the coax to the antenna and secure it to the verticle stand hoolding the pod upright.

Also run a ground wire from the battery to the antenna base plate.

This will stop the Sky dat from alarming and the vert speed/altimeter as well as other things going crazy while transmitting to talk. Also the engine will /can skip and attempt to shut down.

It'll scare the hell outta ya at night or day for that matter..:help:

Hope this will help anyone having this problem that has aggravated me for a while..

Steve

[Harry_S.]

I believe that if you add a flexible braided shield...or better yet, a double braided shield... over the coax cable, from connector to connector, this should solve your interference problem...hopefully.:)


Cheers :)

[RayPierce]

I believe that if you add a flexible braided shield...or better yet, a double braided shield... over the coax cable, from connector to connector, this should solve your interference problem...hopefully.:)


Cheers :)

But! Only connect to one end/connector.
You could form a loop that will itself make the grounding shield act like an antenna instead.
Leaving one end unconnected will not let induction occur.

[JByrd]

The former suggestions have valid merit.

If the problem were mine to solve I would approach it slightly differently. Once the RF is radiated from the antenna, it is still strong and can still affect the engine controls and other instruments. Well chosen placement of the antenna and stood-off routing of the coax from the instrument/engine controls bundle is a good idea.

I would offer as an additional suggestion if the problem persists. Covering a wire with braid reduces the intensity of external fields that enter a wire from an external source.

What would I do? Buy some good quality RG-8U cable (not the foam type) and strip off the outer plastic insulation much longer than the wire bundle you want to shield. Then remove the outer braid sleeve by bunching. It works better if the coax is not the foam dielectric, but the smooth plastic type. This bunching increases the diameter of the braid, maybe enough to slip over your critical engine harness to shield the engine controls and instrumentation bundle from the external RF field. Like one of the fellows indicated, ground on one end.

That may be tougher done than said, but it would by my first approach.

Good luck with the solution, whatever you choose.

Jim B.

[PW_Plack]

Make sure your antenna has a proper ground plane and good connections, and check it with a VSWR meter for proper tuning.

If it's properly matched to the cable and transmitter, more than 95 per cent of the transmitted power flows on the inside of the coax cable's shield. If it's not tuned and matched properly, the outside of the braid becomes an antenna, and you'll get RF into lots of places you don't want it.

[Chris Burgess]

I have similar issues with my Keywest instrument package. Never an engine "skip" but the gauges go nuts when I key the mic. I moved the antenna with no change. I put in a shield inside my instrument pod, no luck. I plan to go after ground issues this winter season. I think I'll try Steve's ideas and the shield over the coax. I also have trouble on the Mentone CTAF only. Seems my instruments send out a signal on that frequency only. While there, I need to move the antenna so my squelch doesn't stay open all the time when someone else transmits. If I key my mike again, the squelch closes. As soon as someone transmits again, my squelch reopens again until I key my mike. Strange but true.

[JByrd]

Hi Chris,

Does your radio have a manual squelch setting or is it one of those infernal "self adjusting squelch" types?

One other suggestion is to place a 0.001 to 0.005 uF ceramic disc capacitor directly across the various instrument meter terminals. This presents a near direct short to 125 MHz RF but will not affect the DC currents required for meter readings or the low audio frequency signal from the rotor tach pickup.

I would start with something not totally critical such as i.e. water/oil temperature first as an experiment. I suspect you will see an improvement when keying the microphone.

All the computer packages (and displays) have a square wave master clock(s) that radiate on many frequencies all the way up throughout the VHF band with generally decreasing amplitude). The only thing I know to do for that is to locate the receive antenna as far away from the computer package as possible, and end on (i.e. trailing aft) from the unit.

Jim B.

[Chris Burgess]

JByrd, yep, it's adjustable and I've done that. It mostly "goofs" with the altimeter and VSI, at least those are the ones that the student picks up on. I guess I've gotten use to it and never paid much attention to the other gauges. I move the antenna when at Mentone but that won't stop the instrument skewing, it only helps the squelch issue. I'll try some of the ideas you have suggested here. My season is nearly stopped and I'll play with it this winter when the snow flys and I don't. :Cry:

[PW_Plack]

Chris, you may have limited luck approaching this as a "radio problem". The bigger issue sounds like it may be poor shielding of the computer guts in the KW system.

If the various wires to the senders, and even the KW's power supply wiring aren't shielded, and filtered at the computer end, you won't be able to move the antenna far enough away from it to solve the problem. This system was known for computer "lock-ups" and other quirks which might have been largely related to EMI/RFI issues.

[Steve McGowan]

Mostly the problem has been with the Sparrow Hawk version of gyros, I even had the problem with the Aussie Sparrow Hawk.

Running the coax 90' of the power leads and shortened is what resolved my problem.

Ya sure don't wanna fly at night when you've had the engine skip on Ya due to transmitting.

Anyway......Mines FIXED

[StanFoster]

Steve: I remember commenting on my SH engine cutting out when I keyed my mike on my handheld radio. I had a few heads twisting until you chimed in saying you had some similar problems. My main radio wont do it,,,my handheld will on occasion. Makes for talking brief important..,,:wacko:


Stan

[mgoroff]

I've had similar issues with my SH, though I have a Stratomaster Enigma EFIS in it. My VSWR is around 2:1 or better throughout the entire freq range, but the antenna is embedded in the front of the cabin, very close to the instrument pod.

I added ferrites to *all* the wires going in and out of the Enigma and the problem was solved. It is a pretty simple solution. As Steve points out, you definately want to keep the antenna coax seperate from all other wires. Also make sure the radio has a seperate ground, preferably directly to the engine block.

The only lingering problem I have is that the GPS antenna on the Enigma came with a long length of cable which is bundled inside the instrument pod. I had to add two ferrites on that cable to stabilize the Enigma, which seems to have screwed up the GPS signal. I ordered a new GPS antenna with a 3' cable and hopefully that should solve this remaining issue.

I initially had a case where the engine sputtered when transmitting. I added a ground braid from the ECU case to the airframe and that problem went away.

Marc

[PW_Plack]

We used to worry about keeping ignition noise out of radios. Now, the computers that run our ignitions are susceptible to radio interference, and the computers add additional interference to radios.

Many small computers are timed by quartz crystals in the 3 - 4 MHz range. Because they handle pulses through logic gates as square waves, they're rich in harmonics. If you bring a shortwave receiver in "SSB" mode near your desktop computer and start tuning, you'll find not only strong signals every 3 - 4 MHz, but also lots of buzzing and other trash in between.

Ferrite beads on a nav, com or GPS receiver's antenna lead will attenuate the wanted signal along with the unwanted. The best answer is to block RF from getting into or out of the computer, whether it's a Key West panel or Subaru ECU.

The ideal grounding scheme for helping cable shields do their jobs would be one, central power system ground point for the entire aircraft, preferably near the battery, with antenna grounds isolated from the DC ground. That's very hard to achieve in a practical wiring harness.

[Caribean_gyro]

i HAVE THE SAME INTRUMENT AND i RUN MY COAX AS PART OF THE WHOLE POWER CABLES BUNDLE. The only thing I get is when I key the mic the altidude drops 20 feet.

Now my antenna ground palne is a copper pcb of 6x8 and the antenna attached to it. on the belly of the twinstarr
ChuckP

[Robert G. Stark]

Is your ground plane 6 x 8 inches?? If so that is not near enough. Also, you should not bundle coax transmittion line with the rest of your wiring.
I assume this is an aircraft band radio. The proper size for your ground plane would be appx. 35 inches square. The ground plane is 1/2 of your antenna.

[Heather Poe]

Evidently Randy Coplen's Sparrowhawk had engine computer problems with push-to-talk. When I rewired it, I did not do anything special with the coax. However, a tip that I got from Greg Richter's pamphlet http://www.bluemountainavionics.com/pdf/aircraft_wiring_04december2004.pdf is that the push-to-talk switch cable MIGHT also act as an antenna. When I rewired the Sparrowhawk I used shielded cable for this function. Based on a tip from the same source, I also sprayed the inside of the instrument pod with a nickel coating http://www.mgchemicals.com/products/841.html, http://www.alliedelec.com/Search/ProductDetail.asp?SKU=661-0610&MPN=841%2D340G&R=661%2D0610&sid=456F70806F8CE17F to reduce RF leakage. This spray is also recommended by Greg to make a ground plane of the composite body. After many months the problem has not reappeared once.

[mgoroff]

Ferrite beads on a nav, com or GPS receiver's antenna lead will attenuate the wanted signal along with the unwanted. The best answer is to block RF from getting into or out of the computer, whether it's a Key West panel or Subaru ECU.


The ferrites are on the leads going in and out of the EFIS, so it is blocking the stray RFI from getting in or out of the computer. The only antenna wire on the EFIS is the GPS. Since replacing the original GPS antenna with a better antenna with a shorter cable length, the system is now stable and GPS performance is very good even with a ferrite on the antenna as its frequency range is very high. This was in accordance with the EFIS manufacturers recommendation.

The Subaru ECU seems pretty RFI immune as long as its metal case is grounded.

Marc