[gereed75]

This is another RFI saga that maybe someone can help with. I suspect others have similar set -ups. Help me, help them.

I have been experiencing some radio transmit problems. My #2 radio has been somewhat intermittent on transmit (always receives OK). Occasional no side tone and no transmit. Occurrences have been getting more frequent. Radio transmissions also affect my fuel pressure indication (EIS 4000). It jumps up to 42 PSI when transmitting. #1 transmits fine and affects fuel pressure only slightly +/- 1 psi.

Here is my set up - #2 nav/comm is a KX155 Nav/Com feeding a plain rod antenna mounted centerline bottom, #1 com is a Garmin GNS300XL GPS/COM feeding a rod antenna mounted on turtle deck. RG400 coax with BNC connection at radios, center conductor connected to antenna via ring connector at antenna with shielding grounded to airframe near antenna via ring connector (ala B Nuckolls). All connections on #2 were checked solid.

To troubleshoot, I switched antennas. Now the KX155 transmits OK on the upper antenna with a much reduced affect on fuel pressure - goes up 8 PSI. #2 (now connected to bottom antenna) also transmits fine with a lesser affect on fuel pressure than the KX155 had.

After discussions with my A&P. he observes that if you are grounded at the radio via the BNC, and grounded at the antenna via the sheild ring loop connector, you have created a ground loop. I check my antenna BNC at the radio, and they are grounded.

Hmm, seems to violate the "ground one end only" convention. So, disconnect the shield ground at the antenna and try again - Radio transmits but RF interference is everywhere!! - Fuel PSI up to 130, oil pressure, fuel quantities etc etc go wild!! Reconnect ground and back to the base line - intermittent transmit and fuel PSI bounces up to 42.

What am I doing here - besides going nuts??? Even with a BNC to a commercial antenna, aren't you creating a ground loop with the shield grounded at both ends??

Avionics gurus please. I went from inquisitive to thinking I was smart to complete frustration all in one 2 hour session. Yikes. Can someone make me smart again??

 

[az_gila]

One definition of a ground loop...

...
What am I doing here - besides going nuts??? Even with a BNC to a commercial antenna, aren't you creating a ground loop with the shield grounded at both ends??
....

...is that it allows an extra, unwanted DC return path. This current can (will) cause a voltage offset between two units.

There should be no DC currents in the BNC cable to your antennae.

Your experiment with disconnecting shielding proved this...

 

[gereed75]

Thanks for the reply Gil. But I am still not sure what I have or how to improve it. Guess I just don;t understand it anywhere near well enough.

 

[PaigeHoffart]

When people talk about ground loops, they're talking about unwanted common mode current. This could occur with or without multiple ground paths; you just need a shared current path.

Let's say I wire my strobe power supply to the battery using two wires. I then decide to save some money and tap off the strobe supply to power my intercom (I can hear people cringing). When the strobe operates it draws current. Since we're not using superconducting wire, there will be some voltage drop. The voltage at the strobe supply terminals will rise and fall every time the strobes fire. The intercom also sees this voltage fluctuation because it's sharing the same wire; the result will probably be a lot of noise in the intercom. Of course no one would intentionally do something like this, but sometimes incorrect installation can have the same effect.

Let's say I decide to ground my strobe power supply (located in the tail) to the airframe. Every time the strobe fires there will be a small voltage drop across the airframe between the strobe and battery ground point. Then I mount my intercom mic jacks to the airframe between the strobe supply and battery and I ground my intercom to the battery. Now every time the strobe fires some current will travel through the mic shield in addition to the airframe (they are in parallel). The voltage drop on the shield is picked up by the interphone amplifier and viola-strobe noise. In this case the best thing you could do is isolate the jack from the airframe. Breaking the shield would eliminate the "loop", but would not eliminate the problem because the airframe at that point still has strobe noise from the ground return that would be superimposed on the mic signal.

What does this have to do with RF? Absolutely nothing. RF needs to be trapped in the coax... it's a transmission line. If you disconnect the shield at either end, that transmission line will turn into an antenna, and you'll have RF everywhere.

My guess is that:

1) The KX155 is putting out more power than the GNC-300.
2) The lower antenna or coax is closer to the EIS4000 wiring.
3) The pigtails that are exposed inside the fuselage on the rod antennas are radiating RF
4) You've developed a bad connection somewhere in the KX155 PTT circuit which is unrelated to the fuel pressure issue.

As for trouble shooting number 4, the easiest way is to monitor the KX155 display. You should have a "T" between your comm. frequencies when you push the PTT. If the T is flashing, or isn't appearing when you push the button, I'd look at PTT wiring, transmit select switch, and tray connectors.

Sorry for the long post, hope it helps.

Paige

 

[az_gila]

Yep... I was just...

Thanks for the reply Gil. But I am still not sure what I have or how to improve it. Guess I just don;t understand it anywhere near well enough.

...trying to explain the gound loop...

However, grounding to the fuselage skin at the antenna base is important. Make sure that the paint/primer is scraped off and fresh star washers - or whatever the antenna manufacture specifies - are used.

Your experiment showed the critical nature of this RF ground at the antenna location....

Here is my set up - #2 nav/comm is a KX155 Nav/Com feeding a plain rod antenna mounted centerline bottom, #1 com is a Garmin GNS300XL GPS/COM feeding a rod antenna mounted on turtle deck. RG400 coax with BNC connection at radios, center conductor connected to antenna via ring connector at antenna with shielding grounded to airframe near antenna via ring connector (ala B Nuckolls). All connections on #2 were checked solid.

How short is this ground pigtail at the rod antenna? The exposed center conductor portion and ground pigtail should both be as short as possible.

I don't understand this bit shielding grounded to airframe near antenna via ring connector

The ring connector usually is the grounding point at the hole that the rod antenna passes through.

Typical instructions are here -

http://i644.photobucket.com/albums/uu166/az_gila/antenna.jpg

 

[gereed75]

Thanks again Paige and Gil. You are right Paige, the KX155 has a higher wattage output than the 300XL. I suspect that it is just overpowering the relatively poor connection (no BNC) at the antenna. This antenna is also much closer to the EIS than the 300XL antenna connection. That all makes sense. (By the way, I always get the Tx light in the KX155, just no side tone and no apparent transmission).

And I see now that it isnt really a ground loop issue. Just poor conduction of the RF energy from the coax to the antenna. Thanks for the drawing Gil. My shield ground is located right next to the antenna but is not concentric to the rod.

It all sounds like this can be overcome by springing for a real antenna connected via a BNC. (As has been suggested in other threads on the subject). The rod and loop connecter may get by on lower power radios, like it almost does with my 300XL, but the higher power KX needs a more efficient conductor. I'm convinced.

Thanks again guys.

 

[Mel]

You're on the right path. These antennas aren't good on high powered radios. I think springing for a "real" antenna is the correct choice. I have one of these "cheapy" antennas on the roof of my hangar, but it is receive most of the time. When I do use it for transmit, it's with my hand held with only a couple of watts.
BTW, the PTT circuit is nothing but a ground. Your problems stem from high power RF, not PTT connections.

 

[az_gila]

How long are the...

.......

And I see now that it isnt really a ground loop issue. Just poor conduction of the RF energy from the coax to the antenna. Thanks for the drawing Gil. My shield ground is located right next to the antenna but is not concentric to the rod.

......

...exposed portions of the shield and center conductor?
How close is "right next"...

These dimensions should be at an absolute minimum.

 

[erich weaver]

I have the same problem regarding increased fuel pressure when I transmit on my SL-40. Not sure that this is really a wiring or antenna problem. I took a handheld radio with rubber duck antenna on board and noted a similar increase in fuel pressure when I transmitted on that, although not as high. Apparently the fuel pressure senders that GRT supplies are just susceptible to RF interference. Have since comunciated with a company called Fair-Rite to see if the problem maght be remedied with a proper ferrite filter, but havent tried their suggested product yet. THey apparently reommend different materials for different frquency ranges. Will report back when I do try it.

regards

erich

 

[PaigeHoffart]

BTW, the PTT circuit is nothing but a ground. Your problems stem from high power RF, not PTT connections.

As I said in my post above, I think there are a couple of different issues going on here.

1) The RF interference with the fuel pressure indication caused by the antenna type.
2) The transmitter and side tone failure

I can see decreased range, or increased RF interference from something on the RF side, but the lack of side tone leads me to believe there is an issue on the mic side of the radio: PTT, mic audio, dirty tray connections, or an internal failure. If the transmit "T" is illuminating and there is no side tone, then PTT isn't the issue.

Replacing the antennas should help the RF interference problems, but if you still can't transmit, start looking on the audio side.

My .02,
Paige