T

tullow

Member
Hello to all,

When I key the mic my ammeter indicates - 35 amps. !!

I have a Garmin SL40 installed and wired per the installation manual, with my mic/phone jacks on the crossbar between the seat backs ( i.e. quite a long wire run from the panel mounted VHF ). The VHF is powered from a 10A switch/breaker thru' it's own 5A fuse. I also have an intercom enable switch on the panel. Both transmission and reception are clear but very weak.
Am I losing signal strength in the long wire run between the jacks and the VHF?
When keying the mic I am obviously grounding something that shouldn't be grounded. If the VHF was really "pulling" 30+ amps I imagine fuses and breakers would have popped. I'm not an avionics installation expert, I just followed the installation instructions i.e. connect pin 2 to pin Z etc.

Any ideas or information would be much appreciated

Thanks in advance

Mike

RV7 (ARAF - almost ready to fly !!)
 
Which ammeter are you using?

The Van's gauges are notorious for picking up noise from RF signals.

Paul
 
When I was testing my radio, my panel went crazy. MP-gage, trim indicators, AMP meter was jumping all over. I just couldn't understand what the problem was. After many hours trying to find a solution, with no luck, I moved on. Finally when I moved the plane to my hangar, I found the problem. Transmitting in an closed environment, like a garage or hangar, the panel (instruments) get overwhelmed by bouncing radiowaves. This is no scientific answer, but I don't se this when I'm outside the hangar transmitting.

Fredrik
 
Use a dummy load during installation testing, it will absorb the RF without radiating it and won't drive the panel crazy. Of course, it also won't transmit, but it will work for testing.

I made a dummy load from 4 5-watt resistors in parallel, it absorbs the vast majority of the signal strength but still bleeds enough out into the air that you can tune it in with a nearby receiver to verify transmit clarity.
 
Ironflight said:
Which ammeter are you using?

The Van's gauges are notorious for picking up noise from RF signals.

Paul
Click to expand...

Paul,
I am using a Vans ammeter. Do you mean that the ammeter is somehow picking up radiated radio energy and indicating this as a discharge ?,would this also explain the weak transmitter and receiver signals?.
Furthermore, does the length of the cable run between my VHF and the mic/phone jacks have any bearing on the weak signals,as I thought the mic jack circuit was only a control circuit for the VHF (telling it when and what to transmit ,as it were)

I must admit I'm out of my depth here

Mike

Fredrik,

Interesting information re transmitting in an enclosed space -- I'll do some tests outside ( when the present storms have blown through) with the engine running and see how it goes

Mike


Greg,
Where do you install this dummy load for the testing that you suggest ?


Thanks


Mike
 
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tullow said:
Paul, I am using a Vans ammeter. Do you mean that the ammeter is somehow picking up radiated radio energy and indicating this as a discharge ?,would this also explain the weak transmitter and receiver signals?.
Furthermore, does the length of the cable run between my VHF and the mic/phone jacks have any bearing on the weak signals,as I thought the mic jack circuit was only a control circuit for the VHF (telling it when and what to transmit ,as it were)

I must admit I'm out of my depth here

Mike
Click to expand...

That gauge can be sensitive to RF but if you also have poor radio performance I would be looking at the coax cable between the radio and the Ant. If the shield is improperly terminated (poor connection at either end) it can turn the coax cable into an ant. Since that cable is routed within the cockpit, it can cause a lot of RF related problems.
 
A bad antenna can also cause this problem. If you remember the old cb days, vswr can cause this. Too much signal is not getting out the antenna because of the antenna or coax and it reradiates back into the radio.

Jay
 
Scott is right on - you are probably plastering all your RF energy throughout the airframe, and creating a high frequency environment for your instruments. Check the Coax, and if you are using a simple bent-wire whip antenna, you leak might be right there - the connection to those things are very had to shield.

Paul
 
Thanks Scott and Paul,
I'll get out to the hangar this afternoon or tomorrow and check out the co-ax connections at the antenna and VHF . I'm using one of those simple bent wire antenna's on the rear upper fuselage and the connection there was quite difficult to do .

I'll keep you up to date on the results

Thanks to all

Mike
 
Dummy Load

You can connect the resistors (50 ohm value) at the termination where the antenna is or a shorter piece of 50 ohm (RG-58 coax) directly to the anrenna jack at the back of the radio. The guys are right with radiating Rf throughout the A/C if you have ANY loose or poor connection or impedence mismatch between the radio and the antenna. Call me if I can be of futher help.
Jerry Fischer
Amateur Radio Extra Class licensee KW4F
cell 678-770-0131
Working on Fuselage RV-7
 
Here's the deal...you can goof around putting resistors and such all over the place...or...just realize this is a common problem with the Van's instruments and even after much, much work you may never get rid of it anyway.

It's been well known and well documented for many years now that this is a real common thing with these guages. So, if it were me, I'd leave it be - or - go buy and engine monitor that costs the same as the round instruments anyway but are far more reliable, accurate and usable.

Cheers.
Stein
 
Jerry Fischer said:
You can connect the resistors (50 ohm value) at the termination where the antenna is or a shorter piece of 50 ohm (RG-58 coax) directly to the anrenna jack at the back of the radio. The guys are right with radiating Rf throughout the A/C if you have ANY loose or poor connection or impedence mismatch between the radio and the antenna. Call me if I can be of futher help.
Jerry Fischer
Amateur Radio Extra Class licensee KW4F
cell 678-770-0131
Working on Fuselage RV-7
Click to expand...

Jerry, thanks for the tip,this will be part of my trouble-shooting when I get up to the hangar tomorrow



Stein,
As usual, good advice with a "historical" slant to it , thanks a lot -- but as a recently retired airline pilot with the last 18 yrs. flying behind EFIS,FMGCs etc. and the last 8yrs on fly-by-wire Airbus, it's nice to have a steam-gauge instrument panel on my RV7, though I do agree that for simplicity and probably reliability ,a " glass cockpit has a lot to be said for it.
However, having seen some of the top end "glass" available for general aviation and homebuilts which are much more complex and sophisticated than any airliner equipement, I have to wonder about the learning curve for the average private pilot with a subsequent loss of "back to basics" reflexes.
Sorry for the slight "thread creep".

Thanks again

Mike
 
SteinAir said:
Here's the deal...you can goof around putting resistors and such all over the place...or...just realize this is a common problem with the Van's instruments and even after much, much work you may never get rid of it anyway.

It's been well known and well documented for many years now that this is a real common thing with these guages. So, if it were me, I'd leave it be - or - go buy and engine monitor that costs the same as the round instruments anyway but are far more reliable, accurate and usable.

Cheers.
Stein
Click to expand...

Stein,
He did say he was getting poor receive and transmit performance so it seems likely there may be an installation issue. I agree these instruments are sensitive to stray RF but it is not a universal problem. I still believe he should look closely at the ant. coax.
 
rvbuilder2002 said:
I still believe he should look closely at the ant. coax.
Click to expand...

Agreed - especially with a wire whip, getting the end of the coax to work with a screw lug is very problematic. I had a serious RF leak when I tried this type of antenna when I was building, and solved it entirely by going with an antenna with a Coax connector - more expensive, but a whole lot less trouble!

Paul
 
I tend to agree too..

Check the coax connections for continuity with a volt/ohm meter. Also make sure something isn't shorted between the center conductor and the braid.

Check the solder connections on both ends of the coax and make sure they're all solid and that they don't look like they were cold. Are they crimp on connections??? If so, get rid of them and solder your own on.

Check the whip on it's own and make sure there isn't a short in the whip between center and outer conductors.

Check to see how well the whip is grounded to the airframe.. Is their paint or some other insulating device that isn't letting the antenna ground properly?

Mentioned before: If you're transmitting indoors or in a metal building, it will affect your vswr and also the amount of energy that can effectively be radiated away from the airplane.

Something you might consider is using a ferrite choke where the wires enter the amp meter. A ferrite choke will act as an RF trap/filter before it enters the amp meter. Some ferrite chokes are available at radio shack, but you might want a larger one.

Here's a photo:
pRS1C-2265709w345.jpg


You'll want to wrap the wire through the ferrite core several times, kinda like this.

SL-choke.jpg


After that, I'd probably try powering the VHF radio with a 12v power supply (or battery - just as long as it's independent of the aircraft's power system) and see if the problem still exist. If it does exist, you know you're battling an RF issue. If it goes away you know you're battling a power system issue or an inaccurate amp meter.
 
The mystery deepens !!

Got up to the hangar this morning to try to use some of your very helpful tips and information. First checked the continuity between the antenna (whip) and airplane structure, it was a direct short !!, I then undid the co-ax connections to the antenna and discovered that there was a short between the shield and the core - second suprise !!. I redid the core and shield connectors ( crimp/solder) and slid the SL40 out of the panel to check the co-ax connectors at the rear of the rack,this connector checked out ok for continuity and isolation.
Reracked the VHF,reconnected the co-ax to the antenna then checked the isolation of the whip antenna from ground and yet again it was shorted !!.
Disconnected the co-ax from the antenna and found that the core and shield were still shorted !!. I finally removed the VHF from the panel and then found that the co-ax checked out perfectly for continuity and isolation at both ends.
In fact when I then checkedthe female co-ax connector at the back of the SL40 on the bench the core is shorted/grounded to the body of the connector !!!.
Now the 64000$ question -- is this normal when the VHF is off and if so would this return to normal when the radio is turned on?
I didn't want to try this just in case it might damage the radio.

So maybe some of you can clarify this so that I can get back to my original problem of erratic ammeter readings and low power performance from the VHF

Thanks to all

Mike
 
What you are seeing is normal. What you have to remember is that an ohm meter checks DC resistance. Comm freqs are a very high freq ac signal. The best way to troubleshot something like this is with a bird watt meter if you can find someone that has one.
 
Hi Mike,

You should expect continuity between the center and outer conductors inside the radio. It's downstream of where the signal leaves the radio that you want them to be isolated. So you want to disconnect the antenna and check for a short at that point.

There are some exceptions to this down stream rule but I dont' think they apply in your case. Those exceptions would only exisit if there is an inductor or capacitor (used to correct for an out-of-tune whip) placed between the two conductors down stream. These devices are usually located at the base of the antenna. I'd be really shocked if the antenna had one, they just aren't something you see on VHF antennas very often. Most of the time they're on HF antennas when someone wants to use an antenna that is too short, but make it appear to the radio as an antenna that is of the proper length.
 
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