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  #1  
Old 01-22-2009, 04:27 AM
tullow tullow is offline
 
Join Date: Nov 2007
Location: Paris ( France )
Posts: 38
Default VHF problem- whats happening ?

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 !!)
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  #2  
Old 01-22-2009, 05:11 AM
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Ironflight Ironflight is offline
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Which ammeter are you using?

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

Paul
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  #3  
Old 01-22-2009, 07:04 AM
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Fredrik Fredrik is offline
 
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Location: Stockholm, Sweden
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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
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  #4  
Old 01-22-2009, 09:43 AM
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airguy airguy is offline
 
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Location: Garden City, Tx
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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.
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  #5  
Old 01-22-2009, 10:54 PM
tullow tullow is offline
 
Join Date: Nov 2007
Location: Paris ( France )
Posts: 38
Default

Quote:
Originally Posted by Ironflight View Post
Which ammeter are you using?

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

Paul
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

Last edited by tullow : 01-22-2009 at 11:03 PM.
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  #6  
Old 01-22-2009, 11:03 PM
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rvbuilder2002 rvbuilder2002 is offline
 
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Location: Hubbard Oregon
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Quote:
Originally Posted by tullow View Post
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
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.
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  #7  
Old 01-23-2009, 03:27 AM
pilot2512 pilot2512 is offline
 
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Location: Spring, TX
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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
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  #8  
Old 01-23-2009, 06:17 AM
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Ironflight Ironflight is offline
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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
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RV-8 - N188PD - "Valkyrie"
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RV-3B - N13PL - "Tsamsiyu"
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  #9  
Old 01-23-2009, 07:10 AM
tullow tullow is offline
 
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Location: Paris ( France )
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Default

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
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  #10  
Old 01-23-2009, 07:43 AM
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Jerry Fischer Jerry Fischer is offline
 
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Location: Winder Ga
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Default 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
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