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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 |
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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 |
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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. |
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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: ![]() You'll want to wrap the wire through the ferrite core several times, kinda like this. ![]() 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.
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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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