What's new
Van's Air Force

Don't miss anything! Register now for full access to the definitive RV support community.

Nav Radio Troubleshooting

Rob Erdos

Well Known Member
Folks,

The Garmin GNC 255A nav/com in our RV-6 has never worked quite right. The comm side of the radio works great, but the nav side doesn't seem to receive anything. The radio functions properly, but it can't receive a VOR ident and won't display a bearing even within a few miles of the transmitter, so I'm inclined to think that I have an antenna problem.

I installed a fancy Bob Archer antenna in the fiberglass left wingtip fairing. A photo of the antenna installation is below [note that I inverted the wingtip fairing and placed it on the wing. The antenna is mounted on the inner-upper surface of the fairing]:

2e6724cbf9066be33a432891bc9ac9dd_zpsy18zshmj.jpg


So, is there anything evidently wrong with this? Do any Electrical Smart People have suggestions for further troubleshooting?

It's a pain to pay so much for a radio and not be able to play with it...

Thanks,
 
Can't comment on the antenna (A. Can't see your pic because my work is blocking the hosting site, and B. I don't have an Archer antenna) but assuming its OK the next thing I'd check is cable continuity from end to end.
 
Looks okay. I would like to see the bare ends of the coax shorter - in the photo it almost looks like the bare parts are touching, but I presume not.
Wingtip screws making good electricl contact (no paint) with the wing?
Close to a station virtually anything should woek as an antenna, so as mentioned, use an ohmmeter to check continuity.
 
I see a lot of solder close to the cable? Make sure it did not melt thru the dielectric to the inner wire!
 
Rob,
A few more questions to help sort this out:

- Is your 255 hooked up to an external CDI (EFIS, HSI, analog CDI etc)?
- When you say bearing do you really mean CDI indication?
- When you say the radio functions properly but can't receive a VOR IDENT can you explain what "is" working on the NAV side?

If the antenna and cable checks out and the 255 itself is not reporting any errors then my guess is there some Nav radio settings in the radio setup pages and/or Nav database issues (for the IDENT problem) that could be the culprits.
 
The power cables for the wingtip lights don't run down the centers of the aluminum strips as called out in the instructions for the antenna. They make that seem pretty important - I'd suggest moving them.

Installing the antenna per plans should let you receive a solid VOR signal at more than 90nm from the station - well above the standard service volume for low-altitude VORs.
 
Similar issues

Hi Rob,

I feel your pain. I have a Bob Archer in each wingtip connected to a VAL Nav 2000 radio which displays on a Trutrak EFIS. My glideslope/localizer works OK, but I've never been able to get VOR reception beyond 30 miles and it keeps flagging inside that radius. I have the same result with either antenna. The VAL uses the same antenna for both VOR and glide slope. When I use my avionics shop's test rig the setup passes.

My EFIS seems to interfere with VOR reception on my Sportys handheld and may be doing the same with my VAL radio, but I've never proven this. If I can borrow a cat whisker antenna, I will connect it up on the ground and see if I get better reception from our VOR which is less than 10 miles away. I think this would be a good test of my Bob Archer.

John
 
jdpowell has the right idea: Substitution is usually the fastest way to troubleshoot.

Replace the coax/antenna with a known-good piece of coax & antenna. If you're getting nothing at all out of the vor rcvr, you could, in a pinch, just substitute a cheap comm whip, carried in the cockpit, to see if you get a response from a nearby vor.

On the flipside, if you have enough slack in the existing ant cable, unhook from the back of the panel mounted radio & attach it to a handheld that has a known-good built-in nav function. You could do both tests on the same flight.

Results of the two tests should tell you where to go next with troubleshooting.

Charlie
 
Hi Rob,

I feel your pain. I have a Bob Archer in each wingtip connected to a VAL Nav 2000 radio which displays on a Trutrak EFIS. My glideslope/localizer works OK, but I've never been able to get VOR reception beyond 30 miles and it keeps flagging inside that radius. I have the same result with either antenna.

John

I'm aware of two specific examples of Nav 2000 radios which were nearly deaf to VOR signals, yet worked fine for ILS.

One of the owners performed extensive testing, side-by-side with another nav receiver. He went as far as to take this entire test rig to another aircraft so he not only eliminated the coax and antenna, he also eliminated his aircraft from the troubleshooting equation entirely. Bottom line result is the King radio would work at ranges several times further from the VOR station than the VAL radio would.

In the second case the aircraft owner had another VOR receiver in the aircraft and swapped antennas/coaxes from one radio to the other, to no avail.

The latest development in this saga is the folks at VAL have worked on both radios. Performance of one radio has improved while the jury is still out on the other one.

Just be aware that all the troubleshooting of your installation may still not yield results and be prepared to go back to the manufacturer of the radio. I'm providing this info simply because I don't want to see somebody unnecessarily spend hours and hours upside down under the panel, or, worse yet, ripping out a perfectly good installation.
 
Handhelds

Charlie and Rob,

Charlie, I've done as you said with respect to my handheld. I keep it connected to one of my Bob Archers, and it receives VORs without problem in flight. For that reason, I've been concentrating on the possible interference issue to solve my reception problem. Things I've tried include adding the second antenna and coax (routed farther from the EFIS than the first antenna), improving grounding of the antenna/radio/EFIS, making sure my RS232 wire from the radio to the EFIS was shielded, adding ferrite beads to the wiring of the radio and EFIS, installing an electronic filter to the power supply wire of the radio and wrapping the radio in copper screen wire. None of these efforts resulted in any improvement.

Sitting on the ground with my handheld tuned to the VOR I get good reception until I turn on my EFIS. Then, handheld reception is usually lost on the handheld. When I turn the EFIS off reception comes back. Not sure what to do next except try the cat whisker antenna.

John
 
Canadian JOY

Canada and Rob,

Thanks for that input. I failed to mention that I sent my radio back to VAL for testing and they reported no problem with the unit. It's still a new product though and there's no telling what issues may exist. I should clarify that the handheld works fine in flight with the EFIS on.

Sorry to highjack your thread Rob.

John
 
Last edited:
John - FYI, both of the cases I've previously cited also sent their radios back to the factory and were told they worked fine. In one case the owner has a handheld radio and found its performance to be substantially better than the Nav 2000 (as in the handheld could receive the VOR from several times the distance as the Nav 2000).

Before changing your nav antenna you may want to do the same testing as the other Nav 2000 owner did and install a "radio in a box" and install a break-out connector in your NAV antenna line (1 male, one female BNC connector). Use this break-out connector to allow you to connect your existing NAV antenna to an external test radio. You don't necessarily have to see the VOR functioning in terms of providing CDI movement - even listening to the IDENT tone will give you an idea of whether or not the NAV 2000 is receiving VOR signal. In the case of the other NAV 2000 owner he was able to discern a very measurable and repeatable difference between the NAV 2000 and the King radio, just based on the ability to hear the ident tone. With the King radio the ident tone was clearly discernible from quite some distance whereas the test aircraft had to close the distance to less than half before the NAV 2000 produced even the most feeble ident tone.

Changing your NAV antenna to whiskers on the tail is not a high-odds bet if you feel your EFIS is producing sufficient EMI to take the Nav 2000 off line. It's likely the EMI is at its greatest level closest to the EFIS, so up at the cockpit. Changing the antenna, multiple feet away, isn't likely to make a difference.

Keep in mind that your handheld radio has very poor shielding from this type of noise and is far more susceptible to it than a rack-mount radio. The metal case of the rack-mount radio gets tied firmly to chassis ground so it makes a nice little cage around all those sensitive electronics - your handheld doesn't benefit from this same type of overall shielding.
 
Looks like there are 3 different problem posters wound up in one thread, with varying symptoms and varying troubleshooting items already ticked off for each.

It would be a lot easier for those trying to assist, if there were 3 threads, so we wouldn't have to sift through who has what symptoms in what conditions and what's been tried, etc.

Just a thought...
 
Keep in mind that your handheld radio has very poor shielding from this type of noise and is far more susceptible to it than a rack-mount radio. The metal case of the rack-mount radio gets tied firmly to chassis ground so it makes a nice little cage around all those sensitive electronics - your handheld doesn't benefit from this same type of overall shielding.

And on the flip side of that, a handheld with an isolated (non)ground might eliminate a problem *caused* by improper/inadvertent grounding issues in the plane.
 
Looking at your wingtip, I would say you don't have a great ground plane for the antenna. 3 or 4 screws providing ground to the airframe likely isn't adequate. I recently did an IFR check on my RV-7 with a calibrated VOR/LOC/GS test box and cranked the attenuation way up on the box and moved it to a position that was forward and to the right of the nose (to simulate intercepting the Localizer - antenna is in left wingtip). The antenna was able to pickup a pretty weak LOC and GS signal. Based on the level of attenuation I used, that should translate to a 10-12 mile reception range for the LOC. I have a long strip of metal between the wingtip and nutplates, which increases the effective ground plane substantially.
The other thing you should check is the continuity of your coax and verify the center conductor isn't shorted to the shield.
 
. I have a long strip of metal between the wingtip and nutplates, which increases the effective ground plane substantially.
.

Actually that extra metal hardly does anything; just millimeters away, thru the fiberglass, is the metal wing.
The ideal ground plane would be a very large piece of metal positioned vertically, running fore-aft and up-down. With the Archer antennas the most important contributor to the ground plane is the metal end rib. A few people have positioned the antenna further outboard, thinking that getting away from the wing was good. It isn't, it needs to be close to that rib. A few screws, if in good contact with the metal, are sufficient for the ground return. Otherwise Bob Archer would have made the ground leg longer. Most of the current flow in the ground plane is induced by the antenna. Having a poor ground plane is a blessing in disguise; if it was really good, you would not get any signals from the other side of the airplane.

The OP has a severe problem. Either bad coax connections, shorted, or the receiver itself.
 
Given your unique symptoms, I am not sure this would help but the instructions want the coax feed at 90 to the element that it attaches to. The diagram shows it at 90* and heading inboard (yours is not 90* and going outboard). If I recall, the instructions call this out as critical.

Best of luck in your diagnosis.

Larry
 
Another consideration, given the likelihood of a grounding issues. Did you scrape the paint out of the countersinks on the 4 wingtip attach screws that attach to the antenna ground plane? This antenna gets its ground from the 4 screws and the screws get the ground from the skin in those countersinks. You have to scrape the paint or you won't get a good ground to the antenna.

Larry
 
Try another antenna in the cowl

Rob, I've got a Val NAV 2000 also. I had the same antenna and did not get a very good signal either. I'm not saying it was the antenna's fault. I think it is the installation. I made a simple dipole with two strips of self stick copper strip inside the cowl. I taped them from just under the prop on the bottom of the cowl out to each side. Each 22.8 inches long and ran the wire into the NAV. Wow what a difference. I'm getting 50NM to VORs when at altitude. This both to and from VORs. I thought I would have trouble going from, but it works great.
 
Back
Top