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  #1  
Old 04-24-2014, 10:45 PM
deltamike197 deltamike197 is offline
 
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Location: Sunny South Florida
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Default SL-30 poor VOR reception

When I first started flying my RV-6A I did some rough VOR range reception checks with the SL-30 and I had to get within 20 miles of a VOR station to get the receiver to lock on and the OBS indicator to stabilize with no flag displayed. I continued with the flight test regimen and getting familiar with the airplane and enjoying it. Since flights have been in VFR conditions around the busy Tampa (Florida) airspace I used the panel mounted moving map (GPS/Comm) to make sure I stayed out of the Class B, there was no need for good VOR reception. The Com portion was the only thing I was using on the SL-30 and that worked very well. Then, a couple months ago I turned on the SL-30 and, nothing, no display or Com so I returned the unit to Garmin for repair. In the meantime I'd been talking to a CFII about working on my instrument proficiency to get back up to FAA standards after a long hiatus. When the SL-30 was repaired and back in the instrument panel I took off on a flight to Ocala to check it out. I climbed up to 4500 feet and watched the OBS and listened for the ID code as I entered the 40 mile radius of the Ocala VOR. Nothing, so I kept going on a course straight for the station. Around 20-25 miles out I started getting activity on the indicator but the flags were intermittent and the radio could not maintain a lock. Finally about 15 miles out the indicators stabilized and the flags stayed off, obviously something is amiss.

I'm using a Comant CI-159 VOR/ILS antenna. This unit consists of a phenolic "puck" with a BNC connector cast into it for the coax connection. There are two fiberglas elements (thin rods about 18 inches long) that screw into the puck to form a "V" shaped antenna. I have it mounted on the bottom of the fuselage in the back of the airplane under the horizontal stabilizer. The BNC sticks through a hole in the belly and appx 25 foot length of RG-58 coax runs inside the aircraft up to the SL-30 receiver. No gasket was supplied with the antenna so I just bolted the puck to the aluminum using the two bolt holes. After the Ocala flight I removed the puck and elements for closer inspection and checked the connector on the coax cable at the antenna end. Nothing wrong that I could tell just by looking at the equipment. There was a bit of engine oil (from the exhaust) on the puck surface facing the skin. I cleaned it off and made a gasket to go between the puck and the skin to minimize the oil build up again. I have not flown the airplane since but I doubt that lack of a gasket was causing the poor reception, maybe someone here can tell me. The other thing I checked was the resistance between the center pin and outside barrel of the BNC connector on the antenna. Resistance was showing basically a dead short (less than 1 ohm, same reading as touching the meter probes together). I'm not an antenna designer so I don't know if this is what the reading should be or not. Aircraft Spruce has the following description for this antenna: " "V" Dipole VOR/Glide Slope Antenna with detachable elements is similar to the CI 158C-3 with the exception of offering 2-hole mount instead of a 4-hole mount. This V Dipole encompasses reduced static capability with the use of P-Stat paint. Integral ferrite balun provides for higher radiation efficiency." . I don't know if the construction of the antenna would cause the meter to show such a low resistance, maybe some here could shed some light on that.

Any ideas, experiences with poor VOR reception here on VAF? Since the radio has been repaired and tested by the factory I'd say it's not the radio. I checked the connector at the back of the radio tray and it appears secure so I'm not sure what else to do. Since VORs are being decommissioned there is going to be less and less need for VOR reception but if I have to use victor airways during instrument operations I need to have good reception at the expected range or I won't be able to comply with ATC clearances. On a side note, a fellow RV-9 builder who is using a simple flat metal antenna in the wingtip, has no problem receiving the VOR 40 miles out, and he didn't pay anywhere near what I did for his antenna. Thanks for the help
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  #2  
Old 04-24-2014, 11:02 PM
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Ironflight Ironflight is offline
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I have had problems with the coax having a good solder joint within the 90 degree connector on the back of the radio rack. It's not fool-proof, and in fact is easy to mess up. It is also a pain to check - but if everything else is good - know that it can fail.
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Last edited by Ironflight : 04-25-2014 at 09:40 AM.
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  #3  
Old 04-24-2014, 11:30 PM
johnf_1 johnf_1 is offline
 
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Default VOR antenna

hi! So, I had a Narco Escort II for a decade in my 4. I used a V of copper foil tape in the back of the canopy with some (3?) toroid coils....amazing range....really simple, reliable and worked great...you are bringing back the memories of navigating by triangulating 2 or 3 radials to fix a (rough) position on the chart, and then fly a wet compass all over the western US...ah, real flying, them days (still have the wet compass...but GPS kinda spoils all of that!) Best, J N95JF
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  #4  
Old 04-25-2014, 03:43 AM
RFazio RFazio is offline
 
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Default I'm building an antenna for my nav now!

I was going to use the foil tape on the inside bottom half of the cowl. I'm planning a vee shape right on the bottom surface. What are the coils you are talking about?
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  #5  
Old 04-25-2014, 06:47 AM
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mikehoover mikehoover is offline
 
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+1 on checking the 90 degree BNC connectors on the back of the tray. My SL-30 com was behaving similarly and I thought I had checked everything - except for the coax pigtail that came with the SL-30. Brand new out of the box, I found it to be the problem. I took the 90 connector apart to find that a single strand of the core wire had not gotten tinned in with the rest and was sticking out enough to contact the connector housing. Quick fix and everything works perfectly.
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Last edited by mikehoover : 04-25-2014 at 08:46 AM. Reason: Spell correct
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  #6  
Old 04-25-2014, 07:14 AM
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GrayHawk GrayHawk is offline
 
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These VOR antennas with 2 rods, may have a balun inside to match impedance. The result of that balun would be a low DC resistance.
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  #7  
Old 04-25-2014, 08:26 AM
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GrayHawk GrayHawk is offline
 
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"appx 25 foot length of RG-58 coax"

The only other thought I had was the cable. The SL-30 installation manual calls for "RG-142B or comparable coax". Cable that is labeled RG-58 has many sources and a wide variation in quality, so we probably would need to know more about the source of the cable and its loss at 115 MHz. Loss in the cable as well as loss due to connector problems described above could reduce the sensitivity of the overall system.

bc

From B&C web:
"The RG-400 is sold by the foot. To purchase 50 feet or more use the product number of RG400/50. This is the product of choice for antenna feedlines in modern aircraft. This is equal in performance to often recommended RG-142. RG-142 has a solid center conductor while the the center conductor of RG-400 is stranded with an insulation layer of teflon. The outer conductor is TWO layers of silver plated copper braid. This coax directly replaces the legacy RG-58 product. PVC and polyethelyene used on RG-58 are WWII era insulations and no longer permitted on new certified aircraft. Qualified to MIL-C-17/128-RG400."
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Last edited by GrayHawk : 04-25-2014 at 08:34 AM.
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  #8  
Old 04-25-2014, 10:22 AM
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GalinHdz GalinHdz is offline
 
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Quote:
Originally Posted by Ironflight View Post
I have had problems with the coax having a good solder joint within the 90 degree connector on the back of the radio rack. It's not fool-proof, and in fact is easy to mess up. It is also a pain to check - but if everything else is good - know that it can fail.
I am assuming the coax you are using is 50 Ohm not 75 Ohm RF impedance. If so check to see the coax isn't shorted out or open. Disconnect both sides of the coax and check with an ohm meter. From the center conductor to the shield you should have a very, very high (Mega Ohms) resistance. Then check that the center conductor has low (almost zero Ohms) resistance from one end of the coax to the other and do the same with the shield. Wiggle the coax at different points when you are doing this in case there is an intermittent failure. Again, make sure both sides of the coax are disconnected while you do this or you will get erroneous readings.

If the checks are good then replace the 90 degree connector with a new one. Make sure the new one is for 50 Ohms RF impedance and rated for at least 300Mhz or it can significantly cut down the signal strength decreasing your effective reception range. Unless you really have to, try not using the 90 degree adapter.

Good luck.


Last edited by GalinHdz : 04-25-2014 at 10:26 AM.
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  #9  
Old 04-25-2014, 10:36 AM
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GrayHawk GrayHawk is offline
 
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Quote:
Originally Posted by RFazio View Post
I was going to use the foil tape on the inside bottom half of the cowl. I'm planning a vee shape right on the bottom surface. What are the coils you are talking about?
This is the 'balun' for matching 2 rods or strips to the single 50 ohm coax.

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  #10  
Old 04-25-2014, 10:38 AM
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DonFromTX DonFromTX is offline
 
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The SL 40 and 30 trays include a 90 degree connector as part of the tray (reduces needed space on the firewall end of the tray). I think this is the 90 degree connector being referenced, and would be hard to eliminate but not impossible..
Quote:
Originally Posted by GalinHdz View Post
If the checks are good then replace the 90 degree connector with a new one. Make sure the new one is for 50 Ohms RF impedance and rated for at least 300Mhz or it can significantly cut down the signal strength decreasing your effective reception range. Unless you really have to, try not using the 90 degree adapter.

Good luck.

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