Old Coax?

MBDiagMan

MBDiagMan

Final Approach
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Doc
With everything going on the last few years, I suspended my instrument rating aspirations. I am now back to it and need to get my marker beacon and VOR's working well.

My VOR's are Narco's, but they came out of the Narco shop with yellow tags just before Narco went belly up. They work well, but have weak signals indicating a likely antenna/cabling/connection problem. Also my Marker Beacon does not work.

Now that I am back into getting it all working, I mentioned the issue to a fellow at the airport who made mention of replacing coax. I had not thought of this because I've repaired lots of coax connectors in my time, but never saw coax go bad unless pulled loose, cut or something like that.

The coax is probably what was put in at the time of the custom panel which was in the late seventies, early eighties. So.... it stands to reason that it might be worth replacing.

This leads to two questions:

o Do you believe that replacing coax to the NavCom and Marker Beacon antennas is a worthwhile endeavor?
o If so, what would be the correct coax to use for each?

Thanks in advance for sharing your knowledge and throughts.
 
Replacing the coax could be worthwhile....if the shield is knicked somewhere along the line, or just aged and cracked, it can definitely cause signal degredation. You don't have to use anything special, some RG-8 or RG-8x should be fine. The only one I might consider for a different thickness coax would be the xpndr. Pretty sure it puts out pretty high power and I could definitely see it causing some rf interference otherwise. Just my from my experiences with ham radio and such. If nothing else, at least inspect the cables as best you can for line breaks and knicks and such if you suspect signal degredation. Good luck!


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FWIW also, I agree that coax if protected doesn't usually go bad. I would suspect connections first. Inspect and/or replace those before you start pulling out cable. Much easier to start there in my opinion...


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MBDiagMan said:
Now that I am back into getting it all working, I mentioned the issue to a fellow at the airport who made mention of replacing coax. I had not thought of this because I've repaired lots of coax connectors in my time, but never saw coax go bad unless pulled loose, cut or something like that.

I agree with that. Unless your coax has been contaminated with petrochemicals (fuel, brake fluid, etc.) the odds of it "going bad" is nearly zero. Out in the open, the polyethylene dielectric will age by attack of smog or ozone, but inside the aircraft, probably not. It is pretty easy to test IF there is a connector at the radio.

Make a cotanger antenna (antenna made out of coat hangers) or brazing rod. Find yourself a reasonably steady weak signal (like an ATIS at an adjoining airport) and see if connecting the cotanger antenna to the radio with a known good hunk of coax makes a significant difference. OR make yourself a hunk of coax to go from your PRESENT antenna to the radio just for testing. If the cotanger antenna is better than your present antenna you have an antenna problem. If the signal stays about the same you have neither a coax or an antenna problem.
The coax is probably what was put in at the time of the custom panel which was in the late seventies, early eighties. So.... it stands to reason that it might be worth replacing.

This leads to two questions:

o Do you believe that replacing coax to the NavCom and Marker Beacon antennas is a worthwhile endeavor?

Not unless you can prove to yourself that it is lossy. "Bad" is not a word most of us use as it means absolutely nothing. And, piping in that new coax with cable ties or wax string is a pain in the labonza that you don't need if you don't have to.

o If so, what would be the correct coax to use for each?

Good old RG-58 with SOLID center conductor (less loss than the stranded stuff). Anybody that tells you that double-shielded is "so much better" in a Cessna 140 Heavy is trying to sell you smoke.

Thanks in advance for sharing your knowledge and throughts.
Click to expand...

No problem. Note the N-number from my C-120.

Jim
 
Well Jim, I guess the day will come, when we are at the same airport and massive confusion ensues when they say "Cessna one-four-victor"........:)


I did a little bit of surveying yesterday and found that the Nav/Com antenna has one antenna lead that feeds VOR1(this is the one with the glideslope and Marker Beacon receiver) and a jumper coax goes from VOR1 to provide antenna connection for VOR2.

Luckily all these connections are the standard removable coax connectors.

What I did not think to check while under there, was the presence of another coax for the Marker Beacon. Isn't the small antenna under the fuselage for the Marker Beacon?

Thanks for the responses.
 
Coax typically doesn't go bad unless exposed to things that destroy it or improperly installed in the first place (radius of bend limits exceeded, no strain relief, connectors installed improperly).

Connectors are almost always the problem, if there is one.

And most importantly...

There's no reason to wonder if it's working. The test gear to test coax at any particular frequency should be available at any good RF workbench and the test takes almost no time at all once you can get to both ends.

If a tech is telling you they want to replace a cable, ask them to show you how they tested it and came to that conclusion. If they say they didn't, or refuse to, run, don't walk, to a better tech.

You've just met the dude who does as much troubleshooting as Geek Squad does on computers. Heh.
 
MBDiagMan said:
I did a little bit of surveying yesterday and found that the Nav/Com antenna has one antenna lead that feeds VOR1(this is the one with the glideslope and Marker Beacon receiver) and a jumper coax goes from VOR1 to provide antenna connection for VOR2.

With no little magic box to do the splitting? Just a coax jumper with no matching network at all? I don't DOUBT you are getting lousy reception with that setup. You can BUY a splitter for fifty bucks or so OR you can make one with two lengths of coax and a penny resistor. Your call.

Luckily all these connections are the standard removable coax connectors.

What I did not think to check while under there, was the presence of another coax for the Marker Beacon. Isn't the small antenna under the fuselage for the Marker Beacon?

Small Cessnas of that era almost always used what they called a "sled" antenna for the marker. That was a 39" lengths of bent stainless rod in a "J" shape. One end was grounded to the airframe and the other end was just left open. A small wire attached to the stainless about 6" from the grounded end and ran to a small porcelain insulator.

Or, somebody could have replaced the sled with a boat. A marker boat is about 10" long and 2" high and wide and is all fiberglass on the outside.

If you are talking about the little spike antenna with a BB on the top, that is the transponder antenna.

Take a picture and I'll tell you what you have.

Jim

Thanks for the responses.
Click to expand...
.....
 
Jim,

Firstly, the Nav/Com antenna jumper appears to be legit. VOR 1 is a NARCO with glideslope and Marker Beacon. VOR 2 is a different model number NARCO that looks the same, less the Marker Beacon and Glideslope. The coax from the Nav/Com antenna goes directly into VOR 1. A coax connector right next to the antenna input on VOR 1 has a short coax that goes to VOR 2. It's not like a T on the coax or anything.

I'm not saying that you are wrong in your assessment of the shared antenna, but it appears to be the way that the shared antenna between both like VOR's was meant to be.

Also Jim, you might have led me to the problem by your antenna descriptions. Now that you pointed out that the short antenna below is the Xponder, and you also described the Marker Beacon antenna, I don't seem to HAVE a Marker Beacon antenna.

The aircraft has two antennas above the doors for the Com's. There is a short antenna on top center of the fuselage which I think is the ELT. It has a horizontal V antenna on the leading portion of the vertical stabilizer that I think is the Nav/Com. It has the short antenna underneath that you point out is the Xponder antenna.

This actually is encouraging. I will look into what antenna connections are available on the tray for VOR 1. If there is a specific Marker Beacon antenna connection, then adding a Marker Beacon antenna will probably sort the Marker Beacon and connections and/or cable should sort the VOR weakness.

Is it possible that at one time someone could have depended on the VOR antenna for the Marker Beacon? If I recall, the Marker Beacon is a drastically dissimilar frequency range than the VOR's.

Thanks.
 
Marker beacons are 75MHz where as your VOR and LOC are in the 110 MHz range. The GS is up higher around 330MHz.
 
MBDiagMan said:
Jim,

Firstly, the Nav/Com antenna jumper appears to be legit. VOR 1 is a NARCO with glideslope and Marker Beacon. VOR 2 is a different model number NARCO that looks the same, less the Marker Beacon and Glideslope. The coax from the Nav/Com antenna goes directly into VOR 1. A coax connector right next to the antenna input on VOR 1 has a short coax that goes to VOR 2. It's not like a T on the coax or anything.

Just for the hell of it, why not give us model numbers instead of "Narco"? I've not seen a Narco since the early days of the successor to the Superhomer (VC27Z) that had an internal marker receiver. Nor one that had an internal splitter to feed #2 from #1. I could be wrong. I'm not a Narco expert, but have seen a few of them in my day.


I'm not saying that you are wrong in your assessment of the shared antenna, but it appears to be the way that the shared antenna between both like VOR's was meant to be.

Also Jim, you might have led me to the problem by your antenna descriptions. Now that you pointed out that the short antenna below is the Xponder, and you also described the Marker Beacon antenna, I don't seem to HAVE a Marker Beacon antenna.

The aircraft has two antennas above the doors for the Com's. There is a short antenna on top center of the fuselage which I think is the ELT. It has a horizontal V antenna on the leading portion of the vertical stabilizer that I think is the Nav/Com.

The horizontal V is for NAV only, no com. The only folks who ever used the rabbit ears for com were Genave in the A-200 series and Narco in the Escort series. It is a lousy way to do a COM antenna for the simple reason that ground stations are vertically polarized.

It has the short antenna underneath that you point out is the Xponder antenna.

Is it possible that at one time someone could have depended on the VOR antenna for the Marker Beacon? If I recall, the Marker Beacon is a drastically dissimilar frequency range than the VOR's.

Drastic is in the mind of the drasticee. I've known some people who successfully split off the marker signal from the nav antenna. It works, marginally. The glide slope split off the nav antenna is trivial; it is a 3/2 wave antenna at the GS frequency and a 1/2 wave antenna at the VOR/LOC frequency.

Thanks.
Click to expand...
.....
 
Sorry 'bout that. I found the NARCO model numbers. I've been incorrectly referring to them as NAV/COM's, but they are actually NAV's.

VOR1 is a NARCO Model NAV-122.
VOR2 is a NARCO Model NAV-121.


I will start trying to get some more details on these units.
 
Well, I started googling and found an installation manual for the NAV 122. It shows two BNC connectors, one for GS and one for NAV. Now, I'm anxious to go to the airport and investigate further.

Edit... Now I found another Installation Manual that covers installation of both of these radios as a pair. I am downloading it now. It might have some really good info.
 
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Well, I'm getting excited now. I tried finding information a few years ago and came up with not much.

Now I find a manual for the installation of both. Problem is that apparently someone has changed around the antenna situation.

To start with, will I be able to use my horizontal V antenna on the tail for BOTH VOR's?

Secondly, there is an antenna connection on VOR 1, for GS, NAV & MKR. Obviously I need a Marker Beacon antenna, but can the horizontal V antenna be used for both GS and NAV?

Thanks for all your answers and help.
 
Okay, in the Installation Manual they talk about some various antenna splitters for different combinations. It sounds as if fixing up the antenna, cable and connections along with the correct splitter might very well sort the VOR's giving me the Glideslope and all. Then adding a Marker Beacon antenna and cabling may very well take care of the Marker Beacon.

Also, the manual mentions a rear mounted splitter. Since the VOR manual shows ant connections mounted in a vertical line and the two on my VOR 1 are in a horizontal line, I'm thinking that it might have a rear mounted splitter that accounts for my daisy chained ant connection.

Got a lot to investigate and think about now.
 
I assume that a splitter contains only passive devices. Are any of these devices capacitors? Capacitors can go bad with age, so I'm just wondering if I should just throw in the towel and go for a new splitter. I found an externally mounted splitter that seems that it would work nicely for the NAV & GS for VOR 1, and a NAV split off for VOR 2.

Do these splitters seem to go bad with age?

Thanks again for all the useful responses.
 
MBDiagMan said:
I assume that a splitter contains only passive devices. Are any of these devices capacitors? Capacitors can go bad with age, so I'm just wondering if I should just throw in the towel and go for a new splitter. I found an externally mounted splitter that seems that it would work nicely for the NAV & GS for VOR 1, and a NAV split off for VOR 2.

Do these splitters seem to go bad with age?

Thanks again for all the useful responses.
Click to expand...

Depends on the design of the splitter, but the ones I design use two pieces of coaxial cable and a carbon film resistor. Glideslope split is with a disk ceramic capacitor.

Everything in the world has a Mean Time Before Failure (MTBF). Some day the rocks in your yard will crumble into dust. Electrolytic capacitors have a rather short MTBF (tens of years) and contain a paste/liquid that dries out. Ceramic capacitors have lives measured in centuries. Coax cable can fail. If properly secured and not subjected to really harsh vibration (like a rocket launch) coax has life measured in centuries. So yes, splitters can fail, especially at connection points. If properly constructed, splitters can have MTBF on the order of hundreds of thousands of hours. Or multiple human lifetimes.

Having said that, old splitters can also be measured by shops with the proper equipment. Sometimes cheaper to just buy new, but then again, infant mortality raises its head.

Proper test equipment is really the only answer.

Thanks,

Jim
 
Thanks a bunch Jim. It sounds as if there's a chance that the existing splitter might be made to work okay, if it has good connections.

I have moved to a different airport since the last time I messed with the VOR's. At the old airport, there was a VOR so close to the airport that I could get a signal on the ground sometimes. I am now about 25 miles away from a VOR. I think I will tune the nearest VOR while on the ground and see if I can get anything at all. I doubt that I will, so I'll get in the air and see if I can get a signal. With that baseline, I will then get under there and do some inspection and connection cleaning, then retry connecting to the VOR.

If that doesn't go anywhere fast, I will just put in a new splitter and cable it accordingly. Hopefully the coax to the NAV antenna is in good shape. I might have to replace the BNC connector on that one, but it seems to have a good connection at the antenna.
 
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I went under there again and I'm now almost certain that a splitter is mounted on the back of NAV 1. I haven't bothered with the VOR's in a long time, so I wanted to get a baseline on their receiving signal strength when I flew a short cross country to an air show Saturday.

I picked up a low VOR from about 30 miles away, no problem. I also picked up a high VOR FROM ABOUT 30 miles away and seemed to lose it after getting about that far out. I was only at about 4,000 feet when tracking the high one, so for all I know I could have held it to 100 miles if I would have gone high enough. I got the signal on both VOR's.

So this leads me to think that the antenna and cable is okay, but the splitter might be why I couldn't pick up the glide slope the last time I tried. When I get the VOR out, I wil be able to tell more about the splitter and see if they were trying to split the marker beacon off that antenna. It seems like I read somewhere that some folks have done that.

If I'm lucky and a good splitter gives me a good glide slope signal, it shouldn't be too much trouble to mount a marker beacon antenna underneath.
 
Why be "lucky"? Get it measured and fixed correctly.

It's like watching someone go to the store for pants not knowing their size and hoping they'll just pick the right paid off the rack...
 
I hangar on a very nice field with 12 hangars, 13 including the Air Ambulance people. There are no maintenance facilities on the field. There is no VOR or other facilities to which I can tune and do some poor boy signal testing without getting in the air. The nearest avionics shop of any known capability is about 160 flight miles away.

Apparently, the splitter mounted on the back of NAV 1, connects to the Marker Beacon as well as the Glideslope and NAV antenna inputs. Because I will be needing access to the Marker Beacon input when installing that antenna, the splitter on the NAV has to go anyway. Since the splitter must be replaced anyway, it seems like a logical step, given what I have (don't have) to work with.

If, however, you have access to test equipment that you can send me on loan, I will be very pleased to follow your suggestion.

Thanks for your comments. As always, all comments, suggestions, advice, no matter how well they do or do not fit the situation are greatly appreciated.
 
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160 miles is what? Just over an hour in most of the aircraft we fly?

Yeah... I'll ship personally owned test gear valued at a few grand to someone on the internet I don't know, who has appropriate maintenance facilities an hour away. Riiiiight.

If you have the time to tinker, cool with me. I'm just reminding you there's s direct path to a proper repair with the exact culprit of the problem known and proven...
 
Hmm.... why the snarkiness? I'm just a poor boy trying to get something sorted out.

If I leave my plane 160 miles away, someone has to go bring me home and then take me back for the plane. It doesn't seem to be too big a deal to try a $40.00 splitter since it must be put in anyway, or maybe you didn't read my explanation about that.

As I said before, all comments are appreciated. Even the snarky ones lead to something positive sometimes.
 
Not being snarky. Being realistic. Why would you leave the plane? It's a thirty minute fix from starting to finishing with the right tech. And you can use the telephone to make sure you have the right tech before making the appointment.

Although I will admit to not batting 1000 on that myself. Found the right tech for some work on my stuff. Right test gear, right knowledge, two years from retiring and lazy as hell.

Wasn't lazy about sending the invoice, though.
 
He's got all the test equipment he needs. A wire antenna and a friend half a mile away with a handheld will tell him everything he needs to know.

Jim
 
Thanks Jim! Yes, that's kind of the way I've been thinking. It's not as if I'm doing a complete radio test and alignment. I'm only dealing with cables and connectors for gosh sakes.

Thanks very much for sharing your knowledge and encouragement. My time is consumed for the next few weeks getting ready for our Veterans day activities at the veterans memorial and at the same time, possibly a hangar move. In the mean time, I'm going to buy a splitter, some solid core RG58 and some connectors. I already have the coat hanger.:)
 
Well, I guess this place is as good as any for THIS possible controversy:

Crimped or threaded BNC connectors?

I dealt with lots of BNC connectors on radar when in the Army. We always used a connector with a soldered center pin and a threaded arrangement for the shield. We had lots of trouble with them too. When there was a flaky signal somewhere, you wiggled on the connectors until you found the intermittent one.

Fast forward from 1970 to 2014 and it seems that crimped connectors are quite common. I think it is safe to assume that the center pin is still soldered and only the shield part is crimped.

I don't mind buying the correct crimping tool and I don't mind taking the time to use threaded connectors. My decision will be determined by which one is the most durable and reliable.

Thanks for any thoughts and experience sharing. I'm hoping to get by the electronics supply and get cable and connectors while running my other errands, so I will be ready to go to work after Veterans Day. They are closing our runway for a seal coat scheduled for ten days, starting Sunday night. If it weren't for the other stuff I have going on, that would be a perfect time to have the plane apart, but the timing on such things never seems to fall in place.
 
MBDiagMan said:
I don't mind buying the correct crimping tool and I don't mind taking the time to use threaded connectors. My decision will be determined by which one is the most durable and reliable.
Click to expand...


If you use threaded connectors they will give you trouble in a couple of years. If you use crimped connectors it will take 24 months. You pays yo money and yo takes yo choice. I personally prefer threaded as they are repairable if and when they go away.

I've taken apart stuff from WWII that used threaded connectors and they were still in perfect shape. My experience with crimped is very limited and restricted to mini-coax (RG-174) where I have not as yet had a failure either.

Jim
 
Thanks Jim. You're answer reminded me about what my dear old Grand Father said about a cold. "Don't do anything for your cold and it'll last a week, or take care of it and it'll only last seven days."

I will buy some threaded connectors.
 
MBDiagMan said:
Thanks Jim. You're answer reminded me about what my dear old Grand Father said about a cold. "Don't do anything for your cold and it'll last a week, or take care of it and it'll only last seven days."

I will buy some threaded connectors.
Click to expand...

Go to the junk shop and buy a 3/8" open end wrench and grind it down to about 100 mils (0.1") thick. You will need it to hold onto the little flat just above the knurled part of the male connector.

BTW, most connectors are sexed by their OUTER shell. RF connectors are sexed by their center pin.

Jim
 
Haven't used a threaded connector in years.

Once I saw the cell sites had gone to crimp with proper connectors and tools at GHz frequencies, I knew the good manufacturers had licked the problems with early cheap crimp connectors.

That said, the proper tools and connectors aren't cheap. For a one-off, if I didn't already have the stuff, I'd do it the "old fashioned" way.

It's amazing that RG-58 is still used for this stuff. LMR 240 ultra flex with proper connectors crimped on, is state of the art, but probably not FAA approved.

Always gotta stay in the 1950's in Aviation...
 
I'm getting ahead of myself, but once the mounted splitter is removed and the remote splitter sorts the NAV's and Glideslope, the MKR, connector will be exposed. I will then be ready to add a Marker Beacon antenna.

When my 140 was restored, I am quite sure that they had a 150 they were wrecking out. There are numerous 150 components on her. If it would work well, I would like to find one of the J antennas that Jim talked about. It would fit the modification theme of the airplane quite well. I've looked a little at the salvage sites and haven't found one yet. I will keep up the search and hopefully I will sort the NAVS and GS so that I can take that next step.
 
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I've been buried into everything except my NAV antenna issue, but finally got things together and got a start while the airport is shut down for runway sealing.

I bought a work bench for the hangar and got connectors, cable, tools including my ground down wrench, a vise, and on and on. I have over two weeks of vacation left to burn before the end of the year, so I planned for today at the hangar.

It turned out to be colder than I expected, but I took my Australian Shepherd and his bed with me. We got in the hangar and closed the door. The sun was shining so it heated the metal hangar well enough to be quite tolerable.

I set up the workbench, mounted the vise and fabricated some brackets to mount the splitter without the need to drill holes. I got it mounted in place and cut enough existing tie wraps to get the antenna cable down where I could clean and inspect it. I had inspected it as best I could before, but once I got it down where I could really get a good look at it, I found that the center pin was not soldered.

The connector was a different type than the ones I have dealt with in the past. It has a very large solder cup for connecting the center pin. The center conductor turned out to be stranded, but larger strands than the RG58C/U that the supplier sent me by mistake (one reason for the delay in getting to this.)

The existing cable appears in good shape and the connection to the antenna itself seems to be quite good. It appears, though, that the center conductor might be too large to fit into the new BNC connectors that I bought. The connector on it, took larger wrenches than 3/8" and by that time the dog and I were ready to come home and thaw out. The sun went away, so the hangar starting getting colder. I brought the connector with me. Once home and I got a good look at it, there appears to be no gold left on the center pin. I had thought I would use it on that cable because of the large solder cup.

My new plan is to catch a day warm enough to get at it again, and strip the cable to see if I can use one of the new connectors on it. Once I get a good connector on it, I will see if I have a stronger signal. If I do, I think that I will fly over and see if the existing splitter and antenna arrangement will pick up the glide slope and marker beacon at a nearby airport with a LOC approach. I will have the cables made up before doing that so that I can simply connect up the new spitter and try that if the existing splitter doesn't work.

I don't like the idea of using the V antenna on the tail for the marker beacon, but it worked at one time. If I can make it work again, alleviating the need to add an antenna, I will go with it. It won't be an elegant solution, but my favorite Einstein quote is: "Elegance is for tailor's, don't fall in love with an elegant solution."
 
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MBDiagMan said:
If I do, I think that I will fly over and see if the existing splitter and antenna arrangement will pick up the glide slope and marker beacon at a nearby airport with a LOC approach.
Click to expand...

If you have an airport with a LOC approach, you will play hell getting a glideslope signal. An ILS approach yes, a LOC approach no.

Thanks,

Jim
 
weirdjim said:
If you have an airport with a LOC approach, you will play hell getting a glideslope signal. An ILS approach yes, a LOC approach no.

Thanks,

Jim
Click to expand...

Whoops, I'm sorry, yes an ILS. There is an airport not far away with an ILS. An ADF is required, but I only want to check the glide slope and marker beacon on a clear day.

BTW, thanks for your help throughout this thread.
 
Okay, they have routed the cracks at the airport and are waiting high enough temps to finish the seal job. Might be Spring for that, but the airport is open again for now.

I used one of my new BNC's on the main antenna cable today. It's probably been 40 years since I put one together so I was careful to get it all stripped and trimmed correctly and thorough in making sure no stray shield strand shorted it. It took me longer than it did when I put these together frequently while in the Army, but I was happy with the results.

I tuned one NAV to a VOR ABOUT 30 miles away and the other to what I THINK is a high VOR that is about 55 NM away. I wasn't 200 feet off the ground and picked up the near one and almost immediately after that, the far one. I was VERY pleased with that.

Now I am hoping for some good flying weather while I'm off for Thanksgiving and I will fly 60NM to an airport that has an ILS APPROACH. I need the flight time anyway, so I will just fly over there and check my glide slope and marker beacon. If they work I will go with what I have. If not, I have the new splitter in place, just need to make up the connector cables. I bought a wire marker beacon antenna off ebay that came off of a 310. If I have to resort to it, I think it will look better with my modifications from the eighties than would one of the dorsal fin type.

I'm really pleased with the antenna fix. Thanks for the help. I will continue to post my progress on this thread.
 
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Every now and again the blind pig inside my mind picks up a stray acorn.

Glad it all worked out for you.

Jim



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