Check antenna connections/condition using VSWR meter?

[bkspero]

I often read postings from people with radio issues, and one common suggestion for investigation is the condition of the antenna connections. This can involve taking apart portions of the interior, and dismounting the antenna.

I make no claim to particular expertise in either RF or antennas, but wouldn't the standing wave ratio be high if a radio problem were due to either a failing antenna or a bad connection or ground? That would be pretty easy to check by removing a radio, connecting a handheld with a SWR meter to the antenna connection on the tray, and measuring the SWR. Then replace the radio.

Very little time and effort to do. But I never read anyone recommending this simpler diagnostic test prior to more intrusive work. What am I missing?

 

[German guy]

[..] What am I missing?

I would think nothing - the average plane owner and I guess many / most mechanics simply never heard of a SWR meter or even an antenna analyzer.

 

[Let'sgoflying!]

I was just talking to one of my avionics guys (yes I have several, why?) and he said just check the center conductor of the antenna from one end to the other with an ohmmeter and a long wire. Be sure to subtract the resistance of the long wire.
It should be 2.1Ω or less.

 

[German guy]

I was just talking to one of my avionics guys (yes I have several, why?) and he said just check the center conductor of the antenna from one end to the other with an ohmmeter and a long wire. Be sure to subtract the resistance of the long wire.
It should be 2.1Ω or less.

This doesn't tell you if there's an issue with the antenna or for example the insulation of the cable.

 

[Let'sgoflying!]

This doesn't tell you if there's an issue with the antenna or for example the insulation of the cable.

No, it doesn't. Perhaps I should have stated that it is designed to test the center electrodes and the wire between them; sorry - thought it obvious.

 

[-KLB-]

An antenna analyzer would be a reasonably good test, though not necessarily 100% complete, but it is a lot better than any multimeter based test. A simple SWR meter requires transmitting to measure reflected power, usually at least 1W to get a reading. The antenna analyzer transmits such a low power that it is difficult to detect, and unlikely to interfere with anything. But sometimes it is tough to justify what can be a $500 tool for fairly rare use.

 

[bkspero]

An antenna analyzer would be a reasonably good test, though not necessarily 100% complete, but it is a lot better than any multimeter based test. A simple SWR meter requires transmitting to measure reflected power, usually at least 1W to get a reading. The antenna analyzer transmits such a low power that it is difficult to detect, and unlikely to interfere with anything. But sometimes it is tough to justify what can be a $500 tool for fairly rare use.

Thanks, all, for your input so far.

Is there an insurmountable downside to using a standard SWR meter and transmitting at non-zero power levels for a few seconds? I see one potential downside of interfering with another person's transmission if the test frequency was one used by others nearby. I was thinking that it would not be difficult to find a frequency not used for, say a 50 mile radius. Wrong?

I ask because what got me to thinking about this issue is that I just recently became aware of the availability of inexpensive SWR meters with pretty good reviews online. I didn't see minimum required power levels in the one I looked at more deeply (Surecom SW-33). If it worked, it overcomes the price problem of an antenna analyzer.

 

[Bell206]

What am I missing?

It's not so much just the connector condition but how the transmit/reflect power travels through the coax and connections to/from the radio and the antenna. Most checks are done with an insertion type meter like a Bird 43 which can check in both directions and measure the actual power to/from the radio. The meter can be conveniently connected inline with the coax at the tray or any accessible coax connection. Most radio problems I've corrected were not connector related but corrosion related at the antenna ground plane. The 2nd was antennas that had seen better days.

 

[bkspero]

...Most radio problems I've corrected were not connector related but corrosion related at the antenna ground plane. The 2nd was antennas that had seen better days.

I was expecting that SWR would highlight those problems (poor antenna ground or deteriorated antenna) as well as poor coax condition/connections? Wouldn't it?

I based that speculation on the thought that anything that didn't cause the radio RF output to either be radiated or dissipated would result in RF energy being reflected back to the radio to increase SWR. In particular the poor antenna ground. If a good antenna with a poor ground does not produce a high SWR, then, I agree, this approach to narrowing the cause of radio issues isn't of much value.

 

[-KLB-]

The tough question is if the antenna ground connection is degraded, does the antenna coax braid act as a good enough counterpoise that the SWR doesn’t really suffer?

 

[chartbundle]

I think a combination of an SWR meter and equally cheap field strength meter would help narrow down a bunch of issues. Maybe it's just me but I don't often see a lot of troubleshooting with electrical/electronics in general aviation, it always seems to be replacing parts rather than pulling out a meter, a scope or similar tools and trying to narrow things down before replacing half the parts it could be.

 

[wilkersk]

I often read postings from people with radio issues, and one common suggestion for investigation is the condition of the antenna connections. This can involve taking apart portions of the interior, and dismounting the antenna.

I make no claim to particular expertise in either RF or antennas, but wouldn't the standing wave ratio be high if a radio problem were due to either a failing antenna or a bad connection or ground? That would be pretty easy to check by removing a radio, connecting a handheld with a SWR meter to the antenna connection on the tray, and measuring the SWR. Then replace the radio.

Very little time and effort to do. But I never read anyone recommending this simpler diagnostic test prior to more intrusive work. What am I missing?

You only mention "radio issues". What kind of problems are we talking about? AN SWR meter is not the first piece of gear that comes mind when someone says "I'm having radio issues".

 

[weirdjim]

I think a combination of an SWR meter and equally cheap field strength meter would help narrow down a bunch of issues. Maybe it's just me but I don't often see a lot of troubleshooting with electrical/electronics in general aviation, it always seems to be replacing parts rather than pulling out a meter, a scope or similar tools and trying to narrow things down before replacing half the parts it could be.

Oh, dear Lord. I see a Kitplanes column in the near future.



Jim

 

[Bell206]

I was expecting that SWR would highlight those problems

It could. But, in my limited knowledge, VSWR is specific to the radio system power, resistance, and frequency so simply removing the radio and sending voltage down the feed line with an off-the-shelf tester will not get you accurate results. Plus if you exceed the system limits you may also damage other items inline to include the antenna in some cases. But as mentioned above, troubleshooting radio problems vary on what the exact problem is and then eliminating the obvious first--like ensuring the mic plug is inserted all the way in the receptacle.

 

[bkspero]

You only mention "radio issues". What kind of problems are we talking about? AN SWR meter is not the first piece of gear that comes mind when someone says "I'm having radio issues".

Wilkersk, I don't know what causes people to respond to questions about radio problems with recommendations to check the antenna system. But maybe a good way to answer your question is to use the expertise of the people and who make such recommendations and say "one way to identify the types of problem I was thinking about is to describe them as ones for which a forum responder recommends checking the condition of of any or all of the antenna, antenna coax, antenna and coax connections, and/or antenna ground."

Or maybe even broaden it beyond online forums to include any problem an avionics tech mentions checking any of those antenna system items I listed above. I know that this is my shop's first response to issues with noisy or weak transmissions, background noise, and other things like that.

I'm not trying to avoid your question. It just that this is the only way that I can think to answer it (not being particularly knowledgeable about radios).

 

[bkspero]

It could. But, in my limited knowledge, VSWR is specific to the radio system power, resistance, and frequency so simply removing the radio and sending voltage down the feed line with an off-the-shelf tester will not get you accurate results. Plus if you exceed the system limits you may also damage other items inline to include the antenna in some cases. But as mentioned above, troubleshooting radio problems vary on what the exact problem is and then eliminating the obvious first--like ensuring the mic plug is inserted all the way in the receptacle.

Bell, I wasn't thinking of sending random voltage down the antenna coax. I thought one could use as the signal source a handheld aviation radio, and an SWR meter that operates on frequencies in the aviation VHF band to measure SWR (the one I mentioned in a posting above works over a range of frequencies from 125 mHz to about 550 mHz, so one could test on any frequency between 125 and 136.975 mHz in the aviation communications band).

I agree that one should check the obvious and easily checked things first. Is the mic plugged into its socket? Is there power to the radio? Did someone sneak into the hangar and break off the antenna stalk? Are there smoke and flames coming out from the radio tray <G>?

But once those have been checked and the next options include taking apart non-trivial parts of the airplane, I was looking for a better diagnostic strategy, instead. To at least indicate or eliminate the antenna system as a potential cause.

 

[bkspero]

I think a combination of an SWR meter and equally cheap field strength meter would help narrow down a bunch of issues. Maybe it's just me but I don't often see a lot of troubleshooting with electrical/electronics in general aviation, it always seems to be replacing parts rather than pulling out a meter, a scope or similar tools and trying to narrow things down before replacing half the parts it could be.

I am not familiar with field strength meters or their use. How would you incorporate those two devices into a troubleshooting strategy instead of parts swapping.

 

[bkspero]

Oh, dear Lord. I see a Kitplanes column in the near future.



Jim

Jim, please have compassion for those of us who are not part of the homebuilding community and give us at least a summary of your thoughts here.

 

[weirdjim]

I was just talking to one of my avionics guys (yes I have several, why?) and he said just check the center conductor of the antenna from one end to the other with an ohmmeter and a long wire. Be sure to subtract the resistance of the long wire.
It should be 2.1Ω or less.

I'd talk to one that knows what they are talking about.

 

[Bell206]

I thought one could use as the signal source a handheld aviation radio

Unfortunately, the installed coax/antenna and any other inline component will be sourced for the installed radio. Calculating the vswr based on your handheld may notgive you an accurate value unless the handheld is designed to operate within the same system resistance and power. Other words your test may tell you its good but its not or visa versa. That is one of the reasons the Bird 43 or oscilloscopes are used with aviation radios. But even with those it's not a sure thing. If I got to the point I needed to check power output, etc. I would just rent a Bird 43 and move forward. However, I usually accessed the antenna connections before that and solved the issue the majority of the time.

 

[weirdjim]

Sure, a handheld and a VSWR (pronounce VIZZwar) meter will give you an absolutely valid way of telling whether the feedline AND the antenna are doing their job. Your only problem is finding the meter that will cover the aircraft com band. Most cheap meters are meant for CB antennas down at 27 MHz., not aircraft antennas 118-136 MHz.. There is a relatively inexpensive antenna analyzer sold by MFJ (nicknamed in the industry Muthur Frikkin Junk) called an MFJ 259 that has been part of my tool bag for the better part of 30 years. They are $250 new, but you only need it for ten minutes -- and it has its own frequency generator, you don't even need your handheld. You may find somebody at your local ham radio club that will either loan you one OR even come to your airplane and show you how to use it.

It would be a kindness to take that person up for a ride with his handheld 2 meter rig for a half-hour of "aero mobile", that most hams never get to see in their lifetime.

Then ask them if they've got a Bird inline wattmeter and you can check the whole setup ... transmitter through to antenna in one swell foop. SOmebody that really knows what they are doing can tell how the modulation is by noting little flicks of the wattmeter needle on loud voice peaks. Again, Birds go for $200 or so used on eBay, but you only need it for a couple of minutes. Maybe you want to extend that joyride for an hour if you are feeling generous.

Jim

73 es CUL

 

[weirdjim]

Unfortunately, the installed coax/antenna and any other inline component will be sourced for the installed radio. Calculating the vswr based on your handheld may notgive you an accurate value unless the handheld is designed to operate within the same system resistance and power. Other words your test may tell you its good but its not or visa versa. That is one of the reasons the Bird 43 or oscilloscopes are used with aviation radios. But even with those it's not a sure thing. If I got to the point I needed to check power output, etc. I would just rent a Bird 43 and move forward. However, I usually accessed the antenna connections before that and solved the issue the majority of the time.

Excuse me, but I call BullFeathers on this answer.


Jim

 

[Bell206]

BullFeathers on this answer.

Always looking to learn. The memory cells aren't always connected in the right sequence, so would sincerely appreciate any correction to the answer. Spark and electron chasing was never my strength.

 

[-KLB-]

Excuse me, but I call BullFeathers on this answer.


Jim

Handhelds aren’t always designed for 50 ohm impedance, as they aren’t intended to be hooked up to a feed line, and their antennas are nearly always compromise antennas. So if the handheld is designed for 75 ohm impedance, you could get unexpected SWR readings using with 50 ohm impedance cable and antennas.

 

[Let'sgoflying!]

I'd talk to one that knows what they are talking about.

Well, he's a successful, trained (not a shadetree) avionics tech.
If you disagree, or have other ideas about this, my conclusion would be that this is like most other things in aviation; there are many approaches to issues and we all have our favorites.
I will say, however, that this gentleman has a more open attitude about others' ideas than you.

 

[bkspero]

...Your only problem is finding the meter that will cover the aircraft com band. Most cheap meters are meant for CB antennas down at 27 MHz., not aircraft antennas 118-136 MHz..

Thanks for the input, Jim. The SWR meter that I mentioned above, Surecom SW-33, is advertised to operate in the VHF band from 125 to about 525 mHz. So, while it's not lots of overlap, it does operate in a portion of the aviation VHF communications band between 125 and 136 mHz. With that device (presuming it works...its reviews are pretty good) should this approach work?

As for giving a local Ham operator an airplane ride, that would be my pleasure just for the opportunity to pick his or her brain. How would you recommend I go about making contact? I did some searching and the best I could find was to go to the ARRL club finder website. There are dozens within about 50 miles. My guess is that I should narrow down to those showing VHF in their "Specialities" (yes/no?). Any other things to look for? Or is there a better way to make contact?

Thanks again.

 

[bkspero]

Handhelds aren’t always designed for 50 ohm impedance, as they aren’t intended to be hooked up to a feed line, and their antennas are nearly always compromise antennas. So if the handheld is designed for 75 ohm impedance, you could get unexpected SWR readings using with 50 ohm impedance cable and antennas.

KLB, do you know aviation handhelds which are not 50 ohm? I'm pretty sure my original Icom handheld was designed to feed 50 ohm coax. I say that because I corresponded with a tech at Icom about building a coax cable to connect between my handheld and the Icom switchbox mounted in the panel of my plane. He told me that I needed 50 ohm coax to make that connection so that's what I used. It works great. And it seems to be working as well with the 2 that have replaced it (a Vertex (i.e., Yaesu) and a Rexon). So they all seem to be designed to at least tolerate 50 ohms.

I would be curious to know which won't, though, in case I every need another replacement.

 

[weirdjim]

Handhelds aren’t always designed for 50 ohm impedance, as they aren’t intended to be hooked up to a feed line, and their antennas are nearly always compromise antennas. So if the handheld is designed for 75 ohm impedance, you could get unexpected SWR readings using with 50 ohm impedance cable and antennas.

This is my 60th year in electronics, 50 of them directly in avionics engineering, and as yet, I've never seen a piece of avionics communications designed for 72/75 ohm load. Besides, the VSWR meter couldn't care less with what impedance it is driven so long as it is RF within the frequency limits of the bridge. So far as I know, since the Brits developed 50 ohm coax just prior to WWII there hasn't been a piece of avionics com gear for anything other than Z=50.

The other side of that coin says that I've never seen a piece of TV or video gear designed for anything other than 72 ohm systems. It can be proven that 72 ohm systems have minimum loss per foot of cable run, 32 ohms for maximum power capability, and 50 ohms because two sizes of British copper plumbing tubing just worked out as a coax system with an average between power handling and loss. Z = sqrt (L/C) = ((60/sqrt(epsilon))*log(e)*(D/d))

Jim

 

[weirdjim]

Unfortunately, the installed coax/antenna and any other inline component will be sourced for the installed radio So you are saying that you have a different type of coax and antenna for each radio installation? That if the manufacturer calls out, for example, a Cal Labs 1234 VHF com antenna that you couldn't use a Com Ant 4321 antenna that meets the same specifications? Or that using generic RG-58 instead of Belden 8240 is going to matter? And what other "inline component" is there between the antenna and the radio other than coax?. Calculating the vswr based on your handheld may notgive you an accurate value unless the handheld is designed to operate within the same system resistance --- that would be impedance, not resistance and power. In general, a VSWR bridge could care less what is driving it so long as there is sufficient RF to drive the bridge to full scale. THe driving impedance of the source is not a factor in the calibration of the bridge. Other words your test may tell you its good but its not or visa versa. That is one of the reasons the Bird 43 or oscilloscopes are used with aviation radios. So tell us, how do you use an o'scope to evaluate an antenna problem? But even with those it's not a sure thing. In 60 years the Bird hasn't lied to me once. Perhaps i just got REAL lucky. If I got to the point I needed to check power output, etc. I would just rent a Bird 43 and move forward. However, I usually accessed the antenna connections before that and solved the issue the majority of the time.

 

[weirdjim]

I was just talking to one of my avionics guys (yes I have several, why?) and he said just check the center conductor of the antenna from one end to the other with an ohmmeter and a long wire. Be sure to subtract the resistance of the long wire.
It should be 2.1Ω or less.

So the coax shielding can be corroded to pieces, the dielectric insulation corrupted and contaminated with sludge, the center conductor bent so that it is shorted to the shield, and so long as the center conductor measures 2.1 (where the hell did THAT precision measurement come from) ohms from one end to the other the transmission line is OK?

Not where I come from.

Jim

 

[Let'sgoflying!]

So the coax shielding can be corroded to pieces, the dielectric insulation corrupted and contaminated with sludge, the center conductor bent so that it is shorted to the shield

Oh, yeah a person is supposed to ignore coax shielding corroded to pieces, dielectric filth, bent conductors and whatever else, when using this test, I forgot to mention this.

I could be wrong but I think one is supposed to use a bit of common sense when using this, or any test.

I have never known this A+P avionics technician to be wrong about such things, he is a great guy......and he had a terrific attitude - yep, I'm going with what he says.

Sometimes I have to wonder, do people post unreasonably argumentative thoughts on the internet to pump up their ego? Or is it some other silliness I am missing.
Over and out.

 

[weirdjim]

Well, he's a successful, trained (not a shadetree) avionics tech.
If you disagree, or have other ideas about this, my conclusion would be that this is like most other things in aviation; there are many approaches to issues and we all have our favorites.
I will say, however, that this gentleman has a more open attitude about others' ideas than you.

You are right. I was an avionics shadetree tech ... when I was 15 and rode my bike to the dirt strip in town with my trusty homebuilt multimeter. Then while I was in college finishing up my semiconductor physics degree the airlines hired me as a college job and taught me a lot about avionics for four years. Then when I got my degree, Teledyne hired me and called me a design engineer on a few "avionics" programs you may have heard of ... Surveyor, Apollo, and Viking. in 1973 I founded my own avionics company and have been designing and manufacturing avionics for the last 46 years. Oh, and about 50 years teaching a college class or two a semester on avionics and electronics technology. My bag, if I have one, is hidden antenna design on plastic airplanes ... you may have heard of Voyager? Most all of Rutan's designs?

Shadetree? Yeah, I got one in the back yard. Great to sit under and ponder the meaning of the square root of minus 1 when it gets too hot in the lab.

Jim

 

[weirdjim]

I think a combination of an SWR meter and equally cheap field strength meter would help narrow down a bunch of issues. Maybe it's just me but I don't often see a lot of troubleshooting with electrical/electronics in general aviation, it always seems to be replacing parts rather than pulling out a meter, a scope or similar tools and trying to narrow things down before replacing half the parts it could be.

Elementary and cheap field strength and modulation "meter".

 

[Bell206]

So you are saying that you have a different type of coax and antenna for each radio installation?

Not at all. The OP readily admitted he had no expertise in RF and antennas. I know aircraft radios are 50 ohm systems, but didn't think he did, so I kept my answers very general.

And what other "inline component" is there between the antenna and the radio other than coax?.

A duplexer comes to mind.

that would be impedance, not resistance and power. In general, a VSWR bridge could care less what is driving it so long as there is sufficient RF to drive the bridge to full scale. THe driving impedance of the source is not a factor in the calibration of the bridge.

Noted. And thanks for the explanation. This is type of answer I was looking for as a 40 year A&P with limited avionics experience--especially from a 50 year avionics engineer.

So tell us, how do you use an o'scope to evaluate an antenna problem?

Don't know. Except based on my experience, every time there was a major radio problem one of the avionics techs had an o'scope as part of his troubleshooting kit.

In 60 years the Bird hasn't lied to me once. Perhaps i just got REAL lucky.

Never lied to me either. But in the context of my statement, I never had a Bird point me to what was the specific part causing the problem. Maybe I was unlucky?

But one thing I have learned in 40 years--time, age, and years of experience don't necessarily make a person the smartest one in the group. Appreciate the reply.

 

[bkspero]

Jim, can you confirm that the original approach should provide useful diagnostic information as long as the SWR meter is designed to operate at a frequency between 118 and 136 mHz (such as the one i was considering which can operate at frequencies between 125 and 136 mHz (and then some)). If the answer to that question is yes, then I will buy one of those devices and see how it works.

 

[weirdjim]

A duplexer comes to mind. But I was to understand we were troubleshooting the COM side of the radio. THe diplexer is in the NAV side. And, (saying this VERY quietly) you will find that a coax diplexer (inside of the metal case) is the **ONLY** place in an aircraft we use 72 ohm coax. Seems that two quarter wave sections of coax of impedance Z*(sqrt 2) will isolate the radios from each other. Sqrt 2 is 1.414 times 50 ohms is 72 ohms. Amazing. That's also a step along the way of proving that 72 ohms has the least loss, but that's more equations than we have room for here.

based on my experience, every time there was a major radio problem one of the avionics techs had an o'scope as part of his troubleshooting kit. Ahem. Bright lights and bells and whistles is part of the mystique of the avionics sparkies. We couldn't command the high prices we do without bells & whistles.

Appreciate the reply. Appreciate the comments.

Jim

 

[Bell206]

THe diplexer is in the NAV side.

That is my understanding also. But I said duplexer which allows 2 COM radios to share a single antenna. I was properly schooled on this difference a number of years ago when I ordered and installed the wrong one--one has extra wires and one doesn't. But an interesting explanation of how a diplexer works. Would sqrt 2 be considered the "filter" then?

Bright lights and bells and whistles is part of the mystique of the avionics sparkies.

I'll grant you the mystique part. But these spark chasers would actually hook the scopes up to the aircraft systems in some cases. After about a 3 minute explanation of what they were doing I usually went and got the coffee.

 

[weirdjim]

That is my understanding also. But I said duplexer which allows 2 COM radios to share a single antenna.

Quite honestly, I've never seen a COM duplexer that has worked as advertised. Much cheaper and more reliable to have two COM antennas.

Wilkenson wrote an IEEE proceedings paper about forty years ago on how to do splitters. Yes, the "2" in sqrt(2) gets you a 2-way split, 3 gets you 3, and so on. Generically, sqrt(n) gets you an n-way splitter.

Two COM antennas met on the top of a Cessna, fell in love, and got married. The wedding wasn't much but the reception was great.

JIm