Radio Frequency Effecting Electronics International Instruments

[Richgj3]

+1 for what gsmx440 posted. Sounds complicated but anything else is shooting in the dark.

When I was a teen I was into Ham Radio. I once had such a bad antenna mismatch that every time I keyed the mic the florescent lights in the basement went on!

Later went on to build microwave stuff. Couldn't do it without a spectrum analyzer.

Checking SWR is pretty easy. I would do that first. Finding out WHY it's not good could be a challenge.



Rich

 

[flymore]

Many thanks to all that have replied with possible solutions and trouble shooting tips and techniques. I own the suspect airplane and am very appreciative for the response. I have an IQ of 10 when it comes to RF and the problem I am experiencing (not much higher for anything else actually) but have enjoyed the discussion immensely. Fortunately "little wing" lives very close and has been willing to help me solve my problem. He has an enormous amount of trouble shooting experience with his own business but has said repeatedly said that he does not know radios or RF. He is very willing to listen to the experts on this forum and others and learn from the experiences. I am the beneficiary of his good nature and ability.

Thank you very much for your inputs.

 

[wireweinie]

Many thanks to all that have replied with possible solutions and trouble shooting tips and techniques. I own the suspect airplane and am very appreciative for the response. I have an IQ of 10 when it comes to RF and the problem I am experiencing (not much higher for anything else actually) but have enjoyed the discussion immensely. Fortunately "little wing" lives very close and has been willing to help me solve my problem. He has an enormous amount of trouble shooting experience with his own business but has said repeatedly said that he does not know radios or RF. He is very willing to listen to the experts on this forum and others and learn from the experiences. I am the beneficiary of his good nature and ability.

Thank you very much for your inputs.

Be sure to let us know what you find as you trouble shoot this.

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[little wing]

Thank you all very much for sharing your hard earned wisdom. Here are a few pics of under the panel. The ground lug has all things grounded to the same bolt through the tab, as in QSMX's description (has the black Fluke test lead attached in some pics). Looks pretty clean, but I have not separated and cleaned yet. You may have to zoom in a little to see, as they are all taken from underneath looking up. Sorry about the framing of the pics, they were taken with my phone and it doesn't offer the benefit of me being able to see the screen for the shot. The VHF coax is pretty much coming out and immediately turning up the window post. First time I've posted pics, so please bare with me if things don't come out right. Any additional shared wisdom is much appreciated. The coax can be seen in the next to last pic with the the encoder box, clear insulation, tape says comm, runs straight up the frame to wing. The black coax that can be seen in the third from the last pic is for the transponder.

 

[qsmx440]

Little wing went back over your posts and the others and didn't see where the SWR was tested. Really it's transmitter problems 101. You start there. A Bird model 43 with the correct slug is the correct instrument for this and someone who knows how to use it. It's a very simple procedure https://www.youtube.com/watch?v=fbj_r6QfN4I (an excellent tutorial) and probably a plus for a mechanic to have one and check at annual. Since the problem came on slowly you are smart to not start moving, cutting and hacking until you find what has changed. The problem is that without verifying proper antenna match and transmitter operation you may be putting bandaids on the problem and it will come back if you even correct it at all. I would never mess with anything (in an old install that had been working) until I figured out what was actually wrong otherwise there are a million solutions with most of them dead ends for "your" specific problems even though they had worked for others at some time or other. Your problems were not from a new installation but from a previously working old installation. Very different from a troubleshooting standpoint. In relation to the other things I said in previous post, if the SWR checks good then another trick to avoid a costly and maybe unneeded trip to the avionics shop is to connect a different radio to the coax and see if the problem follows the radio or the aircraft. Nothing in this RF interference thing is 100% but those procedures will maximize your efforts and dollars. If the radio seems to be the problem it could get costly to repair unless the shop is a depot for the brand.

 

[little wing]

I don't have it at hand, but after watching the video, it looks like the cheapo SWR meter I bought at Radio Shack years ago may work with this one. I think it had a rotary switch to change scales. Will it work, and what is the wattage of a Garmin 420? I think I may have to get a different patch cord, seems like CB stuff has a big screw on ring type connector instead of a B&C. My ignorance made me think that there would be a special "aircraft" version. The most significant RF experience I have was with a CB radio for a serious off road machine about 9 years ago, so things are a little muddy in my mind. I took the time to learn as much as possible about that installation, at that time, because I thought there was a possibility that my life may depend on it's ability to get a signal out where cell phones wouldn't. Great success with it, not necessarily because of my knowledge base on RF theory, but learned a little and followed guidelines from books or internet.

If anyone has time, I've placed some questions and ramblings below. Please answer or correct me if I've got something stuck in my head wrong. It may be a day or two before I can get back over there.



I think I remember reading that you wanted the shortest possible cable running to the antenna, if there was excess, Do Not Coil it.

On the antenna I used, the tip was adjustable (in and Out) to minimize SWR, just get the lowest possible if memory serves???

My antenna had a ground wire, and it was emphasized to ensure a great chassis ground connection for the antenna, or the ground plane would go from great to minimized, I don't recall seeing a ground on this antenna, which is mounted on a fuel tank cover. I saw nothing on the tank cover that made me think that it was supposed to be well grounded to the airframe. i.e. no wire or strap connection, no mounting screw holes with the paint purposely removed to insure a good ground etc. In fact, with the antenna base being plastic, it seems as though they took measures to insulate it from ground.

The pictures I recall seeing regarding ground plane shows the floor of the vehicle as being a really high quality reflector (???) of the RF energy. On our birdcage, or space frame constructed airframes how do they have a good ground plane?

Since the comm antenna looks sealed, how would I go about adjusting SWR for best reading, pull the cap, trim a little at a time until it bottoms out and just starts to rise agian? (no screwing the tip in and out for adjustment, I don't think)

Is going from a RG58 cable to an RG400 a good, bad or indifferent thing, and do the numbers correspond to an impedance? If so, what is the importance of this impedance, cable length is about 6-7 feet.

Theres not much excess, but what there is would be in the fuel tank bay near the antenna, is this ok? I know it was mentioned to use aluminum tape to secure under the boot cowl, this makes me think that excess should be placed there rather than near the antenna, right, wrong, indifferent?

Apologies for this post not being very well laid out, I've cut and added here and there because the more I think about it, the more information creeps back into my head.

Thanks again for your help.

 

[wireweinie]

QSMX

Nothing wrong with checking VSWR but in this case I don't see it being a big deal. There were no complaints about the radio operation and Steve has replaced the com antenna coax with RG 400. There is no 'mismatch' with com radios and antennas as the radios, antennas, and RG 400 cable all use 50 ohm impedance circuits and length of cable has no bearing on antenna efficiency. If there were problems with VSWR it will show up as scratchy reception and very poor range on transmission. As far as 'moving, cutting, and hacking', some rerouting and bundling of wires has been done and it resulted in less interference with the gauges. This is something that should have been done on installation and, if done, may well have prevented this to begin with. As far as troubleshooting techniques, electrical troubleshooting is the elimination of possibilities. When I advise someone to separate wires and move coaxes to note any changes, this is something most owners can do themselves and will save them from having to pay my shop rate for me to do it. Same with cleaning grounds and separating avionics and airframe ground points.

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[wireweinie]

As far as your meter goes, make sure it will work with Aircraft com frequencies and that it will handle AT LEAST 10 watts of power. The leads should be either the RG 58 or RG 400 cable and have ends that will allow you to connect one lead to the radio 'com' antenna connector and the other lead to the antenna coax. You can fabricate these or buy them.

Antenna coax should be reasonably short. A bit of length will need to be there to allow disconnecting and reconnecting. Coiling a coax is not a good idea and if you have some excess in your installation, you can probably work some to the radio end and some to the antenna end.

Aircraft com systems don't use an adjustable circuit for matching impedance. The radio, antenna, and coax cable are all made to a 50 ohm impedance. If there is an impedance mismatch it will be because of an external issue, such as bad grounding of antenna, bad coax connections, wrong cable, etc. Even, in cases of the radio being out of spec, impedance mismatch results in very poor transmission like short range and bad sound quality. RG 400 is the preferred cable as it has a low resistance per foot of length and has a great working life (lasts for years without cracking or pealing).

Ground plane for an antenna means a good conductive surface area. On a Cub, this can be challenging. You stated that this antenna is on the fuel tank cover. You can remove the antenna and make sure that it has good contact with the cover. I like to take a bonding wire brush and clean the antenna side of the mounting screw holes. make sure your antenna base is clean and reinstall. I prefer not to use a gasket under the antenna base and seal around it with a dab of clear sealer after installation. If you are worried about the cover not grounding you can clean around the mounting screw holes before re installation. Remember that the antenna ground plane needs to be a surface not a wire. A wire to the base of an antenna will not work as a ground plane.

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[mike mcs repair]

step 1... redo that ground post and what connects to it by:

clean the tab, new screw & nuts/washers, replace the crimp on ring terminals, reinstall... they may just be getting corroded enough..

 

[OLDCROWE]

Been thinking on this one and several years ago I had a senior radio tech at Bevan-Rebell tell me that Cubs are not a normal install and with any radio issue to pull the radio and clean the contacts on both the radio and bucket with denatured alcohol on a long Q-tip and re-install it before I did anything else...Can't hurt

 

[qsmx440]

The old CB radio testers are for low frequency like up to 30 mhz or so and while some have a pseudo wattmeter setting it is highly inaccurate. The Bird (and some of it's clones, Telewave etc.) are professional instruments that will tell you the truth about power output (total, see limitations below) and your vswr.

The ground plane for a transmitter using a vertical antenna is to be about a 1/4 wavelength (all around the antenna base, 1/2wavelength diameter) plate with the antenna centered on it. No further grounding is required for the transmitter/antenna system so far as RF is concerned. In the case of CB, 27 mhz, about 9 feet in all directions around the antenna base so very inefficient on a standard truck or automobile. The antenna itself should be 9 feet long also. Very hard to build a really efficient CB antenna for a car. For aircraft band say 123 mhz the online wavelength calculator says 2 feet (or 4 foot diameter with antenna centered). The gas tank cover is shy of this four foot by four foot area of course but some tricks with loading and also trimming of the length make it work, just not as efficient as it could be but not nearly as bad as the 3' long CB short antenna on a mirror mount.

As to "the radio works fine so the antenna is fine" there is some truth to that but watts are additive (the old Bird will add up a whole bunch of different frequencies and just tell you total watts, it's weak point kind of) and since it doesn't take much power on your selected channel for communications in an aircraft, a transmitter can put out a ton of garbage and still talk fine. Snow shoe-ed to many mountain top repeater sites to find the customers out in radio land talking fine through the repeater as if nothing was wrong only to discover zero measurable power output from the transmitter. What caught my attention was you said you heard a squeal at the end of a transmission, that smacks of something wrong in the transmitter/feedline/antenna/vswr. Absolutely a bad ground could also be the cause messing with the primary voltage but I entered this discussion after some basic troubleshooting had already been done by Steve and I assume best practices were involved in checking things over so I was offering some opinions based on many many years of doing this stuff for a living with much higher RF powers involved about what might be a better plan toward a speedy resolution. "In my time" (wild a$$ guess as to statistics here) 70-80% of RF interference problems will show up in some kind of bad SWR reading. If a transmitter is putting out spurs (a comb of many frequencies) even a properly tuned antenna will show a high SWR because it (the antenna) is only resonant around a very limited range of frequencies. If radiating spurs many times it shows up in "bothering" nearby electronic equipment which is one of your symptoms.

Anyway you have a lot of seemingly competing suggestions here but the subject is complicated and though there will turn out to be one right answer eventually, how fast you can get there is the question. I don't pretend to know what is actually wrong with your system but you asked for some advice and since some competent people had already looked at this from a basic level and the squeal and that you had attempted to block direct radiation from the antenna with no change, so I told you what I would do next: confirm a squeeky clean transmitter/antenna system.

I don't know what 4313 has to say about grounding but in mobile radio we always used "star" washers between connectors and vehicle chassis grounds. They were knarly things with spikes both external and internal on them that bit into the paint and the connector and made pretty good connections over time even in a trunk with water intrusion.

Here is a funny RF interference thing that used to happen. If you pulled your dump truck up next to a 70's 911? Porsche at a stop light and while waiting at idle you pushed your PTT switch on your Motorola 110 watt low band Micor (Motorola's Idiotic Concept Of Radio), the Porsche would die as the signal overwhelmed the electronic ignition! He'd restart and you'd key up again! Good Times!

 

[wireweinie]

Been thinking on this one and several years ago I had a senior radio tech at Bevan-Rebell tell me that Cubs are not a normal install and with any radio issue to pull the radio and clean the contacts on both the radio and bucket with denatured alcohol on a long Q-tip and re-install it before I did anything else...Can't hurt

That's good routine maintenance for any avionics. Dust, dirt, shavings, and just plain old corrosion can find it's way into the connector areas and cause problems. The older style connectors seem to be worse than the newer. My own observation is that the connectors on the Bendix/King radios and any similar, are more open and allow stuff to slip inside. Newer style components that use the D-sub (computer) connectors are more sealed. But it's still a good idea to clean them like Oldcrowe stated. And I've found that if you have a leaky window or skylight that allows water onto your avionics, those newer style connectors trap that water inside.

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[qsmx440]

That's good routine maintenance for any avionics. Dust, dirt, shavings, and just plain old corrosion can find it's way into the connector areas and cause problems. The older style connectors seem to be worse than the newer. My own observation is that the connectors on the Bendix/King radios and any similar, are more open and allow stuff to slip inside. Newer style components that use the D-sub (computer) connectors are more sealed. But it's still a good idea to clean them like Oldcrowe stated. And I've found that if you have a leaky window or skylight that allows water onto your avionics, those newer style connectors trap that water inside.

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This reminded of a product I had forgotten about. If I remember it's called "DE OX" It was the best electronic cleaner I had ever used. http://www.amazon.com/GC-Electronics-DE-OX-ID-Contact-Cleaner/dp/B004SPJN9W. I don't know if the FAA would approve but it was really good at cleaning corrosion from electronic connections, battery contacts etc. Used to use it extensively on the pager/portable bench. Many times just spraying it on and wiggling the connector corrected the problem.

 

[skywagon8a]

I had an airplane with an assortment of early 60s King tube radios which had sat for a few years and would not run. I pulled all of the tubes and connectors and sprayed ACF-50 on the connectors. http://www.learchem.com/products/acf-50.html Then ran the radios. After about an hour they all started working properly again. Good stuff for cleaning electric connections.

I'm not sure but I believe that Corrosion X http://www.corrosionx.com/corrosionx-aviation.html is a derivative or has some connection to ACF-50.

 

[18180]

I have same problem. Radio on right, instruments on left. Antenna coax runs up the right pillar so can't get them any farther apart. Eagerly awaiting the ultimate fix!

 

[qsmx440]

I had an airplane with an assortment of early 60s King tube radios which had sat for a few years and would not run. I pulled all of the tubes and connectors and sprayed ACF-50 on the connectors. http://www.learchem.com/products/acf-50.html Then ran the radios. After about an hour they all started working properly again. Good stuff for cleaning electric connections.

I'm not sure but I believe that Corrosion X http://www.corrosionx.com/corrosionx-aviation.html is a derivative or has some connection to ACF-50.

Sky also probably due to your radios electrolytic capacitors reforming. They don't like to sit. Common to pull the tubes on old radios, substitute solid state diode rectifiers for the rectifier tube and then bring the power up through a variac over a few days to let the caps reform (if they are not dried out). Lot's of folks fail to pull the tubes and the coating on all the tube filaments gets spoiled during the low voltage period often ruining the tubes prematurely. "Yeah I brought the radio up slowly on the variac and then I replaced the bad tubes" Yep!

 

[little wing]

Thanks for all the suggestions and educational inspiration folks. Been listening carefully which has led me to do some more study on my own. I learned as a young man to diagnose by going from an unknown to a known, or a known to an unknown, using deductive or inductive reasoning along the way. Never just jump in the middle and start checking components or making assumptions. This is what has troubled me about this black magic RF stuff.

With some of the great guidance I've got here, I've made a plan. Bird 43 VSWR, patch cords, and I think the 10 C element (please correct me if I'm wrong) will be ordered in the next day or two. Not sure I want to invest in a spectrum analyzer just yet. We will post the VSWR once we have the data. If nothing is too far out of whack there, I intend for the next step to be to cut all the crap out, organize, connect the remaining backlights, add ferrite doughnuts, shorten the power leads and do a similar ground to what is in my plane, if Flymore wants me to do this. (pics below) Copper plate to act as a ground buss, separate avionics from other loads and run separate ground leads all the way back to the battery. (8ga each?) All but one EI gauge has a large plastic plug, seems like power and ground lines were removed from all the plugs. Are the plugs shielding anything? Should I remove them? The plugs in mine, (pics below), are metal and seem like mil-spec. Getting the supplies will take a few days and I expect the thread to drop off the front page for a few days, unless you guys come up with other things to add to the plan as a "while you're at it" or "do this too." Thank you all very much, nice to know there are some smart people out there who are willing to help the dumb ones like me. Talk soon.

 

[skywagon8a]

I see the copper ground bus. Why is it insulated from grounding to the tubing with a rubber covered adel clamp? Does one of those wires go to a ground point elseware?

 

[Steve Pierce]

I had an airplane with an assortment of early 60s King tube radios which had sat for a few years and would not run. I pulled all of the tubes and connectors and sprayed ACF-50 on the connectors. http://www.learchem.com/products/acf-50.html Then ran the radios. After about an hour they all started working properly again. Good stuff for cleaning electric connections.

I'm not sure but I believe that Corrosion X http://www.corrosionx.com/corrosionx-aviation.html is a derivative or has some connection to ACF-50.

Wouldn't the sticky residue attract dirt? My Dad told me about issues with the radios on the S3 Viking radios. Airplane is hooked up to the catapult and ready for the shot when the radios went dead. Avionics tech pulls a panel at the rear of the fuselage,does something and everything starts working. My Dad asked him what he did. Pulled the radio out, sprayed WD40 on the contacts and shoved it back in. Problem was the oil that didn't eveporate off attracted dirt and created other problems.They ended up leaning all the trays in the squadron and started using a contact cleaner.

 

[skywagon8a]

The ACF-50 doesn't seem to have sticky residue. It is intended for light corrosion removal and protection. One of it's intended uses was to be fogged inside wings and fuselages for corrosion protection.

 

[qsmx440]

Little wing I just looked at my element collection and I have a 25C which is 25 watts and will do just fine for your testing. I don't need my Bird any time soon and I have access to others if I did so I can send you the complete setup to borrow if you want. I will set up BNC cable connections and a jumper on it if that's what you need. It's an old ebay Bird but works fine. I can send it out today if you want regular USPS?
I do ask that you leave the element in it when moving it around and with the arrow pointing midway between the to positions so that the meter is "loaded" (motoring against the short) and that keeps the needle on the meter from slamming around. It will come in the mail that way if it seems confusing. These things are basically bullet proof so not any other worries. Let me know and also this way you can take your time getting your own setup on the cheap instead of having to run out for this job and spend big $.

 

[Steve Pierce]

Billy, the plugs you are seeing under your panel are the Turn Coordinator and Art. Horizon. Your EI gauges are much shorter and above those. I can see the wires.

 

[Steve Pierce]

The ACF-50 doesn't seem to have sticky residue. It is intended for light corrosion removal and protection. One of it's intended uses was to be fogged inside wings and fuselages for corrosion protection.

Your post was just funny because yesterday morning my Dad was telling me the S3 story.

 

[qsmx440]

I just went rooting around in my old test equipment and found an old Motorola TEK 7A Rf voltmeter with probe and I can throw that in the box also and what it is is a sensitive RF voltmeter for the VHF range (aircraft radios among others) and allows you to "sniff" with the probe for different levels of RF on various grounds. wires, chassis etc. It's ok but kind of a barnyard tool compared to the spectrum analyzer. It's what we used to use sometimes for situations like yours. I spotted it in a box at a ham fest and picked it up for nostalgia reasons. I'll send my phone number and we can coordinate to walk you through all of this testing. 90 % sure "we" can find your gremlin in an hour or so and confident if we can't then a trip to the avionics shop is warranted. Be fun to exercise the old brain on this if you want.

 

[little wing]

I see the copper ground bus. Why is it insulated from grounding to the tubing with a rubber covered adel clamp? Does one of those wires go to a ground point elseware?

I am not certain Skywagon, but I don't recall seeing a separate wire that looked large enough to carry the load back to the battery. I took these pics of my panel just for reference. By the time I got home that night my neck had become so weak from looking under Flymore's panel that I didn't spend much time under mine. I did a double take at first, but I think they are using the same tab as Flymore's on one end (side closest to bottom of pic) for a chassis ground, and the adel on the other, just for support.

 

[wireweinie]

The ACF-50 doesn't seem to have sticky residue. It is intended for light corrosion removal and protection. One of it's intended uses was to be fogged inside wings and fuselages for corrosion protection.

Agreed. But both ACF 50 and CorrosionX 'creep'. If you spray it on a seam, it will continue to creep along the overlap. It will do the same with wires. If you spray it on the terminal on the end of the wire, it will migrate up the wire, inside the insulation. It can be good or bad, depending on the situation. De-Ox seems to stay put.

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[skywagon8a]

I see that tab now in the bottom picture. It is difficult to see looking at it edgewise. I was sort of hoping for you that it was something simple.

 

[little wing]

Little wing I just looked at my element collection and I have a 25C which is 25 watts and will do just fine for your testing. I don't need my Bird any time soon and I have access to others if I did so I can send you the complete setup to borrow if you want. I will set up BNC cable connections and a jumper on it if that's what you need. It's an old ebay Bird but works fine. I can send it out today if you want regular USPS?
I do ask that you leave the element in it when moving it around and with the arrow pointing midway between the to positions so that the meter is "loaded" (motoring against the short) and that keeps the needle on the meter from slamming around. It will come in the mail that way if it seems confusing. These things are basically bullet proof so not any other worries. Let me know and also this way you can take your time getting your own setup on the cheap instead of having to run out for this job and spend big $.

This is an incredibly generous offer! I'm floored, most people, understandably, don't even like to lend a screw driver. Though I hate borrowing anything, I know this would be far more expedient than gathering the pieces needed myself. We will take very good care of it, and will pack extremely well for the return trip, along with some dough to cover the shipping costs and your trouble. I can't thank you enough! Yes, he has BNC connectors, I think we need two, terminating in male and female. His coax is too short to reach below the panel.

Will PM with phone and address.

 

[qsmx440]

This is an incredibly generous offer! I'm floored, most people, understandably, don't even like to lend a screw driver. Though I hate borrowing anything, I know this would be far more expedient than gathering the pieces needed myself. We will take very good care of it, and will pack extremely well for the return trip, along with some dough to cover the shipping costs and your trouble. I can't thank you enough! Yes, he has BNC connectors, I think we need two, terminating in male and female. His coax is too short to reach below the panel.

Will PM with phone and address.

Your record seems fine (Steve knows you) so no problems sending it and like anything I give or loan I only give what I can afford to lose so basically a gift until it comes back. Nothing for my time please just return shipping will be fine. Can you tell me how your coax terminates at the base of the antenna (female BNC on the gas cover or etc?). I will also include an RF load to attach there for some of the planned testing. It can be used to isolate the radiation from the antenna as part of the procedure. PM your address and I'll get stuff packed and hopefully in the mail today.

 

[wireweinie]

Littlewing

If you want to ground your avionics all the way back to the battery, a 10 gauge wire will be more than enough.

The plastic plugs on the E.I. gauges are very handy when troubleshooting. Not sure why one would pull the power and ground wires out of them. They are called CPC plugs and the pins and sockets are available from places like Mouser Electronics. I would definitely keep the plugs wired as supplied.

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