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#11
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Aircraft antennas
I would say it isn't the best idea. The silver coat used on the fabric
has tiny aluminum pieces for uV protection. Seems to me this would somewhat shield the antenna and limit the signals into and out of the antenna. Of course, in the real world, it would probably work. Scott Jim Carriere wrote: ccwillwerth wrote: Hi, I am about ready to cover my Cub type airframe, but need a place to attach a com antenna. I was considering brazing a plate to the airframe so that it would be just under the fabric. The antenna is the type that has a ceramic insulator on the bottom of the antenna that insulates the stainless steel antenna from the airframe. Does the antenna need a large plate for a ground plane or will a small plate be sufficient? If a ground plane is required, can the copper foil tape be used on the inside of the fabric as the ground plane? Please excuse my "piggybacking" your question, can anyone with an informed opinion weigh-in on this: http://www.aircraftspruce.com/catalo...nnasystems.php You put it inside a tube and fabric structure. Seems like a good idea at first glance... |
#13
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Aircraft antennas
I would say that you are wrong. Bellanca paid me decent money back in the
early '80s to make the tests to see if we could hide their antennas inside their wood and fabric wings. They actually shipped me a wing from Alex MN to GV California so that I could do the preliminary work out here before I went back there (in the dead of winter, what a mistake THAT was) to hang antennas in a real live airframe and fly them around. THe theory is that the aluminum powder/dust is so broken up into individual particles insulated from each other by a dope binder that they do NOT act as a shield. Test: Put two antennas 30 meters apart. Radiate a signal from one and use a field strength meter to receive at the other (spectrum analyzer). Carefully slip a wing over the transmit antenna. Less than 0.1 dB difference. Slip the same wing over the receive antenna. Same difference. Jim "Scott" wrote in message .. . I would say it isn't the best idea. The silver coat used on the fabric has tiny aluminum pieces for uV protection. Seems to me this would somewhat shield the antenna and limit the signals into and out of the antenna. Of course, in the real world, it would probably work. Scott |
#14
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Aircraft antennas
Not.
Jim "Scott" wrote in message .. . Believe it or not (I checked with trusty wattmeter), I get good SWR across the entire Comm band using a cheap ELT antenna I bought from Chief Aircraft for about $35 several years ago (the one that has a very flexible whip). |
#15
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Aircraft antennas
Sure. The subjective technical term for using a VHF antenna inside a steel
tube fuselage is "not worth a ****". Jim "T o d d P a t t i s t" wrote in message ... On the related subject of mounting an antenna inside a steel tube fuselage, have you done any similar tests or do you know of any? That would seem to be a much different situation, with long conductive frame members, but real world tests can be surprising. Any results or comments you want to share? |
#16
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Aircraft antennas
OK, maybe so. BUT...I STILL vote for putting the antenna outside.
Maybe the wing was acting like a waveguide and the RF was coming out holes at the root end Or maybe Bellance used cheap silver or only put on a layer a micron in thickness Scott RST Engineering wrote: I would say that you are wrong. Bellanca paid me decent money back in the early '80s to make the tests to see if we could hide their antennas inside their wood and fabric wings. They actually shipped me a wing from Alex MN to GV California so that I could do the preliminary work out here before I went back there (in the dead of winter, what a mistake THAT was) to hang antennas in a real live airframe and fly them around. THe theory is that the aluminum powder/dust is so broken up into individual particles insulated from each other by a dope binder that they do NOT act as a shield. Test: Put two antennas 30 meters apart. Radiate a signal from one and use a field strength meter to receive at the other (spectrum analyzer). Carefully slip a wing over the transmit antenna. Less than 0.1 dB difference. Slip the same wing over the receive antenna. Same difference. Jim "Scott" wrote in message .. . I would say it isn't the best idea. The silver coat used on the fabric has tiny aluminum pieces for uV protection. Seems to me this would somewhat shield the antenna and limit the signals into and out of the antenna. Of course, in the real world, it would probably work. Scott |
#17
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Aircraft antennas
I can send you forward and reflected power readings if you wish, say
every 1 MHz from 118 to 136. Now, with that said, that is measured on the ground, not in flight where the whip curves back from all the high speed flying at 75 MPH Scott RST Engineering wrote: Not. Jim "Scott" wrote in message .. . Believe it or not (I checked with trusty wattmeter), I get good SWR across the entire Comm band using a cheap ELT antenna I bought from Chief Aircraft for about $35 several years ago (the one that has a very flexible whip). |
#18
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Aircraft antennas
ExCUSE ME. I don't mind carrying on a technical discussion, but to suggest
that we didn't use standard manufacturing procedures OR that a certificated airplane used "cheap silver" whatever the hell that is or spread it on a micron in thickness and still expected to pass the inspector's muster is just plain stupid. Nor do you give me the credit for knowing how to take polar plots of antennas to meet FAA expectations for certificated aircraft antenna installations. You DO understand dB/relative angle plots, don't you? Quite frankly, I think we have a self-anointed CB radio expert with us who doesn't have a freakin' CLUE about aircraft antennas. Good by, good buddy, 10-4? Jim "Scott" wrote in message .. . OK, maybe so. BUT...I STILL vote for putting the antenna outside. Maybe the wing was acting like a waveguide and the RF was coming out holes at the root end Or maybe Bellance used cheap silver or only put on a layer a micron in thickness Scott |
#19
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Aircraft antennas
First, send me the model of the instrument that you used for the
measurement. Then explain why the trap in the antenna didn't totally mess up the VSWR for the VHF band. You DO understand that an ELT antenna is a trap monopole, don't you? You DO understand that the top 2/3 of the antenna is decoupled from the bottom end by an LC trap, don't you? That a properly operating ELT antenna should be 2:1 or less at 121.5 MHz. and more than 10:1 above 123 and below 119 MHz? My bet is that you used a CB power meter, good buddy, 10-4. Of course, you could be measuring a 51 ohm resistor that somebody put in the antenna to "match" the antenna across the band. That would let the antenna radiate about as well as a limp piece of spaghetti in a copper septic tank. Jim "Scott" wrote in message .. . I can send you forward and reflected power readings if you wish, say every 1 MHz from 118 to 136. Now, with that said, that is measured on the ground, not in flight where the whip curves back from all the high speed flying at 75 MPH Scott |
#20
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Aircraft antennas
OK Jim,
I'll clarify. I'm a ham, I work as a radio tech (up to 7 Ghz). I've bought and built one of your com radio kits...blah blah blah. You are entitled to your opinions. I was just offering some ideas on what may or may not have been going on. I don't work in the aircraft building industry. I don't know how much silver is required on a production aircraft... You used to be a pretty fair guy. Now you seem to immediately attack someone on their first post. You are really coming off as completely arrogant. Yes, you're a big engineer (you are, aren't you?), you write a monthly column in a magazine for homebuilders (who may or may not put 10 coats of silver on their fabric) and you don't listen to anything from anyone who doesn't share your exact thoughts. I used to respect you in the past, but now you seem just plain abusive (and abrasive). On one of my posts on this thread you immediately took this attitude that I'm a hick CBer who doesn't know anything (you make the claim without knowing any facts about me...you just make those assumptions and attack) I admit, I DON'T know it all like you seem to, but I DID measure forward and reflected power with my Telewave wattmeter, my Icom A-22 and the whip ELT antenna I used on my plane. The worst reflected power I saw was 100-150 mW, which is no more than 10% reflected power of the 1.5W forward power (ie 2:1 SWR...not perfect, but acceptable to military standards. I was an avionics comm tech in the USAF). I think you could work on toning down your responses and state what you find to be true without having to resort to name calling. Scott Littfin RST Engineering wrote: ExCUSE ME. I don't mind carrying on a technical discussion, but to suggest that we didn't use standard manufacturing procedures OR that a certificated airplane used "cheap silver" whatever the hell that is or spread it on a micron in thickness and still expected to pass the inspector's muster is just plain stupid. Nor do you give me the credit for knowing how to take polar plots of antennas to meet FAA expectations for certificated aircraft antenna installations. You DO understand dB/relative angle plots, don't you? Quite frankly, I think we have a self-anointed CB radio expert with us who doesn't have a freakin' CLUE about aircraft antennas. Good by, good buddy, 10-4? Jim "Scott" wrote in message .. . OK, maybe so. BUT...I STILL vote for putting the antenna outside. Maybe the wing was acting like a waveguide and the RF was coming out holes at the root end Or maybe Bellance used cheap silver or only put on a layer a micron in thickness Scott |
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