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#21
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Aircraft antennas
See my other post about the equipment used to measure power.
What LC trap? This antenna is a straight, flexible whip. There isn't ANY coil in the whip. 51 Ohm resistor? Could be, who knows...but the antenna radiates and receives fine. The antenna routinely talks air to air over 70 miles or more out here in the midwest where the hills aren't inflated like the hills and egos out in Grass Valley appear to be. Thanks for blowing my whole image of you. You just lost all of the respect I used to have for you. Scott Littfin Bloomer, WI RST Engineering wrote: 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 |
#22
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Aircraft antennas
Here's a link to the ELT antenna picture (the exact antenna I am using
across the com band). You can get as abusive and abrasive and you want. Tell me theory all day long and it still doesn't matter. The fact is, it WORKS in my installation and has been for about 7 years now. http://www.chiefaircraft.com/Aircraf..._05-02-006.gif Tell me more about the LC trap on this antenna. Scott Littfin RST Engineering wrote: 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 |
#23
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Aircraft antennas
"Scott" wrote in message ... 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... OK, Scott, I'll clarify. I'm a ham (originally WB6BHI in '59, now WX6RST). I work as an electronics engineer and college instructor (up to 24 GHz.). I designed, produced, and troubleshot that com radio you built. I don't work in the aircraft building industry either, but I *DO** work in the aircraft maintenance industry and have since 1962. I have a pretty good idea how much aluminum dope is required on any fabric aircraft. In offering ideas (cheap silver, microthin application, "waveguide out the wingtip") what you in essence were saying is that Bellanca and Jim were gaming the system by either using unapproved materials OR not being savvy enough to measure the complete radiation pattern in both E and H fields to see if there was something funny going on. Yes, I take personal affront at that sort of insinuation. You used to be a pretty fair guy. Now you seem to immediately attack someone on their first post. Still am, mostly. And I generally wait until the second or third post unless something completely off the wall comes down the avenue. You are really coming off as completely arrogant. Possibly. Yes, you're a big engineer (you are, aren't you?) Not since I went on my diet. Now I'm a smaller engineer. Blood pressure and all, don'cha know. Got it down to 125/75 for last week's AME physical. , you write a monthly column in a magazine for homebuilders And have for fifteen years or so. (who may or may not put 10 coats of silver on their fabric) If they do, they are damfools and I don't write for damfools. and you don't listen to anything from anyone who doesn't share your exact thoughts. Sure I do, so long as it has some foundation in reality. 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) Don't think I mentioned anything about hick. 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). Telewave makes a good wattmeter; I actually prefer them to the Bird because of the "no slug" design. So it told you that the reflected power was about 10%, which is (as you noted) a skosh below 2:1. Let's take a look and see how that might be achieved. The antenna is an ATX model 05-02-006. From a brief look at the device on page 18 of the current Chief Aircraft catalog #31, it appears from the external view that the radiating element is a single spring steel rubber coated wire of approximately 100 mils diameter including coating. Just for assumption, let's say the wire is half of that diameter and the rubber coating is the other half. That gives us a wire of 50 mils diameter and 25 mils of rubber on both sides. AWG #16 wire. 50 mil wire and 22,000 mil (22") length give us an "aspect ratio" of 440:1. The general rule of thumb (as confirmed by Jasik's "Antennas" and the ITT Handbook For Radio ENgineers (3rd edition)) is that the VSWR bandwidth (a purely subjective term) is given by (6*f)/A, where f is the frequency for which the antenna is cut and A is the aspect ratio of the antenna. Postulating that the antenna is resonant at 121.5 MHz. gives us a VSWR bandwidth of approximately 1.6 MHz. ... hardly the 19 MHz. of the 118-138 MHz. aircraft com band. Now, since you didn't really specify what bandwidth you measured across, it is possible that if you stayed within a MHz. or so of the resonant frequency of that antenna that you will have the results you measured. However, if you truly took it to the band edge limits, we've got to find out how that might be accomplished. Let's examine the TSO specifications for an ELT. The PERP (Peak Effective Radiated Power) as specified in RTCA document DO-183 (which is the technical document that the TSO relies upon) is 50 milliwatts at 121.5 and 243.0 MHz. I draw your attention to PEAK power, not average power. Just like PEP on SSB is the PEAK of the modulating envelope, this is the PEAK of the AM envelope, which for an 80% depth of modulation is about 3:1. The transmitter, then, is only required to put out about 20 milliwatts. Hm. The TSO also requires the antenna to have an EIRP (Effective Isotropic Radiated Power) of 0 dB. If the antenna is truly a quarter wave whip over a ground plane, it has an intrinsic EIRP of 2.14 dB for starters, which reduces the amount of power that we need to radiate another factor of 1.5 or so. Gee, this is getting real easy. Now I have no idea what is in that rubber cone on the base or what is between that input connector and the wire radiating element, but if it was up to me to meet those specifications, I'd sure as hell put in a 5 dB resistive pad made up of quarter watt (and quarter-cent) resistors...and guess what, that would be a 10 dB worst case return loss which will give me a 2:1 VSWR from DC to daylight. I don't say that's what they did. I actually don't have a CLUE what is in the rubber base without buying one and cutting it apart. But that's the only way I know of (other than the aforementioned trap self-resonant coil shown on antennas 3 and 4 of that Chief page) of getting a single element antenna to resonate at both 121.5 and the SECOND harmonic 243.0, where the single wire is exactly ANTIresonant. Would that lossy pad let you transmit and receive over a 70 mile range? Let's take a look at that. If the pad is truly resistive, and if they used quarter watt resistors in a pi-net pad, I'd expect the resistors to fry fairly quickly. ELT antennas of this design weren't meant to be used as COM antennas; that's why they sell COM antennas elsewhere on the pages. Or, if they just said "to hell with it, we don't have to meet the TSO, the ELT has to meet the TSO" then you do have a decent antenna spot on 121.5 MHz. and for about a MHz. on either side. If you are doing a legal chat on 122.75 MHz. you will probably get away with it because the ICOM has a pretty good VSWR protection built in. I'd really like to know what the VSWR is at 243.0 MHz., though, where the single element wire is antiresonant. Just for grins, that 1 watt transmitter and a 1 microvolt receiver has a theoretical free-space range somewhere in the vicinity of a thousand miles, so even if you throttle the 1 watt down to a hundred milliwatts, you still have well over a hundred mile range, horizon not being a factor. (Radio horizon is given in miles as (sqrt(2*h)) where h is your altitude in feet.) At pattern altitude of 1000', this would be 44 miles; if the other guy is at pattern altitude also, this doubles the range to 88 miles, which is about what you are seeing. Howzat? jw |
#24
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Aircraft antennas
Well, like I said, it works for me. I can double check wattmeter
readings since I did them 7 or 8 years ago...maybe I did only go up to 125 or so since I never really saw myself calling on 133.6 or something like that. Regardless of that outcome, the fact remains (at least in my case)...it works. That may, in fact, be due to the fact that 99, no make that 100% of the time I am between 118 and 126 and 99.9% between 122.7 and 123.0. I did NOT want to get in any sort of ****ing contest with anyone, I just wanted to give the original guy another option and post on my experience with it. And for $39, it would be a pretty cheap experiment. If it failed, you can always use it for its intended purpose. Scott RST Engineering wrote: Just for grins, that 1 watt transmitter and a 1 microvolt receiver has a theoretical free-space range somewhere in the vicinity of a thousand miles, so even if you throttle the 1 watt down to a hundred milliwatts, you still have well over a hundred mile range, horizon not being a factor. (Radio horizon is given in miles as (sqrt(2*h)) where h is your altitude in feet.) At pattern altitude of 1000', this would be 44 miles; if the other guy is at pattern altitude also, this doubles the range to 88 miles, which is about what you are seeing. Howzat? jw |
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