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Building Slim Jim for Air Band
Hi all,
New to this forum, I am so impressed with the level of expertise I find here. Thanks for welcoming me. I want to build a Slim Jim antenna for receiving airband (118-136MHz) out of 3/8" or 1/2" soft copper tubing. This antenna won't see any outside use--it will hang away from any metal against my window facing the Hudson River towards Newark Airport, 11 miles away. Amazingly, my little 8" helical rubber duck works as well as a ladder cable-based 125MHz Slim Jim I bought, so I want to step it up...! There are many, many stories floating around the Internet about homebrew Slim Jim projects, but what strikes me is that everybody seems to arrive at different lengths for a 3/4 wave in the 144MHz band (where I find most Slim Jims), even allowing for differing propagation speeds in the various materials these builders use. What am I missing here? I thought the calculation was as straightforward as the wave length in open air adjusted by the velocity factor, about 0.96 for copper tubing. My antenna should center around 125MHz, as the most interesting frequencies, Tower, Ground, tend to group at the lower end of the airband spectrum. I understand that the bigger the diameter of the tubing is, the wider the antenna's bandwidth. What would be a good trade-off diameter? Will the resulting bandwidth be good enough? I am leaning towards 3/8" as happy medium, unless one of you tells me I'm crazy. Anyway, I won't solder any elbows at the two ends; I will loop this tubing using a bending spring. Should I compensate for these round ends in my length calculations, or won't that matter? How critical is the distance between the two parallel sides? I can't find any information on that either. Can I leave the air gap bare, or should I treat the tube endings to some kind of solid, perhaps adjustable, plug? I'll devise a slide for the coax connection so I can adjust it for best matching. I am a complete novice when it comes to building antennas, and for now can only contribute my thanks for your advice. Cheers, Michael |
Building Slim Jim for Air Band
"Michiel Kappeyne" wrote in message ... I want to build a Slim Jim antenna for receiving airband (118-136MHz) out of 3/8" or 1/2" soft copper tubing. This antenna won't see any ouside use--it will hang away from any metal against my window facing the Hudson River towards Newark Airport, 15 miles away. Amzingly, my little 8" helical rubber duck works as well as a ladder cable-based 125MHz Slim Jim I bought, so I want to step it up...! There are many, many stories floating around the Internet about homebrew Slim Jim projects, but what strikes me is that everybody seems to arrive at different lengths for a 3/4 wave in the 144MHz band (where I find most Slim Jims), even allowing for differing propagation speeds in the various materials these builders use. What am I missing here? I thought the calculation was as straightforward as the wave length in open air adjusted by the velocity factor, about 0.96 for copper tubing. My antenna should center around 125MHz, as the most interesting frequencies, Tower, Ground, tend to group at the lower end of the airband spectrum. I understand that the bigger the diameter of the tubing is, the wider the antenna's bandwidth. What would be a good trade-off diameter? Will the resulting bandwidth be good enough? I am leaning towards 3/8" as happy medium, unless one of you tells me I'm crazy. Anyway, I won't solder any elbows at the two ends; I will loop this tubing using a bending spring. Should I compensate for these round ends in my length calculations, or won't that matter? How critical is the distance between the two parallel sides? I can't find any information on that either. Can I leave the air gap bare, or should I treat the tube endings to some kind of solid, perhaps adjustable, plug? I'll devise a slide for the coax connection so I can adjust it for best matching. I am a complete novice when it comes to building antennas, and for now can only contribute my thanks for your advice. Cheers, Michael In simple terms the velocity factor of the antenna is determined by the insulating material around the antenna. There is another factor called the diameter to wavelength. That is how big around the antenna is. That number usually runs from .94 to .98. The larger the diameter the antenna is in relationship to the wavelength will shorten the antenna also. When you go from a wire such as twin lead to copper tubing for the slim jim there is a big ratio change. The graph here may help you see that effect. http://www.radio-electronics.com/inf...on-formula.php So not only does the diameter effect the bandwidth, it also effects the length. For receiving I doubt that you will notice much differance in the bandwidth as far as the signal strength going from a piece of number 14 wire to 3/8 inch tubing. |
Public service radio - aircraft bands - are transmitted with massive amounts of power. Not to mention the fact that the airplanes are in the air when they are mainly transmitting. Not that there is much traffic to listen to anymore, with most things being computerized and digital.
There is no reason to use a 3/4 wave antenna, nor is there any benefit in using a antenna more then 5/8 wave long. Might I suggest you get a copy of the ARRL antenna book, it will cover it better then I can. The bottom line is - if all you want to listen to is local traffic, all you need is a 1/4 wave antenna - mounted outside, in as clear of a place as possible. Some building materials - concrete reinforced steel - being one, is opaque to good radio reception. All effective communications is line of sight. The best scanner antenna for the money would be a Diamond Discone type antenna - which would cover anything from 10 meters to 999 MHz - all in one antenna. Using a good high grade coax and connectors would be key to good reception. As a example, Cleveland Approach is 120 miles from my location, with my Uniden 890XLT - I can hear Cleveland approach and the air traffic between Cleveland / Pittsburgh and Baltimore MD from my location in Central PA. Not to mention the local air traffic. Even a old VHF television antenna would get you some reception... Copper J Poles are great for experimental purposes, but not real good when it comes to reliable reception - even though you are just going to use it inside of your house. They do offer some front to back - directivity in one or more directions, but there is no reason to use a piece of copper pipe if all you want to do is receive. You can acheive practically the same thing with a piece of 300 ohm twin lead - if assembled properly. I have even seen antenna's that were made out of copper foil that worked for what you want to do. |
Building Slim Jim for Air Band
Channel Jumper wrote:
Public service radio - aircraft bands - are transmitted with massive amounts of power. No they are not. Most aircraft radios are around 10 watts and ground stations around 25 watts. Not to mention the fact that the airplanes are in the air when they are mainly transmitting. Pretty much correct. Not that there is much traffic to listen to anymore, with most things being computerized and digital. Nope, aircraft communications is still AM voice and there is lots of it. There is no reason to use a 3/4 wave antenna, nor is there any benefit in using a antenna more then 5/8 wave long. Might I suggest you get a copy of the ARRL antenna book, it will cover it better then I can. The bottom line is - if all you want to listen to is local traffic, all you need is a 1/4 wave antenna - mounted outside, in as clear of a place as possible. Some building materials - concrete reinforced steel - being one, is opaque to good radio reception. All effective communications is line of sight. Well, you got something right anyway. The best scanner antenna for the money would be a Diamond Discone type antenna - which would cover anything from 10 meters to 999 MHz - all in one antenna. Giant overkill just to listen to airplanes. Using a good high grade coax and connectors would be key to good reception. Actually, if the goal is to hear ground stations, the key is antenna height. As a example, Cleveland Approach is 120 miles from my location, with my Uniden 890XLT - I can hear Cleveland approach and the air traffic between Cleveland / Pittsburgh and Baltimore MD from my location in Central PA. You do know that things like Approach are not usually on the airport. Not to mention the local air traffic. You normally hear lots of air traffic but not the ground station. Even a old VHF television antenna would get you some reception... Copper J Poles are great for experimental purposes, but not real good when it comes to reliable reception - even though you are just going to use it inside of your house. There is nothing about a J pole that makes in unrealiable for reception though it is again overkill for the application. They do offer some front to back - directivity in one or more directions, but there is no reason to use a piece of copper pipe if all you want to do is receive. There is no reason to use anything other than a piece of coax with the center conductor stripped out for about 20 inches for receive. You can acheive practically the same thing with a piece of 300 ohm twin lead - if assembled properly. I have even seen antenna's that were made out of copper foil that worked for what you want to do. For listening at home with my aviation HT, I use a piece of RG-58 that has the shield pulled back about 20 inches with about 20 inches of center conductor exposed. The end of the center conductor is soldered into a small loop so I can hang the thing from the ceiling with a piece of string. |
Building Slim Jim for Air Band
In article ,
Channel Jumper wrote: Public service radio - aircraft bands - are transmitted with massive amounts of power. You are an IDIOT, and your understanding of FAA Comms is ludicrous at best. Most FAA Ground Transmitters are less than 100 Watts Output, and Aircraft Radios are usually in the 25 to 50 watt Range, for Commercial and 10 to 25 Watts for General Aviation Aircrat. All UniCom and MultiCom Transmitters are 10 watts, unless an STA has been granted for some very unusual reason. If you consider this "MASSIVE" you are a double Idiot.... Me One who actually KNOWS something about the subject..... |
Building Slim Jim for Air Band
On Thursday, January 31, 2013 11:52:10 PM UTC-6, Michiel Kappeyne wrote:
I want to build a Slim Jim antenna for receiving airband (118-136MHz) out of 3/8" or 1/2" soft copper tubing. I'm not really a fan of those type of antennas. You would likely be as well off to just make a simple 1/4 ground plane with sloping radials. Easy match to coax, and super easy to build. Can be as simple as a SO-239 connector with 5 stiff wires attached. All the bandwidth issues, etc are not worth worrying about for receiving. You will likely never hear the difference between a vertical length of wire, vs a fat copper tube for receiving air band. Another issue with most J-poles, and other antennas of it's ilk are the total lack of decoupling from the feed line. There is no point in worrying about extra gain or bandwidth, when the design of the antenna does not take decoupling of the feed line into consideration. The lack of decoupling will skew the pattern up off the horizon, and make any extra gain from a longer element length apply only to higher angles. This could be good for airplanes in flight, but bad for tower comms. The decoupling of a 1/4 ground plane is not perfect, but it's generally a good bit better than the usual J pole. And it can be improved by adding more radials, or an extra set of radials 1/4 wave below the feed. Myself, I would just build a simple SO-239 ground plane, "google for illustrations", hang up from the ceiling with a short piece of fishing line and a tack, and be done with it. It will likely function as well as most of the J pole designs, with a lot less work involved. And it's a direct match to coax, which means no matching losses. The sloping radial version is a better match, than one with the radials at 90 degrees from the vertical element. And the gain of the sloping radial ground plane will usually be within .3 db of a 1/2 wave vertical. And this assumes the 1/2 wave is properly decoupled. Most are not.. So it would not be totally unusual to see the simple ground plane actually do better in many cases. If one is going to use a 1/2 wave vertical for VHF, one needs to take decoupling of the feed line into consideration. Most don't it seems.. :( |
Building Slim Jim for Air Band
On Fri, 1 Feb 2013 05:52:10 +0000, Michiel Kappeyne
wrote: New to this forum, I am so impressed with the level of expertise I find here. Oh-oh. We're in trouble now. I want to build a Slim Jim antenna for receiving airband (118-136MHz) out of 3/8" or 1/2" soft copper tubing. This antenna won't see any ouside use--it will hang away from any metal against my window facing the Hudson River towards Newark Airport, 15 miles away. Amzingly, my little 8" helical rubber duck works as well as a ladder cable-based 125MHz Slim Jim I bought, so I want to step it up...! Altitude will get you better results than gain in this case. The idea is to get over the ground clutter so that you can hear aircraft and service vehicles on the ground. 15 miles is a bit of a stretch but possible. The trick is to create an antenna that will hear well overhead but also has most of the gain in the direction of the airport. Presumably, your station and the airport are not moving. What I suggest is an aircraft band yagi antenna pointed at the airport. That should take care of the ground stations. However, that does nothing for hearing airplanes flying overhead. For that, I suggest a simple coaxial antenna, as Jim Lux has suggested. A length of coax with 1/4 wave of braid peeled back over the coax will sorta work. If you want something better, a simple 1/4 wave ground plane antenna or a horizontal 1/2 wave dipole. Don't bother with a J-Pole, Slim Jim, or anything with gain. The problem is that such antennas "borrow" gain from the upwards direction, and use it to improve gain at the horizon. That's NOT what you want for the "overhead" antenna. Obviously, it is necessary to combine the two antennas. A Wilkinson combiner will have about 1dB loss on receive, which can be easily tolerated. https://www.google.com/search?q=wilkinson+combiner&tbm=isch One potential problem is that if both antennas receive the same signal 180 degrees otto phase, there will be cancellation. To reduce (but not eliminate) this problem, try to position the overhead antenna where the path to the airport is blocked by some obstruction (i.e. chimney). There are many, many stories floating around the Internet about homebrew Slim Jim projects, but what strikes me is that everybody seems to arrive at different lengths for a 3/4 wave in the 144MHz band (where I find most Slim Jims), even allowing for differing propagation speeds in the various materials these builders use. What am I missing here? The diameter of the elements has some effect on the cut length. So does spacing between elements, coax impedance, where they're measuring from, and how the end of the elements are cut. For example, a square cut tubing will have a different effective length than one with the end crimped and rounded, or with a hemispherical shaped end cap. The idea behind the non-square tubing end is to increase the usable bandwidth of the antenna. I thought the calculation was as straightforward as the wave length in open air adjusted by the velocity factor, about 0.96 for copper tubing. Calculations are nice, but when it comes to actually sweeping an antenna for where the minimum VSWR is located, I often get surprises. If you had a sweep generator, direction coupler, detector, and scope, you could see the VSWR sweep change as the antenna is waved around. The calculations are generally under near ideal conditions. Real installation tend to be not so ideal. My antenna should center around 125MHz, as the most interesting frequencies, Tower, Ground, tend to group at the lower end of the airband spectrum. Yep. However, I think you'll find that with a receive only antenna, you can have quite a bit of VSWR and it will still work just fine. Therefore, the antenna cuts and trims are far less critical for a receive antenna than one that's also used for transmit. You might be trying to over-optimize the design without any obvious benefit. I understand that the bigger the diameter of the tubing is, the wider the antenna's bandwidth. True. What would be a good trade-off diameter? Will the resulting bandwidth be good enough? Well, the -3dB bandwidth of the aircraft band is about: 20/150 = 0.15 = 15% The -3db bandwidth of my 2m J-Pole is about: http://802.11junk.com/jeffl/crud/144-Jpole-50-VSWR.jpg 17/146 = 0.08 = 12% I could recalculate the antenna for 125Mhz, but I'm lazy. The percent bandwidth at 125 would be the same as at 146Mhz. The smaller bandwidth for the J-Pole would seem to be a problem, but isn't because receiver can tolerate a higher VSWR without (much) loss in gain. I am leaning towards 3/8" as happy medium, unless one of you tells me I'm crazy. You're crazy. How will you know that your antenna is actually optimized? Without test equipment, you don't have much of a chance. Better to build something really simple, where the tolerances are not as important or critical. Anyway, I won't solder any elbows at the two ends; I will loop this tubing using a bending spring. Should I compensate for these round ends in my length calculations, or won't that matter? How critical is the distance between the two parallel sides? I can't find any information on that either. Can I leave the air gap bare, or should I treat the tube endings to some kind of solid, perhaps adjustable, plug? The only way I know how to make sure your estimates are correct is to either construct an NEC2 model, or to just build it and measure the antenna gain and VSWR. Since a computer model tends to be less expensive than copper, methinks that would be best. I'll devise a slide for the coax connection so I can adjust it for best matching. Cut-n-Try? Well, good luck. I am a complete novice when it comes to building antennas, and for now can only contribute my thanks for your advice. Well, you're off to a bad start. Got any RF test equipment? -- Jeff Liebermann 150 Felker St #D http://www.LearnByDestroying.com Santa Cruz CA 95060 http://802.11junk.com Skype: JeffLiebermann AE6KS 831-336-2558 |
Building Slim Jim for Air Band
In message , Michiel Kappeyne
writes Hi all, New to this forum, I am so impressed with the level of expertise I find here. Thanks for welcoming me. I want to build a Slim Jim antenna for receiving airband (118-136MHz) out of 3/8" or 1/2" soft copper tubing. This antenna won't see any ouside use--it will hang away from any metal against my window facing the Hudson River towards Newark Airport, 15 miles away. Amzingly, my little 8" helical rubber duck works as well as a ladder cable-based 125MHz Slim Jim I bought, so I want to step it up...! There are many, many stories floating around the Internet about homebrew Slim Jim projects, but what strikes me is that everybody seems to arrive at different lengths for a 3/4 wave in the 144MHz band (where I find most Slim Jims), even allowing for differing propagation speeds in the various materials these builders use. What am I missing here? I thought the calculation was as straightforward as the wave length in open air adjusted by the velocity factor, about 0.96 for copper tubing. My antenna should center around 125MHz, as the most interesting frequencies, Tower, Ground, tend to group at the lower end of the airband spectrum. I understand that the bigger the diameter of the tubing is, the wider the antenna's bandwidth. What would be a good trade-off diameter? Will the resulting bandwidth be good enough? I am leaning towards 3/8" as happy medium, unless one of you tells me I'm crazy. Anyway, I won't solder any elbows at the two ends; I will loop this tubing using a bending spring. Should I compensate for these round ends in my length calculations, or won't that matter? How critical is the distance between the two parallel sides? I can't find any information on that either. Can I leave the air gap bare, or should I treat the tube endings to some kind of solid, perhaps adjustable, plug? I'll devise a slide for the coax connection so I can adjust it for best matching. I am a complete novice when it comes to building antennas, and for now can only contribute my thanks for your advice. Some of the guys in this NG seem to be making things unnecessarily complicated and confusing. The Slim Jim / J-Pole is essentially just a halfwave, endfed via a quarterwave stub (in HF terms, a "Zepp" antenna). It's a pretty good antenna. For receiving, the dimensions shouldn't be that critical. However, a quick Google brings up this calculator: http://www.m0ukd.com/Calculators/Slim_Jim/index.php -- Ian |
Building Slim Jim for Air Band
On Sat, 2 Feb 2013 22:46:41 +0000, Ian Jackson
wrote: Some of the guys in this NG seem to be making things unnecessarily complicated and confusing. That would be me. Thanks. The Slim Jim / J-Pole is essentially just a halfwave, endfed via a quarterwave stub (in HF terms, a "Zepp" antenna). Sorta. The original Zepp antenna was intended for use on a Zeppelin dirigible, because the entire antenna is at DC ground and the voltage peak is as far away from the gas bags as possible. It was in the form of a J-Pole, with the 5/8 wave section dangling downward. I just Googled for pictures of Zeppelins but never found one showing a trailing Zepp antenna. I don't have a clue how the Slim Jim design arrived, but my guess(tm) is that it coincided when people started making J-Poles out of twinlead, and found that the extra wire would be tolerated. Using Google (search by date range), it seems to have arrived in about 1990. It's a pretty good antenna. I prefer a folded dipole and balun or a collinear dipole (as in AMOS/Franklin) and balun antenna. I consider the complexity of construction about the same, although I will concede that end fed antennas are much easier to mount than center feed antennas. For receiving, the dimensions shouldn't be that critical. Agreed. You could make the antenna out of barbed wire and it will work. The problem is that you won't know how good or bad the antenna operates without measurements, simulations, or comparisons. For comparison, I carry a simple telescoping dipole with me. When someone claims that they're "system" isn't working well, I compare it with the dipole. I'm often impressed with the ability of some antennas (mostly mobile antennas) to send most of the RF towards the sky, while leaving little towards the horizon. However, that shouldn't matter here, where a hemispherical pattern, that covers the entire sky, is the goal. Maybe a discone? However, a quick Google brings up this calculator: http://www.m0ukd.com/Calculators/Slim_Jim/index.php Ahem. Although the author mentions using twinlead and ladder line, there's nothing in the calculator to compensate for the velocity factor. The design shown is apparently for one built out of rod and tubing. Incidentally, our local radio club (K6BJ) had an antenna construction session during the Oct 2011 meeting. The theme was how to build an emergency antenna. The Slim Jim variety was the most common form. There were numerous construction articles found online and in books and magazines. Various pieces of test equipment were available. I didn't participate because I wasn't sure I would arrive on time, but eventually showed up near the end. I could almost cry. Most of the antennas were hung from the suspended ceiling, which was metal. The most sensitive point of the Slim Jim is near the top, causing frequency sweeps to change drastically when moved. Several people used the same design, the same roll of ladderline, but had various interpretations of how to cut the antenna. That had some effect on VHF, but was fatal at UHF. I gave a fast demonstration of how proximity to metal, and construction variables can have a big effect. There was plenty of head scratching and head shaking, but eventually everyone threw together something that worked. I had an entertaining diversion diagnosing a very nicely built Slim Jim, that had conductive shrink tube slipped over the gap. The meeting was over before I had a chance to connect these antennas to an HT and run a live test to a distant repeater. My guess(tm) is that they wall would have worked, but to varying degrees. -- Jeff Liebermann 150 Felker St #D http://www.LearnByDestroying.com Santa Cruz CA 95060 http://802.11junk.com Skype: JeffLiebermann AE6KS 831-336-2558 |
Building Slim Jim for Air Band
On Saturday, February 2, 2013 6:10:44 PM UTC-6, Jeff Liebermann wrote:
I'm often impressed with the ability of some antennas (mostly mobile antennas) to send most of the RF towards the sky, while leaving little towards the horizon. However, that shouldn't matter here, where a hemispherical pattern, that covers the entire sky, is the goal. Maybe a discone? Dunno.. I've done a lot of air band listening, and myself, I think it's best to concentrate of the tower, and letting the airborne chips fall as they may. Unless one is real close to the airport, the tower, ATIS, and other ground signals will usually be the toughest to hear. The idea of using a *short* yagi actually works quite well in most cases. You can hear the ground stations, and still most of the others in the air also. Most of the airborne signals will be fairly stout, and a rubber ducky would pick them up. The more gain the yagi has, and the tighter the pattern, the better the tower, but the worse the airborne. This is why I would stick with short 2-3 el yagi's if going that route. You don't want too much gain. I've got a simple 3 el yagi cut for 2m, and I'd often use it for air band. I could tweak the direction for best tower signals, and with the antenna out of tune, the pattern is not sharp, and would pick up most everything well enough. Using that, I could receive ACARS from cruising jets up to about 300 miles. From Houston, to about the OK border or so.. I could follow Southwest jets from KHOU to KDAL, and I could hear them going into Dallas until they descended to about 9000 feet or so. And that's with the 2m yagi in no particular direction, or aimed to favor Hobby.. With an out of tune yagi, sometimes the max signal does not jive with the normal direction.. Anyway, I've found that it's best to favor the ground signals, and not worry much about airborne. You will hear those with little trouble. And this is the main reason I take the decoupling into consideration if using a simple vertical. You will want good decoupling to do well on the low angle ground signals. A sky warmer will not be your friend in this case. |
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