| Page 1 of 2 | 1 | 2 |
|
|
Open Stub fed J antenna
Does anyone know of where I could find an example of a homebrew open stub
fed j antenna. I want to construct a unit for 915 MHz using SMA plug and coaxial cable. Thanks in advance |
David wrote:
Does anyone know of where I could find an example of a homebrew open stub fed j antenna. I want to construct a unit for 915 MHz using SMA plug and coaxial cable. An open stub J-pole is the same as a Zepp. 1/2WL element end fed by 1/4WL balanced series section. The coax connects to the end of the series section. Arrow Antenna makes one for 2m. Plans are available that could possibly be scaled to 915 MHz. -- 73, Cecil http://www.qsl.net/w5dxp ----== Posted via Newsfeeds.Com - Unlimited-Uncensored-Secure Usenet News==---- http://www.newsfeeds.com The #1 Newsgroup Service in the World! 100,000 Newsgroups ---= East/West-Coast Server Farms - Total Privacy via Encryption =--- |
Cecil,
Thanks for the info. The open stub J looks like it should be suitable for my application being ground independent, omni directional and having low radiation angle. Cecil Moore wrote: David wrote: Does anyone know of where I could find an example of a homebrew open stub fed j antenna. I want to construct a unit for 915 MHz using SMA plug and coaxial cable. An open stub J-pole is the same as a Zepp. 1/2WL element end fed by 1/4WL balanced series section. The coax connects to the end of the series section. Arrow Antenna makes one for 2m. Plans are available that could possibly be scaled to 915 MHz. |
Dave Have you considered making a coaxial version of the J-pole? http://www.ansoft.com/news/articles/04.05_MWJ.pdf Jerry "David" wrote in message ... Cecil, Thanks for the info. The open stub J looks like it should be suitable for my application being ground independent, omni directional and having low radiation angle. Cecil Moore wrote: David wrote: Does anyone know of where I could find an example of a homebrew open stub fed j antenna. I want to construct a unit for 915 MHz using SMA plug and coaxial cable. An open stub J-pole is the same as a Zepp. 1/2WL element end fed by 1/4WL balanced series section. The coax connects to the end of the series section. Arrow Antenna makes one for 2m. Plans are available that could possibly be scaled to 915 MHz. |
David wrote:
Thanks for the info. The open stub J looks like it should be suitable for my application being ground independent, omni directional and having low radiation angle. Be sure to include a choking function at the feedpoint. -- 73, Cecil http://www.qsl.net/w5dxp ----== Posted via Newsfeeds.Com - Unlimited-Uncensored-Secure Usenet News==---- http://www.newsfeeds.com The #1 Newsgroup Service in the World! 120,000+ Newsgroups ----= East and West-Coast Server Farms - Total Privacy via Encryption =---- |
Thanks for the feedback everyone.
The article Jerry pointed me to starts looking like the commercial antenna I was initially studying. Their unit though only had a 1/4 wave above the coaxial sleeve. From the SMA plug, the coax coiled around the radome must have been the choke to help reduce RF currents radiating from the earth braid. Then the 1/4 wave length coax forms the match followed by the expose radiating section. It looks like an open stub fed J-pole where the stub is enclosing the inner element rather than being constructed as a rod next to it. Does this seem reasonable ? (PS. I'm a beginner with regards to antenna theory and would like to understand what is happening so that I can experiment with some kind degree of success). Cecil Moore wrote: David wrote: Thanks for the info. The open stub J looks like it should be suitable for my application being ground independent, omni directional and having low radiation angle. Be sure to include a choking function at the feedpoint. |
In article ,
David wrote: The article Jerry pointed me to starts looking like the commercial antenna I was initially studying. Their unit though only had a 1/4 wave above the coaxial sleeve. From the SMA plug, the coax coiled around the radome must have been the choke to help reduce RF currents radiating from the earth braid. Then the 1/4 wave length coax forms the match followed by the expose radiating section. It looks like an open stub fed J-pole where the stub is enclosing the inner element rather than being constructed as a rod next to it. Does this seem reasonable ? (PS. I'm a beginner with regards to antenna theory and would like to understand what is happening so that I can experiment with some kind degree of success). Yes, it does. Take a look at the following: http://download.antennex.com/hws/ws1002/sperrtof.pdf A Sperrtof, in effect, is a J-pole whose matching section is a coaxial tube rather than a single rod or wire. It sounds rather like what you're describing. As with all such (I think), the radiating section is 1/2 wavelength long, give or take a smidge, and behaves as an end-fed 1/2-wave dipole. The matching section isn't supposed to radiate significantly. The overall radiation pattern would, I expect, be essentially the same as other J-poles and other end-fed 1/2-wave radiators - similar to a center-fed 1/2-wave dipole, but tilted a bit "upwards" away from the feedpoint. You can distinguish a Sperrtof-type antenna from one of the coaxial dipoles Jerry referred you to, by the length of the single-wire radiator - it's 1/2-wave for a Sperrtof and 1/4-wave for a coaxial dipole (which is really a center-fed dipole). There's an interesting dual-band 2m/440 antenna which was written up in QST in October 2000 - ARRL members can get the article at http://www.arrl.org/members-only/tis...df/0010050.pdf It's interesting because it's _called_ a J-pole, _looks_ like a J-pole... but electrically it isn't. It's actually a center-fed vertical, not a Zepp. The stub at the bottom acts as a choke/decoupler, not as an impedance transformer. -- Dave Platt AE6EO Hosting the Jade Warrior home page: http://www.radagast.org/jade-warrior I do _not_ wish to receive unsolicited commercial email, and I will boycott any company which has the gall to send me such ads! |
Dave,
If I scale that up to 915MHz, the dimensions are almost perfect for using RG58 coax instead of the copper tubing.The outer tube would be 5.5mm and inner conductor 1.6mm. Being 50 Ohm coax, I assume therefore I do not need to slide the tap point up as their design did. I suppose I should also add the choke coil at the antenna base for additional isolation of ground currents. The article mentions the top part as 1/2 wave and bottom at 1/4 wave. If the bottom part of their design is 505mm then the antenna must be used for 2m band. If this is the case, how come the radiating element is 1480 long ? The Coaxial antenna mentioned in the article mentioned by Jerry looks good. The version they mention with a GAP would be simple to construct. It is a 1/4 wave radiating element at the top (earth braid stripped back), then 1/4 wave of full coax, then a "small" gap, then the transmission line. They do not mention the size of the gap but I assume any small gap has the same effect of isolating the reverse current ? Should the actual lengths of these sections be modified by "K" depending on diameter of conductor or are they exact 1/4 wave length cuts ? Thanks heaps. Dave Platt wrote: In article , David wrote: The article Jerry pointed me to starts looking like the commercial antenna I was initially studying. Their unit though only had a 1/4 wave above the coaxial sleeve. From the SMA plug, the coax coiled around the radome must have been the choke to help reduce RF currents radiating from the earth braid. Then the 1/4 wave length coax forms the match followed by the expose radiating section. It looks like an open stub fed J-pole where the stub is enclosing the inner element rather than being constructed as a rod next to it. Does this seem reasonable ? (PS. I'm a beginner with regards to antenna theory and would like to understand what is happening so that I can experiment with some kind degree of success). Yes, it does. Take a look at the following: http://download.antennex.com/hws/ws1002/sperrtof.pdf A Sperrtof, in effect, is a J-pole whose matching section is a coaxial tube rather than a single rod or wire. It sounds rather like what you're describing. As with all such (I think), the radiating section is 1/2 wavelength long, give or take a smidge, and behaves as an end-fed 1/2-wave dipole. The matching section isn't supposed to radiate significantly. The overall radiation pattern would, I expect, be essentially the same as other J-poles and other end-fed 1/2-wave radiators - similar to a center-fed 1/2-wave dipole, but tilted a bit "upwards" away from the feedpoint. You can distinguish a Sperrtof-type antenna from one of the coaxial dipoles Jerry referred you to, by the length of the single-wire radiator - it's 1/2-wave for a Sperrtof and 1/4-wave for a coaxial dipole (which is really a center-fed dipole). There's an interesting dual-band 2m/440 antenna which was written up in QST in October 2000 - ARRL members can get the article at http://www.arrl.org/members-only/tis...df/0010050.pdf It's interesting because it's _called_ a J-pole, _looks_ like a J-pole... but electrically it isn't. It's actually a center-fed vertical, not a Zepp. The stub at the bottom acts as a choke/decoupler, not as an impedance transformer. |
David wrote:
If I scale that up to 915MHz, the dimensions are almost perfect for using RG58 coax ... I wouldn't use RG58 for anything above HF. It has 20 dB matched line loss per 100 ft at that frequency. -- 73, Cecil http://www.qsl.net/w5dxp ----== Posted via Newsfeeds.Com - Unlimited-Uncensored-Secure Usenet News==---- http://www.newsfeeds.com The #1 Newsgroup Service in the World! 120,000+ Newsgroups ----= East and West-Coast Server Farms - Total Privacy via Encryption =---- |
So for the 1/4 wave (81mm) section of the antenna the loss would be
about 0.05dB ? at 915 MHz. I typically have between 150mm (When antenna mounted on enclosure) and 2m length (when antenna external to enclosure) cable between transmitter and Antenna. (Around 1.2dB max. loss in coax between transmitter and antenna). Would this usually be acceptable or do you aim for much lower loss in the transmission line ? Max. power is 1 Watt. Most of the transceivers I play around with are around 10mW. Thanks Cecil Moore wrote: David wrote: If I scale that up to 915MHz, the dimensions are almost perfect for using RG58 coax ... I wouldn't use RG58 for anything above HF. It has 20 dB matched line loss per 100 ft at that frequency. |
"David" wrote in message ... So for the 1/4 wave (81mm) section of the antenna the loss would be about 0.05dB ? at 915 MHz. 20dB/100' MATCHED line loss. W4OP |
David wrote:
So for the 1/4 wave (81mm) section of the antenna the loss would be about 0.05dB ? at 915 MHz. Depends upon whether you leave the insulation on the radiating part or not. But what I was concerned about is the transmission line. You lose about half of your power every 15 feet. Until your last posting, you didn't tell us the length of the transmission line but personally I find 1.2 dB matched line loss in six feet to be unacceptable. Any SWR above 1:1 and the losses are even greater. Cecil Moore wrote: I wouldn't use RG58 for anything above HF. It has 20 dB matched line loss per 100 ft at that frequency. -- 73, Cecil http://www.qsl.net/w5dxp ----== Posted via Newsfeeds.Com - Unlimited-Uncensored-Secure Usenet News==---- http://www.newsfeeds.com The #1 Newsgroup Service in the World! 100,000 Newsgroups ---= East/West-Coast Server Farms - Total Privacy via Encryption =--- |
In article ,
David wrote: Dave, If I scale that up to 915MHz, the dimensions are almost perfect for using RG58 coax instead of the copper tubing.The outer tube would be 5.5mm and inner conductor 1.6mm. Being 50 Ohm coax, I assume therefore I do not need to slide the tap point up as their design did. Bad assumption, I think. The portion of the matching section between the tap point and the bottom is a shorted stub, which adds some inductive reactance at the feedpoint. If you don't have it, you won't get a good match. Another approach I've seen to creating the necessary shorted stub is to just use RG-58 and a simple BNC "T" connector. If you look at the Sperrtopf antenna design, picture it made this way: - one long piece of RG-58 1380 mm long, with the upper part of the outer shield removed... that's your radiator, and the portion of the matching section above the feedpoint. - A second section of RG-58, about 100 mm long, with one end shorted. That's the stub. - The feedline RG-58. Now, just connect all three together... radiator/matching section, stub, and feedline. You can solder 'em, or add BNC plugs and use a BNC "T". If you do the latter, remember to include the lengths of the plugs and "T" into account. I suppose I should also add the choke coil at the antenna base for additional isolation of ground currents. Certainly would not hurt to do so. You could either coil the coax below the bottom of the matching section, or add a quarter-wave choke sleeve, or just run the coax through a few ferrite beads. The article mentions the top part as 1/2 wave and bottom at 1/4 wave. If the bottom part of their design is 505mm then the antenna must be used for 2m band. If this is the case, how come the radiating element is 1480 long ? I think there's some confusion in the antenna schematic diagram, compared with the text and parts list. The diagram appears to show the upper (radiating) portion of the center element being 1480 mm, but the parts list indicates that the *total* length of the inner element is 1480 mm. The latter seems more correct to me... it'd give you a radiating element of (1480 - 505) = 975 mm, which is a bit less than 1/2 physical wavelength at 2 meters. This seems reasonable to me given that the radiating element is fairly thick (10mm) rather than a thin wire. The Coaxial antenna mentioned in the article mentioned by Jerry looks good. The version they mention with a GAP would be simple to construct. It is a 1/4 wave radiating element at the top (earth braid stripped back), then 1/4 wave of full coax, then a "small" gap, then the transmission line. They do not mention the size of the gap but I assume any small gap has the same effect of isolating the reverse current ? I'm not clear about the gap configuration either. The article sort of implies that its size has to be set experimentally, so that the leakage current through the gap is the right amount to cancel out the signal propagating back down the outside of the upper section. The article doesn't say whether the gap is supposed to go around the full circumference of the coax (completely isolating the upper and lower portions of the shield) or only partially around... I infer that it's the latter, because I don't see how the antenna could work with the shield sections fully separated. Should the actual lengths of these sections be modified by "K" depending on diameter of conductor or are they exact 1/4 wave length cuts ? Well, I think it's a bit of a compromise. The upper section (the coax center conductor and its surrounding dielectric) is going to have a velocity factor of not much less than 1.0. Cutting to 1/4 physical wavelength, and then trimming a bit, seems likely to work. I think the lower section is likely to be a bit trickier, since there are actually two velocity factors involved. The radiating is done by the RF travelling back down the outside of the coax braid from the feedpoint, and this (like the upper section) will have a velocity factor close to 1.0, which suggests that the gap-to-feedpoint distance should be close to 1/4 physical wavelength. HOWEVER: the signal travelling up the *inside* of the braid, like that travelling up the inner conductor towards the feedpoint, will be travelling more slowly - the velocity factor will be somewhere around ..66 - .8 depending on cable type. This means that there will be more than 180 degrees of phase delay between the signal passing the gap going upwards (inside the cable) and the signal reaching the gap from above (travelling back down the outside of the cable). Hence, the gap leakage won't result in full cancellation of the signal travelling down the outside of the coax past the gap. To get the two signals into accurate 180-degree phase opposition, you'd need to make the distance from the feedpoint to the gap a bit less than 1/4 physical wavelength, but a bit more than 1/4 of the coax's usual electrical wavelength. Splitting the difference might work fairly well. I'm not sure what this will do to the antenna's feedpoint impedance / return loss, though, and it'll probably tilt the antenna's radiation pattern somewhat. You will probably need to experiment to get the best compromise between antenna pattern and feedpoint impedance / return loss / SWR. The same is true of the gap size and configuration. -- Dave Platt AE6EO Hosting the Jade Warrior home page: http://www.radagast.org/jade-warrior I do _not_ wish to receive unsolicited commercial email, and I will boycott any company which has the gall to send me such ads! |
|
|
Suggest you do a google search for baluns coupled with each of the following
Maxwell, Cebik, and Lewellen. You will become enlightened grasshopper... wrote in message ... On Thu, 15 Sep 2005 18:37:48 -0000, (Dave Platt) wrote: = If the antenna feed point is a matched condition what is the RF on the coax shield from? It should be very small. |
Fred W4JLE wrote:
Suggest you do a google search for baluns coupled with each of the following Maxwell, Cebik, and Lewellen. . . . Searching for Lewallen will probably be more fruitful than Lewellen. Roy Lewallen, W7EL |
Mea Culpa, my fingers slipped when typing, and I in a senior moment failed
to proof. In any event your piece on baluns is part of my reference library. "Roy Lewallen" wrote in message ... Fred W4JLE wrote: Suggest you do a google search for baluns coupled with each of the following Maxwell, Cebik, and Lewellen. . . . Searching for Lewallen will probably be more fruitful than Lewellen. Roy Lewallen, W7EL |
Thanks everyone for your feedback to my posting.
I have now been given a lot of information but am starting to go around in circles. It seems many people are used to operating at lower frequencies with huge lengths of wire and hundred of watts of power. In my application, I am looking for an omni-directional antenna with reasonable "gain" and fairly low radiation angle. The application operates with Data at 920 MHz where power levels are below 1 watt. The antenna must either fix directly to the enclosure (which can be plastic or metal and is usually only around 100mm x 60mm x 40mm) or within 2-3m of the enclosure. Therefore, a unit that does not require ground plane style antenna would be good. The end-fed antenna style is good because I can build it into the end of a SMA connector and use 1/4" nylon tubing as the Radome. Now I just need what goes in the Radome to complete the unit. It is difficult to install Tee BNC connectors and mid tap points on coils or transmission lines etc. Stripping coax cable and coiling the coax on the outside of the Radome is fine as I can cover the unit later with heatshrink. In my search it seemed the J-pole was a good contender due to its performance and low radiation angle and ground independence. The shorted stub version was troublesome but the open stub looked promising. It appeared as though I could simply use coaxial cable to create the antenna and fold back part of the coax over 1/4 wave section. Some have suggested coiling cable to act as a choke at the feed point and others have mentioned not to do this. Would anyone by chance have details of a whip antenna that is proven to work and is similar to what I am trying to construct that I could use as a starting point ? Thanks in advance. Roy Lewallen wrote: Fred W4JLE wrote: Suggest you do a google search for baluns coupled with each of the following Maxwell, Cebik, and Lewellen. . . . Searching for Lewallen will probably be more fruitful than Lewellen. Roy Lewallen, W7EL |
David wrote:
Would anyone by chance have details of a whip antenna that is proven to work and is similar to what I am trying to construct that I could use as a starting point ? In "Antennas For All Applications", by Kraus and Marhefka, third edition, page 824, a 1/2WL over 1/4WL collinear array vertical is described that allegedly has a gain of 6.4 dBi. There is a 180 deg. phase reversing coil between the bottom 1/4WL and the top 1/2WL. The entire vertical should be less than one foot long. The 180 deg. phase reversing coil is the tricky part. -- 73, Cecil http://www.qsl.net/w5dxp ----== Posted via Newsfeeds.Com - Unlimited-Uncensored-Secure Usenet News==---- http://www.newsfeeds.com The #1 Newsgroup Service in the World! 100,000 Newsgroups ---= East/West-Coast Server Farms - Total Privacy via Encryption =--- |
Cecil,
Thanks for the book reference. I searched amazon for it but they are charging $187 for it. Do you know where I might find a second hand copy ? Regards David Cecil Moore wrote: David wrote: Would anyone by chance have details of a whip antenna that is proven to work and is similar to what I am trying to construct that I could use as a starting point ? In "Antennas For All Applications", by Kraus and Marhefka, third edition, page 824, a 1/2WL over 1/4WL collinear array vertical is described that allegedly has a gain of 6.4 dBi. There is a 180 deg. phase reversing coil between the bottom 1/4WL and the top 1/2WL. The entire vertical should be less than one foot long. The 180 deg. phase reversing coil is the tricky part. |
Cecil, W5DXP wrote:
"The 180 deg. phase reversing coil is the tricky part." For UHF, you might prefer to use a 1/4-wave short-circuited stub in place of a coil to reverse the phase. My 19th edition of the ARRL Antenna Book shows such an antenna, "the super J-pole on page 16-25. At other frequencies, this might be called a "Franklin Antenna". It`s a 1/2-wave in-phase with another 1/2-wave, one mounted directly over the other. The super J-pole is designed for 144 MHz, but can be scaled for any frequency with proper mechanical allowances. Gain is about 6 dB over a 1/4-wave whip. Best regards, Richard Harrison, KB5WZZI |
Richard,
I have the 20th edition. Looking at the picture, I would have difficulty building this into the end of a SMA plug to sit on top of the radio modem. Regards David Richard Harrison wrote: Cecil, W5DXP wrote: "The 180 deg. phase reversing coil is the tricky part." For UHF, you might prefer to use a 1/4-wave short-circuited stub in place of a coil to reverse the phase. My 19th edition of the ARRL Antenna Book shows such an antenna, "the super J-pole on page 16-25. At other frequencies, this might be called a "Franklin Antenna". It`s a 1/2-wave in-phase with another 1/2-wave, one mounted directly over the other. The super J-pole is designed for 144 MHz, but can be scaled for any frequency with proper mechanical allowances. Gain is about 6 dB over a 1/4-wave whip. Best regards, Richard Harrison, KB5WZZI |
On Fri, 16 Sep 2005 02:26:25 GMT, David wrote:
Do you know where I might find a second hand copy ? Hi David, Amazon is possibly the worst source for used titles. Instead, try: http://www.bookfinder.com/search/?ac...71 2546_2:2:2 where there are roughly 40 copies available ranging from $28 to Amazon's gold plated offer. 73's Richard Clark, KB7QHC |
Richard Harrison wrote:
Cecil, W5DXP wrote: "The 180 deg. phase reversing coil is the tricky part." For UHF, you might prefer to use a 1/4-wave short-circuited stub in place of a coil to reverse the phase. My 19th edition of the ARRL Antenna Book shows such an antenna, "the super J-pole on page 16-25. At other frequencies, this might be called a "Franklin Antenna". It`s a 1/2-wave in-phase with another 1/2-wave, one mounted directly over the other. The stub is a good idea and can be mechanically self-supporting at 920 MHz. However, making the bottom section 1/4WL (as Kraus suggests) instead of 1/2WL would make for a low feedpoint impedance The super J-pole is designed for 144 MHz, but can be scaled for any frequency with proper mechanical allowances. Gain is about 6 dB over a 1/4-wave whip. -- 73, Cecil http://www.qsl.net/w5dxp ----== Posted via Newsfeeds.Com - Unlimited-Uncensored-Secure Usenet News==---- http://www.newsfeeds.com The #1 Newsgroup Service in the World! 100,000 Newsgroups ---= East/West-Coast Server Farms - Total Privacy via Encryption =--- |
Searching Maxwell on baluns on Google will turn up mostly baluns for
sale by various dealers, not how they work. Searching Lewallen will turn up good data in answering your question on how does the current get onto the outside of the shield braid. If you want Maxwell's data on how the current gets there go to his web page at www.w2du.com, Click on 'Read Chapters from Reflections 2', and then click on Chapter 21, "Some Aspects of the Balun Problem." Walt, W2DU On Thu, 15 Sep 2005 17:57:11 -0400, "Fred W4JLE" wrote: Suggest you do a google search for baluns coupled with each of the following Maxwell, Cebik, and Lewellen. You will become enlightened grasshopper... |
David wrote:
Do you know where I might find a second hand copy ? Google "used books". I've had good luck with www.abebooks.com I have also run across foreign published books that are about half priced and new. Don't recall the URL. But I've given you all the information in Kraus' book about that particular antenna. Laid on its side: | |----------/////-------------------- | 1/4WL coil 1/2WL The coil is a 180 deg. phase reversing coil. If I remember correctly, that means it is self-resonant. If that's right, it probably means that a trap would work instead of a coil. The impedance at the bottom and top of the coil is very high so it seems to me that instead of blocking the signal, like an ordinary trap, a trap would merely reverse the phase. -- 73, Cecil, http://www.qsl.net/w5dxp ----== Posted via Newsfeeds.Com - Unlimited-Uncensored-Secure Usenet News==---- http://www.newsfeeds.com The #1 Newsgroup Service in the World! 100,000 Newsgroups ---= East/West-Coast Server Farms - Total Privacy via Encryption =--- |
Cecil Moore wrote:
I have also run across foreign published books that are about half priced and new. Don't recall the URL. Doing a search on the following webpage for "John Kraus" and "Antennas for all Applications" yields an "as new" international soft cover version for $28.50. http://www.abebooks.com/ It would be nice if everyone on this newsgroup had one of these as a reference. -- 73, Cecil, http://www.qsl.net/w5dxp ----== Posted via Newsfeeds.Com - Unlimited-Uncensored-Secure Usenet News==---- http://www.newsfeeds.com The #1 Newsgroup Service in the World! 100,000 Newsgroups ---= East/West-Coast Server Farms - Total Privacy via Encryption =--- |
Cecil, W5DXP wrote:
"It would be nice if everyone on this newsgtoup had one of these as a refgerence." I second that motion. At $28.50, it is about 1/2 the cost of mine which was a bargain at that. Kraus` "Antennas" is without peer. As Kraus is now dead, there won`t likely be a new improved edition as in the past. Best regards, Richard Harrison, KB5WZI |
"David" wrote in message
... Dave, ... The article [Sperrtopf ] mentions the top part as 1/2 wave ... If this is the case, how come the radiating element is 1480 long ? Dave, If you read his shopping list you see that his drawing is wrong. The _TOTAL_ length of the small pipe is 1480. Still a vertical half wave radiator. I see no one took this part so I will address the question of what these antennas are, but second. I also read your later post describing your application very well and make this comment first. If you want to simply 'build' the antenna from physical dimensions derived from another frequency or antenna or just from "scratch", I see two significant problems. One is that if you have no measuring equipment for 915 MHz, you run a real risk of not getting the size close enough regardless which type of antenna you choose. This could wipe out all your desire to get "reasonable gain". While RF is commonly called "Black magic" there are strays that can be understood only to some extent and therefore they can easily get you off your target. The other is that the plastic "Radome" *will* have an effect on the resonant frequency (lowering the frequency), so comparing it to an "open air" version moves you off your target as well. A common issue relating to both of these is that as you make an antenna with more and more gain, the dimensions become more and more critical to get the best performance. As a conclusion to this I would recommend setting up the system with a simple, good old quarter wave ground plane and see if you really need "reasonable gain". Now, on to the "what-is-this-antenna" question you asked earlier. A "J-Pole" (or as I consider, more correctly) a "J" antenna is a vertical half wave antenna (radiating element). It is fed at the bottom (end fed) with an approximately quarter wave long, shorted line, or matching section, or matching stub - call it what you wish. This matching section is constructed of open, parallel wire transmission line. The match to 50 ohms is obtained by connecting the feed line to a point on the matching section closest to 50 ohms. As a simplification you can look at the classical quarter wave shorted stub. Where the impedance goes from zero at the shorted end to a very high value at the open end. Connecting the high impedance (end of the) half wave radiator to the high impedance end of the stub leads us to conclude that we ought to be able to find any impedance we want (between "high" and zero) somewhere along the length of the stub - and we basically do. .. The length of the matching section and the point where the t-line connects both can be adjusted to get to the best match - though this may not be easy given the construction method used. It is called "J" because when you put the matching section in line with the radiator (and there is nothing requiring this) it has the physical shape of a "J". The matching section could be at right angles to the radiator destroying the "J" shape and the concepts described here are the same (there are antennas done this way). I described it as I did so I can replace the open wire matching section with a coax matching section of the same properties and get the Sperrtopf antenna exactly. So the Sperrtopf can be called a "Sleeve J-Pole" if you like. Or, for those enamored with the "pole", perhaps a "Sleeve-Pole". The "sleeve" antenna, or "sleeve dipole" I also a vertical half wave antenna, but it is center fed . Start with an ordinary half wave, center fed dipole. Turn it vertically. Feed it with coax (yes, I know there is a balanced to un-balanced issue, but stay with me). Now, convert one half of the dipole (one quarter wave length side) from a wire to a pipe. Make this the half which has the coax shield connected to it. Then, take the coax and rather than running it away at 90 degrees, stuff it down the center of the pipe, out the end and to the transmitter. This is the sleeve dipole. Some may consider the sleeve simply to be a "choke" keeping current off the t-line and there are usually several models which can be used to explain one configuration. The referenced Microwaves article shows one type of these, the ordinary sleeve dipole. Looking at Fig 3 and the Appendix B picture, Antenna 3 is clearly the classical vertical sleeve dipole. If I read this correctly, the sleeve dipole closely mimics the ordinary dipole as one would expect. The "spaced gap" antenna (#2) appears close with possibly a little less gain due to the flattened pattern (more radiation at 30 & 150 degrees). Either the #2 or #3 look like they are easy to construct, with the cautions I mentioned above. Finally, the antenna you described taking apart, looks to me like the #1 antenna (Extended Inner" with a choke 1/4 wave back from the shield end. Too bad the Microwave paper didn't try this. 73, Steve, K;9.D,C'I |
"Cecil Moore" wrote in message ... ... But I've given you all the information in Kraus' book about that particular antenna. Laid on its side: | + - + |----------/////-------------------- | 1/4WL coil 1/2WL The coil is a 180 deg. phase reversing coil. [snip] 73, Cecil, Yep and this makes it a "phasing" coil and the antenna a colinear. The phasing coil causes the current in the two sections of the antenna to be in phase. Note my addition of polarity indicators to the above diagram. The right end of both elements are the same = in phase currents. Now you have a half wave and a quarter wave in phase, yielding gain by a flatter pattern (whicn is vertical in this drawing). You can stack half waves up like this and this is done in common cellular gain antennas - probably WiMax as well. There are several ways to do the phasing... Coils like this, quarter-wave shorted stubs (parallel-line), reverse connecting alternate sections of coaxial t-line as elements, phasing harnesses on the feed line... Probably more. 73, Steve, K;9'D.C,I |
I would like to express my appreciation for all those who have taken the
time to respond to my post. Being new to antenna design and construction I understand some of my questions and comments may seem a bit "silly" and it would be easy for people to suggest that I go learn more first or simply use trial and error. I have a practical requirement for a flexible whip antenna that results in "good" gain that works but at the same time I want to learn what I can because it is my intention to later experiment with other style of antenna (Yagi at 434MHz is next on my list). Now, back to the 1/4 wave, phasing coil, 1/2 wave design. Does this means for a practical antenna I could have... SMA plug centre pin - 1/4 length of wire - several turns of wire around the Radome - 1/2 length of wire ? (ie. coax earth braid stops at SMA and rest of antenna is made up of inner conductor and dielectric only). Is it that "simple" ? If so, that would be easy to construct in a flexible whip (which is my aim) compared to many of the other suggestions. Is the gain the same as a dipole ? Also, I have EZNEC 4. but have not had a chance to learn it yet. Can you model the coil in the program or can you only enter segments ? Thanks. Steve Nosko wrote: "Cecil Moore" wrote in message ... ... But I've given you all the information in Kraus' book about that particular antenna. Laid on its side: | + - + |----------/////-------------------- | 1/4WL coil 1/2WL The coil is a 180 deg. phase reversing coil. [snip] 73, Cecil, Yep and this makes it a "phasing" coil and the antenna a colinear. The phasing coil causes the current in the two sections of the antenna to be in phase. Note my addition of polarity indicators to the above diagram. The right end of both elements are the same = in phase currents. Now you have a half wave and a quarter wave in phase, yielding gain by a flatter pattern (whicn is vertical in this drawing). You can stack half waves up like this and this is done in common cellular gain antennas - probably WiMax as well. There are several ways to do the phasing... Coils like this, quarter-wave shorted stubs (parallel-line), reverse connecting alternate sections of coaxial t-line as elements, phasing harnesses on the feed line... Probably more. 73, Steve, K;9'D.C,I |
David wrote:
Is it that "simple" ? If so, that would be easy to construct in a flexible whip (which is my aim) compared to many of the other suggestions. The non-simple part is designing the 180 deg. phase shifting coil. I think I have one of those on my Diamond mobile antenna. Is the gain the same as a dipole ? Higher than a dipole - close to 6 dBi compared to the 1/4WL ground plane at *about* 0 dBi. -- 73, Cecil http://www.qsl.net/w5dxp ----== Posted via Newsfeeds.Com - Unlimited-Uncensored-Secure Usenet News==---- http://www.newsfeeds.com The #1 Newsgroup Service in the World! 120,000+ Newsgroups ----= East and West-Coast Server Farms - Total Privacy via Encryption =---- |
"Richard Harrison" wrote in message ... Cecil, W5DXP wrote: "The 180 deg. phase reversing coil is the tricky part." For UHF, you might prefer to use a 1/4-wave short-circuited stub in place of a coil to reverse the phase. My 19th edition of the ARRL Antenna Book shows such an antenna, "the super J-pole on page 16-25. At other frequencies, this might be called a "Franklin Antenna". It`s a 1/2-wave in-phase with another 1/2-wave, one mounted directly over the other. The super J-pole is designed for 144 MHz, but can be scaled for any frequency with proper mechanical allowances. Gain is about 6 dB over a 1/4-wave whip. Best regards, Richard Harrison, KB5WZZI Richard I'd have expected the "gain" to be closer to 4 1/2 db over the 1/4 wave stub over a ground. is it easy to show where i've missed something? Jerry |
I have constructed a 915 MHz 1/2 wave, end fed antenna that fits onto
the end of a SMA plug based on help from you guys. I have some pics of the construction if anyone is interested in taking a look. Where is the appropriate place to post these jpg files ? I am accessing this newsgroup from my email client presently. I am not sure how I can optimize this yet. My antenna Analyzer is the MJF unit that only goes to 440MHz. I have a 1GHz spec analyzer, 1GHz Sig Gen and a Telonic VSWR kit that goes to 2.5GHz but I think the drive required to the VSWR "Rho-Tector" needs to be higher than the 10dBm from the sig gen. Could I optimize the design with this equipment, or maybe would I be better to use RSSI levels from a receiver placed say 3m away from Antenna under test ? If the antenna works I will be very excited as it is very simple to construct in around 10 minutes and at a cost of around $3 (no counting labour). Thanks for any help. Jerry Martes wrote: "Richard Harrison" wrote in message ... Cecil, W5DXP wrote: "The 180 deg. phase reversing coil is the tricky part." For UHF, you might prefer to use a 1/4-wave short-circuited stub in place of a coil to reverse the phase. My 19th edition of the ARRL Antenna Book shows such an antenna, "the super J-pole on page 16-25. At other frequencies, this might be called a "Franklin Antenna". It`s a 1/2-wave in-phase with another 1/2-wave, one mounted directly over the other. The super J-pole is designed for 144 MHz, but can be scaled for any frequency with proper mechanical allowances. Gain is about 6 dB over a 1/4-wave whip. Best regards, Richard Harrison, KB5WZZI Richard I'd have expected the "gain" to be closer to 4 1/2 db over the 1/4 wave stub over a ground. is it easy to show where i've missed something? Jerry |
David wrote:
I have constructed a 915 MHz 1/2 wave, end fed antenna that fits onto the end of a SMA plug based on help from you guys. How are you matching the very high feedpoint impedance? I have some pics of the construction if anyone is interested in taking a look. Where is the appropriate place to post these jpg files ? Some of us have web pages from qsl.net for that purpose. There is also a netnews group for that purpose. I think it is alt.binaries. -- 73, Cecil http://www.qsl.net/w5dxp ----== Posted via Newsfeeds.Com - Unlimited-Uncensored-Secure Usenet News==---- http://www.newsfeeds.com The #1 Newsgroup Service in the World! 100,000 Newsgroups ---= East/West-Coast Server Farms - Total Privacy via Encryption =--- |
"Cecil Moore" wrote in message ... David wrote: I have constructed a 915 MHz 1/2 wave, end fed antenna that fits onto the end of a SMA plug based on help from you guys. How are you matching the very high feedpoint impedance? I have some pics of the construction if anyone is interested in taking a look. Where is the appropriate place to post these jpg files ? Some of us have web pages from qsl.net for that purpose. There is also a netnews group for that purpose. I think it is alt.binaries. -- 73, Cecil http://www.qsl.net/w5dxp ----== Posted via Newsfeeds.Com - Unlimited-Uncensored-Secure Usenet News==---- http://www.newsfeeds.com The #1 Newsgroup Service in the World! 100,000 Newsgroups ---= East/West-Coast Server Farms - Total Privacy via Encryption =--- Cecil He is feeding the 1/2 wave antenna in the center, where the impedance will be somewhere around 70 ohms. His coax feed line is located within one half of the antenna and exits the 1/2 wave, center excited, dipole at the "high impedance" end of the dipole. David sent me some pictures. The antenna looks quite good. The effectiveness of the "choke" that attempts to disconnect the dipole from the feed line may be less than ideal, but it is yet unknown how good this antenna needs to be. Jerry |
Jerry Martes wrote:
I'd have expected the "gain" to be closer to 4 1/2 db over the 1/4 wave stub over a ground. is it easy to show where i've missed something? I think it should be more like 3 dB, but hadn't said anything until I had a chance to model it. The quarter wave stub doesn't radiate significantly, so it can be ignored. A half wavelength element should have about 1.5 dB gain over a quarter wave. Two of them would give another 3 dB if it weren't for mutual coupling, but the mutual coupling of collinear elements reduces the gain to about 1.5 dB over a single element. (See for example Fig. 39 on p. 8-35 of the ARRL Antenna Book, 20th Edition; look up Collinear, Gain and directivity in the index of other editions; or model it with your favorite program.) Roy Lewallen, W7EL |
"Roy Lewallen" wrote in message ... Jerry Martes wrote: I'd have expected the "gain" to be closer to 4 1/2 db over the 1/4 wave stub over a ground. is it easy to show where i've missed something? I think it should be more like 3 dB, but hadn't said anything until I had a chance to model it. The quarter wave stub doesn't radiate significantly, so it can be ignored. A half wavelength element should have about 1.5 dB gain over a quarter wave. Two of them would give another 3 dB if it weren't for mutual coupling, but the mutual coupling of collinear elements reduces the gain to about 1.5 dB over a single element. (See for example Fig. 39 on p. 8-35 of the ARRL Antenna Book, 20th Edition; look up Collinear, Gain and directivity in the index of other editions; or model it with your favorite program.) Roy Lewallen, W7EL Thanks Roy Your posts always add alot to my understanding. I appreciate your taking time to evaluate this antenna and then pass the information on to the group. Jerry |
Jerry Martes wrote:
"Roy Lewallen" wrote in message I think it should be more like 3 dB, but hadn't said anything until I had a chance to model it. The quarter wave stub doesn't radiate significantly, so it can be ignored. A half wavelength element should have about 1.5 dB gain over a quarter wave. Two of them would give another 3 dB if it weren't for mutual coupling, but the mutual coupling of collinear elements reduces the gain to about 1.5 dB over a single element. (See for example Fig. 39 on p. 8-35 of the ARRL Antenna Book, 20th Edition; look up Collinear, Gain and directivity in the index of other editions; or model it with your favorite program.) Roy Lewallen, W7EL Thanks Roy Your posts always add alot to my understanding. I appreciate your taking time to evaluate this antenna and then pass the information on to the group. Jerry I'm sorry, I see I wasn't clear in saying that I haven't yet had a chance to model it. There might be a flaw in my reasoning, and the antenna might have more gain than I think. I'll try to get to it as soon as I can -- unless someone else wants to take it on. But I'm extremely busy just now. Roy Lewallen, W7EL |
| All times are GMT +1. The time now is 12:56 AM. |
|
Powered by vBulletin® Copyright ©2000 - 2024, Jelsoft Enterprises Ltd.
RadioBanter.com