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#1
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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 |
#2
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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 =--- |
#3
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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. |
#4
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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. |
#5
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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 =---- |
#6
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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. |
#7
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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! |
#8
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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. |
#9
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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 =---- |
#10
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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. |
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