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#21
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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. |
#22
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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 |
#23
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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 |
#24
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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 |
#25
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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 =--- |
#26
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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... |
#27
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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 =--- |
#28
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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 =--- |
#29
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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 |
#30
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"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 |
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