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Log periodic antenna design
"JIMMIE" wrote in message oups.com... On Jun 28, 1:06 pm, "Jerry Martes" wrote: "JIMMIE" wrote in message ups.com... On Jun 28, 10:36 am, Richard Clark wrote: On Thu, 28 Jun 2007 06:19:44 -0700, JIMMIE wrote: While pondering the design of a broadband antenna that would cover from 50 to 450 Mhz I considered a log periodic array. This would serve my purpose but the size would be inconvienent. Then I had a thought, Could I remove the elements that are not required for amateur band coverage? In my case I am only interested in 50, 144,and 432 MHz bands, Would this allow me to shorten the boom by an amout equal to the space occupied by the removed elements and have a gain of 7Dbd. Could I cover all 3 bands with just the elements for 6 and 2m? Any thoughts on this would be greatly appreciaated. Hi Jimmie, What you are then asking for is NOT a broadband antenna; but rather a multiband antenna, or even three, interlaced single band antennas. Don't try this at home with a gaussian bundle. To answer your question more directly, yes you can remove those elements that do not contribute to the three bands. Unfortunately, to maintain its LDPA aspects, you still have to maintain some outliers for each band. This doesn't necessarily shorten it, because an LPDA's length is driven by many other considerations such as bandwidth and gain, not just simply extremes (yes, that too is part of the mix, simply a complex part of the mix). No to your last question. The matter of resonance is still a first principle. However, the 2M band is loosely related to the 70cM band, and if you are building an LPDA (and sweep the elements, chevron style) there may be a chance of it performing there too. All-in-all, don't expect remarkable gain after having gone through all these exceptions. 73's Richard Clark, KB7QHC Thanks this is what I was expecting but I think I am going to try it anyway. SNIP Jimmie Hi Jimmie Have you figured out which elements to remove and which will remain? Jerry- Hide quoted text - - Show quoted text - In reality it will probably be designed as if it were 2 or 3 single band lpda antennas. Cebik has a couple of designs like that and I will just put them on one boom trying them spead out and with the space removed. For now I am thinking four four element cells for each band. Im also curious to find out how a swept element 2m antenna will work on 70cm. Jimmie Hi Jimmie I dont mean to discourage you from studdying LPDA, but, the "LP" aspect will be completely lost when the antenna is redesigned to operate on only your 3 bands. Correct me if I am wrong. It is my uinderstanding that a Log Periodic Dipole Array is designed to generate a null in the dirrection *away from* where you want to communicate. That means that you want to minimize the radiation in the dirrection from the short elements toward the longer elements. It is also my understanding that the dipoles that do the majority of the radiating are choosen by their ability to match the Zo of the line connecting successive dipoles. The "V" aspect of the LPV is an attempt to take advantage of the fact that the dipoles will have impedances appropriate for matching the line when they are 1 1/2 wave long as well as when they are 1/2 wave long. Each of the V'd, one and a half wave long dipoles will have a radiation pattern that is far different from the halfwave dipole. But, when the "V" angle is correct that resultant pattern is appropriate for this application. It might be, as Richard Clark has pointed out, that you could build a 6 meter band antenna that outperforms the redisigned LPDA. It seems almost certain that antennas for the 2 meter and 70 cm bands can made using *other* design concepts. Jerry |
Log periodic antenna design
"Jerry Martes" wrote in message news:z3Ygi.3873$RZ1.2306@trnddc05... "JIMMIE" wrote in message oups.com... On Jun 28, 1:06 pm, "Jerry Martes" wrote: "JIMMIE" wrote in message ups.com... On Jun 28, 10:36 am, Richard Clark wrote: On Thu, 28 Jun 2007 06:19:44 -0700, JIMMIE wrote: While pondering the design of a broadband antenna that would cover from 50 to 450 Mhz I considered a log periodic array. This would serve my purpose but the size would be inconvienent. Then I had a thought, Could I remove the elements that are not required for amateur band coverage? In my case I am only interested in 50, 144,and 432 MHz bands, Would this allow me to shorten the boom by an amout equal to the space occupied by the removed elements and have a gain of 7Dbd. Could I cover all 3 bands with just the elements for 6 and 2m? Any thoughts on this would be greatly appreciaated. Hi Jimmie, What you are then asking for is NOT a broadband antenna; but rather a multiband antenna, or even three, interlaced single band antennas. Don't try this at home with a gaussian bundle. To answer your question more directly, yes you can remove those elements that do not contribute to the three bands. Unfortunately, to maintain its LDPA aspects, you still have to maintain some outliers for each band. This doesn't necessarily shorten it, because an LPDA's length is driven by many other considerations such as bandwidth and gain, not just simply extremes (yes, that too is part of the mix, simply a complex part of the mix). No to your last question. The matter of resonance is still a first principle. However, the 2M band is loosely related to the 70cM band, and if you are building an LPDA (and sweep the elements, chevron style) there may be a chance of it performing there too. All-in-all, don't expect remarkable gain after having gone through all these exceptions. 73's Richard Clark, KB7QHC Thanks this is what I was expecting but I think I am going to try it anyway. SNIP Jimmie Hi Jimmie Have you figured out which elements to remove and which will remain? Jerry- Hide quoted text - - Show quoted text - In reality it will probably be designed as if it were 2 or 3 single band lpda antennas. Cebik has a couple of designs like that and I will just put them on one boom trying them spead out and with the space removed. For now I am thinking four four element cells for each band. Im also curious to find out how a swept element 2m antenna will work on 70cm. Jimmie Hi Jimmie I dont mean to discourage you from studdying LPDA, but, the "LP" aspect will be completely lost when the antenna is redesigned to operate on only your 3 bands. Correct me if I am wrong. It is my uinderstanding that a Log Periodic Dipole Array is designed to generate a null in the dirrection *away from* where you want to communicate. That means that you want to minimize the radiation in the dirrection from the short elements toward the longer elements. It is also my understanding that the dipoles that do the majority of the radiating are choosen by their ability to match the Zo of the line connecting successive dipoles. The "V" aspect of the LPV is an attempt to take advantage of the fact that the dipoles will have impedances appropriate for matching the line when they are 1 1/2 wave long as well as when they are 1/2 wave long. Each of the V'd, one and a half wave long dipoles will have a radiation pattern that is far different from the halfwave dipole. But, when the "V" angle is correct that resultant pattern is appropriate for this application. It might be, as Richard Clark has pointed out, that you could build a 6 meter band antenna that outperforms the redisigned LPDA. It seems almost certain that antennas for the 2 meter and 70 cm bands can made using *other* design concepts. Jerry I absolutely agree, building a 4 element Yagi for each band of interest would be the most resonable thing to do but I came across a couple of old LPDA TV antennas and I am itching to do something with them. They are both a little beat up but firstI am going to try to make repairs to them and sweep them with my antenna analyzer and graph their impedance. Jimmie |
Log periodic antenna design
On Thu, 28 Jun 2007 21:34:23 -0400, "Jimmie D"
wrote: I came across a couple of old LPDA TV antennas and I am itching to do something with them. They are both a little beat up but firstI am going to try to make repairs to them and sweep them with my antenna analyzer and graph their impedance. Hi Jimmie, Give us the results when you've done that, would you? 73's Richard Clark, KB7QHC |
Log periodic antenna design
Hi Guys
I just noticed you guys talking about impedance of yagi and LPDA tv antennas, I've used a program QY4, I found on the web, and designed a 5 director uhf-band yagi around a center frequency of 636mhz. i used 1/2" copper tubing for the directors, driver and reflector, attached on a 1/4" threaded rod, ofcourse the driver is split to allow the terminals, then i added 4 copper 90s to complete a folded dipole. I am interested in figuring out the impedance as well bridging a 1 director vhf-band yagi around 168mhz also using a folded dipole driver? Would a multimeter or a 20khz oscilliscope help in tweaking the antenna's impedance? Also we have had very interesting results using a paper towel roll and a peice of 22awg wire as a folded dipole. |
Log periodic antenna design
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