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Question 1/4 Wave Ground Plane Measurements
I am making a simple 1/4 wave ground plane antenna and have two rather
silly question regarding its measurements. 1. I've attached an SO-239 connecter to a 5x5 inch aluminum plate. When measuring the length of the radials, I measure from the center conductor of the SO-239 to the end of the radial taking into account the plate, correct? 2. If I make a loop in the vertical radiating element so that I may hang the antenna, does the circumference of the loop count in the measurement of the element or the diameter? (e.g., if I need 20 inches for the element, do I measure from the bottom of the SO-239 to the top of the loop or do I take a 20 inch wire and make a loop not caring that it has shortened the element slightly?) Thanks Bam Bam |
#2
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Question 1/4 Wave Ground Plane Measurements
On Jul 25, 11:47 am, Bam Bam wrote:
I am making a simple 1/4 wave ground plane antenna and have two rather silly question regarding its measurements. 1. I've attached an SO-239 connecter to a 5x5 inch aluminum plate. When measuring the length of the radials, I measure from the center conductor of the SO-239 to the end of the radial taking into account the plate, correct? 2. If I make a loop in the vertical radiating element so that I may hang the antenna, does the circumference of the loop count in the measurement of the element or the diameter? (e.g., if I need 20 inches for the element, do I measure from the bottom of the SO-239 to the top of the loop or do I take a 20 inch wire and make a loop not caring that it has shortened the element slightly?) Thanks Bam Bam You need a grid dip meter or a good SWR bridge and get ready to tune "a little" here is why.... The PLATE adds capacitance and that will make the 'need' for your radial lengths to actually wind up shorter than you would calculate. Also the loop at the top of the antenna will add a little effective length because of the 'capacitance' of the loop (see capacitive hats in any antenna book) so rule of thumb ... is go ahead and use the formulas for 1/4 antenna wire, make the antenna that length and then 'prune' to get a better match at the frequency you plan to use. Also, you can change the angle of the radials in respect to the plate to change the impedance of the antenna. If you lower the radials (from straight out 90 deg) you will increase the antennas impedance and vice versa. This is done 'last' after you get the wire as best match after puning. Then experiment with the radials to further match the antenna. Donot worry about getting a 1:1 match. your time will be wasted with little improvement in performance I found less than 2:1 is usually very good and most solid state finals work with this well. Good Luck and have fun ... KD4Q Dean |
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Question 1/4 Wave Ground Plane Measurements
On Jul 25, 11:47 am, Bam Bam wrote:
I am making a simple 1/4 wave ground plane antenna and have two rather silly question regarding its measurements. 1. I've attached an SO-239 connecter to a 5x5 inch aluminum plate. When measuring the length of the radials, I measure from the center conductor of the SO-239 to the end of the radial taking into account the plate, correct? 2. If I make a loop in the vertical radiating element so that I may hang the antenna, does the circumference of the loop count in the measurement of the element or the diameter? (e.g., if I need 20 inches for the element, do I measure from the bottom of the SO-239 to the top of the loop or do I take a 20 inch wire and make a loop not caring that it has shortened the element slightly?) Thanks Bam Bam PS I would only measure to the top of the loop... not counting the 'fold over' that makes up the loop. The wire actually folds back down the main radiator and thus cancels its lenghting effect (except for the small capacitance I talked about). |
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Question 1/4 Wave Ground Plane Measurements
DeanO wrote in news:1185380995.439253.125410
@d55g2000hsg.googlegroups.com: .... PS I would only measure to the top of the loop... not counting the 'fold over' that makes up the loop. The wire actually folds back down the main radiator and thus cancels its lenghting effect (except for the small capacitance I talked about). Isn't this somewhat similar to linear loading... which does lower the resonant frequency? Owen |
#5
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Question 1/4 Wave Ground Plane Measurements
On Jul 25, 9:24 am, DeanO wrote:
Thanks very much for the advice Dean! I took your advice and just cut everything as per the normal calculations. As luck would have it, the first time I fired it up I had a 1:3 SWR. Thanks again. On Jul 25, 11:47 am, Bam Bam wrote: I am making a simple 1/4 wave ground plane antenna and have two rather silly question regarding its measurements. 1. I've attached an SO-239 connecter to a 5x5 inch aluminum plate. When measuring the length of the radials, I measure from the center conductor of the SO-239 to the end of the radial taking into account the plate, correct? 2. If I make a loop in the vertical radiating element so that I may hang the antenna, does the circumference of the loop count in the measurement of the element or the diameter? (e.g., if I need 20 inches for the element, do I measure from the bottom of the SO-239 to the top of the loop or do I take a 20 inch wire and make a loop not caring that it has shortened the element slightly?) Thanks Bam Bam You need a grid dip meter or a good SWR bridge and get ready to tune "a little" here is why.... The PLATE adds capacitance and that will make the 'need' for your radial lengths to actually wind up shorter than you would calculate. Also the loop at the top of the antenna will add a little effective length because of the 'capacitance' of the loop (see capacitive hats in any antenna book) so rule of thumb ... is go ahead and use the formulas for 1/4 antenna wire, make the antenna that length and then 'prune' to get a better match at the frequency you plan to use. Also, you can change the angle of the radials in respect to the plate to change the impedance of the antenna. If you lower the radials (from straight out 90 deg) you will increase the antennas impedance and vice versa. This is done 'last' after you get the wire as best match after puning. Then experiment with the radials to further match the antenna. Donot worry about getting a 1:1 match. your time will be wasted with little improvement in performance I found less than 2:1 is usually very good and most solid state finals work with this well. Good Luck and have fun ... KD4Q Dean- Hide quoted text - - Show quoted text - |
#6
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Question 1/4 Wave Ground Plane Measurements
You mean 1.3:1 SWR?
"Bam Bam" wrote in message oups.com... On Jul 25, 9:24 am, DeanO wrote: Thanks very much for the advice Dean! I took your advice and just cut everything as per the normal calculations. As luck would have it, the first time I fired it up I had a 1:3 SWR. Thanks again. On Jul 25, 11:47 am, Bam Bam wrote: I am making a simple 1/4 wave ground plane antenna and have two rather silly question regarding its measurements. 1. I've attached an SO-239 connecter to a 5x5 inch aluminum plate. When measuring the length of the radials, I measure from the center conductor of the SO-239 to the end of the radial taking into account the plate, correct? 2. If I make a loop in the vertical radiating element so that I may hang the antenna, does the circumference of the loop count in the measurement of the element or the diameter? (e.g., if I need 20 inches for the element, do I measure from the bottom of the SO-239 to the top of the loop or do I take a 20 inch wire and make a loop not caring that it has shortened the element slightly?) Thanks Bam Bam You need a grid dip meter or a good SWR bridge and get ready to tune "a little" here is why.... The PLATE adds capacitance and that will make the 'need' for your radial lengths to actually wind up shorter than you would calculate. Also the loop at the top of the antenna will add a little effective length because of the 'capacitance' of the loop (see capacitive hats in any antenna book) so rule of thumb ... is go ahead and use the formulas for 1/4 antenna wire, make the antenna that length and then 'prune' to get a better match at the frequency you plan to use. Also, you can change the angle of the radials in respect to the plate to change the impedance of the antenna. If you lower the radials (from straight out 90 deg) you will increase the antennas impedance and vice versa. This is done 'last' after you get the wire as best match after puning. Then experiment with the radials to further match the antenna. Donot worry about getting a 1:1 match. your time will be wasted with little improvement in performance I found less than 2:1 is usually very good and most solid state finals work with this well. Good Luck and have fun ... KD4Q Dean- Hide quoted text - - Show quoted text - |
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