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40/15M dipole question
Let's say I build a horizontal dipole, trimmed for minimum SWR @
7.200 mhz. I'll probably use 14-16 gauge insulated house wiring for this antenna. It'd be a "nominal' quarter wavelength per leg. This dipole will be fed with 50 ohm coax, and it will be erected horizontally, 30 feet high over "average" ground. What SWR reading could I expect to see from this antenna if I tried to use it at 21.300 mhz........in the same environment as stated above? Thanks, folks...... FjLee KA0FPJ |
40/15M dipole question
On Apr 6, 5:56*am, " wrote:
What SWR *reading could I expect to see from this antenna if I tried to use it at 21.300 mhz........in the same environment as stated above? EZNEC sez 11:1 with a feedpoint impedance of 100-j200 ohms. -- 73, Cecil, w5dxp.com |
40/15M dipole question
On Wed, 6 Apr 2011 03:56:21 -0700 (PDT), "
wrote: What SWR reading could I expect to see...? 15:1 73's Richard Clark, KB7QHC |
40/15M dipole question
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40/15M dipole question
On 4/6/2011 6:28 AM, Cecil Moore wrote:
On Apr 6, 5:56 am, wrote: What SWR reading could I expect to see from this antenna if I tried to use it at 21.300 mhz........in the same environment as stated above? EZNEC sez 11:1 with a feedpoint impedance of 100-j200 ohms. -- 73, Cecil, w5dxp.com Hey Cecil - I got: Impedance = 85.26 - J 186.3 ohms SWR (50 ohm system) = 10.337 (75 ohm system) = 7.308 Let's see if we can determine why they don't match. I adjusted the wire length for no reactance at 7.2 MHz. I used 99 segments of #14 wire. I did not include insulation effects. How about yours? John |
40/15M dipole question
On 4/6/2011 2:51 PM, John - KD5YI wrote:
On 4/6/2011 6:28 AM, Cecil Moore wrote: On Apr 6, 5:56 am, wrote: What SWR reading could I expect to see from this antenna if I tried to use it at 21.300 mhz........in the same environment as stated above? EZNEC sez 11:1 with a feedpoint impedance of 100-j200 ohms. -- 73, Cecil, w5dxp.com Hey Cecil - I got: Impedance = 85.26 - J 186.3 ohms SWR (50 ohm system) = 10.337 (75 ohm system) = 7.308 Let's see if we can determine why they don't match. I adjusted the wire length for no reactance at 7.2 MHz. I used 99 segments of #14 wire. I did not include insulation effects. How about yours? John Oops! I just noticed that I had zero wire resistance. Changed it to copper but only got a slight change: Impedance = 87.47 - J 184 ohms SWR (50 ohm system) = 9.960 (75 ohm system) = 7.042 |
40/15M dipole question
John - KD5YI wrote:
On 4/6/2011 2:51 PM, John - KD5YI wrote: On 4/6/2011 6:28 AM, Cecil Moore wrote: On Apr 6, 5:56 am, wrote: What SWR reading could I expect to see from this antenna if I tried to use it at 21.300 mhz........in the same environment as stated above? EZNEC sez 11:1 with a feedpoint impedance of 100-j200 ohms. -- 73, Cecil, w5dxp.com Hey Cecil - I got: Impedance = 85.26 - J 186.3 ohms SWR (50 ohm system) = 10.337 (75 ohm system) = 7.308 Let's see if we can determine why they don't match. I adjusted the wire length for no reactance at 7.2 MHz. I used 99 segments of #14 wire. I did not include insulation effects. How about yours? John Oops! I just noticed that I had zero wire resistance. Changed it to copper but only got a slight change: Impedance = 87.47 - J 184 ohms SWR (50 ohm system) = 9.960 (75 ohm system) = 7.042 If you tune it down a bit so you resonate at perhaps 7.15, then the third harmonic will be closer to 21.3. It's a dipole, so not real high Q, particularly on the fundamental, so maybe what you do is go for a compromise tuning.. A bit low on 40 and a bit high on 15. |
40/15M dipole question
On 4/6/2011 7:57 PM, Jim Lux wrote:
John - KD5YI wrote: On 4/6/2011 2:51 PM, John - KD5YI wrote: On 4/6/2011 6:28 AM, Cecil Moore wrote: On Apr 6, 5:56 am, wrote: What SWR reading could I expect to see from this antenna if I tried to use it at 21.300 mhz........in the same environment as stated above? EZNEC sez 11:1 with a feedpoint impedance of 100-j200 ohms. -- 73, Cecil, w5dxp.com Hey Cecil - I got: Impedance = 85.26 - J 186.3 ohms SWR (50 ohm system) = 10.337 (75 ohm system) = 7.308 Let's see if we can determine why they don't match. I adjusted the wire length for no reactance at 7.2 MHz. I used 99 segments of #14 wire. I did not include insulation effects. How about yours? John Oops! I just noticed that I had zero wire resistance. Changed it to copper but only got a slight change: Impedance = 87.47 - J 184 ohms SWR (50 ohm system) = 9.960 (75 ohm system) = 7.042 If you tune it down a bit so you resonate at perhaps 7.15, then the third harmonic will be closer to 21.3. It's a dipole, so not real high Q, particularly on the fundamental, so maybe what you do is go for a compromise tuning.. A bit low on 40 and a bit high on 15. Well, yeah, I know. But, I'm trying to simulate based on the OP's info and Cecil's results. John |
40/15M dipole question
On 4/6/2011 2:51 PM, John - KD5YI wrote:
On 4/6/2011 6:28 AM, Cecil Moore wrote: On Apr 6, 5:56 am, wrote: What SWR reading could I expect to see from this antenna if I tried to use it at 21.300 mhz........in the same environment as stated above? EZNEC sez 11:1 with a feedpoint impedance of 100-j200 ohms. -- 73, Cecil, w5dxp.com Hey Cecil - I got: Impedance = 85.26 - J 186.3 ohms SWR (50 ohm system) = 10.337 (75 ohm system) = 7.308 Let's see if we can determine why they don't match. I adjusted the wire length for no reactance at 7.2 MHz. I used 99 segments of #14 wire. I did not include insulation effects. How about yours? John Are you using 9 segments? John |
40/15M dipole question
On 4/6/2011 8:16 PM, John - KD5YI wrote:
On 4/6/2011 2:51 PM, John - KD5YI wrote: Hey Cecil - I got: Impedance = 85.26 - J 186.3 ohms SWR (50 ohm system) = 10.337 (75 ohm system) = 7.308 Let's see if we can determine why they don't match. I adjusted the wire length for no reactance at 7.2 MHz. I used 99 segments of #14 wire. I did not include insulation effects. How about yours? John Are you using 9 segments? John I think may have missed something on how segmentation works. I thought you needed an even number of segments to split a dipole evenly. So while 99 is close to centered by my logic, if 9 was used it might skew things a bit. If I'm off base please tell me so I can adjust my model of how NEC works. Thanks. tom K0TAR |
40/15M dipole question
On 4/6/2011 8:57 PM, tom wrote:
On 4/6/2011 8:16 PM, John - KD5YI wrote: On 4/6/2011 2:51 PM, John - KD5YI wrote: Hey Cecil - I got: Impedance = 85.26 - J 186.3 ohms SWR (50 ohm system) = 10.337 (75 ohm system) = 7.308 Let's see if we can determine why they don't match. I adjusted the wire length for no reactance at 7.2 MHz. I used 99 segments of #14 wire. I did not include insulation effects. How about yours? John Are you using 9 segments? John I think may have missed something on how segmentation works. I thought you needed an even number of segments to split a dipole evenly. So while 99 is close to centered by my logic, if 9 was used it might skew things a bit. If I'm off base please tell me so I can adjust my model of how NEC works. Thanks. tom K0TAR Hi, Tom - As far as I am aware, using an odd number of segments in a wire allows the source to be placed in the center of a segment with equal numbers of segments on each side. Think of it this way: a source needs to be in the middle of a segment. So, you need an even number of segments on each side to keep everything balanced. If so, then the total number of segments needs to be odd. If this does not make sense, please ask again and I'll try with more detail. Cheers, John |
40/15M dipole question
On Wed, 6 Apr 2011 03:56:21 -0700 (PDT), "
wrote: Let's say I build a horizontal dipole, trimmed for minimum SWR @ 7.200 mhz. I'll probably use 14-16 gauge insulated house wiring for this antenna. It'd be a "nominal' quarter wavelength per leg. This dipole will be fed with 50 ohm coax, and it will be erected horizontally, 30 feet high over "average" ground. What SWR reading could I expect to see from this antenna if I tried to use it at 21.300 mhz........in the same environment as stated above? Thanks, folks...... FjLee KA0FPJ Just put it up and see what happens. Slap a tuner in the line. Or check out a fan dipole. Or buy a pair of 15 meter traps and use the design sheet that comes with them to build a 40-15 trap dipole. http://www.unadilla.com/?p=34 |
40/15M dipole question
On Apr 6, 2:51*pm, John - KD5YI wrote:
How about yours? I used an EZ file that I already had. It was 40 feet high and I rounded off the feedpoint impedance. I thought that would be a close enough approximation. -- 73, Cecil, w5dxp.com |
40/15M dipole question
On 4/6/2011 9:07 PM, John - KD5YI wrote:
Hi, Tom - As far as I am aware, using an odd number of segments in a wire allows the source to be placed in the center of a segment with equal numbers of segments on each side. Think of it this way: a source needs to be in the middle of a segment. So, you need an even number of segments on each side to keep everything balanced. If so, then the total number of segments needs to be odd. If this does not make sense, please ask again and I'll try with more detail. Cheers, John Thanks! I guess I'll have to look into the docs a bit to educate myself. tom K0TAR |
40/15M dipole question
On 4/6/2011 11:19 PM, Cecil Moore wrote:
On Apr 6, 2:51 pm, John - wrote: How about yours? I used an EZ file that I already had. It was 40 feet high and I rounded off the feedpoint impedance. I thought that would be a close enough approximation. -- 73, Cecil, w5dxp.com I understand. Of course, it's close enough. I was just wondering why my answers did not agree with yours, but now I do. Thanks, John |
40/15M dipole question
tom wrote:
On 4/6/2011 8:16 PM, John - KD5YI wrote: On 4/6/2011 2:51 PM, John - KD5YI wrote: Hey Cecil - I got: Impedance = 85.26 - J 186.3 ohms SWR (50 ohm system) = 10.337 (75 ohm system) = 7.308 Let's see if we can determine why they don't match. I adjusted the wire length for no reactance at 7.2 MHz. I used 99 segments of #14 wire. I did not include insulation effects. How about yours? John Are you using 9 segments? John I think may have missed something on how segmentation works. I thought you needed an even number of segments to split a dipole evenly. So while 99 is close to centered by my logic, if 9 was used it might skew things a bit. If I'm off base please tell me so I can adjust my model of how NEC works. I always use an odd number of segments, and the middle segment is where I feed it. The voltage source is essentially in series with the segment, and if your segments are short enough, the current distribution along the segment is effectively uniform, so it makes no difference. OTOH, I'm running NEC4 with a lot of memory in double precision, so using hundreds of segments isn't a big deal. |
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