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#1
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Problem matching real Log Periodic to NEC
On Sep 30, 12:28*am, Owen wrote:
On 30/09/10 15:23, Dave D wrote: Thanks guys... I've never had that problem before. Any tips for what clamp-on ferrites might work best, or how many? Or the best way to configure them? I meant also to ask how you dressed the coax away from the feedpoint. I often see pics of coax tie wrapped to one leg of the transmission line spine of the LP. Guaranteed to create common mode current. Thanks Owen, In this case I had it down the middle between the haves, through clips. |
#2
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Problem matching real Log Periodic to NEC
On 01/10/10 05:06, Dave D wrote:
On Sep 30, 12:28 am, wrote: On 30/09/10 15:23, Dave D wrote: Thanks guys... I've never had that problem before. Any tips for what clamp-on ferrites might work best, or how many? Or the best way to configure them? I meant also to ask how you dressed the coax away from the feedpoint. I often see pics of coax tie wrapped to one leg of the transmission line spine of the LP. Guaranteed to create common mode current. Thanks Owen, In this case I had it down the middle between the haves, through clips. Does that disturb the characteristic impedance of the spine transmission line? If so, does that contribute to the problems you observe? If this LP is mounted to a mast somewhere forward of the rear element, I would consider dressing the feedline away from the feed point at right angles to the spine, installing a common mode choke near the feedpoint, and bringing the feedline to the mast some distance below the spine. If you were to perform a test where the LP is mounted a wavelength or more above ground, pointing to the zenith, feedline falling downwards from the feedpoint, common mode choke near the feedpoint, it would reveal whether the array is working as intended. If your actual mounting causes it to work differently, you know where the problem is. Owen |
#3
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Problem matching real Log Periodic to NEC
On Sep 30, 12:38*pm, Owen wrote:
On 01/10/10 05:06, Dave D wrote: On Sep 30, 12:28 am, *wrote: On 30/09/10 15:23, Dave D wrote: Thanks guys... I've never had that problem before. Any tips for what clamp-on ferrites might work best, or how many? Or the best way to configure them? I meant also to ask how you dressed the coax away from the feedpoint. I often see pics of coax tie wrapped to one leg of the transmission line spine of the LP. Guaranteed to create common mode current. Thanks Owen, In this case I had it down the middle between the haves, through clips. Does that disturb the characteristic impedance of the spine transmission line? If so, does that contribute to the problems you observe? If this LP is mounted to a mast somewhere forward of the rear element, I would consider dressing the feedline away from the feed point at right angles to the spine, installing a common mode choke near the feedpoint, and bringing the feedline to the mast some distance below the spine. If you were to perform a test where the LP is mounted a wavelength or more above ground, pointing to the zenith, feedline falling downwards from the feedpoint, common mode choke near the feedpoint, it would reveal whether the array is working as intended. If your actual mounting causes it to work differently, you know where the problem is. Owen Well, it does different things based on how the cable is routed, but none are really better than the other... just different. I tried coming off the back, and also at the mid point... It changed, but not better or worse overall... The points of good and bad just changed position across the range. I'm going to try the ferrites. What do you think about these? http://www.mouser.com/ProductDetail/...PQp4FXwkz9g%3d |
#4
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Problem matching real Log Periodic to NEC
On 9/30/2010 4:56 PM, Dave D wrote:
On Sep 30, 12:38 pm, wrote: On 01/10/10 05:06, Dave D wrote: On Sep 30, 12:28 am, wrote: On 30/09/10 15:23, Dave D wrote: Thanks guys... I've never had that problem before. Any tips for what clamp-on ferrites might work best, or how many? Or the best way to configure them? I meant also to ask how you dressed the coax away from the feedpoint. I often see pics of coax tie wrapped to one leg of the transmission line spine of the LP. Guaranteed to create common mode current. Thanks Owen, In this case I had it down the middle between the haves, through clips. Does that disturb the characteristic impedance of the spine transmission line? If so, does that contribute to the problems you observe? If this LP is mounted to a mast somewhere forward of the rear element, I would consider dressing the feedline away from the feed point at right angles to the spine, installing a common mode choke near the feedpoint, and bringing the feedline to the mast some distance below the spine. If you were to perform a test where the LP is mounted a wavelength or more above ground, pointing to the zenith, feedline falling downwards from the feedpoint, common mode choke near the feedpoint, it would reveal whether the array is working as intended. If your actual mounting causes it to work differently, you know where the problem is. Owen Well, it does different things based on how the cable is routed, but none are really better than the other... just different. I tried coming off the back, and also at the mid point... It changed, but not better or worse overall... The points of good and bad just changed position across the range. I'm going to try the ferrites. What do you think about these? http://www.mouser.com/ProductDetail/...PQp4FXwkz9g%3d The one question I've not seen asked is how close to what you built is your model? Everything is critical at those frequencies and as little as 1/32" can make a difference. If your model doesn't match exactly the antenna then it's no wonder it's not working as expected. To me modeling the phasing lines between the elements be they separate from the boom or are the boom is one of the trickiest parts. As an example if you used a 1" square boom and modeled it as a 1" diameter wire there would be a problem. They are not equal. John W3JXP |
#5
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Problem matching real Log Periodic to NEC
On 01/10/10 08:03, John Passaneau wrote:
.... are the boom is one of the trickiest parts. As an example if you used a 1" square boom and modeled it as a 1" diameter wire there would be a problem. They are not equal. Here is a little tool for designing practical square section spines: http://vk1od.net/calc/tstl.htm . Owen |
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