Re: Topband: Question on common mode chokes
Tnx, all, for the comments...learning more and more From: charlie-cunning...@nc.rr.com To: k...@pacbell.net; topband@contesting.com Date: Wed, 3 Jun 2015 21:57:27 -0400 Subject: Re: Topband: Question on common mode chokes I agree. We really don't want the coax shield to be part of the antenna, regardless of whether the antenna is vertical or horizontal. If the coax shield is part of the antenna, it can seriously distort the resonance and the driving point impedance - hence the need for some common-mode isolation. Regards, Charlie, K4OTV -Original Message- From: Topband [mailto:topband-boun...@contesting.com] On Behalf Of Robert Harmon Sent: Wednesday, June 03, 2015 4:21 PM To: Top Band Contesting Subject: Re: Topband: Question on common mode chokes Jim, It will be interesting to see what others say but my take on this is that the tuner function is to provide an impedance match to the vertical and doesnt provide any choking. You still should have the choke. 73, Bob K6UJ On Jun 3, 2015, at 9:42 AM, James Rodenkirch rodenkirch_...@msn.com wrote: I seem to recall, while reading up on common mode chokes, where I wouldn't need one if I employ an autotuner at the base of the antenna...is that true/factual? I have an LDG Z11ProII autotuner right at the base of my vertical - I did add a common choke at the input to the autotuner (21' of my LMR-400 coax wound around a piece of 4.5 PVC for the low bands) but wonder if that is really necessary. 72, Jim Rodenkirch K9JWV _ Topband Reflector Archives - http://www.contesting.com/_topband _ Topband Reflector Archives - http://www.contesting.com/_topband _ Topband Reflector Archives - http://www.contesting.com/_topband _ Topband Reflector Archives - http://www.contesting.com/_topband
Re: Topband: Question on common mode chokes
Jim, It will be interesting to see what others say but my take on this is that the tuner function is to provide an impedance match to the vertical and doesnt provide any choking. You still should have the choke. 73, Bob K6UJ On Jun 3, 2015, at 9:42 AM, James Rodenkirch rodenkirch_...@msn.com wrote: I seem to recall, while reading up on common mode chokes, where I wouldn't need one if I employ an autotuner at the base of the antenna...is that true/factual? I have an LDG Z11ProII autotuner right at the base of my vertical - I did add a common choke at the input to the autotuner (21' of my LMR-400 coax wound around a piece of 4.5 PVC for the low bands) but wonder if that is really necessary. 72, Jim Rodenkirch K9JWV _ Topband Reflector Archives - http://www.contesting.com/_topband _ Topband Reflector Archives - http://www.contesting.com/_topband
Re: Topband: Question on common mode chokes
Those would be transmitted signal strength concerns. On the receive side, if one ever listens on one's transmit antenna, the feed coax shield is a potential path for RF noise from the house. Any noise voltage allowed to go from the coax shield to the radials becomes a differential voltage versus the vertical wire which now travels back to the shack *inside* the coax. Common mode choke on the coax near the antenna takes care of that. The remote tuner likely works against a common case ground which directly connects the coax shield to the radials. 73, Guy K2AV On Wednesday, June 3, 2015, Jim Brown j...@audiosystemsgroup.com wrote: On Wed,6/3/2015 9:42 AM, James Rodenkirch wrote: I seem to recall, while reading up on common mode chokes, where I wouldn't need one if I employ an autotuner at the base of the antenna...is that true/factual? The function of a choke on the feedline is to prevent it from becoming part of the radial system. This matters most when the radial system is relatively limited, but is quite important when it is elevated. See N6LF's work on this, published as a 2-part piece in QEX several years ago, and available on his website. Google his call to find it. The executive summary is that ground losses are least when the current is equally divided between many radials, which is affected by their length, their number, soil quality, and proximity to the earth. The loss in a radial is I squared R, where R is loss coupled from the earth. The more radials present, the greater the division of the base current between them, thus the smaller the I. And because power is I squared, lost power falls in proportion to the number of radials. When there are only a few radials, current distribution will be strongly affected by the nature of the earth under them, which can vary a lot over a radial field. In this case, radials that carry greater current will dissipate more power, and the total power loss will be greater. The significance of the feedline choke is that it prevents the feedline from disturbing that balance. 73, Jim K9YC _ Topband Reflector Archives - http://www.contesting.com/_topband -- Sent via Gmail Mobile on my iPhone _ Topband Reflector Archives - http://www.contesting.com/_topband
Re: Topband: Question on common mode chokes
On Wed,6/3/2015 9:42 AM, James Rodenkirch wrote: I seem to recall, while reading up on common mode chokes, where I wouldn't need one if I employ an autotuner at the base of the antenna...is that true/factual? The function of a choke on the feedline is to prevent it from becoming part of the radial system. This matters most when the radial system is relatively limited, but is quite important when it is elevated. See N6LF's work on this, published as a 2-part piece in QEX several years ago, and available on his website. Google his call to find it. The executive summary is that ground losses are least when the current is equally divided between many radials, which is affected by their length, their number, soil quality, and proximity to the earth. The loss in a radial is I squared R, where R is loss coupled from the earth. The more radials present, the greater the division of the base current between them, thus the smaller the I. And because power is I squared, lost power falls in proportion to the number of radials. When there are only a few radials, current distribution will be strongly affected by the nature of the earth under them, which can vary a lot over a radial field. In this case, radials that carry greater current will dissipate more power, and the total power loss will be greater. The significance of the feedline choke is that it prevents the feedline from disturbing that balance. 73, Jim K9YC _ Topband Reflector Archives - http://www.contesting.com/_topband
Topband: Question on common mode chokes
I seem to recall, while reading up on common mode chokes, where I wouldn't need one if I employ an autotuner at the base of the antenna...is that true/factual? I have an LDG Z11ProII autotuner right at the base of my vertical - I did add a common choke at the input to the autotuner (21' of my LMR-400 coax wound around a piece of 4.5 PVC for the low bands) but wonder if that is really necessary. 72, Jim Rodenkirch K9JWV _ Topband Reflector Archives - http://www.contesting.com/_topband
Re: Topband: Question on common mode chokes
I agree. We really don't want the coax shield to be part of the antenna, regardless of whether the antenna is vertical or horizontal. If the coax shield is part of the antenna, it can seriously distort the resonance and the driving point impedance - hence the need for some common-mode isolation. Regards, Charlie, K4OTV -Original Message- From: Topband [mailto:topband-boun...@contesting.com] On Behalf Of Robert Harmon Sent: Wednesday, June 03, 2015 4:21 PM To: Top Band Contesting Subject: Re: Topband: Question on common mode chokes Jim, It will be interesting to see what others say but my take on this is that the tuner function is to provide an impedance match to the vertical and doesnt provide any choking. You still should have the choke. 73, Bob K6UJ On Jun 3, 2015, at 9:42 AM, James Rodenkirch rodenkirch_...@msn.com wrote: I seem to recall, while reading up on common mode chokes, where I wouldn't need one if I employ an autotuner at the base of the antenna...is that true/factual? I have an LDG Z11ProII autotuner right at the base of my vertical - I did add a common choke at the input to the autotuner (21' of my LMR-400 coax wound around a piece of 4.5 PVC for the low bands) but wonder if that is really necessary. 72, Jim Rodenkirch K9JWV _ Topband Reflector Archives - http://www.contesting.com/_topband _ Topband Reflector Archives - http://www.contesting.com/_topband _ Topband Reflector Archives - http://www.contesting.com/_topband
