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 <[email protected]> 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
