Well written Ron! On Tuesday 13 November 2007 19:48, Ron D'Eau Claire wrote: > Consider a center fed wire. At the frequency where the wire is at 1/2 > wavelength long the impedance at the center is usually about 50 ohms (in > free space it would be 75 ohms, but as one moves close to the earth, the > impedance drops). At the frequency where the wire is 1 wavelength long, the > impedance at the center feed point will be in the range of 3500 to 4000 > ohms, depending again on the height. As it becomes longer in wavelengths, > those impedance extremes decrease, so the maximum range of impedance values > are from about 50 to 4000 ohms.
Moving the feedpoint away from the center of the dipole will also reduce the impedance extremes. Yes, this will introduce some imbalance, but lets bust another myth. The center fed dipole in practical deployment is rarely balanced. Most dipoles do not have symmetrical surroundings or ground characteristics, nor do they have feedlines that run at a 90 degree angle to the antenna straight down to the shack. So what we are really talking about is the degree of imbalance. Like SWR, imbalance is not necessarily a bad thing. > The other reason for loss at high SWRs is ohmic loss due to the extremely > high currents at the current loops (points of maximum current). At even > moderate power levels very large currents will occur there just as very > high voltages appear at the voltage loops 1/4 wavelength away. RF flows > only in the very surface of a conductor so the RF resistance of a conductor > is much, much higher than its d-c resistance (unless the conductor is a > very, very thin tube - probably too thin to handle). And it is these very current loops and nodes that can manifest themselves in RF in the shack problems. Antennas with a greater degree of imbalance are often blamed for the RF in the shack problems, and while greater imbalance does contribute to the problem, it is really much more of a feedline length problem. Again, it is rare to find, in practical deployment, a feedline with perfectly balanced currents. Change the feedline length and the current loops move. The trick in a multiband antenna system is to find the right length for all the bands the antenna system is to cover. In the old days, when open wire feed was the norm, sections of feedline were often switched in and out for the different bands. The right lengths resulted in manageable impedances for the tuner and kept RF out of the shack. And every station had a tuner, be it the pi network on the output of the tube amplifier or a purpose built wide range tuner. With the advent of solid state finals, manufacturers left out the tuners. This made the price of the fancy new rigs less, and since many hams used 50 ohm matched antennas systems such as the triband trap yagi, they could get by without the tuner. Interesting to see that the manufacturers have added tuners back into the rigs, at least as options. Most of these tuners are similar in matching abilities to the old tube pi networks. The Elecraft tuners are an exception to this as they have a wide range of impedance matching. Now we just have to remember the old wisdom to switch feedline lengths for different bands on a multiband antenna. The KRC2 is a nifty device to control relays to do just that. Yes, multiband antennas can be a challenge, but then again, any antenna is still better than no antenna. 73, Darrell VA7TO K2 #5093 -- Darrell Bellerive Amateur Radio Stations VA7TO and VE7CLA Grand Forks, British Columbia, Canada _______________________________________________ Elecraft mailing list Post to: Elecraft@mailman.qth.net You must be a subscriber to post to the list. Subscriber Info (Addr. Change, sub, unsub etc.): http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/subscribers.htm Elecraft web page: http://www.elecraft.com
