On February 20, 2007 02:17 pm, rohre wrote: > Even simpler is a 80m dipole fed with balanced line to a tuner for all band > use. Perhaps simpler, but not necessarily a good solution.
> The window line is less costly than coax. Usually. > A good quality tuner is > less lossy in multiband use than coax/ tuner balun, etc.. A link coupled tuner is the most efficient. Proper balanced tuners with baluns at the input should also be efficient. Any tuner with a balun on the output is going to stress the balun on bands where the antenna is an even multiple of a half wavelength. You could easily see thousands of ohms of impedance which is an impossible situation for a balun. Yes, many amateurs use this configuration, and make lots of contacts. The 100 watts output at the rig may only be 10 watts of effective radiated power. Any balun manufacturers care to publish the measured efficiency of their baluns terminated by an impedance of 2500 -j3300 ohms? How about it Elecraft: how efficient are your baluns from 500 kHz to 54 MHz when terminated by an impedance of 2500 -j3300 ohms? How does the efficiency vary with applied power? How effective are they with common mode currents? > Balanced > antennas have fewer problems than off center feeds. Like what? Very few centre fed dipoles are balanced. You must have symmetrical surrounding terrain and objects within at least a half wavelength, preferably two wavelengths from the antenna, and also symmetrical ground losses. The feedline must run at 90 degrees to the antenna. Very few amateurs have such a perfect site for an antenna. So even with a centre fed antenna, balance is usually not achieved. It becomes a matter of degree of balance. An off-centre fed dipole will present a lower impedance on more bands, and therefore will place less demands on the tuner and balun. Use a good Guanella type balun should be used. What happens when the balance of an antenna is upset due to surrounding objects, uneven terrain, a feedline that does not run at 90 degrees to the antenna, or an offset feed? Typically feedline radiation. That is not necessarily a bad thing. The feedline radiation can improve the radiation pattern as it may add vertically polarized components and fill in where the nulls occur from the antenna. The downside is the possibility of RFI. However this will usually only be evident when an unbalanced current loop exists near the shack. Changing the feedline length will usually help reduce the RFI. Let's face it a multiband antenna is a compromise. As long as we understand the compromise, and can live with the results, there is no problem. Unfortunately most of what we are taught is from modeled antennas that do not take into account asymmetrical surroundings and ground losses, and feedline routing. There are very few, if any, measurements of efficiency of tuners and baluns in high impedance (including high reactance) situations. Therefore many amateurs are simply unaware of the losses that occur in a multiband antenna system. However, any antenna is still better than no antenna. -- 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
