The only important point about resonance is that the *system*, including any matching networks, is resonant (i.e. presents a purely resistive, non-reactive load to the transmitter) for optimum power transfer to the antenna. With modern transmitters, we usually want the resistive impedance to be 50 ohms since that's what they are designed to handle. Also, if any significant length of 50 ohm coaxial cable is used to connect the system to the transmitter, keeping the impedance at 50 ohms minimizes the transmission line losses too. That's why using a built in tuner in a rig to correct for anything but small mismatches usually is not a good practice.
However, even with a matching network, antenna feed point impedance can have a huge effect on the antenna system efficiency when using an end-fed antenna - vertical, 'long wire' or whatever. That's because the current at the feed point is shared between the radiator and the "ground" system - counterpoise, radials, or whatever. Any impedance in the ground system will consume RF power that would otherwise be radiated by the antenna. Half wave end fed antennas are extremely efficient because they present a very high impedance at the feed point, meaning very low RF currents flow in either the antenna or ground system. Such an antenna can be more than 90% efficient even if the ground system has an impedance of 300 ohms or so (a fairly punk, but common ground system impedance). But, as the radiator is made physically shorter (or the frequency lowered) the impedance at the feed point drops and the current goes up, meaning the ground system resistance consumes a larger and larger share of the RF. It is common for a short whip antenna to have an efficiency of less than 10%, meaning that 90% of the RF is consumed as heat in the ground system. An electrically short (<1/2 wave) antenna that exhibits a broad bandwidth (low SWR over a large range of frequencies) almost certainly has lots of loss. A radiator always shows a change of reactance (SWR) with frequency. A resistive dummy load (like a punk ground or other resistance in the system) will be "flat" over a broad range. A useful experiment, if you have EZNEC or something similar, is to model your antenna and check the SWR bandwidth (or impedance at two frequencies straddling the resonant frequency) then compare that to your actual antenna. Any additional bandwidth is a sign of additional loss in the system. There is no "magic" way to raise the antenna's inherent impedance other than to make it physically larger. Loading coils, top hats, etc., may cause an antenna to become self resonant at a particular frequency, but the resistive part of the impedance that governs its efficiency with a given ground system remains as low as ever. Going the other way, though, the efficiency improves at frequencies where the current maxima occur as the antenna is longer than 1/2 wavelength. The impedance is never as low as it is at 1/4 wavelength or below. Indeed, as you go higher the lowest impedance goes up, and the highest impedance goes down (making the system easier to match). Now this doesn't say anything about radiation pattern, etc. Only about the efficiency of radiating RF. But even a full-size dipole doesn't show significant directivity unless it's at least 1/2 wavelength above the ground. A doublet avoids the problem of the ground system, but requires twice as much wire up in the air. It will work very well down to the point where the radiator is 1/4 wave long, as long as the low-loss (open wire) feeders are at least 1/4 wave long, making each side a full 1/2 wave. Of course having half of the antenna in the open wire feed line means that half of it does not radiate, but that only costs about 1 or 2 dB. That's the only consideration concerning the length of a doublet (my favorite antenna too). I believe by "Cebik" doublet you are referring to his favorite length of 42 feet. He recommended that as the longest antenna that will still have good low-angle lobes on 10 meters, not because it's more efficient at a radiator. If you make it longer the antenna will exhibit a very strong lobe at 90 degrees (straight up). 73, Ron AC7AC -----Original Message----- Hello Mike. My EFHW vertical is resonant only at 20 meters. The *antenna* SWR is lousy everywhere else according to my MJF259 which stops reporting at 25:1. -- just as bad as my non-resonant-by-design 44' Cebik doublet with which I've worked the world, at 5 watts and below. The internal tuners in my KX1, K1, and K2 all tune the antenna between 1:1 and 1:4. I've used this vertical with good results on 40 meters and *great* results on 30, 20, 17,12, and 10. ______________________________________________________________ Elecraft mailing list Home: http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/mmfaq.htm Post: mailto:Elecraft@mailman.qth.net This list hosted by: http://www.qsl.net Please help support this email list: http://www.qsl.net/donate.html
