Chuck, K2CG wrote: I enjoy putting together and playing with simple wire antennas for field operations. I have also been fortunate to enjoy moderate success with my efforts.
However I am looking to refine my approach in an effort to make my systems more efficient. My approach in the past has been to follow designs and dimensions for established dipole's and doublets, then check my effort by using an MFJ-259 to verify that they fall within acceptable limits for my tuner and desired frequency range. I am not looking to build an antenna that will give me 1:1.0 SWR but rather try to change the feed-line impedance to produce a better load transfer. Currently I am using a 135' doublet with 50' of 450 ohm ladder line, but I am looking at trying an 88' Doublet and different types of balanced transmission lines to vary the input impedance, by using 300 ohm, 450 ohm ladder line or even trying some home built 650 ohm ladder line. ------------------------- In truth, there isn't much to it! That's why simple antennas aren't belabored in the literature. It only gets complicated when you want passive antenna systems that show a low SWR on many bands or which has a lot of directivity, etc. The first requirement for an efficient antenna is low reactance at the connection to the transmitter. Low reactance is required for maximum power transfer. Almost 200 years ago Heinrich Hertz discovered that a wire that is electrically 1/2 wave long is naturally resonant; that is, it shows minimum reactance to the signal applied. That's the shortest length of antenna that is "naturally" or "self" resonant when operated independently (e.g. free space). Marconi had a huge problem as he started tinkering with antennas. To be self-resonant at the frequencies he was using in his early work - perhaps 50 or 100 kHz and probably less - he'd need an antenna at least 40,000 feet long. Even Gugliemo's mom must have balked at that idea! (He was still living at home.) He tinkered and worked out that if he connected one end of his antenna system to the earth, it would resonate at half the length of a Hertzian antenna. Well, that did save 20,000 feet of wire! (Now, I confess I'm telling tall tales here. It's not likely Heinrich or Gugliemo really understood resonance or, as Gugliemo like to call it 'synchronicity'. But as we better understood the effects of reactance on an antenna system in later years, what Heinrich and Gugliemo were doing by trial and error became evident.) When an antenna is longer than its natural resonant length, it'll have inductive reactance. If it's shorter, it'll have capacitive reactance. If it's not convenient to make the antenna the right length, we can add suitable inductance or capacitance and bring the system to resonance. Our antenna tuners will do that for us, if they have the required range to match our antenna at the frequency we want. That's one of the two things a "tuner" must do. We'll touch on the other in a moment. With the right amount of inductive or capacitive reactance, you could, on paper, have an antenna 1 inch long that would radiate as well as any wire ever launched into space, except for one problem: resistance. Here's why resistance is a problem. A resonant antenna looks like a resistor to the transmitter. If it has zero losses, that resistance is a fictional amount representing the RF that is radiated as electromagnetic waves. Since we're dealing with alternating currents here (at RF frequencies) we don't say resistance but impedance. Since the antenna is resonant (no reactance) the impedance is equal to the resistance. The value of that impedance varies according to the length of the antenna. If the antenna is 1/2 wave long, the value of that impedance at the end may be several thousand ohms. At the center it's about 75 ohms if the antenna is in free space. (Getting it close to the earth were most of us have to put them brings it down closer to 50 ohms). But, as you make the antenna shorter than 1/2 wave the impedance drops. At 1/4 wave (the length Marconi used) it's 35 ohms. As you go shorter the impedance plummets like a lead garden gnome dropped from that 90-foot tower you dream of owning. Many short antennas, like mobile antennas, have an impedance of less than 1 ohm, sometimes down in the range of 0.1 or even 0.01 ohms! That alone isn't a problem except for the fact that the antenna wire and the wire in any inductors we use to make it resonant also have resistance. The power is shared between the resistance of the wire and the radiation resistance. And don't forget that the RF resistance of a wire is much greater than the DC resistance because RF only flows along the skin of conductors. (That's why some RF conductors are silver plated for best efficiency.) Also, your feed line is part of that antenna system, so if it has a very high SWR it'll show high ohmic losses at the points where the RF currents are high. Even open wire line will do that under extreme conditions. I tend to use about #12 or larger conductors in my open wire lines that have a significant SWR on them. The main thing about open wire line is that it has a higher impedance than "coaxial" lines. Higher impedances mean, normally, a lower SWR over the maximum range of impedances you're likely to encounter and so lower losses because the maximum RF currents are less. So if you have a short whip with a radiation resistance of .1 ohm and 10 ohms of resistance in the loading coil and antenna wire itself, you have a total resistance of 10.1 ohms. >99% percent of the RF will be dissipated as heat in the wire and less than 1% remains to be radiated in the 0.1 Ohms of radiation resistance! You have the same problem with a "Marconi" antenna working against ground, only now you have the ground resistance to add to the system. Getting the resistance of a ground connection below 50 or 100 ohms is tough. That's why commercial broadcast stations typically use 120 0.2 wavelength radials buried and still prefer to site the antennas in a salt marsh! That brings us to the second job of the antenna tuner (or, as some like to call it, the "Matching Network"). It transforms the impedance presented by the antenna to 50 ohms for your transmitter. There's nothing "magic" about that 50 ohms. It's just a convenient value that has become "standard" because it is a good match for a simple center fed dipole and a large range of coaxial cables are made with that value. So, for maximum efficiency, you want your antenna as long as practical. Basically 1/2 wave is a good goal for an optimum antenna. If it has to be shorter, make it as long as possible and try to use the most efficient loading inductors or, if it must work against ground, try to bury as much metal as possible for the ground. As much as some of the antenna manufacturers would like you to believe they've found a way around these basics, it's not happened yet. Those are the basic for considering changes for a more efficient antenna. There are some other issues relating to directivity, earth losses (not from the ground loss mentioned above but from currents induced in the ground by the antenna) etc. But that's a subject for a different question <G>. Ron AC7AC _______________________________________________ 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
