[Elecraft] Wire Antennas and feedline for the KX1
Hi, Chuck. I usually don't add the transmission line in the EZNEC model, just the source. I then take the source data for the frequency of interest and plug it into this tool where I can play around with different kinds of feedline, add a balun, calculate optimal feedline lenghts, etc: http://www.ac6la.com/xlzizl.html 73 Martin. ___ 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
[Elecraft] Wire Antennas and feedline for the KX1
I do the same as Martin writes. Modeling transmission lines with Eznec is difficult for me... You could also use TLA by N6BV. 72, -- ___ Surf the Web in a faster, safer and easier way: Download Opera 9 at http://www.opera.com Powered by Outblaze ___ 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
RE: [Elecraft] Wire Antennas and feedline for the KX1
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
[Elecraft] Wire Antennas and feedline for the KX1
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. I have read several articles and books on antenna building but have missed anything that addresses this aspect of antenna construction. That is not to say that it hasn’t been discussed only that I have not seen it. The following links and articles have been very helpful: L.B. Cebrik’s W4RNL’s technical articles “10 Frequency Asked Questions about the All-Band Doublet” http://www.cebik.com/wire/abd.html “My Top Five Backyard Multi-band Wire HF Antennas” http://www.cebik.com/fdim/fdim9.pdf “Some (Old) Notes on Home-Brew Parallel Transmission Lines” http://www.cebik.com/trans/par.html “The Minimalist Backpack Antenna System” by Bonnie Crystal KQ6XA http://www.qsl.net/kq6xa/antenna/ I have downloaded and tried the trial version of EZ NEC only to be totally confuses with my effort. I would appreciate some feedback on my thought process, am I running down a rabbit trail that has already been explored as a wasted effort? Is there someone on the reflector that understands owns and understands EZ NEC that could provide me feedback on what the modeling results would be with a 135’ doublet or 88’ doublet with 300 ohm, or 450 ohm or 650 ohm ladder line on 80M, 40M, 30M and 20M at a height of 45’ to 65’. Of course if I am all wet then please be kind enough to respond off the reflector and spare me the embarrassment of appearing to be a complete moron. 73 K2CG Chuck G. ___ 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
Re: [Elecraft] Wire Antennas and feedline for the KX1
[EMAIL PROTECTED] wrote: ... I have read several articles and books on antenna building but have missed anything that addresses this aspect of antenna construction. That is not to say that it hasn’t been discussed only that I have not seen it. You might benefit from a visit to the DX Engineering web site (http://www.dxengineering.com/) where your path to Nirvana is Base Station Antennas - Multi-Band Dipoles. These guys do a great job of explaining the reasons for certain designs, choices of antenna lengths, feed-line types and lengths, the use of suitable choke-type baluns, and so on. And they sell great hardware. Their prices indicate that they are proud of their products, as they should be, but I'm happy that I purchased some of the critical components of my 80-10 meter doublet from them. Good luck with your antenna system, Gus Hansen KB0YH ___ 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
