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shortened dipole loaded
October's QST has an article "Designing a Shortened Antenna" pp 28-32. It
gives an example of a shortened dipole for 40 meters at 7007 khz. Dipole length of the half-sized dipole [p30 "a second example"] is 10.61 meters. This is 20 feet off ground using #12 wire. The formulas give a solution of XL= +j1776 ohms or an inductance of 40 microhenries at 30 degrees from each leg. I tried to simulate this antenna on EZNEC. A 10.61 meter antenna at 7007 khz gives impedance =11.33 - j 881.2 ohms. If EZNEC is correct wouldn t I need an inductance of +j881.2 on each leg of the dipole, rather than +j1776? -- Jerry |
Jerry wrote:
October's QST has an article "Designing a Shortened Antenna" pp 28-32. It gives an example of a shortened dipole for 40 meters at 7007 khz. Dipole length of the half-sized dipole [p30 "a second example"] is 10.61 meters. This is 20 feet off ground using #12 wire. The formulas give a solution of XL= +j1776 ohms or an inductance of 40 microhenries at 30 degrees from each leg. I tried to simulate this antenna on EZNEC. A 10.61 meter antenna at 7007 khz gives impedance =11.33 - j 881.2 ohms. If EZNEC is correct wouldn t I need an inductance of +j881.2 on each leg of the dipole, rather than +j1776? Get a copy of VE3ERP's Hamcalc,it includes a program that lets you set the total length and "slide" the loading coil anywhere inside that length.. Hamcalc is available on the CQ magazine website. 73 de VE3JUA.... |
On Sat, 27 Sep 2003 10:05:06 -0500, "Jerry" wrote:
I tried to simulate this antenna on EZNEC. A 10.61 meter antenna at 7007 khz gives impedance =11.33 - j 881.2 ohms. If EZNEC is correct wouldn t I need an inductance of +j881.2 on each leg of the dipole, rather than +j1776? -- Jerry Hi Jerry, I can confirm your first pass analysis is close enough (probably differs only by wire gauge from mine). I would suggest you simply add the loads you speculate and find out for yourself, which is, after all, the point of modeling. 73's Richard Clark, KB7QHC |
On Sat, 27 Sep 2003 19:18:26 GMT, "Bob Walker" wrote:
Jerry, I tried an EZNEC simulation also and did not have the same results. I got 74 ohms at 1912 ohms inductive reactive. I resonated the simulated antenna with 28 uH, getting an SWR of 2:1 I will send my simulation to you. Please send me yours as we are not getting the same thing from the QST numbers. Bob, Hi Bob, For a 40M dipole (short or long) that is only 20' off the ground? 73's Richard Clark, KB7QHC |
Jerry wrote:
October's QST has an article "Designing a Shortened Antenna" pp 28-32. It gives an example of a shortened dipole for 40 meters at 7007 khz. Dipole length of the half-sized dipole [p30 "a second example"] is 10.61 meters. This is 20 feet off ground using #12 wire. The formulas give a solution of XL= +j1776 ohms or an inductance of 40 microhenries at 30 degrees from each leg. I tried to simulate this antenna on EZNEC. A 10.61 meter antenna at 7007 khz gives impedance =11.33 - j 881.2 ohms. If EZNEC is correct wouldn t I need an inductance of +j881.2 on each leg of the dipole, rather than +j1776? Yes, if you were going to put the loading coils at the center of the antenna. That is akin to a base-loaded mobile antenna. But you are going to put the coils in the center of each leg. That is akin to a center-loaded mobile antenna and that requires about double the inductive reactance that a base- loading coil requires. -- 73, Cecil http://www.qsl.net/w5dxp -----= Posted via Newsfeeds.Com, Uncensored Usenet News =----- http://www.newsfeeds.com - The #1 Newsgroup Service in the World! -----== Over 100,000 Newsgroups - 19 Different Servers! =----- |
For design of short coil-loaded centre-fed dipoles begin with a short loaded
1/4-wave vertical and, in-effect, connect a pair of them back to back. Download programs VERTLOAD or LOADCOIL or ADDALOAD. For even shorter loaded dipoles download MIDLOAD. All these programs will tell you how may turns are needed on a former of given length and diameter at whatever position along the wire you chose to locate it. In most cases the length of the coil is taken into account in the overall length. When using the design of short-verticals to design short-dipoles the ground loss estimates can be assumed zero. Some of these programs include assistance with bottom-end tuner design which, of course, is of no use when a pair is connected back to back. It should be remembered the feedpoint resistance of a short resonant dipole will be considerably less than the desirable 50 ohms and a poor match to a coax feedline will exist. But the mismatch may not be excessive. The main disadvantage is single-band working whereas an unloaded short dipole can usually be effective on several bands by using a 450-ohm or 600-ohm feedline and a good tuner. But you will need a tuner with whatever length of dipole, loaded or unloaded, you end up with. As for myself, when using short antennas, I prefer not to use heavy-weight loading coils in antenna wires but concentrate on keeping home-brew tuner losses to a minimum. Programs can be downloaded in a few seconds and run immediately. As were Bolton & Watt's condensing steam engines - Made in Birmingham. ---- ======================= Regards from Reg, G4FGQ For Free Radio Design Software go to http://www.g4fgq.com ======================= |
Jerry,
I ran EZNEC on this, using the QST dimensions, and got what seem to be reasonable results. Resonant frequency = ~7.11 MHz Impedance = 21.15 + j1.17 For those who obviously did not read the QST article, the 25 uH loading coils are 1.7 meters from the ends. Total length is 10.64 meters, and height is 6 meters. Over real ground, maximum gain is about 6dbi, straight up, and about 3 dbi at 30 degrees elevation. Tam/WB2TT "Jerry" wrote in message ... October's QST has an article "Designing a Shortened Antenna" pp 28-32. It gives an example of a shortened dipole for 40 meters at 7007 khz. Dipole length of the half-sized dipole [p30 "a second example"] is 10.61 meters. This is 20 feet off ground using #12 wire. The formulas give a solution of XL= +j1776 ohms or an inductance of 40 microhenries at 30 degrees from each leg. I tried to simulate this antenna on EZNEC. A 10.61 meter antenna at 7007 khz gives impedance =11.33 - j 881.2 ohms. If EZNEC is correct wouldn t I need an inductance of +j881.2 on each leg of the dipole, rather than +j1776? -- Jerry |
Whoops, I see I did "Building the Antenna", not "Second Example". There is
something wrong with "Second Example", because the dimensions are about the same as "Building", but the coil is almost twice as big. Tam/WB2TT "Tarmo Tammaru" wrote in message ... Jerry, I ran EZNEC on this, using the QST dimensions, and got what seem to be reasonable results. Resonant frequency = ~7.11 MHz Impedance = 21.15 + j1.17 For those who obviously did not read the QST article, the 25 uH loading coils are 1.7 meters from the ends. Total length is 10.64 meters, and height is 6 meters. Over real ground, maximum gain is about 6dbi, straight up, and about 3 dbi at 30 degrees elevation. Tam/WB2TT "Jerry" wrote in message ... October's QST has an article "Designing a Shortened Antenna" pp 28-32. It gives an example of a shortened dipole for 40 meters at 7007 khz. Dipole length of the half-sized dipole [p30 "a second example"] is 10.61 meters. This is 20 feet off ground using #12 wire. The formulas give a solution of XL= +j1776 ohms or an inductance of 40 microhenries at 30 degrees from each leg. I tried to simulate this antenna on EZNEC. A 10.61 meter antenna at 7007 khz gives impedance =11.33 - j 881.2 ohms. If EZNEC is correct wouldn t I need an inductance of +j881.2 on each leg of the dipole, rather than +j1776? -- Jerry |
It's disappointing to see that people have gotten so many different
results modeling such a simple antenna with EZNEC. Jerry's results are correct. You'll get slight variations with differing segmentation and ground conductivity, by they won't be large differences. I haven't followed through the article's math, but there's either a computational error or something fundamentally wrong with the equations to produce a value of j1776 for the antenna's reactance. The fact that the author's reactance is just about twice what it should be points to a likely error in computation. Jerry, what the EZNEC result means is that you'd need a *total* of j881 ohms at the feedpoint to resonate the antenna. You could do this by adding half the amount to each leg, or the total to one leg. Or, you could move the coils out toward the end, but you'd then need more inductance. Of course, you've still got a feedpoint resistance of about 11 ohms, plus coil resistance, to transform into something close enough to 50 ohms to make your rig happy. Roy Lewallen, W7EL Jerry wrote: October's QST has an article "Designing a Shortened Antenna" pp 28-32. It gives an example of a shortened dipole for 40 meters at 7007 khz. Dipole length of the half-sized dipole [p30 "a second example"] is 10.61 meters. This is 20 feet off ground using #12 wire. The formulas give a solution of XL= +j1776 ohms or an inductance of 40 microhenries at 30 degrees from each leg. I tried to simulate this antenna on EZNEC. A 10.61 meter antenna at 7007 khz gives impedance =11.33 - j 881.2 ohms. If EZNEC is correct wouldn t I need an inductance of +j881.2 on each leg of the dipole, rather than +j1776? |
By coincidence, one of the tutorial examples for the MultiNEC program
deals with the subject of loading coils on a shorty dipole. With MultiNEC you can define the antenna model using variables for both the amount of reactance in the coils and for the placement of the coils along the wire. Each time you change the variable that controls the placement of the coil you can use the MultiNEC "Resonate" function which will automatically invoke EZNEC in an iterative manner and adjust the reactance of the coils until the antenna is resonant. The example was written to illustrate and explain various capabilities of MultiNEC, of course, and not to address the particular scenario of this thread. It's available in standalone form at www.qsl.net/ac6la/mnex3.html. Dan |
For given location of loading coil along a short antenna wire, of given overall length, of given wire, rod or mast diameter, the following programs will calculate the inductance needed to resonate it to 1/4-wavelength. Two loaded wires connected back-to-back form a 1/2-wave resonant dipole. Program ADDALOAD will tell just that. It will also tell you the value and position of a loading capacitance if antenna length requires a resonating capacitor. Program LOADCOIL will do likewise and also tell you how many turns of what gauge wire to wind on a coil of given former length and diameter. Program VERTLOAD does a similar job insofar as dipoles are concerned. Program MIDLOAD is dedicated to very short centre-loaded dipoles. All coil and wire dimensions and number of turns are taken into account. Such antennas can be fed via either coax or balanced-pair feedlines of any impedance. Program SOLNOID2 will design any coil of any proportions for any given dimensions and required inductance, plus a variety of other performance characteristics including temperature rise for given applied RF volts. Nice for tank coils, loading coils and antenna traps. All these programs are used in a "What-if " user-friendly mode. USA citizens may be at a slight disadvantage - metric measurement units are used although in some cases a translation is provided. Once one of these simple loaded anennas has been designed, provided you have a BIG, UNOBSTRUCTED back-yard, EZNEC may be used to estimate its receiving and radiating pattern. Otherwise your guess is as good as mine. However, it should be borne in mind, a coil-loaded short antenna is essentially a single-band device and a tuner is nearly always needed even for the single band. With a tuner it may be goaded into doing something useful on one or two higher frequency bands. For optimum efficiency the location of a loading coil is about 2/3 or 3/4 of the way along the wire length. The bigger the physical size of the loading coil, the higher the Q, the smaller the coil loss and the nearer it can be placed towards the end of the wire. But location is VERY non-critical. If the wire is not very much shorter than self-resonance then efficiency can be maximised by NOT inserting a coil in the wire and depending entirely on a high-impedance balanced transmission line and a tuner. Without a high-in-the-air loading coil the antenna reverts advantageously to multiband operation. In general, good efficiency is possible with a 450 or 600-ohm feed-line provided the overall length of an unloaded dipole is not less than 0.3 wavelengths at the lowest frequency of interest. One antenna with its feedline, from coax to open wire, or both types in tandem, can be compared efficiency-wise with another by using program DIPOLE3. All the above small programs, one self-contained file only, can be downloaded in a few seconds from the following website and run immediately. No training course needed. Free to USA citizens. ;o) ---- ======================= Regards from Reg, G4FGQ For Free Radio Design Software made in the original Birmingham, go to http://www.g4fgq.com ======================= |
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