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Designing an antenna for the 5000m band
On Feb 16, 11:22*pm, Frnak McKenney
wrote: Back in December I posted a question about ways to receive LF/VLF radio signals. *Based on the suggestions made by a number of people here I decided to use my existing Heathkit Mohican receiver and add this upconverter kit from Jackson Harbor: *http://jacksonharbor.home.att.net/lfconv.htm The kit arrived and was half assembled before I turned on the Mohican, its first power-up in some years; the horrible squeal that erupted from the speaker put a bit of a damper on things. *It now appears that replacing the two output transistors (Germanium, no less!) *with NTE102As from Mouser will fix that, so I'm thinking about an antenna that might be a little more snesitive to LF signals than the Mohican's built-in whip. Along those lines, I have a couple of (what I hope are) simple questions that I'm hoping someone can help me get started with. First, the need for impedance matching between an antenna and a receiver. *My understanding is that a resonant halfwave dipole will have an impedance around 73 Ohms; unfortunately, unless I can obtain research funding from the just-passed Congressional Economic Stimulus bill I'm going to have trouble paying for 2.5km of copper wire, some towers, a crateload or two of porcelain insulators,and the land to build it on. *(Hey, I promise to dump it back into the economy ASAP. *Really! *grin!) So any non-loop antenna I can construct will necessarily be a "short wire" or "electrically small" antenna (two useful search terms). But how does one go about calculating the impedance of a coat hanger or an extension cord ("short piece of wire")? I've done Google seaarches and read what seemed like the relevant sections of the 2004 ARRL Radio Handbook and their Antenna Book; unfortunately, most authors restrict their discussion to quarter- wave or longer antennae. *Any starting points, hints, or references on impedance calculations for less-than-1/10-wavelength antennas will be appreciated. My other question has to do with how to interpret signal strength. The first "standard reference" transmitter I'll be attempting to receive will be WWVB out of Fort Collins, Colorado (60kHz/5000m). Per the NIST documentation at: * NIST Special Publication 250-67: NIST Time and Frequency Radio * * * * * * Stations: WWV, WWVH, and WWVB *http://ts.nist.gov/MeasurementServices/Calibrations/ * * * * * * Upload/SP250-67.pdf figure 4.5 seems to say that I could reasonably expect to see a signal of at least 100uV/m. *Does this mean that I should expect to see 100uV from any one-meter hunk of wire strung out horizontally in the optimum direction? Or is there something more subtle going on I need to be aware of? Frank McKenney A field strength measured in Volts/meter is just that, but the problem getting the energy out of the air and into a receiver. A short linear antenna has a very low radiation resistance ( 1 ohm) which is a poor match to a practical transmission line, whose characteristic impedance is typically 1000's of times larger. The radiation resistance of an antenna is the component of its complex impedance that is associated with the power captured. Poor impedance matching is equivalent to low energy efficiency, in this case very low. One solution is to use a small circular loop antenna whose low radiation resistance can be increased by adding turns. Balanis (Antenna Theory Analysis & Design (1997), p.209) gives a formula for the radiation resistance of a loop smaller than 1/25 wavelength: R = 20 * pi^2 * (C/L)^4 * N^2 ohms where C is the circumference of the loop, L is the wavelength and N is the number of turns. Better still is to use a ferrite loop antenna. You may be able to get one out of an old AM radio and adapt it to your receiver. The resulting formula is identical to the above, multiplied by the relative permeability of the core, u (SQUARED !), so you can use a very small-diameter loop and/or fewer turns, getting improved selectivity and sensitivity (i.e. high Q) in a tuned circuit: R = 20 * pi^2 * (C/L)^4 * N^2 * u^2 ohms -- Joe |
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