4
$\begingroup$

I am building a ground and wall penetrating frequency modulated continuous wave radar. The only components that I have not been able to track down are the antennas. I need two antennas (one for transmit and one for receive) that cover the frequency range of 100MHz to 1GHz (or as much of that range as possible). I have searched a lot of different sites and was hoping someone here might be able to suggest something I haven't come across yet.

This is for a university project, so I am unfortunately limited by budget and an imperfect understanding of what exactly I need.

Basically what I am trying to do is transmit a ramped signal from the low end of my range (100MHz) up to the high end of the range (1000MHz) into the ground and then be able to receive the signals back to calculate if they are bouncing off of anything buried underground. The lower frequencies allow me to see objects at greater depths. I am hoping to find something that will cover the whole range, but so far there is a plethora of options in the 1GHz and above ranges and almost nothing ultra wideband in the lower than 1GHz range.

Specifications I have so far:

  • Freq Range: 100MHz-1000Mhz
  • Impedance: I planned on an external network to match, so whatever is available.
  • Size: Need to fit 2 antennas in an approximate area of 2x3 ft maximum
  • Price: Budget for 2 antennas $100
$\endgroup$
  • 1
    $\begingroup$ Please give us more details, re: your requirements / specifications / constraints: gain, impedance, size, weight, cost, etc. $\endgroup$ – Brian K1LI Sep 25 '18 at 21:11
  • $\begingroup$ A conventional over-the-air TV antenna covers a lot of your spectrum of interest. $\endgroup$ – Brian K1LI Sep 25 '18 at 21:22
  • $\begingroup$ From what I've seen, they cover about 170 to 240MHz and then 470 to 860MHz. I was hoping for something that would cover more of the lower frequencies. Since they are sold as receive antennas, is there anything about them that would keep me from transmitting out of one? $\endgroup$ – cheovy Sep 25 '18 at 21:46
  • 2
    $\begingroup$ An x-to-10x frequency range generally means specialized wideband antenna designs. Do you specifically need that for your application, or are you just hoping for nice general-purpose specs? Please edit your question to clarify. (Regarding transmit versus receive, generally yes but there are caveats; see ham.stackexchange.com/questions/5880/… for an answer to that question.) $\endgroup$ – Kevin Reid AG6YO Sep 25 '18 at 22:07
  • $\begingroup$ Some discones are advertised to cover roughly that order of frequency range. For short term indoor use, you might be able to build 4 cones (2per antenna) out of chicken wire or mesh. $\endgroup$ – hotpaw2 Sep 25 '18 at 22:30
1
$\begingroup$

You didn't say if you need something directional, but I'll assume you do. There are a number of trivially fractal antennas that have this bandwidth.

The most popular of those is the log periodic antenna; you can look at it as a series of stacked parallel mutually driven dipoles. Each dipole will need to be roughly a half wavelength long. at 1G, that would be about 15cm, at 100mhz 1.5m, and the boom to hold the elements might also be about that long. (In other words, it'll be a bit bigger than your size requirements.) However, there's ways to make log periodic antennas that fold or disassemble easily, so if this size limit is for storage, it may still work.

The next antenna I'd suggest is a (log periodic?) spiral antenna. It has a fairly wide radiation pattern if it's flat, but if you put the spiral on a cone, it becomes highly directional. Again, the size of the spiral will approach a half wavelength in diameter, so it may be too large for the lower frequencies.

More basic antennas (which would probably need to be larger to be effective) would include a horn or a corner reflector.

You might be able to cheat a bit and use a smaller versions of the above antennas that would work on the lower frequencies, but at a reduced effectiveness and probably a worse match.

IF you didn't want something directional, stacked discones might work. Again, the lowest frequency will be large, but there are ways to fold it for storage. A single discone would also work, but the radiation pattern is ugly for the higher frequencies (which might be ok for receive but is unlikely to be useful for transmit). The higher frequency discones you might want to make out of solid sheet metal; the lower frequencies could be skeletonized with just stiff wires arranged in a cone and joined at the narrow end.

It might also be possible to combine some of the above with "loaded" elements that are shortened by adding coils in the dipole element. However I've not seen any designs that combine bandwidth with loaded elements, so you'd probably have to find someone to design it for you and you might end up with discrete frequencies in the range rather than a full spectrum.

Most of the larger consumer grade outdoor TV antennas include both a log periodic and corner reflector antenna.

The only difference between a transmitting antenna and a receiving antenna is that matching matters more for transmitting antennas, and receiving antennas are sometimes made cheaper by using thinner wires.

$\endgroup$
1
$\begingroup$

A log-periodic dipole array on printed-circuit board from WA5VJB might provide an acceptable off-the-shelf solution. Note, though, that size is dictated by the low-frequency response. Dropping the 400MHz low end of his largest unit to the 100MHz you desire will drive a 4X increase in at least one dimension.

$\endgroup$
  • $\begingroup$ Not that it impacts this particular application but I am surprised at the relatively low gain/high AF of that LPDA. $\endgroup$ – Glenn W9IQ Sep 29 '18 at 16:58

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.