Is the open source antenna or cheap price antenna cover ELF-ULF even ELF-VLF? - Page 1
Forum section: electronics / rf-microwave

author: nicegirl
author_rank: Contributor
timestamp: March 06, 2022, 04:25:26 pm
content: Looking for antenna cover 1HZ-9kHZ,Aaronia and Narda has very good product but price is not affordable.Is there open source or cheap price antenna cover 10HZ-9KHZ?If YES,any hint is welcome.

author: geggi1
author_rank: Frequent Contributor
timestamp: March 06, 2022, 05:41:20 pm
content: This is basically AF area.If you google natural radio you might find something that fits.Basically it would be a ferrite rod antenna with plenty of tuning capacitor and a anplifier.The amplifier is very often an opamp to get a high input impedanze.

author: coppercone2
author_rank: Super Contributor
timestamp: March 07, 2022, 09:59:49 pm
content: www.vlf.itCan you give us some details so the thread is at least entertaining? I wanted to build some stuff for this for a long time but I worry I might be bored. E-field active dipole is easiest and cheapest. H-field probe is big and expensive.

author: nicegirl
author_rank: Contributor
timestamp: March 11, 2022, 08:19:20 am
content: I am planning to record impulse signal around 10hz-9khz.When I have antenna cover ELF-ULF even ELF-VLF,I can use SDR+ upconverter to record the IQ data then analyze it.

author: profdc9
author_rank: Frequent Contributor
timestamp: March 11, 2022, 08:04:21 pm
content: At these very low frequencies, less than 30 kHz, the antenna is going to be much smaller than the wavelength of radiation that you are receiving (30 kHz = 10 km wavelength). Also, there will be many electrical noise sources, especially manmade noise if you are in a city. Perhaps you are trying to measure the impulses due to lightning strikes?Typically the type of antenna that would be used would be a loop antenna because it will be more immune to picking up electric field noise in the "near field" which is the region less than a quarter wavelength from the antenna (for example, for 30 kHz that would be less than 2.5 km from the antenna). The loop is often wrapped around a high permeability material like a ferrite rod, because this multiplies the area of the loop effectively by the permeability of the ferrite. When making an antenna, especially a low frequency antenna, the resistance of the copper wire used to make the loop must be kept low, because there is an effective impedance of the antenna, and if the resistance of the wire is much greater than the impedance of the antenna, the wire will absorb most of the RF energy that is received. The impedance of the antenna scales with the loop area squared and linearly with the permeability of the material in the loop so there is a big advantage to making the loop bigger and adding the ferrite. By making the loop area physically small, but effectively very large by using a ferrite rod, you can use a short length of copper wire around the ferrite and therefore make the resistance of the copper smaller relative to the large effective area of the loop.There are two ways to use the antenna. The easier way is to place a capacitor across the antenna (usually a variable capacitor such as an air variable capacitor) so that you can tune the antenna to resonance as a parallel resonant circuit. At the resonant frequency, the impedance of the antenna will be high enough to drive the input of the SDR efficiently, which has an input impedance of 50 ohms, and so you can use the SDR to demodulate and capture the signal. However, the signal that will be very narrowband. This kind of approach is used for AM radios and the 60 kHz WWV time signal, or you are trying to pick up some faraway VLF station and need to reject local noise well. The resistance of the wire used is important and often ferrite rods are used to minimize the wire length needed to resonate the antenna at the desired frequency.You can also use a loop antenna nonresonantly. The antenna then looks like a RF source with a very low output resistance in series with an inductance. Because of this large inductance, the effective impedance of the antenna is high. You can build a simple circuit using a low-noise op-amp or a JFET input amplifier to amplify the signal and provide a 50 ohms output impedance suitable for the SDR. The LT1115 or LT1028 may be a good choice for op-amp, and LM4562 would be ok too, but because of the semiconductor shortage, these may not be available. I see that the LT1115CN and LT1028CN are available at Mouser for example:https://www.mouser.com/ProductDetail/Analog-Devices/LT1115CN8PBF?qs=ytflclh7QUWk8x4PZzUG3w%3D%3Dhttps://www.mouser.com/ProductDetail/Analog-Devices/LT1028CN8PBF?qs=ytflclh7QUX4b0rlevS%2F%2FA%3D%3DA generic op amp like a LM358 will not work well.A basic circuit like that is described here can amplify this loop:http://www.vlf.it/easyloop/_easyloop.htmThis uses a large multi-turn loop with a electrostatic shield on it and no ferrite in the loop.Hopefully this is enough information to get started.Dan

author: cdev
author_rank: Super Contributor
timestamp: March 13, 2022, 07:50:59 pm
content: In my experience a mini-whip e-field antenna works well at VLF, well enough to pick up VLF beacons, etc.They can be made for just a few bucks with widely available fairly common parts..And a piece of copper clad PCB and a hobby knife..This can sometimes be a very good antennna for 30 MHz and below. In other words, as a general purpose receiving antenna.They use almost no current, but they do require a bit of power. That said, People arealwayssurprised at how well they work.One thing, however, It needs to be outdoors,It doesn't work at all indoors.On a mast made of PVC or wood is fine.Ideally in free space. The location should be the quietest one you can find.Seehttp://dl1dbc.net/SAQ/miniwhip.html

author: coppercone2
author_rank: Super Contributor
timestamp: March 13, 2022, 08:52:19 pm
content: I will just link this in incase anyone is interested in where I was going with low frequency antennashttps://www.eevblog.com/forum/rf-microwave/best-op-amp-for-active-biased-dipole/msg3435528/#msg3435528

author: cdev
author_rank: Super Contributor
timestamp: March 22, 2022, 05:57:23 pm
content: Why not use a decent computer sound card?You could use Spectrum Lab softwarehttps://www.qsl.net/dl4yhf/spectra1.html

author: cdev
author_rank: Super Contributor
timestamp: March 22, 2022, 07:28:26 pm
content: Why not use a decent computer sound card?You could use Spectrum Lab softwarehttps://www.qsl.net/dl4yhf/spectra1.html