Someone mentioned making your own antennas. Is it just as good to do that?
If so, what’s the best way? I’ve got plenty of copper wire around of various gauges. I also have a couple 24" /62cm antennas from MHz frequency scanners. They have female BNC connectors on them. I don’t know what size connectors, but the male jack measures 1cm across.
I’m trying to get the whole range of frequencies, and want to make sure I’m getting the lower half of the MHz spectrum.
Can one antenna do this, or does it require multiples?
I’ll be getting my Lime soon. Usb A. Any recommendations on the cables to get? One person mentioned getting better results with a certain kind of Usb cable over the other, but I not sure where that post is.
Oh, the Pandora’s Box you are opening!
Wideband antennas are out there but they don’t solve the problem with wavelengths under 30 MHz being more than 10 meters long. Log periodic dipole arrays are higher gain and directional. Discones are like upside-down monopoles and are omnidirectional. The lower frequency sets how big they are and they get really, really big for 10 MHz and below.
I would recommend to read a chapter dedicated to antennas in “The Hobbyist’s Guide To RTL-SDR, Third Edition” book. It’s brief, practical and describes a few wideband antennas you are interested in. The book is available for purchase from Amazon and eBay. Naturally most likely there are pirated versions available online for free, but this book is so good you may want to have it on your bookshelf.
I’m left wondering different about different devices. I have a MHz and lower GHz frequency counter. It measures the full range. How can it do this with an antenna at 24" ? If an antenna needs to be fairly long below 30Mhz for example?
The same for other frequency generating software like signal generators and frequency therapeutic devices for instance, which might only have a wire going to a short electrode.
Great questions
It depends what you are trying to get out of the signal. The more information that is in the signal, the more you have to separate the information from the noise. If you are only trying to measure the frequency you don’t need much information to do that. When you are trying to receive a signal with 20 KHz bandwidth of information you need to have a better spread between the noise and the signal so you can detect the differences better. 60MHz, even harder. A tuned antenna will react to the signal in a favorable fashion while rejecting energy not in it’s band. This makes more clear the information in the signal. This is the big trade off in wideband antennas. You get a stronger signal of the frequency that you want but are also getting stuff you don’t want that will probable be noise to you. If you are not trying to get information out of the signal other than the frequency you can take the small signal and amplify it and filter out the noise. The more information, the harder to filter out noise.
For transmitting the situation is different. You want to efficiently couple the energy from your generator to the load. If a big pipe of water flow comes to a smaller section it has a reflected wave bounce back. This is a resistance and an extra load on the source. the same thing actually happens if it comes to a larger section suddenly. RF behaves in a similar way. A good antenna changes the current and voltage in a transmission line to electromagnetic waves radiating away at just the right load to seem transparent to the transmitter. These resonances are like strings vibrating to make sounds. There are harmonics and you can filter them and you can have things like cymbals that vibrate at many frequencies just as well. You can yell into a cymbal and hear your voice in it, but a tuning fork will just return one tone. Antennas are like that.
An isotropic antenna, which doesn’t exist, radiates in all directions equally in a spherical shape. If you have a real antenna that concentrates its pattern in a direction it has gain. Antennas can be made to focus their pattern into a cone like a flashlight beam, and are even called beam antennas. These have high gain. A dummy load looks like a perfect load to the transmitter but the signal just turns into heat in it. It has no gain ;). So if you want the RF to do work, It will be transmitted like radar or absorbed like a microwave oven.
ARRL used to have an antenna manual that was packed full of information, theory, formulas, projects, examples, etc. I have one from 25 years ago, there may be a new one.
If you come up with an infinity tuning compact wideband beam antenna be sure to put it up here first.
Thanks guys, some great information here! Axeman, thanks for the in-depth yet noob-friendly explanation.
I’ve been reading about this, and watching youtube videos. I ordered some UFL to SMA pigtails (5) from the interwebs and they should be here today.
Does a 1/8th wave antenna work well? Or even a 1/16th for that matter? Or how much difference it there from that to a 1/4 or 1/2 wave? Or should I stick with the bigger one?
At 75Mhz (probably the lowest I have to go), a quarter wave I think is about 3 feet. So I wondered if cutting it in half for portability and maneuverability would be ok. I still will need to find some jacks and the thinner coax I guess to make the rest of an antenna. But will likely need to make a few different ones to experiment.
It would be cool if could have a group of different antennas and Lime could just pick the best signal for what you are scanning.
What would be the best kind of cable, jacks or adapters to put a wire or rod antenna on the SMA connections? I’m used to soldering and working with cable to make Tesla-based equipment.
I figure I can go to lowes and see what they have too.
The science of it is interesting, but not easy to find clear answers. I like research, but I’ve been too bogged down lately to do the in depth study I normally do. I wish it was easier. where you could just pick out what you need for X frequencies and plug it in.
Oh, before I forget, I saw someone had a big coil of wire as an HF antenna. There might have been two coils, zip tied together in a big loop each. Does that work? Can an antenna be bent or coiled up and still work well? That would save a lot of space.
If 75 MHz will be your lowest frequency, keep the 1/4 wavelength (3 feet) as this is the lowest electrical length you can use to radiate that frequency at resonance. If you’re looking to reduce the size of it, you can use a lower gauge wire (20 AWG) on a fiberglass, plastic, or bamboo rod and wind the antenna to be physically shorter (from 36" to something around 20" or less), but electrically the same length. You’d need to keep the windings to about 1/4" apart as you wind it and this will build a ‘loaded’ antenna. Loaded antennas do not perform as well as a straight electrical length antenna, but you’re trading that small loss in performance for compactness in the antenna design - it’ll still radiate. If you bunch up the windings a little (use 1/8" spacing instead of 1/4" spacing between winds) you can achieve a longer wavelength (maybe something on the order of 1/2 wave or 6 feet of wire) and radiate more energy, but the length will be 36". So it won’t be as compact as a straight electrical length antenna at 36", but it’ll perform better than the 1/4 wavelength antenna because you’re at a 1/2 wavelength. Antennas are a real science - there’s a lot of tricks you can play with to get what you want, but you have to be ready for trade-offs as a result of something you need (compactness) to radiate at the frequency of interest.
Have fun with it and let us all know if you need anything else pertaining to antenna physics -
I found this youtube video in one of the comments on hackaday, and this demonstration from 1981 on antennas is nothing short of astounding. Well worth a watch, even if you think you are very knowledgeable about antennas: Everything you always wanted to know About Antennas (but were afraid to ask) || Frank Rutter K3AW (it is just over an hour long, and it is well worth investing that time).
This is fascinating. So I can get 1/2 wave performance in shorter antenna by winding the longer 1/2 wave wire at 1/8" spacing around a non-conductive rod.
Yep - Antennas are probably the most flexible medium you can design, you just have to be aware that when you design an antenna to be portable/compact there will be deficiencies in the design because you’re not making it the electrical length that a normal antenna would be. By winding an antenna to be physically shorter than the electrical length, you will encounter some deficiency by the windings - but not so much that the antenna itself can’t work, it just won’t be as efficient as a normal electrical length antenna. You will experience loss winding to winding (becoming more inductive by design and the capacitive distribution losses winding to winding) but those are generally small and most of your transmitted power will actually get radiated - it just won’t be as efficient as a straight-wire antenna cut for the resonance point (in this case 75 MHz). But don’t let winding a physically shorter antenna scare you - as I mentioned, if you build it for a 1/2 wavelength it will deliver more of your signal into the air than a 1/4 wavelength - so build it 6’ long and you will get better performance in receive and transmit.
And @mzs is right, do check out that video - it’ll light your mind on fire with ideas for antennas. If you have any other questions just ask away - we’re all here to help.
Thanks, I was exactly aiming for in-depth and noob-friendly in my reply.
Antennas can receive either the electric field or the magnetic field. The changing electrical potential in the antenna’s space changes the charge at the feedpoint, similar to a capacitor. Or, the changing magnetic field can push electrons around in the antenna, similar to a transformer. The coil you saw was probably a type of magnetic antenna. These are not used a lot for transmitting but they can make good receive antennas.
The Lime was made to be a very broadband receiver and in its original use more for the VHF, UHF, and SHF (WiFi, Cellular, and the like) bands. It was found to be a pretty good HF receiver early on using the RX1_LO or RX2_LO ports and even further an inductor was removed to make the Lime on par with some of the best SDRs out there. One could use the RX1_LO port for the purpose of HF (with the inductor removed) and just leave the RX1_HI, RX1_WB, RX2_HI, RX2_WB ports completely alone and have a receiver that’s capable of 0.5 MHz to 3.8 GHz reception - - better than most SDRs that are out there. And I haven’t even discussed the transmit ports or the reference ports on the Lime yet…!
Start with one antenna to see what your results will be - best to prototype with one design and play around with it until you get it the way you want - then build a second one. Actually, it would be better to build one antenna that’s good for your Lowband work and have another made for the Highband work you’ll be doing so you have a lot more spectra to play with.
Wooden dowels are fine to start out with, but if these will be used outside you’ll want to have Fiberglass rods instead because they won’t degrade in moisture. Also, winding some black electrical tape over the winds of the antenna won’t cause any issues, will hold the winding spacing, and will also allow you weatherproof the antenna for outside use (temporarily - again, Fiberglass works best when you can find that kind of rod).
No issues with Tesla effects - the power from the LimeSDR is too small to create that in transmit. But if you use these antennas with an amplifier above 50W then there would be issues - more on that later.
Good reading in either book - just one more recent than the other, but still really good information that doesn’t change too much (it’s physics, you know)…
Varnished wire is actually preferred than bare wire for antennas, and once you have an equal spacing for your winds please do put a layer of tape over it to keep them in line and that they won’t move.
I made this last night. Fiberglass was too far away so went with the dowel rods. I was a little surprised how much I was able to wind on the dowel. Its 1 yard long btw. It got 2, one for Tx and Rx. Figured I’d do one first and see how it goes.
Using #19 magnet wire I had laying around (who doesn’t?), I started winding at 1/4" lengths. at 22.5" I got 13 feet wrapped on there. A full wavelength according to www.wavelengthcalculator.com/
I kinda figured it would use the whole dowel, but there’s a foot left.
Next to it, is the SMA with some kind of Coax. I got those from a surplus store. I couldn’t find clear info in which cables go best with SMA jacks or which tool to crimp them with, so I went with some pre-made cables. It was the only skinny coax they had.
I figure I can use some regular electronic connector pairs you get from lowes, to connect the magnet wire to the SMA coax, and then to the pigtail which will go to the LimeSDR.
I still have to make a case for the Lime and find some USB cables.
So what do you guys think?
This pic is otherwise horizontal. Not sure why the forum turns it sideways.
For a half wave antenna you would use two (a little longer than) quarter wave lengths and start in the center (trim for low SWR). The winding should be the opposite for each leg. You will end up with a balanced antenna so you will need a balun to hook up coax. The antenna will be about 300 ohms so it needs to be stepped down. Fortunately there are 4:1 baluns that are used all of the time for this purpose. 75 ohms is close enough. Old style TV antennas were adapted from the parallel wire lines to coax with these cheap and easy devices, but they won’t transmit without burning up. The pattern of these dipoles is directional to the sides. If you stand it up vertically it is omnidirectional with a null top and bottom. A great old article explaining baluns.
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