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Cloning the famous ARA-2000 antenna for SDR use

Several years ago the company behind the original ARA-2000 antenna, Dressler Hochfrequenztechnik, closed.The ARA series of antennas, like many other products by this company never got a patent, instead it was trusted that no one would bother to replicate it.A bit like the Swiss Army knife, many tried to copy it, none really managed to match the original quality.There is quite a bit of hacking still going on for this antenna, most projects though seem to be abandoned at the time of writing this.I am currently trying to figure out how to create an entire clone that everyone who knows how to properly use a soldering iron can build.There is a lot to consider here...The active element is of quite unusual shape and needs to be wound around a cylinder of a pretty accurate diameter.My initial tests showed that for example aluminium foil with some unavoidable wrinkles already has a negative effect.And a change in diameter of just 2mm means the entire antenna only performs badly for the entire band.This part is thankfully already solved to my satisfaction using thin copper sheets and some stiff plastic sheet.Quite a pain though is the MMIC part - the amplifier that makes the antenna active.There is a ton of MMIC blobs available, either solo or as a ready to go amplifier.Downside is that without really knowing any characteristics of the original is comes down to guesswork.And as most of the cheap SDR dongles won't provide a BIAS TEE I will opt for an external power supply for the amp.I might provide the option for a inline use a bit later though.Why clone the ARA-2000 antenna?For starters you need to forget the mythical stories you might have heard about this antenna."Picks up even the weakest signals!", "Totally linear over the entire bandwidth" and so on...Without the amplifier the antenna is actually not even average in therms of reception performance.My initial tests with a network analyser showed that the anteanna actually is behaving really weird (without the amp!).Although this first bit needs further testing, it seems that most, if not all of the work in the 1.5-2GHz range is done by just the straight connecting strup going from the amp, or in my test case the coax, to the wrapped antenna part.For anything in the more interesting frequency bands it seems that the antenna is not using anything like a discone, whip or ground plane antenna.Instead the 3rd harmonics of a given frequency provide the max power output from the antenna but it arrives at the cable at the right frequency.Especially in the lower frequencies, below 200MHz there is also quite some phase shifting happening.As a passive antenna it seems to be almost impossible to find a frequency to transmit on without using some matching trickery first.But when it comes to size or looks, the ARA outperforms everything you can think of unless you want to constantly adjust the length of your whip antenna.And if you check what is available in real (user) data in terms of noise and signal quality than most other antenna types are far worse.The design provides a wide frequency range with very little noise, almost like a build in filter.Considering that mostly harmonic frequencies are used not that surprising.Getting hooked on SDR means you start little and then you want more and more.Unless you really need the low frequency HAM bands below 50MHz the ARA is a good choice that just makes sense.What is quite surprising in the original is the total lack of protection for strong signals.Sure, we might never need a lightning arrestor because all is enclosed in plastic and has little attraction lightning, but someone hittiing the transmit button close by....I will have to do some more checks to determine whether or not more protection is required.What is the problem with amplifier?For starters, no one really knows what was used in the original - they all just guess based on how well the real design matches some datasheet.Means whatever was used might as well be a custom made solution.I checked a few datasheets for MMIC amps but could not find any useful reference to the handling of things like negative gain, phase shift or a constantly changing impedance.Some however state that a 50 or 75Ohm signal is provided at the output.If I interpret that correctly than those MMIC's not only amplify but also do some matching.In most cases you won't need an amp that works outside what the antenna can provide.Problem is that I don't like regretting things later on ;)So IMHO it would be best to use a wideband MMIC covering all from about 1MHz to a few GHz.Additional filters can then cut off what is not required or where the antenna starts to fail.What is clear by the original design is that the cable shield acts as a ground and most likely also has a balancing function.It would make sense to add a ferrite trap close the the receiver to filter out what the cable might otherwise mess up.Can the frequency range be lowered to get even the low HAM bands?The answer is YES and NO.It is not a big problem to extend the cone shape and then hope to come much lower.Issue with this is the helical, long periodic design.As basically only the 3rd harmonics are used for all interesting frequencies any ARA type antenna going much lower would end to be really long.You can't just make it longer!One thing is to have a full and even number of turns.The original only had two, three turns is bad, four means the entire antenna is slightly longer than your average downpipe for your roof gutters....Other, seemingly logical alternative would be to stick to two turns and to increase the diameter.Apart from the size problem here we would also change the shape of the foil quite a bit and I have not done enough tests with that to provide a conclusion.Are there alternative design options?As it turns out copper pipe is available in 80mm diameters for the use in chimneys as well as downpipes.With a proper machine it would be pretty straight forward to remove what is not used as the active element.Milling a pipe or rod is these days a common thing in many good workshops.But on a hobby level and low budget....One of the best options for cheap test antennas of this design is to use tinting foil - the cheapest you can find ;)Just read the lable and make sure it does not use a metalised film.If it has no UV protection and no tinit at all it is best but hard to find.A little less stiff is the stuff to cover school books or cupboards.Vinyl is bad though!If you look for copper foil in the cheap online places you mostly find the suff used for shielding in rolls of 200x1000mm.Unless you have a really sharp knife or really suitable sissors this stuff is a pain to cut as the glue tends to stick very good to whatever you use to cut through.Don't ever try one of these blade type cutters for paper and pictures unless you put a slight oil film on all cutting surfaces first....In some hobby shops you can get copper foil without any glue in different thicknesses - this stuff is the prefered option.Not only cheaper than the China rolls with glue but you invest a bit more and get a thickness that does not wrinkle right away when working with it ;)Cheap, steel downpipe and cutters or nibblers?I though about and I tried - and I failed LOLUnless you use a pin type nibbler and custom made rig the result is quite bad - at least mine was.What works though is to use thin aluminium sheets, cut them and then bend them around a suitable template.But I ran out of old laminated sings to salvage and the duble sided ones I have left are too much work.What comes next?Well, I have a few rolls of copper sheets coming next month, the cheap glue covered type.This time however I will leave the plastic cover on and use tape to secure the foil to the pipe.A two-stage amp with external power supply is coming too so I can do some more tests in this area.For the time being I will opt for some 3D printed end caps but with a bit of luck can find something easier next time I have time to waste in the hardware store.Excluding cable and a cheap USB or 12V power supply, the current costs of building the anteanna are around $40US.About half of that if you don't cennectors and attach the coax directly.Another experiement I am working on is to use copper tape, 12mm wide, to create the antenna in a semi-fractal style.I am hoping this will provide a high enough gain so the antenna is usable without an amplifier.Right now the biggest issue is to find a really SDR suitable way to deal with strong signal close by.I will keep you update here when I start with the new antenna and upload some pics along the was of building it.

Topic by Downunder35m    |  last reply

TV Aerial Modification

Hi, I have a Yaggi style tv aerial (vertical polarisation, wideband) in the loft I pick up the main signals from a sub-transmitter (forgotten the right word) to the front of the aerial - good reception. I receive some weak signals, depending upon the weather conditions, from the main transmitter at the rear of the aerial. I receive these signals by reflections from the landscape, etc. I am thinking of attempting to boost the reception from the rear by placing horizontal directors alongside the vertical directors. Do you think this idea will increase, decrease, or make little difference to the reflected/direct receptions. Thanks A PS.  Before you ask, Yes I have tried pointing the aerial towards the main transmitter, but I get no signals, due to geographical considerations. I get better reception from the main transmitter when the aerial points 90 degreesaway from it.

Topic by anon_private  

Could an O2 sensor from an automobile be used inside the combustion chamber of a potato gun?

So I was shooting my potato gun after talking about car repair today, when I got the idea. Could an O2 sensor from an automobile be used inside the combustion chamber of a potato gun to achieve optimal fuel/air ratio? Finding the chemistry of the aerosol fuel would be difficult, and I wouldn't know where to start with the computer stuff, but could it be done? If so, has anyone done it? I searched instructables and no one has even mentioned it. I found on a mazda forum that a wideband O2 sensor may detect the air/fuel ration when using propane, but what about aerosol? Propane in a plastic gun is just begging for injury. I use Right Guard sport deodorant (Without antiperspirant) as the fuel, I think it works the best. The primary ingredient is Denatured Alcohol.

Question by MistaStokes  

TV antennas and what is possible indoors

Back in my young years a TV antenna was on the roof, quite big and only required to get 3 or 4 channels at best.The basic design of these Yagi antennas has not really changed since then.Same for the antennas required to get UHF and VHF channels at the same time.They either comes as two seperate antennas with a mixer or as a UHF antenna with some added loops for UHF.To talk some basics:For UHF you can get away with quite small dimensions for your antenna bits.After all a quarter wavelengths here is only around 15cm long.With VHF though we get already 30 to 40cm here depending on the channels.So called bradband antennas claim to be able to get a wide range of frequencies.That however is only true for their dedicated frequency band.If you take a normal whip style antenna then this becomes obvious - just compare the lenghts required and you realise "going the middle way" will mean neither UHF nor VHF would get any usable reception.For a long time now there is the option of so called "fractal antennas" to get better TV reception....Fractal antenna? What the heck is that?The theories behind them are about as old as fractal antenna designs that were not even considered to be fractals.Let me explain with the example of a standard FM radio antenna.For them we use frequencies from about 88 to 108MHz.And of course, for our cars they are always on the roof or mounted somewhere around the wheel arches.Some cars however does not seem to have any antenna depsite having a working radio.In this special case rest ensured they do have one, usually behind the plastic bumper bar ;)I like to go off road every now and then and lost count how many times I had to replace my antenna after getting to close to some bushes and trees.As a solution I used some thin wire to create a L-style anteanna on the inside of my windscreen.And funny enough it picks up more stations than a "proper" antenna outside.Really old design and if you think about it then an antenna with a kink that looks like L is ""self similar" - the definition of a fractal ;)More complex shapes like the Sierpinski models do exactly the same just in one more dimension.Cell phones use them as well as you WiFi router that has no external antenna anymore.Their designs and looks are so different that it is next to impossible to list the currently used shapes here.What it comes down to, when you just take a very basic flat or wire shape is that the straight single segment of the active antenna is split into one or many shorter segments that change the angle.They all have similar relations to the wavelenght still.Means really long segments might be half of a wavelength while the shortest bits are just 1/64 of a wavelength.Think of it (far too simple of course) like amny matched antennas combined into a single one.With that often comes a destinct radiation pattern, usually with a quite narrow beam angle that provides a quite high gain.Are fractal antennas any better than for example a proper Yagi antenna?You can ask 100 people and you get 100 different opinions for this one.This is mostly due to some simple facts like:Not caring about actual comparisons.Not being accurate enough when building the fractal antenna.Using just very basic equippment to compare results.For example when you have a 30 year old and quite small TV antenna on your roof and struggle with digital TV reception than you might want to just replace it all.In some case even the old rabbit ears on your TV set will work better.But take modern Yagi antenna with 12 or more segments and even station that are 100 miles away have a good chance.Comes down to possible gain, frequency match and of course how well the antenna is aimed at the transmitter.Then why is it that some fractal antennas still seem to perform so much better?The big antenna scam....If you paid attention during the intial DVB-T rollout in your country then noticed two things.a) You were told you need a TV or set top box capable of receiving DVB-T signals.b) You need a suitable antenna.Well, A is obvious but for B a lot of people got scammed badly.You see, in almost all cases the frequencies allocated for TV signal did not change at all.And an antenna really does not care if the incoming signal is analog or digital - it only cares about the frequency.But more and more local stations often meant that the trnsmit power is limited.After all it makes no sense to have a 500kW transmitter if the area to cover is only about 20 square km big.So people were told they need new antennas once the noticed digital TV is more like playing Tetris, blocks everywhere....You analog TV had no problem with a slightly weak signal, you did not really notice it unless the weather was really bad as well.All the benefits of digital TV however only work with a proper signal strength.If your areas still has both analog and digital TV channels then have a look on how your local transmitter provides the signals ;)Part of the big scam was hiding a very simple fact:All analog TV is transmitted in a horizontal orientation while almost all digital TV is transmitted in a VERTICAL orientation.Ok, we already gain and ignored it but why would the orientation be important?Your radio antenna is vertical because the FM signal is transmitted the same way - the "waves" if you like go up and down like a sine wave.You still get reception if you turn the antenna 90° to make it a horizontal one but you will realise that weak station are no longer available.The same happened with digital TV.And if there is only digital TV left in your area then simple turning your antenna 90 on the mast to have the single elements vertical instead of horizontal gives you top reception again ;)If you struggle to get all station then just try it out before buying a new antenna ;)Modern fractal TV antennas come paper thin and often with rediculous claims in terms of gain and range.On Fleabuy you can even find models claiming to get stations from over 2000 miles away LOLSome even come with the fake claim of being omnidirectional, meaning it does not matter where you place them or how you orient them.A fractal antenna can't change basic phsyics though...High gain mean highly directional or at least with a very flat radiation pattern instead of being more like a sphere.A high bandwidth means you compromise on the gain and/or the phsical dimensions.For example a thin whip antenna is quite limited in the optimum bandwidth while a piece of tubing provides amuch higher bandwidth but at the expense of gain (and some other problems).A fractal antenna can compromise between the two worlds much better than anything straight.And you would even need to make the traces on the plastic extremely wide because you can simply add different lengths for different frequencies.Means a wideband fractal antenna for FM, VHF and UHF basically is three different antennas combined into a single one.It also means that there is usually always more than just one single element of the antenna that is receiving a signal.If you experimented with WiFi antennas for your router or modem than you certainly found the famous Pringles can antenna - highly directional and with an insane amount of gain.Works exactly like a simple directional microphone.And you could, at least in theory do the same with a TV antenna - if there wouldn't be the problem of the wavelenght.A pringles can works for WiFi because the wavelenght only requires an antenna of about 3cm in lenght.You might not want a row of steel drums with a diameter of around 80cm on your roof just to watch the news though...Ok, if I need a new antenna anyway then should I get a fractal antenna or not?I try to make it simple:Roof option not possible or too costly AND your transmitter is not too far away then try one of the cheap paper thin antennas you find online.If you already have an antenna on the roof that is suitable for the frequencies you need then check it to know if it is still any good.With age and weather comes corrosion and some antennas are connected badly in this regard.Sometimes you are lucky and all it takes is to cut off a few cm of cable to get back to clean copper and to give the terminals on the antenna a good fine sanding or a wire brush treatment.In other cases you might find an amplifier under the roof that has faulty power supply ;)And well, certain animals love to chew through your cable...If all is goo from antenna to TV then as said try to flip the antenna 90° to get it vertical.No, I did not mean to point it up to the sky..... ;)I mean rotate it around the x-axis if X is pointing in the direction of the transmitter...

Topic by Downunder35m