Cloning the famous ARA-2000 antenna for SDR use Answered
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 LOL
Unless 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.