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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.

Discussions

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John T MacF Mood
John T MacF Mood

8 months ago

Your article is fascinating, but having not seen the original, difficult to visualize. I'm imagining a discone with the bottom part skirted rather than elements approximating the discone ground plane.

If you manage to finish the project, I'm eager to see the finished project and see if I visualized it badly or not! Best of luck on this one, it sounds great.

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Downunder35m
Downunder35m

Reply 8 months ago

It is not an antenna with radiating elements of the classic desing.
The ARA is a metal-foil "strip" wrapped around a cylinder.
Just type "ara-2000" into your favouite search engine and check the image from the results.
There are quite a few showing the inner workings.

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EricJ97
EricJ97

11 months ago

As for the LNA, maybe an existing design can be adapted to a circular circuit board similar to the original, and the design could be uploaded to oshpark so that anyone can order the circuit board from them and the components from a normal electronics supplier and assemble at home?

The SPF5189Z seems easy enough to implement and you could probably make workable layout where it is the only surface mount component, for easy home assembly.

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Downunder35m
Downunder35m

Reply 11 months ago

It is not so much a question of the shape of the circuit board but a question of matching the LNA to the antenna.
You could as well place the LNA into a metal box and have it connected with some coax...
The original LNA was a purpose made design for the ARA, what I found on the market so far is all generic and requires more or less complex filters to be included.
Did some more tests with the few LNA's I have so far and it seems overloading is a major issue.
For example, when a strong UHF signal is present it causes the LNA to go haywire well outside this strong signal.
Sometime well dwn into the HAM frequencies.
Waiting to get a proper frequency analyser borrowed for a few days.

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dpavliv
dpavliv

1 year ago

Very interesting! Any news?

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Downunder35m
Downunder35m

Reply 12 months ago

There are some news indeed, just not really much time on my end :(

Modern LNA's seem to struggle a bit with the antenna itself.
I can't get my hands on an original, due to their prices and limited numbers.
But I have to assume that even the original was not as perfect and inear as it was claimed.
For example: The bandwidth is mainly limited by the LNA, not the antenna shape or length.
It is quite possible to get this antenna to work in the 2Ghz bands and above, but not in a really usable form as the noise will get too much.
Going low into the Ham frequencies is possible as well but makes the antenna quite huge and costly.
Means the original design seems to be a compromise to keep dimensions small while being able to use a dedicated LNA to amplify whatever goes into the antenna.
Sadly in my tests I struggle with interference from strong stations.
Going to think it is the LNA to blame here so I am waiting for some different types to test.

Design wise I made a tiny progress as well.
Aluminium foil works about as good as copper foil.
But in return it is pain to keep wrinkle free and to make a proper solder connection without making a hole in the foil.
Anyways, using the copper foil with the protection still on the glue makes things much easier.
For both methods however you need a perfect match of template and cylinder diameter.
Just one mm more on the diameter and it is next to impossible to set the foil correctly :(
With the new LAN's I will try to customise the original desing to use a standard PVC pipe of 50mm instead of creating a tube from plastic sheet.
It won't perform, so I work my way up in the diameters until I get acceptable values out.

Last but not least antenna material.....
Trust me it is quite costly and time intensive to make several iterations of this antenna just to figure out how to make it work close enough to original performance specs.
For obvious reasons I would like the final project to be easily replicated even by people without trade skills and a corresponding workshop.
My current idea is to use template on the cylinder to cover the UNUSED areas.
The actual antenna would then be a matter of spray on metal protection - Zink or aluminium spray as used to protect metals from corrosion.
Solves all the soldering problems on the plastic as well because we can just "paint" over the required contacts ;)
The LNA placement seems to be cruicial by design as well.
It really needs to be on a metal plate - those cheap ones on a normal circuit board won't cut it!
Additionally it also matters how the LNA is connected to the cable.
Antenna (LNA) ground and cable shield are connected and result in more like a dipol configuration of sorts.
When I placed the connector with the LNA inside the tube (electronics above the bottom end of the antenna foil) the output was bad and full of noise.
Placing it too far down means the signal strength goes dow badly.
Right now I am tending to include a length of metal tube connected to the base plate with the cable inside.
Provides some independent ground and solves the problem of placing the connector at the right height.

Just to be clear:
So far the performance ofmy test antennas was everywhere between rather crap and quite good.
But never over the entire frequency range as stated for the original.
Means I need to work on the tuning to find out what part of the foil shape is responsible for certain frequency ranges.
The omnidirectional style however makes these rather complicated.
In my tests I often got a much stronger signal just by rotating the antenna a bit.
Some initial tests with just copper strips forming the outer lines of the antenna got basically the same reception properties.
If i find the time I will test if a circular strip antenna in the ARA shape would be sufficient.
Assuming it is then costs would be going down quite a bit :)

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Downunder35m
Downunder35m

1 year ago

I finally got all the parts together to make another clone.
Currently working on the Bias Tee and LNA to make it somehow fit for external and Bias Tee power supply.
Gain will be adjustable through the phantom voltage - from 5-12V it make 0 to 32DB.
For the box with the Bias Tee there will be an adjustable attenuator to lower the signal strenght by 0- 20DB.
I decided on this option to have it as flexible as possible for everyone.
Be it fixed 12V power supply for the antenna with the attenuator, Bias Tee fed antenna with or without attenuator or all in one box on the receiver side.

I decided to use "ready to go" modules for the LNA and Bias Tee assuming a lot more people will be interested if the soldering of SMD parts is reduced to a minimum.
As there will be left overs from the copper foil I will try to include these in the design for the boxes to have proper shielding.
Metal enclosures are of course the best option but again not always feasable for the hobby builder.

Right now I am still struggeling a bit with the matching antenna and LNA.
If the original claims are true than the antenna must provide a half decent signal throughout the bandwidth.
In terms of my clone for about 50-2000MHz.
Slight variations in the alignment when placing the foil result in often weird behavior.
Trying to find the "sweet spot" that works equally good for all frequencies is a challenge.
Same for the required resistor between antenna and LNA input.
Both must be a close enough match or there is "black spots" in several frequency areas.
I hope I get it close enough over the next few weeks to provide some easy to follow measurements and templates.

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Downunder35m
Downunder35m

1 year ago

Great news!
Just bought 5 variable attenuators (20db) on Ebay :)
Sometimes you just need to be lucky to find these old manual gadgets.
20db might not be much but should be enough to deal with stronger signals.
Size is basically identical with what Dressler used in their bias tee to power the amplifier.