Stampfl Active dipole

In the fall of 2016, I was able to buy an antenna that had been completely forgotten. The Datong AD370. The AD370 is a so-called active dipole and was developed and built in the 1980s. I was really lucky, because this AD370 was only operated under roof and was in mint condition. It was used at the road traffic office St.Gallen in Switzerland to receive the weather messages from the German weather service DWD. This was of course in the 1980's when they didn't have the internet. So in winter the snow clearing vehicles could be made ready in time.
I was very curious about the reception performance of the AD370. Before I could put the AD370 into operation, I had to modify it first. From the factory, the AD370 has a soldered coaxial cable that was only 10m long. Because my antenna feed lines are about 30m long, I built a BNC connector into the AD370. So I could mount it on my 7m high mast. Reception on shortwave was quite good, but did not match the performance of an ALA1530 or Bluewave. On long and medium wave, the AD370 was very noisy and clouded the joy. Something could not be right! The antenna line was fine and everything else was fine. So I tried another Bias Tee. I took the Bias Tee of the Bonito MegaLoop "CPI1000". And lo and behold! The AD370 showed up as a completely different antenna. The LW and MW reception was suddenly crystal clear and could inspire. Not only that, even VLF reception was possible with the new Bias Tee. And that from 10KHz. The alpha signals from Russia arrived here with a clarity that I had not heard before. The limiting factor of the AD370 was the Bias Tee. Incredibly what this inconspicuous part can make.
The AD370 had me so excited that it was my favorite antenna for a long time. Because the AD370 was connected to my online SDR, I got many requests for the antenna. Because of the great interest, I asked Heinz Stampfl once, whether he would not be willing to build an active dipole amplifier similar to the AD370. It was not long before the first prototype amplifier was produced. As with the Bluewave, a small team with a lot of "hamspirit" and heart and soul was involved. Heinz Stampfl built the amplifiers, Günter Mandel DL4ZAO, gave valuable tips for the circuit and I tested the active dipole amplifiers. After several months of development and testing, the final version of the Stampfl active dipole was ready. The Stampfl active dipole is designed as a kit. Only the Bias Tee is built into an elegant, blue anodized aluminum housing. During development, emphasis was placed on transparency and repairability. The amplifier as well as the Bias Tee were equipped with conventional components. The circuit diagram is supplied.
 

The kit comes well packaged in an attractive cardboard box. This is almost too good to dispose of. To store small parts it is perfectly suitable.

In the picture below you can see a case with small parts to build an active dipole from the kit. These parts are not included in the delivery and must be obtained yourself. However, a few screws and washers are missing. The dealer had none left in stock...

Installation in a housing

On the pictures below you can see the finished active dipole. It was not easy to find a suitable and high quality housing for the amplifier. The one shown here is a waterproof IP65 enclosure with neoprene seal. The holes for the radiator holders as well as for the mounting clamp have to be sealed carefully. Teflon tape from the sanitary sector or self-sealing adhesive tape is suitable for this purpose. All components except the mounting clamp are made of stainless steel. The radiator holders are self-made and manufactured by myself. The two antenna radiators are made of stainless spring steel and are each 1m long. During the construction of the housing, great importance was attached to highest stability and durability. All holes on the housing are reinforced from the inside and outside with additional washers to distribute the tension of the screw connection over a larger area. Therefore, this housing variant is not cheap. The sticker "Fenu-Radio.ch" refers of course only to the housing :-)
 

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Stampfl active dipole in comparison with the Datong AD370

The circuit of the Stampfl active dipole is not exactly the same as the AD370. The Stampfl active dipole amplifier consists of two balanced MiniWhip antennas connected together. The Datong AD370 works according to the transformer negative feedback. A detailed description of the Datong AD370 by Günter Mandel DL4ZAO can be found >here<.
Why an active dipole? An active dipole has the big advantage that it works symmetrically. That means, by its second radiator the antenna has its own counterweight and is not dependent on the earth (mass). Due to the horizontal alignment, it also emits much less noise than in vertical alignment. If the antenna is aligned horizontally, it has a clear directivity in the lower frequencies. So you can turn it into the preferred direction as needed. If one listens more on long and medium wave, a rotor would offer itself, in order to be able to turn the antenna.
Both antennas are basically designed for horizontal alignment. However, it is also possible to operate them in vertical orientation. In the vertical orientation, the active dipole receives like an ordinary whip. The difference is that in the vertical orientation the active dipole offers omnidirectional reception with more level. However, the vertical orientation has two disadvantages! Because of the increased level, it noises more. And, the installation of the antenna is more complex. The coaxial cable, which is connected to the active dipole, must be laid horizontally 1-2m to the rear, starting from the connection. Only then can the cable be routed as desired. If the cable goes directly down from the housing connection and is very close to the radiator, the coaxial cable acts as a counterweight and the active dipole thus loses its special properties. Then the active dipole reacts like a MiniWhip with all its disadvantages. See here. >Test UniWhip<
The Stampfl active dipole is like the Datong AD370 an E-field antenna. That means it uses the electric part of the electromagnetic field. Therefore, this type of antenna is more suitable for a low interference environment. In densely built-up areas such as residential neighborhoods or downtown areas, an H-field (magnetic) antenna may be the better choice.
 

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Below you can see both active dipoles on a mast boom. The radiators of the Datong AD370 are 1.2m long. Those of the Stampfl active dipole are 1m. This picture is only for illustration of the size of the antennas. When the antennas are in operation, both are about 1.2m apart. One is then attached to the rear mast boom.

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The reception direction

During the test phase I noticed a strange reception behavior. I noticed that the receiving direction did not correspond to the theory of the dipole! The theory says that the reception maximum is displaced 90° to the radiator axis. According to the picture below left. But with the Stampfl active dipole it was different! The receiving direction was like with a magnetic loop, In the radiator axis. Picture below right. This reception behavior caused a discussion among hobby colleagues, among them also people from the professional environment. No one could say for sure why the reception behavior of the Stampfl active dipole was contrary to theory. Other hobby colleagues reported a normal reception behavior, thus as the theory states. So I compared the Stampfl active dipole with the Datong AD370 and two prototypes of the NTi active dipole I still had there, regarding directivity. All three antennas showed exactly the same reception behavior as the Stampfl active dipole. After further clarifications and inquiries also with manufacturers the following came out. Active dipoles with strongly shortened radiators can show this phenomenon under certain conditions. The exact cause I could not clarify yet conclusively. It must have something to do with the location, ground & space wave and the angle of incidence of the radio waves. Later a hobby colleague contacted me, who noticed exactly the same reception behavior as here with me.
 

Receiving direction as according to the theory of the dipole. 90° offset to the emitter axis.

Receive direction like a loop. Receive maximum in the emitter axis. Like a magnetic loop.

The reception with the Stampfl active dipole

I don't really need to say much about the reception. The Stampfl active dipole is practically on par with the Datong AD370. The reception over the whole long, medium and short wave range was equal with both antennas. There were differences at the fringes. The "modified" Datong AD370 performed significantly better below 30KHz than the Stampfl active dipole. The alpha signals on 11.9KHz were only recordable with the AD370. SAQ at 17.2KHz, however, came up smoothly with the Stampfl Aktivdipol. The picture below is a snapshot from May 1, 2018.
 

The long and medium wave ranges were virtually the same for both. Below a zoomed section from 0-2.1MHz. The Datong AD370 gave about 3-4dB more level in these ranges than the Stampf active dipole. The important SNR, however, was about the same for both.

4 o'clock UTC

Below is a screenshot over the entire frequency range. It is easy to see that the Stampfl active dipole increases in level with increasing frequency. In the upper frequency ranges, the Stampfl active dipole brings 3-4dB more than the Datong AD370.

18 o'clock UTC

Conclusion:

The Stampfl active dipole is an asset to the hobby of long, medium and shortwave reception! In the horizontal orientation, for which it is intended, it receives very low noise and can easily compete with the AD370. It is a real alternative to the well-known MiniWhip's, because it is independent of the earth (mass) and less complex in the structure. What is not so pleasing is the not too good IP2 +43dBm- value! This is not very high and makes the antenna susceptible to intermodulation. This only has an effect in the evening hours, when very strong signals are present in the broadcast bands. If signals are higher than S9+30dB, the same station is audible on twice the frequency. This behavior was also shown by the Datong AD370. However, with somewhat weaker intermodulations.

The Stampfl active dipole can be regarded as the successor of the old Datong AD370.

It can be obtained from Heinz Stampfl directly or from the German Funkamateur.

posted: 04/07/2019