Insight into Integra FODU Design

292020 January

Integra series products come in two different types of full outdoor units (FODU): a fanless design and one equipped with one or two cooling fans on both sides of the handle. How come there is a need for two types at all? To answer the question of why there is such a division, one should get an insight into how the cooling of the inner components is carried out and what makes the Integra FODU superior in terms of corrosion resistance.

Trade-off between corrosion and over-heating

Prevention of over-heating and corrosion are the two factors that have mainly defined the design of the Integra FODU form factor and materials it consists of. Achieving higher transmit power is one of the key tasks to make the product competitive in the microwave industry. In return, however, it produces a necessity to deal with higher power consumption and its associated heat dissipation.

In this case the most obvious solution appeared to be a fully metal enclosure produced with the die-casting method since it has excellent heat dissipation properties. This technology has one important drawback though – the die-casting method can produce gas cavities and/or porosity inside the material which causes internal corrosion and disrupts the hermetic integrity of the enclosure.

Fig. 1 An example of corroded die-casted SAF FODU (old design)

As a result, a seemingly simpler solution turned out to be a design limitation: a fully metal casing used to dissipate excessive heat more efficiently made the radio more vulnerable to corrosion and deterioration of the enclosure material. So instead of going with the metal case we made a choice in favor of an innovative plastic with metal threads for electromagnetic compatibility (EMC). The plastic case provides a more durable design that is not affected by corrosion as it's built from a UV stabilized ABS (hard composite plastic) that contains stainless steel particles.

The Integra FODU was developed to consist of two combined parts: a plastic lower enclosure for housing the components and an upper cover milled from aluminum sheet for heat dissipation. Milling instead of die-casting helps to avoid the formation of inner cavities – making the Integra FODU essentially fully corrosion-proof.

Fig. 2 Integra FODU main parts (new design)

The lower frequency ‘case’

Integra series models were initially specified as FODU without any moving mechanical parts inside. Integra-G and -W models in 15 GHz and higher bands continue to comply with this requirement. However, while developing radios in the 6-13 GHz bands we also had to meet market requirements for high transmit power levels. This initially led to larger casings to house these larger power consuming components and to provide the surface area necessary to dissipate the additional heat.

At one point during development we found out that these lower frequency radios would overheat during thermo-chamber tests at high temperatures. To resolve the over-temperature issue, an additional cooling system was incorporated into the product.

Fans using diaphragm technology which creates pulsating jets of air were discontinued due to the inability to withstand the environmental tests. And so we decided to stick to a more common type of rotating coolers.

Reliable cooling system

To address the usual concerns related to the durability of the components containing rotating parts and their resistance to harsh environments, it's also worth mentioning that the chosen cooling fan model is rated IP68 (which equals to the ability of being fully immersed in water without going out of order) and thus fully fits the outdoor utilization requirements. The cable connection for the cooling system is waterproof as well (IP67).

Moreover, to account for coastal climate conditions, Integra fan-equipped FODUs have successfully passed both of the environmental salt tests:

  • ISO 9227:2006(E) (NSS Test);
  • MIL-STD-810G (TM 509.5 Salt Fog).

In addition, Integra radios were tested by TUV NORD Baltik LLC, LVD Testing Laboratory against:

  • LVS EN 60950-1 Information technology equipment – Safety;
  • LVS EN 60950-22 Information technology equipment – Safety – Equipment installed outdoors.

The conformity to IP66 of the whole radio's housing was acknowledged by these tests.

Fig. 3 Testing of Integra FODU cooling fans in a salt spray chamber

Looking at the statistics of the known repair cases among the FODUs with cooling fans, the failure rate of this component is less than 0.1%. The fans operate according to a specifically programmed algorithm, which allows them to turn on during short pre-defined periods of time: 2 minutes once in a 24 hour period – or when system-critical temperature is reached. This threshold is defined by the system's internal temperature; the critical value is 60°C /140°F. Ambient conditions causing the system temperature to raise that high are uncommon. That's why the expected lifespan of the fans for continuous use can be extended even more at a global scale – since they are only being operated during a small fraction of the time.

The cooling system and its parts currently used in the SAF Integra series products are reliable and support outdoor operation fully in every respect. For now there are no indications that an upgrade is needed – unless new and even more reliable components become available on the market!

SAF Tehnika is a pioneer of compact full outdoor microwave radio technology. Integra is an unbelievably light, energy-efficient, carrier-grade system that exemplifies an outstanding return on smart engineering – the synergy of high competence in radio electronics and materials science. Integration of the next generation microwave radio with high and super high performance antennas into a single unit translates into lower total cost of ownership, as well as less time spent on the installation site, and better reliability of the link even in densely served areas.

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Anton Bezdel

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