small logo Guest Editorial

SETI Receivers: Internal or External?

by Milan Hudecek
Managing Director, WiNRADiO Communications

WiNRADiO Communications manufactures a range of internal and external PC-based VHF/UHF receivers. Either approach (internal or external) seems to have its own advantages and disadvantages. But which type is more susceptible to noise?

On the surface of it, it would appear that internal cards must be more prone to PC-generated interference because they reside inside a PC. In practice, however, the issue is a little more complex.

Most PC-based receivers, such as WiNRADiO, are very well shielded against directly radiated interference. Most of the noise generated by the PC enters the receiver by conduction to the antenna, especially if the antenna is unbalanced or not properly decoupled from common mode feedline currents. Noise can be also injected into the ground loop formed by the power leads, which then radiates back to the antenna.

The biggest culprit in computer-generated interference is nearly always the monitor. Because an internal receiver card resides inside the metal box of a PC, one could almost say that it is actually better shielded from PC-generated RFI than an external receiver!

It is also worth remembering that an external receiver also needs an external power adaptor, so it is more prone to ground-loop earthing problems.

After having sold several thousand units of both types, judging from the feedback we are receiving from customers, it would appear that there is no real practical difference between the internal or external models in terms of susceptibility to PC-generated noise. What really does matter are the details of the user's overall system - in particular, earthing, power supply filtering, antenna placement and its impedance matching to the receiver.

We recommend observing good RF engineering practices when installing the antenna and feedline. (One useful practical trick is to wrap 10-15 turns of the feedline through a large toroid located at the receiver end of the feedline.)

If you are using an external receiver, be mindful of the potential noise generated by the AC plug pack, which is usually included with the unit by the manufacturer (we do include multivoltage adaptors as a standard accessory with all our external receivers). These are usually mass produced low-cost switching units, which need to meet (at least) FCC, UL and CE specifications. But there is a difference between meeting specifications, and being "noise-free" enough to suit weak signal applications. We do recommend replacing these power supplies with traditional linear ones - although much larger, heavier and more expensive, at least one potential source of noise is easily removed!

In any case, no matter what receiver you use, do spend some time experimenting with good RF filtering placed between the PC power inlet and the mains. A little time and effort investing here usually helps greatly to reduce noise caused by those nasty ground loops.

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