Useful information regarding antennas for SDR products.
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Post by glovisol » Mon Jul 22, 2019 4:57 pm

Now that luckily a lot of confusing information on this thread has been removed, as a service to those who did not follow the cancelled discussion and may be worried by the bits of controversy left over, but still need to build and use the Antenna Swich in question, I can confirm that this switch will work perfectly well and with excellent reliability.

The doubt that was introduced is that since the relay contact, when making and holding in the "receive" position, would not pass any current (this is called a relay contact with "dry current" and it is known by experience that sometimes contacts not passing any current may fail and present high resistance because of surface oxidation with age) then the relay would fail.

While the above is true in the general sense, we must put reasonable boundaries to our problem, these being: (a) projected lifetime of the device and (b) relationship between cost and lifetime.

The relay used in this project is a general purpose encapsulated relay having the following characteristics:

Manufaturer: Songles
Contact material = Silver alloy AgCdO、AgSnO2、AgNi
Contact resistance = 100mΩ(1A 6VDC)
Sealed construction shown by the -SL- designation.

Sealed means that the relay is potted, so the contacs are well protected by external atmosphere. It was stated that these relays have a small hole in the case, but this detail is not true for the relay in question and I doubt this to be true in general, as the hole would defeat the purpose of encapsulation. This relay, complete with driver circuit and mounted on a PC board retails for $1.50 on Ebay.

Even though "potted" is not "hermetically sealed", still this enclosure effectively insulates the contacts for a period of several years. Apart from this, the shelf life of these relays, with the contact alloys listed above, has a rating of 200,000 hours, or in excess of 22 years. While the relay is stored, no current flows through the contacts, but the contacts still exhibit the rated low resistance throughout the rated life. So at the first instance, we can project a lifetime of 22 years.
If we now consider the number of switching operations, this GP relay is rated for a minimum of 100,000 and it can be easily calculated that, even with a large number of operations, the relay will last in perfect operation for a minimum of 4 years . Furthermore, when and if the relay will fail, the only detrimental effect will be de-sensitisation of the companion SDR receiver (but no damage to it) and a replacement relay (costing $1.50) will solve the problem.

Doubt was also passed on the insertion loss of the device. Of course this parameter has been carefully tested in the course of the construction phase and has been measured to be less that 0.2 dB in the range 0.5 to 30 MHz.

Finally it is true that a more expensive relay, with Silver-Palladium contacts and hermetically sealed will last longer than 22/4 years, but certainly at a significantly higher cost, so my conclusion is that the antenna relay under scrutiny is a perfectly good design, not requiring any "improvement" whatsoever.
Last edited by glovisol on Thu Jan 01, 1970 12:00 am, edited 0 times in total.
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