A FOOLPROOF ANTENNA SWITCH

Useful information regarding antennas for SDR products.
Tech_Support
Posts: 491
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Re: A FOOLPROOF ANTENNA SWITCH

Post by Tech_Support » Mon Jul 15, 2019 1:49 pm

Once again, I have had to delete posts that I feel are inflammatory or likely to raise personal tensions. It is a great pity because this debate has been interesting and informative and in my view it is more educational when people are not actually all agreeing. Debate and disagreement feeds new knowledge as long as it doesn't become personal.

Once again I am issuing a plea to please avoid personal attacks or anything that may be perceived to be insulting.

Thank you

Tech_Support

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glovisol
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Location: Piedmont, Italy

Re: A FOOLPROOF ANTENNA SWITCH

Post by glovisol » Mon Jul 15, 2019 1:53 pm

Thanks again, Tech_Support, for defending our Forum,

Gianfranco

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sdrom33
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Re: A FOOLPROOF ANTENNA SWITCH

Post by sdrom33 » Mon Jul 15, 2019 7:41 pm

Hi Tech_Support, I am glad you have eliminated posts with controversial arguments that do nothing but creating confusion and ill feelings among forum members. With sincere appreciation,

Thomas

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vk7jj
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Location: Tasmania

Re: A FOOLPROOF ANTENNA SWITCH

Post by vk7jj » Mon Jul 15, 2019 11:52 pm

This looks like the data sheet for Glovisol's relay, follow the link below and then click the "Download PDF" button

https://www.radiolocman.com/datasheet/pdf.html?di=135545

Contact material = Silver alloy AgCdO、AgSnO2、AgNi
Contact resistance = 100mΩ(1A 6VDC)
Sealed construction as opposed to just "dust proof" is shown by the -SL- designation on the relay case.

I can't see if my Songles are potted or how they are sealed because they come attached to PC boards with the drivers on, maybe Glovisol could turn his upside down and see what it actually looks like?

The mechanical and electrical endurance numbers look like overkill for the modest number of switching movements when used as an antenna switch and the contact resistance remains as spec. for the given endurance.

No wetting current is specified as being required so it's not a case of being designed to require current flowing through it, that's not an issue anyway because Glovisol's circuit shows plenty of Tx RF flows between J1 and J2 or R1.

https://en.wikipedia.org/wiki/Wetting_current

The worst case scenario as pointed out by Ian is that the Rx might be rendered deaf with rising contact resistance caused by oxidation and he's alerted us to that, which is good.

In the absence of any evidence to contradict the manufacturer's spec. it must stand.

So what does that leave us with? A really helpful and neatly designed foolproof antenna switch.

Phil

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glovisol
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Location: Piedmont, Italy

Re: A FOOLPROOF ANTENNA SWITCH

Post by glovisol » Tue Jul 16, 2019 6:48 am

THE HAIR IN THE EGG METHOD

It is a long time that I had in mind to write what follows, but I needed a third party who would independently confirm that there is nothing wrong with the "Foolproof Antenna Switch" which was erroneously labelled "A Fool's Antenna Switch" by one contributor to this thread. Now Phil has done what anyone with common sense and good faith should have done BEFORE condemning without appeal this design: study the relay specifications and/or simply ask me to produce them. But this never happened.

We have a way of saying in Italy: when someone wants to criticise and find fault at all costs, we say: "cerca il pelo nell'uovo" which in English means: "he is looking for a small hair in the egg". This is common millennary winsdom: you cannot open the egg, otherwise you break it, so you do not have to prove that the small hair is there (which is of course impossible) you just declare it is there, this is enough.

What could be simpler and more effective than using an ad hoc amplified USB cable to place the RSP even 30 m away from the laptop and near the antenna outlet? A guy comes along and starts writing several page long treatises describing the untold dangers posed by lightning stealing into the house and killing the reckless users of such a dangerous device. This was simply grotesque, the hair in the egg being, in this case, the danger coming from lightning. So if you have to criticise and have no arguments, you change the technical grounds on which to exert your action.

The same happened with the "cousin" of the first guy. Now the hair in the egg is the danger represented by dry contacts (which are not dry, by the way, because they switch RF currents when diverting TX power away from the RSP into a load resistor). So pages of comments, heaps of references, own biographical details just to try and find the mitycal hair in the egg. When the second guy leaves the Forum for good on "false antenna pretences", here comes the Defender and we have the improper exchanges of past days, fortunately cancelled by Tech_Support. Phil has gone to the heart of the matter: nothing wrong with criticizing a post on technical grounds, but if technical is, technical must be and before looking for the hair, one should at least look for the spec.

Yes, Phil, these relays, I have plenty in my bin, are hermetically sealed with a voidless moulding from top to bottom. Finally I have been using these same relays on board of my sailing boat since 2010: they are used in critical circuits like the drive logic for the anchor capstan, or for ON/OFF switching my fridge, or for ON/OFF switching of my LED night sailing lights on the mast top, or for controlling my Autopilot by means of the GPS and I never had a failure. What better test for relays than operating them in the corrosive sea environment?

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Mike2459
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Re: A FOOLPROOF ANTENNA SWITCH

Post by Mike2459 » Tue Jul 16, 2019 8:40 am

Hello Phil & govisol. Hope there are no hard feelings but in any event I had to chime in on this discussion. Hope you don't mind. So here goes.

You wrote: "which are not dry, by the way, because they switch RF currents when diverting TX power away from the RSP into a load resistor" These are not the pair of contacts I was referring to (although I don't think Tx will last much longer than the Rx) . My main concern is the pair of NO (normally open) contacts that supply the Rx. These are not the same contacts. The common stationary contact set has a contact on each side of it's mounting. It engages the NO with one side, the NC with the other). Every small form C relay that I have seen, as well as auxillary contact sets on power contactors use this arrangement. It allows for a simpler design and increased reliability. In some relays they are mounted horizontally in others vertically like the relay below.
relay.JPG
relay.JPG (46.91 KiB) Viewed 571 times
IMO these relays they will probably never receive a signal that will satisfy the minimum switching requirement. Which to no surprise, Songle does not show in their data sheet. However for a Panasonic DS relay, which is marketed for telecommunications, measuring devices, office equipment, computers and related equipment:

Min. switching capacity (Reference value)*1 10µA 10mVDC
1* This value can change due to the switching frequency, environmental conditions, and desired reliability level, therefore it is recommended
to check this with the actual load. (TX/TX-S/TX-D relay AgPd contact types are available for low level load switching [10V DC, 10mA max.level])

I doubt if the Songle can even come close to that low value.

As to hermetic sealing as you just wrote again. These relays are not hermetically sealed they are epoxy sealed at the base of the relay. True they are classed as a sealed relay. But that does not imply they are air tight. If you look carefully at the top of an actual relay or the photo in the Songle datasheet you will see a vent small hole in the top left corner of the relay. I have several GP relays that I've accumulated over the years. All the epoxy sealed ones have a small vent hole on the top of the plastic case. A piece of 28 ga. solid wire will pass through it easily.

That's it for me guys.

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glovisol
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Location: Piedmont, Italy

Re: A FOOLPROOF ANTENNA SWITCH

Post by glovisol » Tue Jul 16, 2019 10:09 am

Hi Mike,

No hard feelings at all, in fact I am very glad we can seriously discuss this antenna switch. First of all we must clear possible misunderstandings.

1) This antenna switch is NOT an antenna commutator in the usual sense. THERE IS NO CONNECTION AT ALL TO A TRANSMITTER OUTPUT. What the relay does is to DISCONNECT the RSP input from ITS OWN ANTENNA and SHORT CIRCUIT TO GROUND, through a limiting resistor, the RF power that might be coupled from the separate transmitting antenna to the RSP receiving antenna.

2) Thus we have ONLY ONE relay contact, the one shown in all schematics.

DRY CURRENT
From the above I can agree with you that, although some current will always flow when the transmitter is active, due to the RF current caused by the coupling between transmit and receive antenna, no current is ever flowing during receive, so there is the theoretical possibility that in the future the contact making to the receiver input will develop a high resistance.

CHEMICAL FAILURE
I do not know and nobody really knows, probably not even the manufacturers, what is the degree of isolation from the external atmospere the relay has. This is important, because it is only through chemical attack and hence oxidation, that the relay contact may fail. You have noticed a tiny hole on similar type relays. I confess I have never seen one, but I shall check when I have a chance. On the other hand, as already explained, I have been using this relay in a salt, marine atmosphere with no problems for several years.

So, to conclude, yes, there is the theoretical possibility of contact failure due to contact oxidation and zero current, but this relay is not being used on an aircraft or on a spaceship, it is used in a radio shack and can be easily replaced. Furthermore is is very easy to eliminate dry current operation. Let us add two 24 KOhm resistors (in red) and have a 0.5 mA current thru the contact all the time. The 12 KOhm resistors have of course no influence on system RF operation.
Attachments
Modified antenna switch.jpg
Modified antenna switch.jpg (90.06 KiB) Viewed 545 times

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Mike2459
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Re: A FOOLPROOF ANTENNA SWITCH

Post by Mike2459 » Tue Jul 16, 2019 4:39 pm

glovisol:

This would certainly help solve the contact resistance problem. However I see the need for two DC blocks (one at Rx In, the second at Rx Out) to prevent the relay contact biasing current from being shunted to ground by any upstream or downstream shunt inductors or RF transformers in the antenna circuit.
Secondly this will necessitate a well filtered 12 volt supply for the antenna switch.

Mike

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sdrom33
Posts: 123
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Re: A FOOLPROOF ANTENNA SWITCH

Post by sdrom33 » Tue Jul 16, 2019 6:42 pm

Hi glovisol, I agree with Mike that two series capacitors are needed to avoid the danger of shorting the bias voltage. I also note that with the circuit as is now the antenna common lead is shorted to the 12 V supply common lead and this could bring noise in. It would be advisable to put a choke in series with this lead to leave the common antenna lead floating. Further agreed that a simple 12 V supply is necessary, surely a linear type, even though voltage stabilization is preferable, but not mandatory. Switch mode PSU's to be carefully avoided.

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vk7jj
Posts: 206
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Location: Tasmania

Re: A FOOLPROOF ANTENNA SWITCH

Post by vk7jj » Wed Jul 17, 2019 7:53 am

I wonder how the MFJ-1708 antenna Tx/Rx switch (and it's variants) would stand up to the same level of scrutiny, relays included.

Featuring prominently in these pages and with an application note on the SDRPlay site, no less!
MFJ-1708.png
MFJ-1708.png (86.91 KiB) Viewed 435 times
I've several MFJ products in the shack and while they have become a ham household word for providing lots of affordable gear and pioneering popular products like their first antenna analysers, that household word has included the phrase...

https://groups.google.com/forum/#!topic ... s0Z7MVOAjg

Phil

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