help me find a 4 position switch - Page 1
Forum section: electronics / projects

author: Flump
author_rank: Frequent Contributor
timestamp: November 07, 2014, 04:23:50 pm
content: I need a 4 way Antenna switch for my Ham radio setupbut they are expensive to buy, I have the sockets andenclosure so just need a suitable switch.The switch needs 1 input and 4 outputs and I need to be ableto ground the 3 outputs that are not in use.Frequency range is 0-50Mhzand Max RF power is 100wThis is the kind of thing I want to makehttp://www.ebay.co.uk/itm/Antenna-Switch-Watson-CX-SW4PL-4-Way-Coax-Switch-with-SO239-PL259-Sockets-/371155357366?pt=UK_Mobile_Phones_Communication_Radio_Antennas&hash=item566a954eb6Can anyone suggest a reasonably priced suitable switch ?

author: PlagueDoctor
author_rank: Contributor
timestamp: November 07, 2014, 05:25:09 pm
content: I'm not quite sure what would be Ideal. but ere are some options (if I understand your problem correctly)if you have a common ground you could try a guitar pickup switch.(something likehttp://www.fender.com/en-CA/guitar-bass-parts/controls-and-switches/4-position-custom-shop-telecaster-pickup-selector-switch/)(you may need to add extra shielding yourself)you can get a cheap tv input switch and modify it(http://www.aliexpress.com/item/4-way-AV-Audio-Video-Game-Multi-Selector-Switch-AV-4-Input-1-Output-Switcher/316569350.html?currencyType=CAD&af=ppc&isdl=y&src=Google&albch=Google&aff_short_key=6vYvMRFf&gclid=Cj0KEQiA-PGiBRDRz4jH9o39yZwBEiQAWCBZNWmO9zkKwAJmW4yg41eHl4b7D-VJ3kWcjLO-wr39BYIaArlM8P8HAQ)(though you might want to try and find one that uses coax..)I suppose you could tear apart a cheap voltmeter and use the rotary switch in there (again, it probably has fairly high noise, and it'll look ugly)just some Idea's-Note, I'm not (yet) an electrical engineer. I've only applied for the engineering course, I Have not yet been excepted. so take everything I say with a grain of salt

author: Rufus
author_rank: Super Contributor
timestamp: November 07, 2014, 05:53:29 pm
content: Quote from: Flump on November 07, 2014, 04:23:50 pmThe things you are trying not to buy are suitable switches.A switch which maintains a 50 ohm transmission line through its contacts has to be designed for that purpose and will come with 50 ohm coaxial connectors.If you don't care about transmission line discontinuities then any switch with enough voltage and current rating will do.At up to 50MHz you can get away with some discontinuity - how much I don't know. If it is practical to make your own some HAMs will have done it so I would be searching the web for "homebrew antenna switch".

author: Paul Price
author_rank: Super Contributor
timestamp: November 08, 2014, 09:10:36 am
content: You might consider using reed relays for low power not too much into UHF, if you can find them cheap enough.

author: German_EE
author_rank: Super Contributor
timestamp: November 08, 2014, 09:48:46 am
content: As you need to ground the unused antenna inputs you have two alternatives:1) Use a four pole four way switch. Each switch bank has three grounded connections and one connection to the output with the antenna connected to the wiper. A switch like this that can handle 100W is VERY expensive.2) Use a single pole four way switch and four relays. Each antenna is connected to the relay common connection and the normally closed contact is grounded. All of the normally open contacts are connected together to form the output connection. 100W into 50 ohms is 70,7V RMS 1,4A so finding relays that can handle this should be easy, even at 50 MHz.

author: Flump
author_rank: Frequent Contributor
timestamp: November 09, 2014, 10:44:59 am
content: thats for the ideasif the output of the radio goes to each relay and some are closed and one is openthen the radio output is going to be grounded all the time ?

author: German_EE
author_rank: Super Contributor
timestamp: November 09, 2014, 08:13:01 pm
content: 1) Each antenna connects to the common connection of one relay, there are four relays.2) The normally closed contact on every relay is connected to ground.3) The normally open contact on every relay is connected together and to the transceiver.4) The four relay coils are controlled by a four-way rotary switch.In normal use only one relay will be active, with the common connection connected to the transceiver. All other relays are inactive with the antenna connections grounded through the normally closed connection. When you close down for the night and turn off the power all four relays are inactive and all four antennas are grounded.

author: calexanian
author_rank: Super Contributor
timestamp: November 09, 2014, 09:20:43 pm
content: For HF range basically any switch will work PROVIDED! you never switch it under load. Also watch your breakdown voltage for difficult loads and coil loaded antennas. Mains relay rated over 240 volts tend to have longer breakdown paths, also use at least 5 amp contacts. It gets the mini ones out of the equation. Keep you lead lengths straight. Straight and longer is better than short and all bent up for these frequencies, but HF is rather forgiving. Relays driven by small switches can be cheaper than a proper rotary switch, or cruise ebay. Also cheap ones for CB radio use work fine for HF. Just don't expect the quality to be anything of mention. And DO NOT use a high impedance circuit with a driver transistor anywhere near the unit to run relays. transistors go nuts with unbalanced high power RF nearby. switch the relays right at coil voltage. 12VDC or 24VAC, etc.....

author: Flump
author_rank: Frequent Contributor
timestamp: November 10, 2014, 09:53:07 pm
content: Hi calexanian, thanks very much for the infoGerman_EEhas sent me the Schematic below which looks perfect for what i neednow I know nothing about ARM and Arduino but could I use something like this or shouldI be looking for seperate relays and build from scratch ?http://www.ebay.co.uk/itm/5V-4-Channel-Relay-Module-Shield-for-Arduino-ARM-PIC-AVR-DSP-/181515137470?pt=UK_BOI_Electrical_Components_Supplies_ET&hash=item2a432531be

author: calexanian
author_rank: Super Contributor
timestamp: November 10, 2014, 11:03:20 pm
content: No need for anything digital there. Just a cheap selector switch to apply coil voltage to the relay you want.

author: Flump
author_rank: Frequent Contributor
timestamp: November 10, 2014, 11:56:48 pm
content: Ok are any of these good for the switch ?10 x 10 Amp Mini Power PCB Relay 1 Pole Change Over SPDT 12VDC HRMH-S-DC12V-Chttp://www.ebay.co.uk/itm/10-x-10-Amp-Mini-Power-PCB-Relay-1-Pole-Change-Over-SPDT-12VDC-HRMH-S-DC12V-C-/181261123157?pt=UK_BOI_Electrical_Components_Supplies_ET&hash=item2a34013e5510 x 20 Amp PCB Power Relay 1 Pole Change Over SPDT 12VDC CMP7-S-DC12V-R-Chttp://www.ebay.co.uk/itm/10-x-20-Amp-PCB-Power-Relay-1-Pole-Change-Over-SPDT-12VDC-CMP7-S-DC12V-R-C-/171173312980?pt=UK_BOI_Electrical_Components_Supplies_ET&hash=item27dab979d45 x 30A PCB Automotive Relay 1 Pole Change Over 12VDC CMA31-S-DC12V-CPhttp://www.ebay.co.uk/itm/5-x-30A-PCB-Automotive-Relay-1-Pole-Change-Over-12VDC-CMA31-S-DC12V-CP-/181261802044?pt=UK_BOI_Electrical_Components_Supplies_ET&hash=item2a340b9a3c

author: German_EE
author_rank: Super Contributor
timestamp: November 11, 2014, 12:34:03 pm
content: For HF 100W service the 10A relays will be good enough. A hundred watts into fifty ohms is only 1,4A @ 71V. As for controlling the relays with a CPU you need to be very careful here because of:1) RF from your transmitter getting into the Arduino2) RF noise from the Arduino getting into the receiver3) A program fault causing more than one relay to be energized at once, this would be very badA four way rotary switch is a much better option.

author: Richard Crowley
author_rank: Super Contributor
timestamp: November 11, 2014, 12:40:20 pm
content: By the time you cobble together four relays or some inappropriate switch(es) together, you will have a non-impedance controlled kludge.For the same money you could have simply have bought the proper RF switch and have something suitable for RF signals.I virtually always prefer to make things for myself. But RF switching high power is not that easy if you want to maintain impedance, etc.

author: German_EE
author_rank: Super Contributor
timestamp: November 12, 2014, 08:33:41 am
content: RichardWhilst your comment is technically correct it does not really encourage the home brew spirit of ham radio. I know that the use of relays will create a small impedance mismatch but at HF and 6m this will be small enough to deal with, after all, most automatic antenna tuners use relays in their construction and they work just fine.As radio amateurs we are sometimes accused these days of being 'appliance operators'. Well, if he follows the schematic diagram Flump will have an antenna switch in his shack that he can point to and say "I made that". Will it work better than a commercial unit from (say) MFJ? Maybe, at least all the joints will be soldered and the screws tightened

author: Flump
author_rank: Frequent Contributor
timestamp: November 12, 2014, 10:33:42 am
content: I appreciate your comment Richard but I have not been able to findcheap RF switches and I like making things anywayOkay so I have ordered some of thesehttp://www.ebay.co.uk/itm/10-x-10-Amp-Mini-Power-PCB-Relay-1-Pole-Change-Over-SPDT-12VDC-HRMH-S-DC12V-C-/181261123157?pt=UK_BOI_Electrical_Components_Supplies_ET&hash=item2a34013e55This is the closest data sheet I can findhttp://www.farnell.com/datasheets/1306703.pdfDo I need to add diodes across the coils or just build it exactly as the drawing is ?

author: richard.cs
author_rank: Super Contributor
timestamp: November 12, 2014, 10:52:07 am
content: Those relays look fine, a miniature type is good because they should have less stray inductance compared to somethign like an octal-based relay with it's long wires to the contacts. Normally for an application like this where switching speed is unimportant I would use a diode but here it has the potential to rectify stary RF picked up by the coil, maybe not enough to pull it in but quite possibly the 5mA or so needed to hold it.Given this I would put a 560 Ohm resistor across the coil instead, this will limit the inductive spike to under 40V (so keeps your switch safe) but will dissipate a quarter Watt when the relay is closed so use a half-Watt resistor here, or a higher resistance if the switch you're using to control it all will be happy with the increased voltage.

author: Flump
author_rank: Frequent Contributor
timestamp: November 12, 2014, 12:16:02 pm
content: Thanks richard.csdo the resistors need to be a certain type like ceramic or with wirewound be okay ?editsomething like these be okay ?http://www.ebay.co.uk/itm/50-pcs-0-5-Watt-Carbon-Film-Resistor-Fifty-0-5W-Resistors-100-Ohm-to-8K2-Range-/120845261287?pt=UK_BOI_Electrical_Components_Supplies_ET&var=&hash=item1c22f059e7

author: richard.cs
author_rank: Super Contributor
timestamp: November 12, 2014, 01:32:29 pm
content: These resistors only see the control signals rather than any (significant amount) of RF. Those you linked are fine, as are any other resistors of the right value and power rating.

author: Flump
author_rank: Frequent Contributor
timestamp: November 12, 2014, 02:16:36 pm
content: Thanks, I have ordered themI will post pictures when I do the build