RF Switchbox - Page 1
Forum section: electronics / projects

author: Bicurico
author_rank: Super Contributor
timestamp: September 03, 2017, 07:16:29 pm
content: Hi,I am considering building an RF Switchbox.I have 8 cables providing different signals (SAT, CATV, UHF/VHF, amateur radio, etc.).All use 75 Ohm cable. i would like to have a box where all these 8 cables are permanently connected to and that would switch one of those 8 inputs to one output connector (perhaps two in parallel - one in the back, one in the front).The buttons could be mechanical, but it would be nicer to have touch buttons with signal led. They could be controlled by an Arduino, which on top could be controlled alternatively through RS232/USB by the computer.Questions:- What kind of relais should I use for signals up to 2GHz?- How would a schematic best look like?- Any considerations, advice?This is just an initial idea and I am brainstorming...Regards,Vitor

author: Benta
author_rank: Super Contributor
timestamp: September 03, 2017, 09:05:19 pm
content: A bit difficult, because you mention CATV, SAT and VHF/UHF, which are very low power, but then mention amateur radio which could be several watts.Can you be more specific? Is it only amateur radio reception?

author: Bicurico
author_rank: Super Contributor
timestamp: September 03, 2017, 09:19:29 pm
content: Yes.The amateur bands are just for reception using a broadband antenna.All signals are under about -40dBm.

author: Benta
author_rank: Super Contributor
timestamp: September 03, 2017, 09:36:41 pm
content: I'd use low power SPDT reed relays. Double-throw to short unselected input signals through 75 ohm termination to ground.Reed relays (either SIP or DIP) have straight-through signal flow and generally work very well in RF applications.

author: BigBoss
author_rank: Regular Contributor
timestamp: September 03, 2017, 09:37:35 pm
content: -All bands can not be combined with the same characteristic impedance because while SAT,CATV are using 75 Ohm, Radio Amateur bands use 50 Ohm.-SAT Reception needs DC thru because of LNB supply ( 13/18 V and 0/22kHz )-Radio Amateur Transmitter outputs are pretty high and using silicon switches wouldn't work due to very high RF powers.Instead, mechanical relays are suitable ( Teledyne) for this purpose but they are extremely expensive.-Insertion Loss of that combiner/switch will be involved directly into Noise Figure so sensitivity will be degraded by insertion loss ( dB ).

author: Benta
author_rank: Super Contributor
timestamp: September 03, 2017, 09:45:38 pm
content: Quote from: BigBoss on September 03, 2017, 09:37:35 pm1: it's only for reception. A 50/75 ohm mismatch won't warrant impedance transformation here, and will probably give more loss than the mismatch.2: easily fixed, an L-filter DC path can be made.3: please read the thread.It's only for reception.4: I think everyone is aware that any circuitry will degrade performance.

author: Neomys Sapiens
author_rank: Super Contributor
timestamp: September 03, 2017, 10:35:29 pm
content: Quote from: Benta on September 03, 2017, 09:45:38 pmI can only corroborate this statement, as I had to use a 'leftover' radio antenna on a roof as well as a car radio antenna as auxiliary antennas for a scanner sometimes. Better nothing than a matching pad.

author: BrianHG
author_rank: Super Contributor
timestamp: September 04, 2017, 03:06:23 am
content: RF relays aren't too expensive, pick from here:https://www.digikey.com/products/en/relays/signal-relays-up-to-2-amps/189?FV=ffe000bd%2Ca700004&mnonly=0&ColumnSort=1000011&page=1&stock=1&pbfree=0&rohs=0&cad=0&datasheet=0&nstock=1&photo=0&nonrohs=0&newproducts=0&k=relays&quantity=&ptm=0&fid=0&pageSize=250&pkeyword=relaysThis is the cheapest SPST...https://www.digikey.com/product-detail/en/coto-technology/9002-05-00/306-1059-ND/301693This cheapest quality SPDT with 75 ohm shield:https://www.digikey.com/products/en?keywords=ARE134H

author: Bicurico
author_rank: Super Contributor
timestamp: September 04, 2017, 07:39:39 am
content: Thanks so far for the input.Regarding the signal cable from the amateur band antenna: yes, that cable is 50 Ohm and goes normally into a scanner. However, I do like to connect it alternatively or in parallel to a spectrum analyzer or a field meter with spectrum analyzer, to overview what is happening. The 50 Ohm/75Ohm matching is to be neglected in this case - losses are minimal.Also, this antenna provides low power - it is safe to connect any field meter or spectrum analyzer I have.Regarding the 13V/18V and 0/22kHz pass-through for satellite signals - correct. I assume that a relais switching this signal path would be bidirectional and I would not expect any issues.Of course such a device would attenuate the signal, but so does any cable joint.I will look into the linked relais - thank you!However, I am still thinking this over:If I have such a switching box, I would still have to connect a cable on the OUTPUT and from there to the desired test equipment. Even if the cable was permanently plugged into the OUTPUT, I would have to plug at least one end, whenever I switch devices.So why not do just a 19" front with 8 RF F-type connectors on both sides. From behind I attach all signal cables and on the front I connect the cable that would go to the test equipment. In this case I would have to plug two ends, on every change. The advantage is that I could connect several test equipment in parallel, provided they use different sources.So as stupid this may sound, I think the simpler, passive solution is actually better.All I actually need is an old 19" rack and have a new front machined (can be arranged easily).What do you think? Am I missing something?Regards,Vitor