Antenna Switching insight - Page 1
Forum section: electronics / projects

author: ElectroWarrior
author_rank: Contributor
timestamp: October 28, 2013, 02:28:18 pm
content: Hi guys.Im working on a design involving GPS. I need to switch the GPS reciever signal between two GPS patch antennas with integrated amplifier.Do any of you have some idea on the following problems:- Which kinds of transistors, can be used for this purpose?- Which affect on the 50Ohm impedence can such switching transistor have?- other thoughs on the idea?

author: DutchGert
author_rank: Frequent Contributor
timestamp: October 28, 2013, 02:40:11 pm
content: What if u use a low impedance analog switch?http://www.ti.com/lsds/ti/analog/switches-multiplexers/overview.page

author: ElectroWarrior
author_rank: Contributor
timestamp: October 28, 2013, 02:50:34 pm
content: I looked into it, and it seems as a very good choice!Thank you very much!

author: Fraser
author_rank: Super Contributor
timestamp: October 28, 2013, 02:54:37 pm
content: You have the DC feed to also consider. Presume you intend to use a BIAS-T for powering each antenna then switching just the RF signal ? Trying to switch the DC and RF signal would likely need a physical RF relay.How often is the change in antenna happening ? This will have a significant influence on the solution. Relays don't like high speed switching. For RF only switching Google 'PIN Switch'http://en.wikipedia.org/wiki/PIN_diodeYou can make or buy Pin switches. Mini Circuits make many types. They can switch quickly if needed and are solid stateBIAS-T'shttp://217.34.103.131/products/BiasTees.shtmlPIN switcheshttp://217.34.103.131/products/Switches.shtmlRemember, you are switching a signal with a carrier frequency of almost 1.6GHz. Microwave construction rules apply.

author: KJDS
author_rank: Super Contributor
timestamp: October 28, 2013, 03:09:21 pm
content: Hittite make good switcheshttp://www.hittite.com/products/index.html/category/305A custom PIN solution will provide lower loss but with higher component count and a much greater design time. As it's narrow band then you can just apply the DC feed via quarter wavelength line.If you're not used to RF design, get someone who is to do it for you or be prepared for a few iterations to get a good solution.

author: w2aew
author_rank: Super Contributor
timestamp: October 28, 2013, 03:27:39 pm
content: Quote from: ElectroWarrior on October 28, 2013, 02:28:18 pmRF signal path switching is often done using switching diodes or PIN diodes. Usually cheaper than doing it with transistors, etc. PIN diodes can have very nice low impedance when "on" at RF frequencies, so they can simply be inline with your 50 ohm path. Bias is provided through RF chokes or inductors, with current limiting via resistor, etc.Here's a video on the basics:

author: ConKbot
author_rank: Super Contributor
timestamp: October 28, 2013, 04:20:22 pm
content: +1 to W2AEW, if you want to switch it use a diode switch, you can even use the 3.3 and/or 5V that powers the internal amp can be used to bias the diode also, so that way you can shut off the unused amp when its not needed.How 'integrated' are you trying to go? Just using mini-circuits components and coax to connect stuff? Getting a board made with mini-circuits parts on it? Doing your own RF layout with coupling capacitors/chokes/PIN diodes etc?Also, what are you trying to do with the different antennas? measure 2 points on a vehicle for orientation? Could you integrate the GPS receiver onto the back of a patch antenna and just feed serial data back to your main processor? The cable would be more rugged, and signal integrity would be less of a problem (especially if you used a differential serial format)

author: ElectroWarrior
author_rank: Contributor
timestamp: October 28, 2013, 06:52:51 pm
content: Thank you for all your replies. I will look into it now..My system consists of a GPS module, a accelerometer and two GPS patch antennas mounted on opposide sides of the PCB. The plan is to activate the GPS antenna that is pointing most towards the sky by using the accelerometers gravity direction. The antenna switching will not be fast or frequent since the GPS reciever will only be activated for periodes of 20 min once every 5 days. During the measurement periodes the circuit will not be rotated or flipped much, therefore antenna switching during on time will be very seldom.I really appreciate the help everybody at forum provides!

author: SeanB
author_rank: Super Contributor
timestamp: October 28, 2013, 07:16:08 pm
content: A latching RF relay will do then, low power use to switch and zero power use when not changing state. Use a DPDT unit so you can have state feedback. They are available in coaxial versions that will do 2GHz.

author: ConKbot
author_rank: Super Contributor
timestamp: October 28, 2013, 08:08:41 pm
content: If you have it in your link budget (you probably do, using amplified antennas with the antennas mounted very close, i.e. not driving 10's feet of coax, as long as your receiver doesn't have a garbage -130dbm sensitivity)) and dont need super precise accuracy, consider using a splitter/combiner that passes the DC. If you have a patch that covers most of 180 degrees in elevation, and if your antennas are literally mounted on the PCB as a ground plane you can use coax(or stripline) as a delay line to adjust the phase. (9.5cm 1/2 wavelength * 62% velocity factor = make one coax 5.9cm longer than the other). If the antennas are spaced apart, then just use the coax to adjust the phase delay to match themIf there is no back-lobe on your antenna, great, if it does have one, then with them 1/2 wavelength apart(electrically) and 180 degrees out of phase mechanically, the signals will come out in phase, where they are combined. If the unit is on its side then your 2 180 degree lobes still provide full sky coverage and the RF coming down will be in phase. Stuff at oblique angles may be a bit wierd, but but still workable. The mini-circuits SP-2G+ is $0.96, and is narrowband around GPS L1 so it should help keep other stuff from bleeding in. It doesnt specify if its DC coupled, but you can just use a cap and choke to add your amp bias between the combiner and the antenna connectors for each antenna.Passive solution and it now gets full sky coverage on its side