433MHz Bandpass filter - Page 1
Forum section: electronics / beginners

author: IO390
author_rank: Regular Contributor
timestamp: September 14, 2014, 09:11:00 pm
content: RF is not something I've played with, so I probably shouldn't be venturing here at all. But I thought it shouldn't hurt to ask...I dabble in FPV model flying, and I've got a 433MHz UHF R/C system. It's a fairly cheap one and it seems to be throwing out all sorts of crap and interfering with my 2.4GHz video link (I flew on 35MHz before without the interference, so it must be this 433MHz system). You can buy bandpass filters to go on the transmitter, and these apparently solve the problems (like thishttp://www.foxtechfpv.com/433mhz-band-pass-filter-p-1220.html), but I'm just wondering if it's at all practical to build my own. I don't have access to a spectrum analyser, and currently I only have a scope with 60MHz bandwith, so I couldn't measure it if I did build one. I do have a signal strength indicator on my receiver, so I can see if it's impeding performance or not.I understand the basics of RC, LC etc.. filters. My plan was to build it point to point (I can work with SMD components) and put it in a machined brass housing as a shield (I have a lathe and mill). So is this doable?

author: KJDS
author_rank: Super Contributor
timestamp: September 14, 2014, 10:22:36 pm
content: It's possible.I'm about to move house, so remind me in a few days and I'll come up with a design that should function.

author: IO390
author_rank: Regular Contributor
timestamp: September 14, 2014, 11:26:49 pm
content: That would be great, thanks.

author: David Hess
author_rank: Super Contributor
timestamp: September 15, 2014, 03:57:26 am
content: One simple way to do this is to add a shorted quarter wave coaxial stub in parallel with the antenna. It acts as a rough bandpass filter for the fundamental frequency and odd harmonics.The velocity factor of the coaxial cable needs to be taken into account and will typically make the coaxial line about 33% shorter.

author: German_EE
author_rank: Super Contributor
timestamp: September 15, 2014, 06:24:06 am
content: Yes, it is possible but the inductors and capacitors are very small values, you may also need to take care to use the correct PCB material as thickness and type can affect the impedance. A simpler method may be to use a coaxial stub and the following link gives you an idea of what is involved:http://www.ifwtech.co.uk/g3sek/swxfiltr/swxfiltr.htmIt may be better to just buy what you need. Search for 'helical filter' on Google, these are VHF and UHF filters that are easily adjustable.

author: IO390
author_rank: Regular Contributor
timestamp: September 15, 2014, 10:13:07 pm
content: Well I really need it to be as small as possible. This would make a coaxial stub filter impractical. I'll have a go with one when my new coax arrives though.

author: VK5RC
author_rank: Unknown
timestamp: September 15, 2014, 11:05:43 pm
content: The coax could be pretty small diameter eg rg174, depends on the size of the model.

author: Alexei.Polkhanov
author_rank: Frequent Contributor
timestamp: September 16, 2014, 12:44:10 am
content: Get a SAW filter like this:http://www.digikey.ca/product-detail/en/B39431B3791Z810/495-3749-1-ND/1812324. Make sure max power, central frequency and bandwidth matches your requirements. They are very popular in cell phones dues to size.You can also find a helical filter like thishttp://www.digikey.ca/product-search/en?pv139=447&k=helical+filter&mnonly=0&newproducts=0&ColumnSort=0&page=1&quantity=0&ptm=0&fid=0&pageSize=25for your requirements, but these are bigger and more expensive.If you want to design your own filters using lumped elements you can use software like Elsie fromhttp://www.tonnesoftware.com/, also you need to make sure that components (presumably SMD) that you use are of high quality (have high Q) and you will need spectrum analyser with tracking gen or VNA to test. By using home made inductor coils you may be able to tune it a bit by changing distance between turns on coils. Do not use inductors with ferrite cores - unless special ferrite material is used they are not suitable for these frequencies.