Can anyone recommend a low pass filter to deal with harmonics and spurious emissions from an FM transmitter? I looked on ebay but I have no idea which one to get or how much I should spend.
A low pass filter lets only LOW PASS FREQUENCIES past it. If it's rated to let all pass besides up to 2Ghz then I doubt it would be of any help for any FM broadcasting on the standard bands. In fact I am pretty sure the filter mechanism reduces output power so using something that literally has no effect at the frequency you are using would be detrimental and not at all useful.
As far as low pass filters (LPFs) for FM, I would recommend something with a range from whatever to more than the frequency you are using, or at least < 108MHz.
Bit of tech talk: It is true especially with class C amplifiers that the harmonics become a problem when not filtered at their odd frequencies. So that usually puts harmonics in bands you don't want usually well above the broadcast FM band.
I still believe a bandpass filter is the way to go. It takes out the issues with high frequencies used by hams and police. It's always best to filter lower frequencies also, a lot of people forget this. Those lower bands are still in use by some services and why not filter them if you can?
So go bandpass. Want to know what it cost? This is a black arts hobby. You can't just go out and buy a box that does these things. You need them made for the frequency you run. So either learn the math and get the equipment to do it, or let someone else help you out. There are lots of people out there that can help out with filter design.
Can anyone recommend a low pass filter to deal with harmonics and spurious emissions from an FM transmitter? I looked on ebay but I have no idea which one to get or how much I should spend.
Thanks
As far as what to recommend? Just as Kage stated, the one which will work in the range you are broadcasting. Also, be sure it will handle the power you are radiating.
Broadcasting without one is asking for trouble. Without a spectrum analyzer, you really don't know what you're sending down the pipe, and from experience I can tell you that cheap transmitters can send out all sorts of spurious emissions. Particularly in the lower bands where TV sound still resides on the analog spectrum. It won't take long for someone to complain if you start trampling on their air.
So far as how much to spend? Well, budget may be an option, but the FCC Notice of Forfeiture will cost a few dollars more than a decent LPF. Just sayin'