RF interference filtering...help!

Yep, you need a HPF. Probably at least a 5-node one. See some filter design applet, such as this really good one:

At those frequencies you may get unrealistic component values, that means you may need to use the impedance tansform options to get reasonable values.

Another option is to BUY an already designed filter, they're not too expensive, see

Unfortunately that won't help if it's harmonics or spurious signals in the 430MHz range generated by the transmitter. The OP mentioned that he can't change that unit. If it is one of those pre-cert deals that would include the antenna so you can't (legally) connect a lowpass filter between transmitter and antenna.

True, you can't do anything there, but it's much more likely its due to the large transmitter signal getting into the front end and getting cross-modded there. Since he can't do anything on the transmitter end, we might as well try to improve the area we can modify.

Then a tuned trap on the transmit frequency would do more good than a HP filter.

Possibly. Also, it is a good policy to look at all the connections. Dissimilar metals can caused enough non-linearity to create intermodulation. And back to back diodes to GND to protect an input transistor are an absolute no-no here.

Michael's suggestion of providing a trap is a very good one.

If the device is supposed to be FCC certified, and it's causing spurious emissions at its tenth and eleventh harmonics and so, then he has a bone to pick with the manufacturer of the device.

If the 40 MHz is just swamping the receiver, then a HPF, but I have nothing to offer as to the design of a filter at those kinds of frequencies. One thing that springs to mind is a parallel resonant LC at 40 MHz, in series with the antenna lead, but for more than that, you'll have to see a VHF guy. :-)

Good Luck! Rich

Hello Rich,

Not necessarily. Even the smallest spec of noise will show up if you have the antennas right in the face of each other. Yet the emissions can still be below legal limits.

Better see a UHF guy :-)

The best approach is usually a nice sharp bandpass for the frequency you are receiving at, not trying to muffle other noises one by one. Even when you manage to get it quiet, next thing you know they put a cell tower right next to it. This happened to a client. Or the Fedex freighter comes lumbering in and there is a lengthy chat with the tower. Happened to me after they re-routed Mather field approach.

The WACX TV transmitter site in Orange City has several unusable C-band earth stations. Right after they were built, AT&T built a microwave c-band relay right over the station, wiping out all 24 channels by being on the frequencies we needed to receive. We ended having to use the old pair of microwave links from the original studio site and a dial up DTMF decoder to select the desired receiver and feed it to the old transmitter site on 11 GHz (CARS band) then another hop at

Thanks for the guidance everyone. Looks like the problems solved...put a high pass LC on the 433 front end (10pf and 22nH) which killed the 40 stone dead but left the 433 apparently unnaffected. Can now put the antennas almost together before there is any problem. The simulation also says I have a 1.75dB signal gain at 433 so this really is a result! Pico

Hello Pico,

Great, but I'd still consider a proper bandpass. It might work now but what if someone with a walkie-talkie passes by or the cell phone in his pocket rings?