Here are the schematics and bandpass vs return loss charts for for two different Butterworth Low Pass Filters (LPF's).  Having one of these devices is nearly a MUST for DXing the Longwave Beacon Bands, especially if you have any Medium Wave (MW) Broadcast Stations anywhere near you.

These filters, when placed between your antenna and receiver should eliminate all Intermod Products (IMD)  that are generated in the front end of your receiver by MW overload.

There are two different versions here.  The first is a 5 Pole (two coils and three capacitors) filter that should be sufficient for nearly any DXer who doesn't have very powerful Mediumwave Broadcast bands nearby.  However, if you live in "RF Alley" and have one or more 50 kW behemouths nearby you will probably need the 7 Pole filter.  If in doubt, build the 7 Pole unit as the difference in insertion loss and cost is minimal while the rolloff of the 7 pole filter is far steeper..

Although these are billed as 510 KHz LP Filters, that doesn't mean that you won't hear a beacon on 530 KHz, the rolloff is gradual at the onset.

Below the schematics are the graphs showing the attenuation and return loss (VSWR) curves of each of the filters.  The third graph is an overlay of the two so that you may see a direct comparison.  If you notice, the 7 pole filter has far greater attenuation throughout the Mediumwave Broadcast band.
 
The filter should be built in a metal shielded box with a  coax jumper between the receiver and filter to prevent signal pickup on the jumper cable.  OR, one of these may be built as part of a homebrewed preamplifier. 
View LF Preamp Article

The filters were designed and modeled using the AADE Filter program.

The input and output impedance is 50 Ohms.

7 Pole Filter

   
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