Seven  element Chebyshev Low Pass Filter for 500 kHz

The 500 kHz band extends from 501  to 504 kHz.  Although the allowable radiated power is only 100mW, there is considerable risk of interference to medium wave broadcasters due to second and third harmonic radiation. Being able to reduce harmonic radiation is particularly important to those who live in built up urban areas where there may only be 15 - 20m between the antenna and a neighbour trying to receive their favourite medium wave radio station.

The following LPF is a seven element Chebyshev which provides attenuation above 600kHz. 
The capacitor values calculated have been adjusted slightly to the nearest preferred values.
The cut off frequency was set to 600kHz to allow for errors / tolerances in winding the inductors.
This is a REAL filter, not just a computer simulation. The graph below are measured values.
The filter was built into a small 100mm long diecast aluminium box fitted with an SO239 socket at each end.

The graph above plotted using Excel shows the LPF characteristics.  Note the last result at -80dB is slightly compressed as it does not have 100kHz spacing.

The -3dB frequency of the prototype is 613kHz.      1000kHz is -41dB  while 1500 kHz is -66dB. 
Ultimate rejection of the low pass filter is -88dB. This is due to the limits of the physical construction. To achieve higher attenuation would require each of the filter elements to be separated by individual metal screens.

Circuit

C1 + C4      4400pF   made from 2  x  2200pF  in parallel                    G3WOE picture from Network analyser
                                                                                                                                    close up from 200k to 800k. phase and dB loss
C2 + C3      10nF

L1 + L3      20.6 microhenrys   49 turns 0.56mm dia on T-94-2

L2              24.3 microhenrys   54 turns  on T-94-2 

The capacitors used are Polypropylene.   ( RS Components )
The wire used for L2 was 0.8mm dia. However this was really to large as some turns had to be overlapped. 0.56mm would be OK for this winding.

Modifications.......   Power is limited by the size of core and the 0.56mm wire. 100W is probably the limit. To increase the power capability you need to increase the wire diameter to 0.8mm and the cores to T-130-2.   That combination should be OK to 400 Watts

Another potential, but untried, idea would be to place a 1000pF capacitor across (in parallel with) L2. This forms a second harmonic trap. However, I don't anticipate needing that with my 100mW ERP limit.

Have fun!!!

Update:  First QSO  503 kHz  9/April/07  G3XIZ   Sent 589  received 579.    Thanks Chris