[Elecraft] Appliance Operator gets gift

[Vic Rosenthal]

Bill Coleman wrote:
> 
> To be fair, a design like the DX-40 ought to be fully compliant with all
> FCC regulations for purity of transmissions, even though it runs Class C.
> 
> Why? Because the pi network on the output has sufficient Q to supress
> unwanted energy. The high impedance of the vacuum tubes make this a
> simple process. (whereas the relatively low impedance of transistor
> output stages usually complicates it, and typically requires switched
> low-pass filters to clean up)

The DX40 is reasonably well-shielded and filtered to reduce harmonic radiation
from the case.  However, the pi-net will not sufficiently attenuate harmonics to
eliminate TVI on low VHF channels in nearby TV sets.  For example, 10-meter
operation will almost certainly wipe out channel 2.

The solution is a low-pass filter with a cutoff somewhere between 30 and 50
MHz.  These are quite common at hamfests and EBay.  Johnson, Drake, etc. made
good ones.

Once you have added a low-pass filter and are sure that the cabinet is correctly
buttoned up, you should have no more chance of TVI with the DX40 than a modern
rig.  Of course, you still have to worry about problems caused by your
fundamental frequency, just like with any transmitter.

73,
Vic, K2VCO
Fresno CA