WW2 HF Filter, AP56152

I believe I bought this RF filter in 1961 after I'd built my first transmitter and suffered from really bad TVI. I don't think it was ever pressed into service though as it couldn't easily match into a long wire antenna and it's just been hanging around for years, lastly in a rather damp caravan which is slowly returning to nature and merging into the background. I fitted a pair of PL259 sockets in place of the dreadful old Pye coax plugs and removed the label before rubbing down the case prior to repainting. Of course, "AP" means Admiralty Pattern and I'm reliably informed it was used in front of the B28 (Marconi CR100) and the B40 series of receivers to eliminate RF interference originating from ship-bourne equipments.

Opened up you'll see it carries six stages of matching/filtering and employs beehive trimmers each soldered into position.

Below is the finished filter painted in matt black. The label says it was made in 1944.

Here is a set of pictures taken from my spectrum analyser proving the filter was well designed and has stood the test of time

The horizontal scale reads 0 to 60 MHz and the curve dives down from 30MHz and flattens out at about 36MHz

I set the marker to roughly the 3dB point which is 30.6MHz

Below is a scan from 0 to 500MHz... pretty good for a 70 year old piece of government surplus kit

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