I purchased the FF-501DX LPF along with a number of other items. It belonged to a very good friend, sadly it was part of his SK sale! As my interests are very much VHF upwards, along with the odd session on 10m, this LPF has sat gathering dust for the best part of four years now. With conditions improving and the ES season well underway I’ve been more active on 10m and this got me to look at the FF-501DX LPF again.

There was no paperwork with it so I had no idea of the specification. It has a cast alloy housing, weighs a reasonable amount and has the dreaded SO-259 connectors (it is HF  🙂 )

Curiosity got the better of me and I had to take a look inside…

I have to say I was impressed with the build quality.

The next step having reassembled it was to see what it looked like on a spectrum analyser with tracking generator.

FF501DX Filter Characteristics

The results were equally impressive.

I then looked at the Return Loss / VSWR using an early version of the NanoVNA. The LPF was connected directly to a Bird 50R dummy load.

I added markers for 3.5MHz, 14MHz and 28MHz and did a sweep from 1MHz to 30MHz.

VSWR 1MHz to 30MHz

Overall the results were much better than I had expected.

In summary I measured:

Insertion Loss:   Less than 0.2dB

Cutoff:   35MHz

Attenuation above 70MHz:   Better than 80dB

Return Loss:   Better than -24dB

Searching for “FF-501DX LPF” on the Internet returned what seemed to me to be surprisingly few results. It did however produce a single link to a copy of what appears to be the original one-page information sheet, which can be found here. I also found a copy of an old radio magazine from 1985 that was advertising the FF-501DX for the sum of GBP33.00

Comparing my results with the original specifications shows that I have a good one here. Not that I would expect anything different from my old pal, although thinking back, he was the founder of the 30 Volt Club!   🙂