Cavity Filters in General

General
1

I’ve been getting DMs and emails asking me something to the effect of “**Lothar, what’s up with your recent cavity filter monomania? **”
Several things really:

  1. KevinE leading me down MOAR rabbit holes! He better not lead me into a cave though. I heard there is a special bunny in there.
  2. Cavity filters generally have better (lower) insertion loss characteristics that SAW (Surface Acoustic Wave) or LC (Lumped constant) filters.
  3. Cavity filters are inherently more temperature stable and drift less in temperature extremes. This is important for outdoor uses and becomes even more important as the filter bandwidth is narrowed. I would never expose a SAW or a Flight Aware Blue tube LC filter to extreme temperatures up on a tower in the sun or in the arctic, or a closed outdoor box on a roof for example.
  4. Cavity filters are inherently DC grounded devices, meaning that static buildup never gets through to the front end of the SDR, except in the case of really close lightning. A cavity filter looks like a dead short at DC. The front end of many SDRs is protected only with a breakdown diode that WILL fail eventually. The AirNav Radar Box green sticks are notorious not only for heat failures, but for that diode to go short, thus making a deaf SDR.
  5. Simpler in general. The FA blue tube filter has about 30 SMD caps, coils and resistors soldered down to a board. That is at least 60 solder joints. A Cavity filter has 4 solder points.
  6. A cavity filter has higher signal level / power handling and thus lower potential for passive IMD (Intermodulation distortion).
  7. Put one or two in a gym tube sock and you have an excellent self-defense weapon too! Just like my metal cased IBM buckling spring keyboard!
  8. Now you might object that SAW filters are used on spacecraft, but those filters are really expensive and you will never find that grade of SAW in any consumer product. Plus, see 7 above. They can’t afford to use cavity filters on spacecraft because of the mass and cost to orbit. They are used on the TX side of many satellites though.

Now stop DM ing me about this matter. LOL.

2

I’ve mentioned the possibility of an ACARS / VDL cavity filter to a few of my feeders and they are all very interested - as am I.

Looking forward to hearing more about what that filter might look like, its size (for up pole mounting) and its performance specs.

The most common question I’ve been getting is 'do we need to find a VHF pre-amp to put in front of the filter?"
Given I don’t know the db drop over the filter and the locations current noise floor - its a tough one for any of us to answer. Cleaning up the signal might well be more than the db loss through the filter?
Personally, I’ve not found any VHF LNAs that I have been happy with - yet.

While very few of us are focused on price, the performance boost we hope to see at VHF is causing most of us to be looking for the hammer to smash the piggy bank and send some pre-order bucks your way when the time comes.

3

See the new topic TBG