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BPFF – The Guess-timated Scale and actual Guess-timates – Part 6

Part 6 of the W4KAZ filter project series discusses the actual measured S-meter calibration, and the filter attenuation estimated based on S-meter measurements.

As I meandered through Part 5 of this group of posts, I needed to find a way to calibrate the S-meter scale on the FT-920 to a 6DB reference. Lacking any real test equipment, this will allow me to do relative tests on the band pass filters to measure the filter attenuation on the harmonics and sub harmonic. So I used the attenuator pad(6,12, and 18db) to measure the delta between each S-unit from S-0 to S-9, “10 over S-9” and “20 over S-9”.

Big shock(NOT!): The S-meter on the FT-920 is definitely not linear.

Actual Big shock: The S-meter actually IS linear over part of its scale. I was a bit surprised by that.

The S-meter on the FT-920 was “measured” by using the attenuator pad, inserting attenuation and noting the S-meter drop. It came out to something like this:

  • S0-S4 – 6db
  • S4-S5 -6db
  • S5-S6 – 6db
  • S6-S7 – 6db
  • S7-S8 – 6db
  • S8-S9 – ~9db
  • S9-10 over 9 – ~12db
  • 10 over to 20 over – **Not measured**.

It was hard to decide if the drop from 20 over 9 to 10 over 9 was 12db or more, so I didn’t do any testing with any signals that strong. For the sake of an example, when the original signal was reading S4, adding 6db attenuation dropped the reading to S0.

It seems noteworthy that the spread from S0 to S4 is only 6db. I can often work stations that are down around S0, and almost always work anything higher in decent conditions. I guess to me it emphasized how important just a few db difference might be to making a contact. Maybe a 1db loss throught the filters is more siginficant than it appears. To paraphrase the OM’s, “every db counts…”.

Armed with that calibration scale, there now is a way to make an educated guess at the amount of attenuation a filter is providing on its harmonics and sub harmonics. By injecting a signal on the harmonic band, comparing the S-meter readings with and without the filter gives an easy way to approximate the filter’s attenuation on that band. It won’t be surprising to find that the accuracy of the measurements will be poor when compared to lab measurements, but it gives a rule of thumb guideline. Better than nothing.

Amendment, 2009.06.21– Somewhere along the way I misplaced my notes with the measurements made during mid-May. It looks like I won’t have time to re-test the filters for a couple of months, so here are a few notes from an e-mail to N4YDU. The executive summary….

K4VX filters – worst case is about 30 to 35 db of attenuation, through the 20m filter. The best cases are probably 35 to 40 db attenuation on the second harmonics.

NVARC Ugly filters – Woooweee! These puppies may have a bit of loss, but they sure do a great job on the harmonics. All bands showed 6 to 12 db better attenuation on the second harmonics than their K4VX counterparts. An S9 signal is dropped to S0, still readable. An S7 signal becomes barely audible at the noise floor of the F-920. The guestimate here is better than 40db attenuation on the second harmonics. Higher order harmonics were disappeared, so no ideas on the attenuation there, except that is “Enough!”.

Previous in series: Part 5 Guess-timating the filter efficacy

Start from the beginning at the W4KAZ Band Pass Filter series.

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