30M LPF response? #lpf

Adrian Scripcă

Hello all,

I needed a 10MHz LPF for a project so I dug out a 30 meter QRPLabs LPF kit I purchased a while back and quickly put together. Measuring the filter response implies I can't get lower than -65dB for the third harmonic which is a long way from the -80dB approx measured by Ulli DC8SE, shown on the kit page:

Anybody else had the chance to measure the 30 meter LPF? I have the feeling it's me and my build and not the components. I attached some photos of the filter and it's measured response.

73! Adrian, YO6SSW 


I cannot get below about 36 dB down at ~20Mhz. Using a antuino to measure

Hans Summers

I don't think one should neglect the impact of measurement equipment on the accuracy of results. 

Test equipment is very important, in some cases. When making any measurement, you have to always spare a thought for the capabilities and limitations of your test equipment. Not just measurement equipment but also grounding, shielding, enclosures, cables... when you get down 30, 40, 50dB you are dealing with very small signals. It doesn't take a lot of leakage to bypass the filter and significantly alter the apparent results. 

Furthermore - do expect some variation from build to build, as inductors are wound differently and of course capacitors have tolerances too. 

73 Hans G0UPL

On Mon, Oct 28, 2019 at 5:32 PM ohwenzelph via Groups.Io <> wrote:
I cannot get below about 36 dB down at ~20Mhz. Using a antuino to measure


When one gets to the -60db level and lower how the test setup is assembled
is a factor along with the cables used.

A good example is a engineer in the lab not a hardware type doing tests in
the low UHF region and could not get the expected rejections of from the filter
he was testing.  I walked over with a SMA torque wrench and tightend his
connections and then replaced a bad cable (results changed id it was moved)
and then results exceeded.

As it stands likely 65db is good, but review your work as it doesn't take much.
FYI the hint is above 30mhz the rejection is degrading unless you manage to
get a real odd angle in the photo.




The Antuino may have limitations or your setup has errors.
First is inset a known attenuator in the same path and see if
40DB of attenuator gets 40db on the Antuino.  If it does keep
adding attenuation until it hits the system limit (likely better
than 60 db).  If it doesn't your Antuino has issues.

Substitution using attenuators is a solidly reliable way to verify 
what your seeing.  Of course it requires a set of fixed and
maybe even a step attenuator but those are easy to build.
There are multiple kits for making them as well.



Yes, sometimes details matter. With my Antuino using the SNA function, passing the signal thru the 30m QCX LPF, at ~10mhz the signal was down one dB fro a dead short, at ~20mhz it was 36 dB less but nothing I did ( with much fiddling and knashing of teeth) would make it any lower. Using the PWR function I measured with a dummy load, tap attenuator and variable attenuator at 10.12mhz, and then at each of twice, three, and four times that frequency. These peaks each measured 61, 71, and 75 dB less than the fundamental. Good enough for me, I just wanted to be sure I was legal. Just 36 down would not have been ok, looks like that method and the setup used were not up to the task.
 Thanks for the help and advice.
jerry aa1of