2M Low Pass Filter from RG142 (For Harmonics)

Having "failed" to build a suitable bandpass filter with RG142 I tried the low pass filter based on stubs concept from here. While it does little to improve reception where pager intermod's are a problem it will provide peace of mind when transmitting. RG142 is easy to work with since you don't have to worry about inner conductors gong soft while soldering and you can solder the shield without needing pigtails. RG58 or RG213 are much harder to work with.

With the nanoVNA construction is simple. I attached the three stubs (for RG142 I started with 180mm, 120mm and 90mm) and the inductor.

Looking at a graph of S21 versus frequency it is easy to see where the stubs are resonant.With the Australian band plan most of my FM transmitting will be around 146.5MHz. So with markers placed at 293MHz, 439.5MHz and 586MHz I carefully trimmed each stub so peak attenuation fell on each marker. Changing the frequency span to zoom in to these frequencies is helpful as those markers move toward the dip. By careful I mean as small a cut as you can manage as you get close. I always overshot on the higher harmonics.

My first attempt:

 

I didn't trim the stubs very well but it showed promise.

The final result:

After replacing the coax I had trimmed too much I once again tried to get it spot on. This time I got closer and the biggest challenge was getting (take your pick of terminology) the input return loss, S11, or VSWR down. 

Initially the input return loss was not as good as the initial version. This was due to the stubs, now trimmed to a different length, having a different capacitance at 2m. After experimenting with different inductors and looking at the Smith Chart of S11 I concluded I needed some capacitance on the output pin so the inductor could tune the input correctly. Once I discovered this it was plain sailing. I used a 1-10pF variable capacitor but I also had good results with a piece of copper foil spaced closely over the ground plane.It just wasn't as good mechanically so I switched to the variable capacitor.

Yes, you can tune for a ridiculously low return loss

With careful trimming but I still cut too much off the shortest stub

Conclusion

Easy to achieve a 40db reduction or more in the second and third harmonics with negligible loss on 2m. That harmonic rejection is not as good as the bandpass filter made from LDF4-50 unless you tune it with absolute precision. However, it has a loss of under 0.1dB while the LDF4-50 filter had 0.7dB of loss. Perhaps that is just bragging rights. Either filter will make a difference to harmonics levels.

It is straight forward but some test equipment helps. I could just have easily tuned this with a signal generator and a scanner with a S meter. But it does make me realise just how profound the step change in home brew has been with the nanoVNA.

I like that the stubs made of RG142 cannot be detuned by touching them except if you put your finger right on the open end.

It worked so well it is now permanently installed. I will give thought to housing it in a suitable metal enclosure to improve the installation. I own a couple of the Chinese "Harmonics in a box" handhelds. I never have had cause to use them but one day I might. When the day comes one of these filters is peace of mind.