2M Bandpass Filter from LDF4-50

 

To recap, RG142 coax proved unsuitable due to an insertion loss for a narrow filter of around 3dB. RG213, which has a larger diameter, was encouraging with an insertion loss around 1.5dB. LDF4-50 is still larger in diameter. How did it go?

 

Circuit

The filter was very loosely based on the RG142 filter. With all the practice I had tuning up such filters I didn't stick to this religiously. My coax length was a guess, the construction technique more devil may care than finesse and everything was made up as I went along.

Substituting air space copper strips for the 0.7pF capacitor and lengths of RG142 for the coupling capacitors I was surprised when this tuned up and worked without any grief.

Murphy was absent.

I passed the two length of LDF4-50 through holes just wide enough to accommodate the cable. Then I wrapped several turns of copper wire around the copper sheath to stop the LDF from falling back out. A firm result that when soldered was surprisingly robust.

Some more re-purposed copper sheath to connect the tuning capacitors and a section of RG142 for terminating capacitors. The RG142 was a little hard to work with and if I used RG142 again I would have allowed spacing for 5cm of RG142 without such a sharp corner. 

 Initial Results

About 0.7dB loss on 2m, 1dB at the band edges and 7dB at 152MHz. Second harmonic was -40dB and the 3rd harmonic was -58dB. 

 

I haven't finessed this as it is already fit for purpose and proves the concept. No point twiddling those nice variable capacitors too much since that can wear out any gold flashing that exists on the threads.

Results in Practice

I once had a Motorola radio programmed for 2m which was so poor it appeared to be getting cross modulated by broadcast band FM signals when I had it in the car. Stupid varicap tuned printed inductor front ends! My suspicion is there was something wrong so I gave it away for parts.

However, I no longer have any radio which suffers from inter-modulation at the home QTH so I can't confirm if it truly helps. Should I go mobile again I will update you on my findings.

I found it could handle 40W without issue and had no apparent impact on weak signals.

 

Conclusion

LDF4-50 can be used to make outstanding 2m band pass filters. Compared to my best band pass filter made with inductors the difference is a step change. The 3 pole inductor version, while still a good filter, had a passband of 25MHz to keep losses down to 1.3dB so it did nothing to reject pagers.

While cumbersome in size compared to filters made with wire wound inductors I can't make inductors with a Q of around 500 to achieve the same results. 

While 7dB of rejection for pagers doesn't sound much it should be noticeable in practice.