Hi Danish, yes copper disc capacitors work well if you have the space, but in a filter of this size & frequency wouldn't work as I need quite a few puffs. Same goes for piston trimmers I've got a few of these with a maximum of about 6 pf so not enough & again they're too big. Ceramic trimmers work OK, & are fairly stable once set. Best wishes nick

@@ptronix TX out about 1 watt. Hi-Q filter at 100mhz or so. NPO ceramics are OK. But there are cheap alternatives. You can always use some Kapton or Teflon tape to extend HV breakdown on Hi-z discs. Still have my 1997 RSGB hardbound book. Most excellent book. Far better than ARRL. Merry Christmas Nick

@@danishnative9555 hi Danish, I think that's where I got the original design for the helical filter out of the rsgb handbook. That was for an unmentionable project back in the early 90's, a home brew car phone before cell phones were available cheaply. As it was full duplex had to filter the transmit signal heavily as the two antennas were very close on the car, best wishes nick

Hi Diego, I use it and others like it in my two transceivers, so I can listen to what's being received on a pocket radio with earpiece when I'm away from the radio room or out in the garden, they actually produce a fairly good signal 200 meters away, so my neighbors can also enjoy my reception 😄

@@ptronix Oh I see! nice idea! I did the same thing when I was living in Portugal, but used a PMR radio with vox on, in front of the radio speaker. Then go walking the dog around and listen to HF from another PMR in my pocket with earpiece. Satisfying