from FB group Building Transistor Radios 2018-11-26
https://archive.org/stream/ETIA1983/ETI%25201983-03%2520March_djvu.txt
Two 6 uH RF chokes
The photo shows two different RF chokes, each about 6.4 microhenrys. The one on the left is a ferrite bead, made by Fair-Rite, Cat. # 2643000801. These were about 12 cents apiece from Mouser.com. Yes, the wire passes through the small hole only twice and it gives 6.4 uH.
The one on the right is a piece of 3/8″ or 9mm OD Pex plastic water pipe from the big box home improvement store. I bought a short length for a few dollars. I cut off about 32mm or an inch and a quarter, and heated up a straightened out paper clip wire to poke a hole through the plastic for the wire to go through. The wire is 26 AWG solid conductor enameled wire and is about 36 turns, close wound. I will put a piece of shrink tubing over the windings to hold them from moving. But the windings could be secured with clear nail polish or paint.
These will be used for RF chokes which are not critical. The RF interference bead has low copper losses because the wire is only a few cm long. But the ferrite core has some loss at 100 MHz. The plastic tube and air core has very low loss but the wire is much longer and has higher resistance.
The original one, the Z-50, has a SRF of below 100 MHz, also. According to the list it’s 76 MHz.
According to my dipmeter the Pex coil is dipping at 88 MHz. That’s just the coil, with nothing connected to it. The SRF could be raised by reducing the number of turns and increasing the space between turns.
