Here is an updated reprint of a message that I sent to an ebay seller about Ohmite Z-0 VHF RF chokes:
"I bought a lot of (10) of these Ohmite VHF RF chokes from you a while back and have been looking into the use of them. Based on construction articles and radio catalogs I have seen of the time, my guess is that the Ohmite Z series started back in the mid to late 1930s and were produced for amateur radio use in factory built and homebrew receivers and transmitters. The Z-0 choke would have been the highest frequency choke produced at that time and was designed for the old 2 1/2 Meter ham band. Some of these chokes were actually marked "2 1/2 Meters". The 2 1/2 meter band covered the 112 to 120 MHz range in those days (VHF). The next choke in the series was the Z-1 and was made for the 5 meter band - very popular at the time with some ham operators in the mid-1930s. The 5 meter band covered the range of 56 to 60 MHz (VHF). The Z-2 covered the 10 and 20 Meter bands (14-28 MHz); Z-3 covered the 20 and 40 meters bands (7 to 14 MHz); and the Z-4 choke covered the 20, 40, 80 and 160 Meter bands (1.7 to 14 MHz).
This range of Ohmite chokes saw some use in war time radio production as well - mainly the Z-0 and Z-1 VHF chokes - for use in simple transceivers and transmitter-receivers. I have seen the Z-0 chokes used in the famous SCR-522 VHF transmitter-receiver radio unit. This radio was used by ham operators in the new 2 Meter ham band after the war.
In the post-war period, the ham bands got shifted around a bit. There were new services in VHF mobile radiotelephone, FM broadcast radio and Television which led to a change in the frequencies available for use by ham radio operators. It appears Ohmite redesigned their RF chokes slightly for the new band coverages. Also, they started to mark the chokes in recommended frequency of use rather than in wavelength. The Z-0 became the "Z-144" choke. Z-1 became the "Z-50" choke. Z-2, Z-3 and Z-4 were redesigned as Z-28, Z-14 and Z-7 (less 160 meters) respectively. The actual number of micro-Henries used in the revised line of chokes changed a bit but were proportional to the older series. Values given for the Z-0 and Z-1 are about 2.2 uH & 5.4 uH respectively (1938). The post-war Z-144 and Z-50 chokes are smaller in size physically but comparable in values."
73, DeWitt Clay WB6FBF
Subject: Re: Ohmite Z series of RF Chokes 
July 8th 2020, 10:30 am
