This prototype rf amplifier uses two 2SC2640 VHF power transistor wired in push-pull configuration. The 50 ohm impedance of the input is transformed to lower impedance by the 4:1 broadband coaxial cable transformer providing a close match to the base of the two transistor. To operate in class C, the base of the transistor must be at ground potential so the secondary winding of the 4:1 coaxial transformer is grounded at the center position( null). The impedance of both collectors are transformed by the combination of inductors L1 , L2 and the 120pF capacitor and further transformed by the 1:1 coaxial balun.
Both the 4:1 coaxial transformer and the 1:1 balun are made from RG316 teflon coaxial cable and these are cut to a certain length in order to operate at the highest cut-off point which in this case at 110MHz. Actual test of the prototype amplifier, I was able to measure 55 watts output from a 1.5 watts input. ---73 de du1vss
Hello,
ReplyDeleteI think there was a misunderstanding. Coaxial cable RG316 has impedance of 50 ohms. Wouldn’t the cable used have to have 25 ohms?
You are right, rg316 is a 50ohm cable. This cable was intentionally used as a 1:1 coaxial transformer hence no transformation is required. This 50 ohm cable recombines the out of phase output of each transistor into unbalance output.
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DeleteRight, but the cable used in the transformer 4: 1?
DeleteI read in some books that 4:1 transformer is made of 25 ohm coaxial cable but I have also tried several HF amplifier projects that uses binocular ferrite transformer and just wires for its broadband matching transformer and it works just fine. It provides good input and output match in my amplifier so I dont bother looking for a 25 ohm cable. You will also find that most commercial hf transceivers like icom, yaesu and kenwood also uses broadband transformers that are made just with binocular ferrite transformer and some insulated wires. I believe also that the turn ratio between secondary and primary winding determines the impedance ratio of the transformer. Thank you by the way for dropping at my blog, only a few these days are interested in ham radio. 73 de du1vss
DeleteYou are welcome. Do the transistors have equivalent? It is difficult to find them.
ReplyDeleteYou can try the mitsubishi 2sc1946a but this is hard to find now.
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ReplyDeleteHello,
ReplyDeleteI was also in doubt. If I use 25 ohm cable, is the impedance of the transformer will be reduced by half? Another thing: what are the dimensions of input transformer? It was not clear for me to build it.
Re replacement transistors . Why not use the RD15VHF1 MOS and just rehash the bias network. This will give you 16 W or so out Linear and about 20 W FM non linear. The RD15s are about 4 USD each from quality suppliers like UTsource.net. Don't buy 1 dollar specials they will be rubbish.
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ReplyDelete"will also find that most commercial hf transceivers like icom, yaesu and kenwood also uses broadband transformers that are made just with binocular ferrite transformer and some insulated wires." Caution , almost all successful Ferrite binoc HP transformers use copper tube primaries and TE insulated wire secondaries. It is however possible to use a coil and coil construction as in the Mike Kossor IRF510 HF amp. The TE insulated wire is hard to get and generally will have to come off Ebay (I have used a seller called Navships but they come and go) though aircraft repair workshops are known to have it. You can't use PVC insulated wire for these transformers as PVC destroys RF above 10 Mhz and the insulation will overheat and break down. Caution type -61 ferrite is the only ferrite that will work for RF power above about 20 Mhz. It is known good to 4metres (75Mhz) but will be very marginal in terms of losses at 100 Mhz. Timothy Fidler, WRE , NZL
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