BE FM30B Transmitter Part 2

Walk around and initial startup of a Broadcast Electronics FM30B 30kW FM transmitter. Part 2 of 2

Category:

Education

Tags:

• fm transmitter
• radio station
• broadcast electronics
• radio engineering
• engineer
• fm

[ve2zzz]

Thanks for the reply...

My question is about the aluminum cylinder that is attached to the tube's anode.

This cylinder makes the cavity's inner conductor that resonates at the transmitter's frequency. But there are two types of cavities: The quarterwave that is open at one end (the tube side) and shorted to ground on the other end, and the half wave cavity that is open on both ends.

My question is about the top end of the cylinder. Is it left open or is it connected to a DC clock capacitor ?

[norcalengineer]

@ve2zzz Yes, if memory serves this transmitter has the exhaust stack connected to a blocking cap at the tube. It's been a while since I worked on this box. :)

[ve2zzz]

That's the second video i watch on a tube FM transmitter... And on both videos, i have an unansewered question: In the plate cavity, you tightly secure the lower part of the cavity center conductor (chimney) to the tube's plate (and the Ebb RFC)... But how about the TOP end ? Is it an half-wave cavity that has it's top end open ?

[norcalengineer]

@ve2zzz Forgive me if I misunderstood your question, but you connect the tube to the socket (filament, screen, grid voltages are applied), and you connect the HV supply to the anode on the top of the tube. In most of these types of tubes, the anode is the entire outer metal shell on the top of the tube, shared with the heat-dissipating fins. The output RF then conduits through the air (usually about 1-2 inches in distance) to the antenna connection located on the rear of the cavity.

[wb5mgr]

In this transmitter the no blocker is required. The chimney is at high DC potential, and the center tuning section is at ground. The output is capicitivly coupled to the 50 ohm output and is tuned by a corrugated tube within the chimney that functions as an inductor. And the loading is adjusted by a second inductor (visible in part 1 at 1:28 in extreme RH corner of image)

[JCTBone]

Thanks for the very indepth video. This is soo much sweeter than the 100W LP station I look after.