Stub "hybrid" lowpass filter to filter out amplifier harmonics

-


 I needed to knock down the second harmonic from my amplifier as I was getting spotted on 20 when conditions were good, when calling CQ on 40 using my multiband vertical.

At first I was thinking about some sort of low impedance, series resonant circuit on 20, installed between vertical feedpoint and ground, but then I shifted to coax stubs, but I had no SO239 tee's. I wanted to it done right away so I figured I'd install panel mount SO239's on an aluminum box, and make the stub connection on the inside, when I read about W2VJN's hybrid lowpass filter. It's a three pole lowpass, but instead of capacitors, it uses coaxial cables with their ends open. Since they need to be left open, they are cut to 1/4 wave on the frequency that we want to supress. For 20M, this works out to be approx 3,4M.  The series coil is about 1uH to resonate with the coax capacitance. I built this because it was an easy design, and no more work than to drill 4 holes in a 1590B box. I used, after recommendations from OK2ZAW, a coil of 7 turns with a diameter of 30mm for this 40M LPF, with 20M notch

After some tuning of the stubs, and coil adjustment, I managed  a very sharp -76 dB of rejection on 20M, and an insertion loss of 0.11 dB. It can be stagger tuned, to give a wider, but not as sharp, null.

I'm more than pleased. No more false spots, and one more build up the sleeve.











Comments

Post a Comment

Popular posts from this blog

My HP8640 died!

-
Image
 My HP8640 died! This is me catching up on past projects. I did this back in 2022, figured someone would benefit from this information, since the 8640 is still around in great numbers at hams all over the globe. I bought a nice looking HP8640 at our local hamfest back in 2022. It was working when I got it home, but a day or two after I got it home, it started acting erratic, with RF ouput dropping out. eventually, there was no output whatsoever. Did some tests, and got to the conclusion that the cavity oscillator was not oscillating. Following various clues online, I tracked down the issue to the A3 unit (cavity), and a series resistor, which had drifted in value. By measuring the series resistance between the + feed and the A3Q1 socket and the - feed and the A3Q1 socket, I could see that the + lead seemed reasonable at 370ohms, whilst the - lead had drifted to 19Kohms, where I was expecting around 900ohms. The A3Q1 (HP21 pictured below) transistor sits under the small hex cap om t...
Read more

ARC-5 receiver power supply, an in depth look.

-
Image
 The power supply is essentially a vibrator power supply, but with an IC and mosfet instead of the mechanichal vibrator used in the old days. The schematic can be seen below: It´s actually quite straight forward:  The CD4047 is operated as an astable multivibrator, providing two , out of ohase, square waves, driving the gates of the FET´s. The FET´s ground one end of the transformer winding, causing current flow, and induces voltage to the secondary winding (which in this case in the old primary) Switching frequency is determined by capacitor C3 and resistor R4. These values in the schematic gives approx. 50Hz switching frequency.  Optional 28V operation: I included holes on the board to hold a zener diode at the input of the CD4047 (max vcc 18v) in case 28v operation would be used (non-modified filament strings). A different transformer is also needed. There is a transformer that fits the board, 14A-10R-36, by the number we can tell that it´s a centertapped 36v seco...
Read more

The Power Amplifier build. Part 2

-
Image
Power Amplifier Saga – Part 2: The RF Deck So, back in 2020, I was working for an old friend of mine who runs a machine shop. I was operating the CNC laser cutter – a 4 kW fiber laser – and spending my days around a machine that could turn sheet metal into precision cut panels I started planning how to use this in my amplifier build. Then, as we all remember, COVID hit . I got sick, along with the rest of the family, and ended up spending two weeks at home in the middle of winter. Tired, restless, but with just enough energy to sit at a screen, I decided to finally learn Fusion 360 – to take advantage of the situation and make plans for the amplifier. From Components to CAD During those two weeks, I modeled the entire RF deck from scratch. That meant creating accurate models of: Variable capacitors  Roller inductor and fixed coils GU81 tubes and sockets Vacuum relays Stepper motors (luckily, existing models helped here) It was a lot of work – but worth it . On...
Read more