Currently I have a 60W amplifier for the 400-500MHZ band with a 50Ω impedance. In order to prevent jamming other frequencies I will need a low pass filter to block any frequencies above 500MHZ. I just made a full Internet search and haven't found any low pass filter capable of supporting 60W preferably 100W.

Since I'm new to electronics and have no experience in building Low Pass Filters or have an oscilloscope to fine tune. For the 60W amplifier I was thinking in buying separately 12 identically low pass filters (8 watts max each) and then put them in parallel so that each one shares only a load of 6W and all the system a total load of 60W and a theroretical max load of 96W (8w*12units). In case I would need more power it would be just a question of adding more low pass filters in parallel. Feasible?

  • \$\begingroup\$ You cannot connect RF filters in parallel as you suggest. Just think of the impedance of all those filters in parallel. \$\endgroup\$
    – Juancho
    Feb 17, 2012 at 11:02
  • \$\begingroup\$ @Juancho, you have to design them to be higher impedance per so that they show 50 ohms together, but I agree, series with filters that distribute the power is a more common design. \$\endgroup\$
    – Kortuk
    Feb 18, 2012 at 2:20

2 Answers 2


You should maximise purity of the signal fed to your amplifier to start, to make your job easier.

Depending on your output turned circuit and amplifier device topology you may have to deal with 2nd harmonic on upor 3rd on up. This is very standard fare at this sort of power level and you should have no trouble [tm] implementing a single filter.

The output matching stage of an RF amplifier is usually also a low pass filter by design, whether a traditional pi coupler (C-L-C) or LC circuit or other, or strip line or resonator functional equivalent. As you are usually concerned with harmonics (2x, 3x etc the desired fundamental) the requirements are usually not severe.

You'll find relatively few RF output stages with complex explicit low pass filtering.

Around 500 MHz is a bit of an in-between range where lumped element (RC) or waveguide / resonant trough etc type tuning is appropriate.

A good start is to look at Amateur Radio amplifiers that work around that frequency and see what sort of output tanks they use. Common amateur bands of relevance are at 432 MHz (70 cm) and 1296 MHz (23 cm). There is also the "2 metre" Ham band at about 144 MHz but that will be leaning more towards lumped element tanks and filters.

In 1990 Motorola published application note AR347 A compact 1 kW 2-50 MHz solid-state linear amplifier. While this is below your frequency range of interest, this amplifier has become a workhorse starting point for a zillion spinoffs and much can be learned by looking up examples.

432 MHz 110 Watt cct

Amateur amplifier designs up to 1 kiloWatt are common enough so something that works at 432 MHz and 1 kW should be scaleable to your use in the 500-600 MHz range.

Here's a page with Many amateur RF amplfier designs - some very close to your application.

This 432 MHz PA useuses steam power aka a power triode but shows you how simple a design is deeemed acceptable. Plumbing skills will be handy. An antenna tuner may follow this stage - but reading a number of amateur design articles will rapidly introduce you to the subject.

Steam power design from here:

enter image description here

Nice 432 MHz 1500 Watt amplifier pictures only here but you can follow it up if it looks useful

Design using older tech Russian made vacuum tube but useful for its output stage comments article here

They note:

  • The output circuit: The output uses a λ/2 75 Ohm stripline with both tuning and loading at the open end, it is constructed with silvered cooper 1mm thick with 125mm width and 220mm overall length. The line has a 25mm collar to reduce the spacing to ground while the finger stock contacts the tube anode on the lower cooper ring of the anode cooler. The line is fixed at 45mm from ground by ceramic or Teflon insulators (Teflon insulators are recommended). The output on a 7/16 connector (or a good quality N connector) is connected directly to the loading flapper. This flapper is 15mm by 30mm at a distance from 10 to 30mm from the end of the line. A choque connects the output to ground to avoid the presence of high voltage in the case of a flash over at the output flapper. The movable tuning flapper "C1" is 76mm wide by 45mm high, and tunes at 432MHz aprox. at 16mm distance from the line. The fixed flapper "C2" is 76mm wide by 35mm high, and is about 15mm from the inner side of the end of the plate line. The use of a kapton sheet between the flappers and the stripline has eliminated arcing from the flappers to the stripline

One kW at 432 mHz useful discussion and pictures

Many RF amplifier links


Russell's answer that you should first insure that the output circuit of your amplifier is optimized is right on the money.

Since you asked about a low pass filter I will make a suggestion. Have you considered a coaxial low pass filter?

enter image description here

Here is a PDF data sheet with photo from PROCOM for 900 MHz, but it should give you a good idea of how such a filter performs.

Another possibility is a cavity filter.

enter image description here

Both coaxial and cavity filters will perform extremely well in the 400-500 MHz range and designing one to handle 60 W (or even 10 times that) should not be a problem.

  • \$\begingroup\$ Thanks for recomending the PROCOM 900MHZ it seems that they also have the PROCOM LPZ470 link \$\endgroup\$
    – Mykson
    Feb 19, 2012 at 22:16
  • \$\begingroup\$ Thanks for recomending the PROCOM 900MHZ it seems that they also have the PROCOM LPZ470 link Probably I will use this one. A range between 400 to 470MHZ will suit my needs. The only question now will be the price... \$\endgroup\$
    – Mykson
    Feb 19, 2012 at 22:24
  • \$\begingroup\$ You can just make your own, there are straightforward designs. \$\endgroup\$
    – JonnyBoats
    Feb 20, 2012 at 0:03

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