What kind of filter should be used at the output of HF linear amplifier?

Question

I am looking for a output filter to use with a HF linear amplifier called miniPA70 which is available through several online auction sites. The circuit is available here : http://iu3idf.blogspot.com/2017/04/a-small-power-amplifier.html

I am intending to use this amplifier in 80m, 40m and 20m bands. So i need filters for these bands. Most of the filters i saw online were LPF pi-section LC filters. LC filters make sense as they don't dissipate energy. Having a LPF also makes sense as the harmonics will be having frequency as multiples of the useful amplified signal( for 40m band, i can have a LPF of 7.3MHz cutoff; and harmonics will be 14MHz, 21Mhz etc which are filtered out by LPF).

I have the following questions regarding selection of LPF -

1. The LPF filters out the harmonics whereas it allows the fundamental to pass unattenuated into the antenna. Where is the energy from these harmonics dissipated? (if i use a good quality inductors and capacitors, these components wont be able to dissipate much energy)

2. Does the energy of harmonics get reflected back to the final MOSFETs and damage them? (If it is so, i should be able to see a high VSWR between PA output and LPF input)

3. In practice, how much percentage of the energy contained in the harmonics with respect to the fundamental? (I think this depends on what device is used in final. But for a MOSFET, how much should i expect)

4. Is it a good idea to create a Band Pass filter rather than Low Pass Filter. This filter should be such that all the harmonics will be terminated at a dummy load and filtered output will go to the antenna. (Does this make the MOSFET see less reflections from filter?)

Answer 1

LC filters make sense as they don't dissipate energy.

Hm, that's kind of true (at least theoretically), but guess what: that energy that doesn't reach the antenna still has to go somewhere, and will eventually be converted to heat. In the case of LC filters, that might be in some ohmic resistances in components and traces, or in the amplifier output stage.

Does the energy of harmonics get reflected back to the final MOSFETs

Effectively, yes, a LPF like that simply looks like a large resistance, i.e. a mismatch, to higher frequencies.

and damage them?

No, that doesn't necessarily damage your amplifier: Often, amplifiers are designed to be able to dissipate the harmonics it produces themselves.

If they aren't (can't tell you about your specific amp), an option would be to have a high pass "shorten" out the harmonics to a dummy load / ground.

(If it is so, i should be able to see a high VSWR between PA output and LPF input)

I'm not a fan of VSWR as measure. Standing waves aren't measurable "at a single point", the whole point of standing waves is that there's locations of high and low amplitude on the transmission line.

In practice, how much percentage of the energy contained in the harmonics with respect to the fundamental? (I think this depends on what device is used in final. But for a MOSFET, how much should i expect)

That really depends. There's nothing that limits that number in either direction for a MOSFET – you can build a good mixer with MOSFETs (which means that basically all energy is moved to harmonics) or you can build a very linear amplifier (which means that nearly none of the energy is in harmonics).

However, you're buying a linear amplifier – and that should come with very clear datasheets telling you things like second- and third-order intercept points, or acceptable output power ranges for which nonlinearity is limited to a low figure. Nonlinearity is what causes the harmonics!

If you can't get such a datasheet (you really should when buying electronics that someone built – they need to have the measurement equipment to know how well the device is built, and they must tell you, else buy somewhere else), because you're building this yourself, then you'll have to be able to measure the harmonics you get from single- and dual-tone tests yourself. From that, you can calculate the amount of power in the most significant harmonics yourself, and that will allow you to decide what kind of filtering you need – or whether you want to avoid overly complex filtering by predistorting your input so that the output looks linear. (yay, SDR beats classical radio once again.)

Is it a good idea to create a Band Pass filter rather than Low Pass Filter. This filter should be such that all the harmonics will be terminated at a dummy load and filtered output will go to the antenna. (Does this make the MOSFET see less reflections from filter?)

Depends. Usually, that's not necessary or desirable, since band pass filters are hard to design and get right reliably if you need to defined both the passband impedance as well as the stopband impedance over small frequency differences.

Answer 2

RF Power Tools sells 3-band and 7-band low-pass filters modules. You can purchase bare boards to stuff yourself or assembled and tested units. You might also consider using their 100W MRF101 amplifier board.

I do not attest to the specs for the products and I am not affiliated in any way with RF Power Tools.