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Low Pass Filter Placement

The "bottom-line" of "passive" (not "active") low-pass-filters (and to back-up our "chastised" member "DudMuck" here ;-)

All LPF circuits / boxes WILL produce reflected power from any / all harmonics that are being suppressed by the LPF., AND will affect amplifier performance.!

The RF power amplifier (PA) in most all radios, employ a harmonic filter / termination circuit in order to boost its efficiency. It consists of a series of passive resonant circuits and tuned to the desired output frequency.

The harmonics generated by the active output device (the PA) are mostly reflected back to the output of the PA, and causes some amount of parasitic / anti-resonance problems between the harmonic-rejection filter (LPF) and the output termination circuit of the PA, which can degrade the efficiency of the PA itself.

This is why you will typically see, that only the first inductor coil (just after the PA final device(s) output transformer) will have had one of its windings (usually only in its center) just slightly "spread" by the factory.

*Just "google" some internal images of new, NON "cranked & spanked", 10 & 11 meter radios.!! :)

It's VERY similar to an antenna, in that any frequencies outside (above & below) of its resonance WILL be reflected back to the PA output (though, in an LPF circuit, this only effects the frequencies above its design limit).

For an LPF to "ground-out" any of the harmonics, one would have to design the LPF to not only pass-through all the frequencies below its designed upper cut-off limit, but more importantly., it would also have to be designed to be "series LC resonant" to ground at each of the un-wanted harmonics.!

It's NOT similar to an audio low-pass filter (as in electric guitar tone-controls or amplifier tone-controls) in which is simply a "hi-cut" filter, using a capacitor and just two connections of a potentiometer (making it technically a "rheostat"), which does in fact, just grounds-out, frequencies above a certain design-point.

Such a thing of the grounding of un-wanted RF frequencies (regardless of frequency) is usually done with an RF "circulator", wherein any / all reflected power is diverted / circulated into a "dummy-load" of appropriate wattage size. With some circulators being dual or even triple stage, for even further "dumping of watts to ground" before getting back to the PA.

Oh., and one last thing to "drive it home". Here's a "real-world" experiment., for those who are humble enough., to actually prove-out the Truth to themselves...

Take a standard / quality 30 Mhz LPF (ie: a Drake brand, TV-1000, 3000, 3300) and cable it up as follows:

Using an "HF-rig" (that will operate on 54 Mhz, in the 6M band) or better yet, for even greater "stark" results., use a simple 2M, FM rig (at 145 Mhz) and run its output, FIRST into a standard SWR meter, then into the 30 Mhz LPF, then lastly, into a standard 50 ohm, non-inductive dummy-load, and then attempt to take a standard SWR reading (between your transmitter's output, and the input of the 30 Mhz LPF).

Then be honest with your test results...

Eric Klein
Klein Communications
Now you are making me think. I have a 15w GMRS radio that I can try. Will report back.
Now you are making me think. I have a 15w GMRS radio that I can try. Will report back.
Great to hear you'll give it a "real-world" test.!!

Especially if you're using a typical / non-commercial, UHF band / 450 Mhz, GMRS transmitter., you'll certainly see ALL of it being reflected back from a typical 30Mhz LPF (as the Drake TV-xxxx models have a factory stated spec of 80dB rejection above 40 Mhz.!) and most other brands and models having at least 30 dB rejection above 40 Mhz (which even @ 30dB, equals a 1,000:1 reduction of through-put.!)

Just to prove it to a customer one time., I used my "dual-Band" Alinco DR-610 (set to 145 Mhz / VHF) in the medium power setting of just 10w (as to not cause possible harm to the PA)., and what it showed, was not only was ALL of the power being reflected, but that it was only showing about 5w of "forward" power.!

The half forward-power (10w down to 5w) was caused by the fact that ALL forward power was being reflected., thereby GREATLY lowering the PA's efficiency AND the amount of reflections going back & forth (through the cabling) between transmitter's output and the LPF's input.

Yes., let us know.! :)
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I'd like to know the real results as well. Please do report back. I've got a df3000 that was planning to use in the future base set up but this makes me want to rethink.
Well., there's certainly nothing wrong with using your DF-3000 for HF use up to 30 Mhz, as the factory spec-sheet states 60dB @ 60 Mhz (no rating at 40 Mhz) but am quite sure it's at least 40 dB @ 40 Mhz.!

The original theme behind this "thread" was simply the OP asking whether to place the LPF before or after his watt-meter., which the obvious answer is to place it BEFORE the watt-meter, so that the watts the meter is "seeing"., is harmonically-clean power.!

It's just that the thread devolved into bragging about connecting an LPF directly to an amplifier's output (no cable, just a back-to-back adaptor).

The problem with such a configuration (ie: no "reasonable" length of cable in between)., is if the amplifier is prone to producing harmonics., by not having some amount of "buffer" cabling between the two, can cause some problems :-(


I built a rather large, 65v, dual-LDMOS Linear, which incorporated a pair of "dual-pie" LPFs., one for each of the dual amps (the LPFs have very high rejection above 30 Mhz), which are then fed into a 3Kw isolation combiner.

My initial problem was not only having low CW & PEP power output (for a given RF power input), but also a quite low DC power-input to RF power output efficiency rating :-(

Through much research, I found that because I had wired both of the amplifier's output, from the output transformer coax DIRECTLY to the input of the LPFs (1" distance at tops.!) that given the tendency for LDMOS devices to have a VERY wide / high frequency range, that whatever harmonics the amplifiers were outputting into the LPFs (and being reflected back) was interfering with both the RF amplification ratio, and the DC power versus RF power efficiency of the dual amps.!

What resolved the problem., was to add about 40" of coiled RG400 cable between the amplifier outputs and the LPF inputs to start off with, and adjusting / cutting them both shorter (an inch at a time) using the exact same length for each of the dual amps, until I reached maximum RF amplification-ratio, and DC efficiency-ratio., and then did some final "tweaking" of the "dual-pie" LPF coils (2 in each LPF, 4 in total).

Bottom line.??

By just placing the LPF inputs, at a "reasonable" distance from amplifier outputs., so that most (not all) of the reflected harmonics coming back from the LPFs, were now NOT in the exact same "phase-angle" from what was coming out of the dual amps into the LPFs.! :) :)

Hope some of the above explanation, was at least somewhat understandable.....


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