Low Pass Filter Placement

[W6HSA]

I just read a 'Reducing Radio Interference' article where it recommended placing a low pass filter after a watt meter - the last link before the antenna(s). I have always thought that the LPF should be placed before a meter and/or a scope so that any potential spurious signals would be filtered, giving a more accurate reading, and the filtered signal would pass through the meter to the antennas.

I currently have mine before the LP-100a coupler which is the last device before the antenna(s). It sparked a rather lively debate, so I thought I'd ask, "what is the best way based on your experience?"

 

[Captain Kilowatt]

A low pass filter should be placed as close to the transmitter (or amplifier) output as possible using a very short patch cable. For years I had my B&W LPF connected to the output of my radio with a solid 90 degree elbow connector, no coax between it and the radio at all.

 

[mackmobile43]

I have my drake filter connected directly to the output so-239 of the amp with a double male connector.

 

[W6HSA]

Excellent, that's what I always thought. The watt meter comes after. Thanks for that. I thought I'd post this if others had the question.

Best,

W6HSA

 

[mackmobile43]

The meter after the filter shows output less the harmonics the filter is there to eliminate.

 

[W6HSA]

That's what I always thought. The article, as well written as it was, challenged that notion with a few of my friends. Thanks for the reply.

 

[dudmuck]

For normal operation, you would want your low-pass filter directly connected to your transmitter. You want to see your power to antenna without any harmonics.
However, for testing your setup its useful to try your watt-meter between transmitter and LPF to test how much power the filter is reflecting back to your transmitter to make sure its not excessive. It could be excessive reflected from LPF because either you're operating on a frequency its not tuned for, or your transmitter has lots of harmonics.

That being said, the value of low-pass filter is becoming less because the VHF-LO 54-88MHz TV band is very sparsely used now, only a handful of stations actually on air on VHF-LO. And even you were interfering with them, how could anybody tell? All TV there is now digital.

 

[W6HSA]

I actually just used a MFJ antenna analyzer and measured the filter. My antenna match on my beam is 1.1:1 and on the vertical it is 1:2.5:1. The match through the ICE filter is 1.2:1 on the beam and 1.3:1 on the vertical. My SWR when transmitting stays rock steady as measured on the LP-100a. I actually feel pretty good about the installation.

I got the filter along with 1:1 baluns for both antennas to be as clean as possible. I ended up getting some ferrites for a few of my neighbors who had problems on their cheap cordless phones, computer speakers and headphones. I called it my good neighbor policy and they are happy campers.

 

[MrSuburban]

For normal operation, you would want your low-pass filter directly connected to your transmitter. You want to see your power to antenna without any harmonics.
However, for testing your setup its useful to try your watt-meter between transmitter and LPF to test how much power the filter is reflecting back to your transmitter to make sure its not excessive. It could be excessive reflected from LPF because either you're operating on a frequency its not tuned for, or your transmitter has lots of harmonics.

Can you please explain this quote, a lo pass filter doesnt reflect power back to the transmitter if you have harmonics.

 

[Captain Kilowatt]

Can you please explain this quote, a lo pass filter doesnt reflect power back to the transmitter if you have harmonics.

You beat me to it. A LPF actually bypasses the harmonic content to ground in series mode and attenuates it in parallel mode thus preventing it from reaching the antenna . Nothing is reflected back to the transmitter.

 

[dudmuck]

Filters are just capacitors and inductors.
They will only absorb due to loss in the components.
This loss is very low. Such as at the operating frequency of filter the loss is as low as half dB.
But when operating at a higher harmonic frequency, the filter will reflect most of that power back to the source. A small amount of that will be absorbed by the filter because of that same insertion loss twice. But most importantly the harmonic power is much reduced going to the output.

Capacitor-inductor filters only represent a 50 ohm impedance at their designed operating frequency band. When you operate outside of that (such as harmonics to a LPF) then its no longer 50 ohms (at that frequency) and returns most of the power back to the source.

See the dark-blue traces for power reflected by filter (magenta is insertion loss):
Lumped Element Filters - Microwave Encyclopedia - Microwaves101.com

You could google more about "filter reflected harmonics" or LPF reflected harmonics.
Absorbing type do exist, but they are special.

 

[Captain Kilowatt]

Filters are just capacitors and inductors.
They will only absorb due to loss in the components.
This loss is very low. Such as at the operating frequency of filter the loss is as low as half dB.
But when operating at a higher harmonic frequency, the filter will reflect most of that power back to the source. A small amount of that will be absorbed by the filter because of that same insertion loss twice. But most importantly the harmonic power is much reduced going to the output.

Capacitor-inductor filters only represent a 50 ohm impedance at their designed operating frequency band. When you operate outside of that (such as harmonics to a LPF) then its no longer 50 ohms (at that frequency) and returns most of the power back to the source.

See the dark-blue traces for power reflected by filter (magenta is insertion loss):
Lumped Element Filters - Microwave Encyclopedia - Microwaves101.com

You could google more about "filter reflected harmonics" or LPF reflected harmonics.
Absorbing type do exist, but they are special.

The BOLD text quoted above is only half right. The LPF does not present 50 ohms impedance to the harmonics and that I agree with. None of the harmonic energy is reflected back to the source, that being the transmitter. A LPF consists of series and parallel circuits. The series circuits are from the main transmission path to ground and in effect short out the harmonic energy to ground. None of it goes back to the transmitter. The parallel circuit components offer a very high impedance inline with the transmission path and basically force that energy to seek another path and that path is via a series circuit to ground. Again, none of it goes back to the transmitter.


One a note: I noticed an ad that stated the following,

"All RF amplifiers will "react" in some way when a low pass filter is attached. This reaction is mainly caused by reflected harmonics. All RF amplifers generate harmonics. When a low pass filter is attached to the amplifer most of the harmonic energy is reflected back to the transistor causing reduced efficency and output power. The transistor will also run hotter."

Perhaps this piece of misinformation this is a reason many CB'ers don't want to use a LPF on their amps.

 

[loosecannon]

captain kilowatt, i am very curious to know where you found that article.

to me it sounds like some amp builder trying to refute customer complaints about their amps not reaching the advertised output once a low pass filter is put in line.

"dont use them, they will blow you amp!!!" LOL


also, i read somewhere that a low pass filter is not very effective if its case is not grounded to an earth ground.
anyone ever hear this?
LC

 

[Captain Kilowatt]

captain kilowatt, i am very curious to know where you found that article.

to me it sounds like some amp builder trying to refute customer complaints about their amps not reaching the advertised output once a low pass filter is put in line.

"dont use them, they will blow you amp!!!" LOL


also, i read somewhere that a low pass filter is not very effective if its case is not grounded to an earth ground.
anyone ever hear this?
LC

I kick myself for not linking to the article but I added it above on an edit when I was in a hurry. I found it while looking for something unrelated but I don't think it was a regular amp builder. IIRC it was part of an article about amps and filters. It was not so much an ad for amps but was on an online retail site.

As for the not grounding the filter case, it is grounded via the coax cable shield however a direct separate ground is always a good idea for all pieces of gear.




Found the site. Thank God for not clearing my history.

1500W Harmonic Absorbing Filter [LPF1500] - $150.00 : Broadcast Concepts, The Art of FM Amplification

The "reacting" that is mentioned is most often due to changes in output impedance driving into the load either with or without the inline filter and not due to reflected harmonics. How many class C amps have you seen that were upset by adding an LPF to the output other than reducing the apparent power output?

 

[dudmuck]

The BOLDThe series circuits are from the main transmission path to ground and in effect short out the harmonic energy to ground. None of it goes back to the transmitter. The parallel circuit components offer a very high impedance inline with the transmission path and basically force that energy to seek another path and that path is via a series circuit to ground. Again, none of it goes back to the transmitter.

Any part of antenna system: antenna, transmission line, or filters and whatnot.. if they are not 50 ohms then they will reflect power back to the source (the transmitter). I dont know how there could ever be an exception to this rule.

If a low-pass filter is absorbing harmonic power instead of reflect it, then it would provide a near 50 ohm impedance at the harmonic frequencies.

Most amplifiers are usually OK with harmonic energy being reflected back to them, if these amplifiers are not being over-driven to begin with. Perhaps some amplifiers are sensitive to reflected of harmonics because they inherently are unstable with high gain or cannot handle bad vswr in the first place.