Low pass filter use and linear amplifiers

[NightOwl3261]

I always by habit run a quality low pass filter after amplifiers due to being in an urban environment to be neighbor friendly just in case. With a modern HF radio (not a CB or Import radio) is there any reason to run one prior to the amplifier or is the harmonic suppression sufficient from current transmitters?

 

[loosecannon]

as long as the HF transceiver is functioning properly and hasn't been messed with, then there is no reason to install a low pass filter in between it and the input of the amplifier.

just check to make sure the match from the transceiver to the amplifier is good and the input level is correct and you'll be fine.
LC

 

[dave457]

Never hurts as well

 

[Captain Kilowatt]

No harm in running a filter between the radio and amp. In fact, with really broadband amps you actually SHOULD run a good LPF before the amp.

 

[NightOwl3261]

No harm in running a filter between the radio and amp. In fact, with really broadband amps you actually SHOULD run a good LPF before the amp.

I have always done this by habit it the past but technically why on a broad band amplifier?

 

[Justme]

Your broadband ampwill amplify anything it gets in it's input including harmnics.
Good tube amps like my Heathkit SB-1000 tou tune the PA and it will supresss harmonics and have a clean output if you don't overdrive it.

Broadbanded amps don't have that tuning section and amplify anything that gets into them.


I don't run an filter here, my HF radio is clean, i don't overdrive the amp, know how to tune the amp if i use it.
Living on the edge of the city in a build up area i can use all power up to 1000 watts on all bands from 160 - 70 cm without getting angry neighbours knocking on my door

On my station running 6 meters as well on the same port as 160 - 10 a low pass filter is also a nono.

 

[sp5it]

Lowpass ilter should be placed after the amp, not before.

 

[Captain Kilowatt]

Lowpass ilter should be placed after the amp, not before.

The reason for running an LPF before an amp is simple. I have an amp that was designed as a wideband television amplifier module that uses MRF-151G's. I plan to run it on 6m. It is a Larcan. The amp itself is SUPER clean and does not produce harmonics on it's own. By using an LPF between the exciter and the amp I can further reduce any harmonics coming from the radio to produce a super clean output since the amp will indeed amplify any harmonics from the exciter. This also has the added bonus of not requiring a high power filter that will function on 6m. In any event there is NO harm in running one before AND after if you are just running HF.

 

[sp5it]

Modern transceivers (younger than 50yo) have LPF built in unless you are using DIY spark-gap transmitter.
Mike

 

[Mustang 131]

It's really not a bad idea to do both. Less garbage in = Less garbage out.
The plus is less stress on your AMP, specifically in AM.
And a good brick wall on your output is never a bad idea.

 

[Captain Kilowatt]

Modern transceivers (younger than 50yo) have LPF built in unless you are using DIY spark-gap transmitter.
Mike

And yet they still emit harmonics. Hmmmmm................

 

[sp5it]

Lets take IC-7300, very popular rig in USA and rest of the world.
Typical PA stage. Different bands, different LPF.


There is test result of 7300:


Do you really think another LPF is needed after the radio? FCC thought opposite and allowed this rig to be sold in USA.
Link to test file: https://www.ab4oj.com/icom/ic7300/ic7300_dc4ku.pdf
LPF after amplifier spitting garbage is different story.
Mike

 

[dave457]

Wouldn't hurt

 

[jacquelyn S cortes]

Filters take a lot of your power and a lot out of your receive as well I bet you loose a lot of distance Dxing put them in the junk bin 73,

 

[NightOwl3261]

Filters take a lot of your power and a lot out of your receive as well I bet you loose a lot of distance Dxing put them in the junk bin 73,

Insertion loss is very low in high quality low pass filters in the range of .1 -.2 db insertion loss at operating frequencies. Jumpers with pl259s, antenna switches, barrel connectors, swr and power meters,, etc also have insertion loss yet we all use those. You also have insertion loss with an amplifier in stand-by. A little insertion loss to be RF and neighbor friendly is no reason not to put out a clean signal.