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15FB572 Profile
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New Kid On The Block

Registered: 11-2016
Posts: 2
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Low Pass Filter - a must?


Hi all,

Ham Operators use always a Low Pass Filter after an amplifier or the most amplifier have a LP
Filter already build in.
When I watch some CB Activities with a Amplifier (RM Italy) they don't use a LP Filter.
The RM Italy KL-203 Amplifier have not a build in LP Filter. It seems nobody takes care
about the harmonics (Ok, analog TV is mostly dead, so nobody gets disturbed).
Do you use a Lowpass Filter after an Amplifier (Max. 100W Out)?

Many thanks for your opinions!

Ben
 
4/Dec/2016, 4:49 pm Link to this post Send Email to 15FB572   Send PM to 15FB572 Blog
 
Spac3Monk3y Profile
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Freebander

Registered: 06-2014
Location: No Place Like 127.0.0.1
Posts: 252
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Re: Low Pass Filter - a must?


No because i have no use for a amp... emoticon

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31/Dec/2016, 2:35 pm Link to this post Send PM to Spac3Monk3y
 
Hot Fusion Profile
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Management

Registered: 03-2008
Location: South West England
Posts: 1389
Karma: 7 (+7/-0)
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Re: Low Pass Filter - a must?


I suppose the sensible answer would be to say yes, use a low pass filter.

Amps come in 2 basically different types - tuned output, or wideband.

The wideband CB types, for cheapness, tend not to have any filtering and so it is possible for them to radiate significant harmonics.

The tuned output types include all valve amps, and some solid state amps. A little solid state Zetagi b150 I have here has a tuned output stage, so this would tend to reduce harmonics.

One way to tell is to open the case and look for open-wound wire coils - this would probably mean a tuned output. The presence of rings of dark material (ferrite) with wire coils passing through would suggest a wideband output stage, with no filtering.

Hopefully that hasn't confused anyone, but grab a decent Kenwood LF30A low pass filter and all will be OK with the world. emoticon

---
Andy - FB107
1/Jan/2017, 7:46 pm Link to this post Send Email to Hot Fusion   Send PM to Hot Fusion
 
43RC982 Profile
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Freebander

Registered: 12-2020
Posts: 24
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Re: Low Pass Filter - a must?


Just curious, I know very little about Low Pass filters. Would a LF30a work on 27mhz? The add I found for one says "Attenuation: more then 90 dB between 90 and 300 MHz" Does this mean that it only works between 90 and 300 mhz? Probably a noob question but if I don't ask I don't learn
12/Jan/2021, 2:42 am Link to this post Send Email to 43RC982   Send PM to 43RC982 Blog
 
One Slivovitz Profile
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New Kid On The Block

Registered: 05-2021
Posts: 3
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Re: Low Pass Filter - a must?


LPF was almost a 'must have' in the golden days of CB, because if you hung power onto your CB the power would come through on audio systems and occasionally TV sets.

Which got you a visit from the radio interference government thugs.

Nowadays it's not so bad as it's digital everything EXCEPT for some audio equipment (mainly because the speaker leads act like aerials and port your power straight into the amplifier).

Bottom line? If you are going to use power, fit one. The frequencies it lists are typical and it stops your HF signal (often well distorted after amplification)sounding like a loads of bees in the speakers.

I've just set my rig and apart from an ATU, I also included a LPF out of habit.

However, it's unlikely (unless the neighbors complain or if you start interfering with critical services) you'll get a knock.

The frequencies listed are those the LPF protects.

Paul



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MrFidget Profile
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Freebander

Registered: 09-2018
Posts: 107
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Re: Low Pass Filter - a must?


Yeah. Common mode currents through the speaker wires were always a problem for me.

These days, the noise on the lower HF bands is unbelievable here in the city. S7-S9 on 7MHz. But our ACMA doesn’t care about noisy LED lights or solar panel controllers that are putting out measurable milliwatts of noise from 50KHz through into the VHF and UHF part of the spectrum.

Operating portable from a mountain top or a remote beach seems to be the only way to get away from it
27/May/2021, 10:30 pm Link to this post Send Email to MrFidget   Send PM to MrFidget Blog
 
Hot Fusion Profile
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Management

Registered: 03-2008
Location: South West England
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Re: Low Pass Filter - a must?


Bear in mind that audio breakthrough on an amplifier is a function of the transmitter's power output, not the harmonics, so fitting a LPF won't make any difference in this case.

As hinted at earlier, maybe don't use an amplifier, and concentrate on the best antenna you can get or make; that way you avoid putting out a dirty signal.

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Andy - FB107
30/May/2021, 10:09 am Link to this post Send Email to Hot Fusion   Send PM to Hot Fusion
 
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Freebander

Registered: 09-2018
Posts: 107
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Re: Low Pass Filter - a must?


Antennas will always win over an amp, up to about 6 dog Biscuits / 4x power 25W > 100W > 400W but to QSO you need to be able to hear the bloke on the other end too
14/Jun/2021, 12:42 am Link to this post Send Email to MrFidget   Send PM to MrFidget Blog
 
Hot Fusion Profile
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Management

Registered: 03-2008
Location: South West England
Posts: 1389
Karma: 7 (+7/-0)
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Re: Low Pass Filter - a must?


Agreed - the classic scenario of an Italian frying chips, who can be heard all over Europe but is unable to hear the weaker signals. Best situation is for both parties to be running similar power outputs.

However, improve the antenna, and transmit AND receive are improved.

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Andy - FB107
15/Jun/2021, 7:23 am Link to this post Send Email to Hot Fusion   Send PM to Hot Fusion
 
MrFidget Profile
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Freebander

Registered: 09-2018
Posts: 107
Karma: 3 (+3/-0)
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Re: Low Pass Filter - a must?


These days you also have to compete with QRN from all the badly designed switching devices. S7-S9 hash on 40m even with common mode chokes and grounding. I’ll be going back to mag loops. See if I can improve things
16/Jun/2021, 10:38 pm Link to this post Send Email to MrFidget   Send PM to MrFidget Blog
 


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