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Author Topic: Is a low pass filter necessary?  (Read 27296 times)
K0YHV
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Posts: 179




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« on: May 10, 2012, 08:22:16 PM »

I was on an antennas reflector recently asking about the proper placement of a low pass filter and several members asked why am I using one at all.  They said with the modern radios (I have an Icom 706 original) a low pass filter really isn't necessary, that most modern rigs already contain one and the spurrious emissions are already greatly reduced.  So, what is the opinion of forum members-with a modern rig, does having a low pass filter in line really help to solve any additional problems?

John AF5CC
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K5LXP
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« Reply #1 on: May 11, 2012, 06:34:57 AM »

Quote
does having a low pass filter in line really help to solve any additional problems?

What "additional problems" do you believe exist that need to be solved?


Mark K5LXP
Albuquerque, NM




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K8AC
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Posts: 1748




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« Reply #2 on: May 11, 2012, 01:55:24 PM »

The primary purpose of the low pass filter was to attenuate harmonics that fell withing TV channels.  For example, the third harmonic of 21 MHZ fell in TV channel 3.  So, if you lived in an area with fringe reception of channel 3, you could be sure that you'd be causing TVI and a low pass filter may have helped, assuming that the harmonic energy was getting out via the transmitting antenna (not always true).  Since most of the TV channels moved out of the VHF range when the move to digital TV occurred, harmonic interference to TV channels is no longer an issue.  There are no other problems that a low pass filter addresses.  Anyone using one today does so out of ignorance. 

73, Floyd - K8AC
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SP9HZX
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« Reply #3 on: June 09, 2012, 02:06:09 AM »

Hi,

I would recommend using LPF.I use Drake TV-3300-LP  80db. Well done!

 Smiley
Andy

www.qsl.net/sp9hzx
http://spearo.republika.pl
http://tychyangielski.republika.pl
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AA4PB
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Posts: 14304




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« Reply #4 on: June 09, 2012, 11:48:16 AM »

There is normally no need for a low pass filter on modern solid state transmitters. In the 1950's they were quite common, as were pink slips from the FCC for radiation of the second harmonic. It was even common practice to include the band you were using when calling CQ (as in "CQ 75") so that somone on the next higher band didn't try to answer your second harmonic.  Shocked

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NO2A
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Posts: 1185




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« Reply #5 on: June 11, 2012, 10:11:19 AM »

Most all rfi problems today are from signal overload. If your antenna is too close to a neighbor`s house for example. Chances are you stand a greater risk of causing rfi to your own house.  In the days of broadcast reception,I used to tear up our tv and phone on 49 mhz. Since going to cable tv and 5.7? 5.8? ghz phone no more rfi/tvi.
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KC4MOP
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Posts: 957




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« Reply #6 on: July 09, 2012, 04:16:21 AM »

I agree with the others. If your station is being operated conservatively and not all controls full tilt, you'll be ok without an LPF.
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KE7TRP
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« Reply #7 on: July 31, 2012, 03:53:34 PM »

Hi Fred!  hows it going?

I removed my low pass filter on the beam as I found I can get on 6 meters with it.  No isssues here. I dont think it hurts though and if you got it, Why not run it?  You might lower some harmonic that could cause trouble to some piece of Chinese electronics your neighbor purchased at wally world.

C
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N0SYA
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Posts: 402




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« Reply #8 on: August 20, 2012, 12:48:24 PM »

If you live in a metro area with several megawatt tv and fm stas nearby a low pass may help keep all those gigawatts out of your hf rig. I lived in Omaha in los of the tower farm at 72nd and Maple and the rf coming off all them antennas would peg my fs meter attached to a 75ft sloper. Should have tried to rectify it and charge batteries.
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If you have a clumsy child, you make them wear a helmet. If you have death prone children, you keep a few clones of them in your lab.
KC6RHE
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Posts: 4




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« Reply #9 on: August 30, 2012, 07:41:20 PM »

I did allot of reading and asking to seek a resolution to the RFI issues I have, for the most part its proximity, 40 meters makes noise into our surround sound speakers. the worst part is that my neighbors have a china cabinet with a touch sensitive area, when ever I transmit on 75/40/20 the light comes on. My antenna is probably 25 feet from them, the issues is with the cheap touch sensor but nothing I can do about that. My neighbors and really great people and I live in a beach are where living close to each is part of the community.

To resole the cabinet issue I bought them a timer so that the cabinet was only hot from 6pm to 10pm, I ended up making a bigger deal about resolving it then they did, in fact, they thought the china cabinet was haunted and enjoyed the light flickering when they had dinner parties "Dad is at it again". he he. when they told me about it I knew what it was and what was causing it.

I learned some good things though, my feed line where picking up radiation from the antenna and I was able to reduce that changing the angle of the coax across the roof

73's
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N4NUI
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Posts: 3




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« Reply #10 on: August 31, 2012, 05:57:41 PM »

The "problem" is that most consumer devices don't have to demonstrate resistance to incoming radio frequency energy.  Regardless of how "clean" your station's emitted RF is there is always some harmonic component. A "low pass" filter will cut off the VHF/UHF component of those harmonics before they get to your antenna feedline if you put the Low Pass filter as close to the final RF output point of your equipment before it attaches to the antenna feed line.

I you are having problems with comments from neighbors or within your own home an 1:1 isolation balun at the antenna feed point and the coax down to the shack can keep RF off of the shield of the coax. There are times when that RF can couple to AC mains and get into consumer devices through that path.  If you are running an amplifier, a 1:1 isolation balun between the rig and the amp can also help.  In fact "back in the day" we would also put a low pass filter between the TX and the amp.

The main point is that if you are not having problems with comments or complaints, there is no need to use a low pass filter.  No matter how high of quality the filter is, there will be some insertion loss.

My station is currently "state of the art" and very "clean." However, a neighbor 20 yards from my antenna commented that he could hear me on his Radio Shack brand audio system. I dug out my old Drake LP filter and inserted it in the coax line going to the antenna and the "problem" went away.  73
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VE7REN
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Posts: 550




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« Reply #11 on: December 18, 2016, 06:28:40 PM »

whats peoples thoughts on this rfi thing today in 2016? is it worth having one in line for some added help? even with a clean install!!
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WB4SPT
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Posts: 454




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« Reply #12 on: December 21, 2016, 07:46:22 AM »

In the 1950's they were quite common, as were pink slips from the FCC for radiation of the second harmonic.


The current law has no spec for spurs on amateur Tx before 1977 or so.  I wonder what law they were trying to enforce? 
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KF7CG
Member

Posts: 1189




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« Reply #13 on: December 21, 2016, 03:01:02 PM »

Harmonics that fall outside an amateur band were always a target. 3rd harmonic of some of 80-75 meters, second harmonics of anything higher that 14 meter band. Of course most low pass filters had their cut off somewhere above 35MHz so didn't work for 6 meters.

KF7CG
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AA4HA
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Posts: 2375




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« Reply #14 on: December 22, 2016, 12:56:31 PM »

That type of filter is a bi-directional device. If it attenuates 80 dB on frequencies higher than 35 MHz (just using a few specs that have already come up in this thread) then it also attenuates received signals to your transceiver to the same degree. That means that FM broadcast, public safety VHF/UHF are also attenuated before they get in to your receiver.

As long as the "insertion loss" (the loss in dB's in the part of the RF spectrum that you want to use is minimal), (just a dB or two) then it can be a benefit to your setup.

If you do not have problems with interference from frequencies above... or you are not generating interference in the VHF/UHF band then it really is just a useless piece of equipment that robs you of a bit of your transmitter power.

When selecting any type of filter you should get somewhat familiar with the specifications of what a dB is and how it impacts you. Remembering that every +/- 3 dB is a doubling or halving of power and that every 10 dB is a x10 or /10 impact.
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Ms. Tisha Hayes, AA4HA
Lookout Mountain, Alabama
Free space loss (dB) = 32.4 + 20 × log10d + 20 × log10 f
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