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Message |
    
Sitm
Intermediate Member
Username: Sitm
Post Number: 317
Registered: 1-2004
| | Posted on Tuesday, November 10, 2009 - 11:07 am: | 
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Ok, we talked about this last night on 2 meter until people started getting angry.
Where should your low pass filter be put in line with the following equipment.
radio
amp
swr meter
low pass filter.
Some say between the filter and antenna
Some say right after amp before the meter.
What do you guys have to say?
I am tempted to put one after the amp and another after the swr meter just to cover the bases. |
Tech833
Moderator
Username: Tech833
Post Number: 1770
Registered: 8-2002
| | Posted on Tuesday, November 10, 2009 - 1:07 pm: |
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Assuming you are talking CB, the filter should be right behind the radio, before any meters, etc. The reason is, you don't want any harmonics leaking out from cables and other equipment and being radiated. You want to knock out harmonics immediately, with the shortest cable possible between the harmonics source and the filter.
If you are talking about a ham station, the filter should go right behind the amp. All meters, and similar stuff should go after the filter, which is immediately after the amp.
Of course, there is nothing wrong with running more than one filter. An example would be a filter after the radio and another after the amp.
I can give an example as to why.
In my installation at home, I use a lowpass filter for a different reason- To keep RF signals above 30 MHz. OUT of my station. I have an FM broadcast transmitter on my tower, and the strong FM signal tends to desense my HF receivers. So, the lowpass filter keeps the FM signal out of my gear and lets me hear faint signals just as if the powerful FM transmitter wasn't there at all.
So, in this example, let's "reverse engineer" the scenario. We are treating the FM signal near 100 MHz. as we would an unwanted harmonic. If the lowpass filter is located away from the radio with a bunch of meters,etc. between the filter and the radio, then the FM signal will have plenty of places to leak into the cables and equipment, right? Of course it will. However, if the filter is located right at the radio antenna connector, the FM doesn't have a chance to leak in between the filter and the radio, so it is much more effective. |
Sitm
Intermediate Member
Username: Sitm
Post Number: 319
Registered: 1-2004
| | Posted on Tuesday, November 10, 2009 - 2:30 pm: |
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sent this letter to Bencher Inc.
Dear Bencher, Inc,
I have one of your fine low pass filters and would like to know the best
way to put this in line. Would you please let me know of the following
components where in like does the low pass filter belong?
1. Radio
2. Amplifier
3. Meter
4. low pass filter
Is this the way it should be?
Thank you for your help as it seems there is alot of disagreement between
fellow hams how this should be.
GOT THIS REPLY.
Mark - you have it exactly right. Some watt meters are capable of generating TVI, so having the filter after them cleans up that possible source of problems as well.
73
Bob Locher W9KNI |
Bruce
Senior Member
Username: Bruce
Post Number: 5223
Registered: 9-2003
| | Posted on Tuesday, November 10, 2009 - 2:33 pm: |
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I have to agree with 833 ....
At my place I run a lowpass on the ICOM and YASEU HF radio ( DRAKE TV-100LP TYPE ) and on the ts-2000 2 meter 6 meter bandpass filters and a lowpass filter so that no matter what band i'm on I can run of HF 6 and 2 meters without creaming the other radios. I'm waiting on several new ones to replace several mitilary bandpass ones im using on 10 6 in front of the PT-3 preamp and on 2 meters in front of my .5 db NF/25db gain 144 mhz preamp preamp .
I also have a bandpass on the 223 mhz radio.
The only drawback is if your amp is not clean you got to add a filter after it too but done this way you don't have to worry about pagers and other trash out there .....
ICE makes some good bandpass ones if you need them that is what I'm going over to. |
Sitm
Intermediate Member
Username: Sitm
Post Number: 320
Registered: 1-2004
| | Posted on Tuesday, November 10, 2009 - 2:38 pm: |
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I have two low pass filters, a nice 1000 watt drake and a bencher 1.5 so I think that I will place one after the radio and then another after the meter. I should have things covered. I have no tvi now, but still get audio into the stereo if it is on. I figure if I am getting it the surrounding houses are as well. So I will place them one after radio and then another one after the meter before the run to the antenna.
And this is 10 meter with 100 watts RMS. |
Tech833
Moderator
Username: Tech833
Post Number: 1771
Registered: 8-2002
| | Posted on Tuesday, November 10, 2009 - 7:31 pm: |
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Hi Bob.
Truthfully, if your meter is capable of generating harmonic TVI, then it needs to be replaced, it is broken.
With all due respect to the fine people at Bencher, they are just hams. The answer you received proves that they do not understand the issue at all. A harmonic is a multiple of your transmit frequency. Harmonics are generated in transmitters, not in meters. Spurs (and intermod) can be generated anywhere after the transmitter (like in a meter) and NO lowpass filter will fix that.
It is your equipment, so naturally, the choice is yours. It makes no difference to me. Just trying to help you do it right. And trying to expose and refute some of the myths that are perpetuated by those who do not know what they are talking about. |
Patzerozero
Senior Member
Username: Patzerozero
Post Number: 4709
Registered: 7-2004
| | Posted on Tuesday, November 10, 2009 - 9:12 pm: |
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'Assuming you are talking CB, the filter should be right behind the radio, before any meters, etc. The reason is, you don't want any harmonics leaking out from cables and other equipment and being radiated. You want to knock out harmonics immediately, with the shortest cable possible between the harmonics source and the filter.
If you are talking about a ham station, the filter should go right behind the amp.'-so that means ALL cb stations disseminate harmonics & no ham radios do? so that means all ham stations have perfect jumpers that do not leak rf, whereas all cb stations do? hmmmmmmmmm....... |
Tech808
Moderator
Username: Tech808
Post Number: 17211
Registered: 8-2002
| | Posted on Wednesday, November 11, 2009 - 12:04 am: |
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Personally for the last 40+ years I have always placed them as:
Radio / Low Pass Filter / Meter / Antenna
or when using an Amp:
Radio ~ Amp ~ Filter ~ Antenna
I guess Iam very lucky or I have some very dumb  radio's, meter's and antenna's as they have never been able to figure out if it is a CB or a Ham or Mobile or Base station rig sending the signal to my antenna's. 
Lon~Tech808
N9CEF
CEF#808~CVC#002
Radio Enthusiast!
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Tech833
Moderator
Username: Tech833
Post Number: 1772
Registered: 8-2002
| | Posted on Wednesday, November 11, 2009 - 1:02 am: |
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Quote: "so that means ALL cb stations disseminate harmonics & no ham radios do? so that means all ham stations have perfect jumpers that do not leak rf, whereas all cb stations do? hmmmmmmmmm......."
No, that's not what I meant. Re-read what I wrote. For the CB station, there is no amp mentioned. For the ham station, I mention an amp and where the filter goes in that case. The amp was the difference. |
Bruce
Senior Member
Username: Bruce
Post Number: 5224
Registered: 9-2003
| | Posted on Wednesday, November 11, 2009 - 6:41 am: |
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IF yout just trying to hold down TVI filter goes last before antenna.
They also keep radios from seeing other radios in your shack which after number of radios and the antenna farm here got trimed was done here. Otherwise it would be one radio on at a time ..... |
Ironmike
Junior Member
Username: Ironmike
Post Number: 21
Registered: 6-2007
| | Posted on Wednesday, November 11, 2009 - 9:06 am: |
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RFI is one of the most difficult technical problems to solve because it is quite unpredictable.
You put the filter(s) where it solves the particular problem. |
Sitm
Intermediate Member
Username: Sitm
Post Number: 321
Registered: 1-2004
| | Posted on Wednesday, November 11, 2009 - 9:51 am: |
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This is all great advice, I tend to agree with Lon on this one and will place a bencher 1.5 low pass behind the amp and a nice little drake behind the meter just for the S & J's. |
Sitm
Intermediate Member
Username: Sitm
Post Number: 322
Registered: 1-2004
| | Posted on Wednesday, November 11, 2009 - 9:53 am: |
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Lon I remember a photo of your station, did you place the filter right after the amp with a barrel connector?
Moderator Note!
Yes, I use a Copper Part #C70-05002 Elbow Connector or a Copper Part #C70-05012 Double Male Connector depending on the clearence I have behind the radio.
 pictures will be posted as soon as I get my camara fixed.
Good Memory Mark. |
Tech833
Moderator
Username: Tech833
Post Number: 1773
Registered: 8-2002
| | Posted on Wednesday, November 11, 2009 - 10:42 am: |
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Quote: "IF yout just trying to hold down TVI filter goes last before antenna. "
Actually, if you are trying to hold down TVI, you place the filter right at the source. That means right behind the radio or amp, before any meters or other gear.
Again, TVI can have many sources. This topic was brought up asking about harmonic filters. Harmonics are generated in transmitters, NOT in meters or other outboard gear. A faulty meter can cause spurs, and a lowpass (harmonic) filter will not fix that. There is no practical purpose for placing a lowpass filter downstream of a meter to 'clean up the meter'. That is more false CB folklore being spread by those who have no way to prove their faulty opinion.
Lon did it correctly. Follow his example and ignore all other 'advice'. |
Sitm
Intermediate Member
Username: Sitm
Post Number: 323
Registered: 1-2004
| | Posted on Wednesday, November 11, 2009 - 11:40 am: |
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Tech833,
Would I be reading harmonics in the meter before I put the low pass filter in line? After I put the filter, before the meter, will that show in the swr meter? |
Tech833
Moderator
Username: Tech833
Post Number: 1774
Registered: 8-2002
| | Posted on Wednesday, November 11, 2009 - 8:03 pm: |
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I don't really understand your question, I'm sorry. |
Bruce
Senior Member
Username: Bruce
Post Number: 5226
Registered: 9-2003
| | Posted on Wednesday, November 11, 2009 - 8:23 pm: |
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I think he is asking does the meter read harmonic power also and if they are there yes it will |
Tech833
Moderator
Username: Tech833
Post Number: 1776
Registered: 8-2002
| | Posted on Thursday, November 12, 2009 - 11:24 am: |
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Thanks for the 'translation', Bruce. I think I understand now.
If your transmitter is putting out harmonic content, then a wattmeter will show it as additional power from your transmitter.
Example- If your transmitter is putting out 2 watts of fundamental and 1 watt of harmonics, it will show your transmitter putting out 3 watts. |
Sitm
Intermediate Member
Username: Sitm
Post Number: 324
Registered: 1-2004
| | Posted on Thursday, November 12, 2009 - 11:41 am: |
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Tech833,
How will this reflect in the standing wave. Am I correct in the theory that harmonics can read into your standing wave and if so will a low pass filter behind your radio and or amplifier help to reduce your harmonics being read by your standing wave meter? |
Bruce
Senior Member
Username: Bruce
Post Number: 5227
Registered: 9-2003
| | Posted on Thursday, November 12, 2009 - 2:38 pm: |
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I would think you are right the cleaner the signal at the meter the better the reading. |
Tech833
Moderator
Username: Tech833
Post Number: 1778
Registered: 8-2002
| | Posted on Friday, November 13, 2009 - 12:34 am: |
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A meter tells you how much power is going out or in (SWR). It is not a frequency selective device, so it measures ALL power. |
Tech237
Moderator
Username: Tech237
Post Number: 1252
Registered: 4-2004
| | Posted on Saturday, November 14, 2009 - 11:57 am: |
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Bruce, Paul and Lon are all correct and a low pass filter should be as close to the radio as possible, in fact if possible, I use a male-male adaptor to connect them, or failing that the shortest length of the best quality coax I can get.
They are also right about the meters. Power and SWR meters do not care what frequency the signal is, if it's there they will try to measure it. Removing all unwanted signals BEFORE the meter will give you a more accurate reading
Simon
Tech237
N7AUS
.
I thought he said, "there was no rust for the wicked, and I own an MGB"
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Press_man
Member
Username: Press_man
Post Number: 90
Registered: 5-2008
| | Posted on Thursday, December 10, 2009 - 9:24 pm: |
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I agree with 808,833,Bruce,237 I was taught-nasty out nasty in and visa versa. 833 I do have a question since your dealing with FM.
With the switch to HD and virtually no analog and most of TV moving to UHF, some of it here in FL though has been granted to stay lowband on their old channels ie: ch5 WPB, ch6&9 Orlando. Will there still be as bad a problem with TVI whether open broadcast, cable or sat signal. What will work to clean up the recievers. Will the standard highpass filters still work of are we looking at a whole new ball game, possibly a new type of filter with different freqs. to accommodate the the digital pattern.
With so much available over air now some here are dropping cable and sat service. I don't bother mine with a mobile but soon the base will be up and I want to be ready. I've used an antenna for years with very little effect to ch5 but I'm always filtered to the hilt and that won't help a neighbor with old 75ohm at their house.
Wally/Pressman/KC4ZWM
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Tech833
Moderator
Username: Tech833
Post Number: 1795
Registered: 8-2002
| | Posted on Sunday, December 13, 2009 - 10:58 am: |
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Hi Wally.
Good question.
Some of the LPTV stations were granted a reprieve of the HDTV migration, but it is temporary. Eventually, all TV stations will be all digital, there will be no more analog TV over the air broadcast.
The good news is, an HDTV receiver will not demodulate your analog signal. That means, neighbors won't hear you on their TV sets. If you do somehow interfere with their TV set, your RF will simply make the TV signal disappear, or become scrambled looking on their screen.
The biggest threat to CBers now is not TVI, but ASI, which stands for Amplified Speaker system Interference. In other words, those horribly cheap amplified computer speakers found everywhere. No shielding, no filtering, all analog, and the speaker wires make great antennas for picking up your signal and feeding it to the high gain amplifier.
Your radio 'Mythbuster' since 1998
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Press_man
Member
Username: Press_man
Post Number: 93
Registered: 5-2008
| | Posted on Sunday, December 13, 2009 - 8:47 pm: |
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Thanks Paul!
In the beginning it appeared as if all over air HDTV was going to move to UHF. Some though are staying on VHF but have made the switch to digital. I'm concerned that with the buy out of NBC by a cable company they may try to squeeze out broadcast transmition. Hopefully they'll take a clue from Disney who is doing a fine job with ABC but they are an entertainment company.
Back to the subject.
Will we be in the same danger with ASI as we are over a leaky door bell? Or do you think it will be the same as with a cordless phone?
Wally/Pressman/KC4ZWM
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Tech237
Moderator
Username: Tech237
Post Number: 1268
Registered: 4-2004
| | Posted on Monday, December 14, 2009 - 11:19 am: |
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NBC has not been bought out by any one. The cable company concerned (happens to be the one I work for), has worked out a deal where we get to carry their information AT NO COST TO US in exchange for granting them rights to certain in-house created programming.
NBC is still owned by who ever has always owned NBC.
Tech237
N7AUS
Chipmonks roasting on an open fire
Hot sauce dripping from their toes.
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Press_man
Member
Username: Press_man
Post Number: 94
Registered: 5-2008
| | Posted on Monday, December 14, 2009 - 4:00 pm: |
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Simon:
Wasn't trying to start WW3, the fish wrapper here said X cable company was "buying" NBC (No accounting for accuracy.)
There is always a concern when you read any large company "buys" another. Since buyouts and take overs are often ugly and most often a power move, it brings up concerns.
Glad to hear this appears to be a move to improve NBC and your employers entertainment abilities.
Wally/Pressman/KC4ZWM
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Tech833
Moderator
Username: Tech833
Post Number: 1796
Registered: 8-2002
| | Posted on Friday, December 18, 2009 - 2:49 am: |
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ASI is going to be a major problem in densely populated areas.
Your radio 'Mythbuster' since 1998
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Tech237
Moderator
Username: Tech237
Post Number: 1270
Registered: 4-2004
| | Posted on Friday, December 18, 2009 - 2:22 pm: |
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Pressman - no problem and definitely no ww3. Just wanted to make sure the real facts were out there.. I've been fielding irate calls about this all week - lol
Tech237
N7AUS
Chipmonks roasting on an open fire
Hot sauce dripping from their toes.
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Press_man
Intermediate Member
Username: Press_man
Post Number: 100
Registered: 5-2008
| | Posted on Monday, December 21, 2009 - 3:54 pm: |
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Simon
Actually happy you did dispell what the Mullet wrapper said. Hey! Didn't you recently turn 50?
Paul
Will current TVI traps help with the ASI or do we need to petition the FCC for protection such as that against cordless phones or to have filters made available for the speakers or will it need to be internal? Maybe a cap across each speaker?
Everyone be safe over the holidays!
Still no snow in Florida!
Wally/Pressman/KC4ZWM
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Tech833
Moderator
Username: Tech833
Post Number: 1799
Registered: 8-2002
| | Posted on Monday, December 21, 2009 - 11:55 pm: |
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ASI is caused by the presence of a strong signal, period. TVI or lowpass filters will not help.
A cap across the speaker leads is a good fix as long as the RF is not simply being demodulated by the amplifier itself regardless of wires.
Your radio 'Mythbuster' since 1998
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Tech237
Moderator
Username: Tech237
Post Number: 1273
Registered: 4-2004
| | Posted on Tuesday, December 22, 2009 - 10:50 am: |
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Pressman - yes back n June
Tech237
N7AUS
Chipmonks roasting on an open fire
Hot sauce dripping from their toes.
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Joey_migs
Member
Username: Joey_migs
Post Number: 67
Registered: 9-2009
| | Posted on Wednesday, December 23, 2009 - 8:05 am: |
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Would you not want put the filter theoretically at the transmission line to the antenna in CB and/or Ham. This way any spurious signals and /or harmonics are attenuated regardless of the source (jumpers, meters, linear amp. If a piece of equipment in between has a tendency to give off spurious spikes or strong harmonics a filter could be used at its output as well.Am I wrong?
Joe |
Tech808
Moderator
Username: Tech808
Post Number: 17325
Registered: 8-2002
| | Posted on Wednesday, December 23, 2009 - 8:26 am: |
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As posted above in this Topic:
Radio ~ Low Pass Filter ~ Meter ~ Antenna
or when using an Amp:
Radio ~ Amp ~ Filter ~ Antenna.
Hope this help's,
Lon~Tech808
N9CEF
CEF#808~CVC#002
Radio Enthusiast!
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Tech833
Moderator
Username: Tech833
Post Number: 1800
Registered: 8-2002
| | Posted on Saturday, December 26, 2009 - 1:33 pm: |
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808 is right.
Also, let me re-state that lowpass filters do NOTHING to filter out spurious emissions. Only 3rd and above harmonics.
Your radio 'Mythbuster' since 1998
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Joey_migs
Member
Username: Joey_migs
Post Number: 75
Registered: 9-2009
| | Posted on Saturday, December 26, 2009 - 8:53 pm: |
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833, A lowpass filter does not attenuate spurious emissions?
Thanks, I've learned something today.
What about a Butterworth and Chebyshev implementing bandpass, cavity, or elliptical characteristics (the latter with a infinity deep notch at suspected frequency of the superfluous spurious signal (hows that for alliteration) filter?)
Same deal with unwanted spurious spikes? How do amplifier design engineers deal with this particular bugaboo? |
Tech833
Moderator
Username: Tech833
Post Number: 1801
Registered: 8-2002
| | Posted on Sunday, December 27, 2009 - 11:25 am: |
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If an amplifier that has been designed properly with measures to supress parasitics and such will not produce a lot of spurious emissions.
Different filters by name always fall within one of 3 categories-
High or low pass
Bandpass
Notch
The bandpass filter will help knock down spurious emissions, but the others won't help much or at all.
The problem with depending on a bandpass filter to clean up spurs is that they are never narrow enough to be completely effective. Best bet is to clean up the transmitter.
Your radio 'Mythbuster' since 1998
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