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Antenna Design Basics + Amplification

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Doktor digital

I,m only a mere TV technician /antenna installer and not an RF engineer like yourself or James but I interprete from what James is trying to say is that as a typical filter has an insertion loose of around 1-2dB,If you put it after the amp you would get a better carrier to noise at the outputthan if you put it before the amp.

I also interprete that he is suggesting that this is only suitable where unwanted singals will not be so high as to overload the amplifier and he recommends using amplifiers that a very linear so that intermodulation is minimized.

He is not saying that it will remove the "behind signal".Infact he mentions careful positioning of antenna to address this issue.

There is logic in this method especially if you are in a very terminally weak area to squeeze out the last drop from the antenna.

Otherwise you have said some intersting things.

I liked and finally understood your last description of the "behind signal" ,Der finally get your point.

This sounds like the argument of why MER is more important than CNR because it is a measure of the channel and the behind the channel interferrer.

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James...get a life...you have got no idea what you are talking about and now everything is getting out of hand. There is no need to offend me by calling me a troll.

But you are yourself so ignorant of RF that you have not understood one word that was written in my posts.

They are long because if I don't bloody cover every single possible angle, there will always be a smartass like you to pick on a hole that I have left out. This is typical of people that know nothing and profess that they know everything...Like YOU James!

And if you continue to reply on this thread, you will see that you are only showing everyone how silly you are for the simple FACT OF THE MATTER THAT YOU CANNOT EVER ANSWER ANY OF MY QUESTIONS OR CHALLENGES WITH ANY REAL TECHNICAL FACTS... ALL YOU DO IS INSULT!

You either give some constructive arguments so we can debate this like civilized persons or just shut the hell up! Simple like that!

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Unfortunately you have to read the crap that James is writing which has thrown everyone off the beaten track!

Now I will try to explain in another way what the problem is with what James is saying.

Basically what James is trying to say is that he can filter out interference, even after an amplifier.

THIS IS WRONG. Please look back to my previous post where I gave the example of 2 modulators connected together Via a 2 way splitter in reverse and both modulators Output on the same frequency/channel.

This was an example (AND ONLY AN EXAMPLE FOR UNDERSTANDING PURPOSES) so that people can understand that if you do not attack the interference problem before the actual amplifier (in the example case it was a Splitter but the same analogy) you will NOT be able to remove the interference.

If anyone did not believe me, then by using the example given, anyone who is half a technician could have set it up and tested it for themselves for peace of mind. Ie: FACTUAL EVIDENCE!

If you have a real life scenario where there is an interference carrier very close to the original carrier that you wish to distribute, it will be more difficult to do this AFTER an amplifier than before because:

A) The amplifier will clip if the Interference carrier is amplified and is greater than the maximum output of the amplifier spec (could be around 115 - 125 dBuV)

Then because filters can effectively attenuate the particular frequency by around 40dB (Yes sure there are better filters but I have used this number for example purposes) you will not be filtering enough of the interference carrier so that it does not impact on the adjacent carrier (in this particular example).

However if you filter this interference BEFORE the amplifier, you are dealing with a carrier (interference) much lower in power and hence able to nearly, if not completely, eliminate the offending carrier.

Just think, it is easier to filter a 50dB carrier with a 40dB filter than it is to filter an 80dB carrier !

I hope you can understand what I am saying now.

With regards to overdriving the amplifier...remember that to find the maximum input of an amplifier you must use the following formula: Maximum Output - (minus) Gain = Maximum Input

The Output and Gain figures of the amplifier are from the SPECIFICATION of the particular amplifier that you are using. So if the amplifier you will use on this particular example might be Max Out = 120dBuV and Gain = 30dB, then your Maximum Input would equate to 120 - 44 = 76dBuV

This means if you input more than 76dBuV into the INPUT of the amplifier you will overdrive it and it will start to clip/distort etc... the signals. I hope you can see the relevance of the max input when you combine it with the high level interference carrier...

Finally, CNR and MER are sort of the same thing.

If possible, please imagine, CNR = Carrier To Noise Ratio and MER = Modulation Error Ratio

Now think of CNR, MER and BER all Mathematically linked together. If you want the Mathematical formulas to prove this statement I can provide them but I can tell you now it will be a few pages of calculations and will bore the crap out of everyone including myself.

However, where CNR fails in a Digital measurement is when a carrier or interference is present UNDERNEATH or BEHIND the actual carrier that you are wishing to distribute.

If there is an interference, but you can't see it because it is on the same centre frequency of the TV carrier you wish to distribute, you will not be able to get an accurate measurement of the actual carrier to noise.

Whereas with MER, it will measure the carrier, digitally so to speak, and therefore advise of the real scenario. I could go into much more detail here if you like. Just let me know.

In any case, MER is not the most important measurement, it really is BER. BER IS KING and will tell you straight away what sort of signal you are working with, and by the way is an excellent anti-hair-loss product :-)

I hope that this has helped you understand more.

And no, I do NOT know everything, but the little I know I now wish to share with everyone with the hope it can make everyone's life more easyer when installing and without a baffling with bulldust explanation.

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The statement I made was very simple but I'll try to make it even simpler, just for you, but I'll type slower this time.

It is incorrect to state that the filter must always be placed before the amplifier.


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I think you will have to give me your postal address so that I can send to you a complimentary truck load of sorbent toilet paper (no not paper napkin) because you abviously need it to wipe your mouth quite often with all the crap that is coming out of it :blink:

Your debating skills remind me of some Kindergarten children that I know...

And now I will type slower for you to understand...


If you can't show me with an example, you are just confirming to me and everyone that is reading this now very silly thread, just how ignorant you are of RF.

All you know how to do is say that I am wrong but you don't give any realistic proof to back up your claims.

The least we could do is debate it like civilized humans with realistic & logical examples and maybe we can all learn something out of it, however sadly this is not the case.

All you are telling me and everyone is that you just don't know and you are shooting your mouth off in a last bid of desperation because you have realised that you are not as intelligent as you think you are and now everybody knows it!


Give it a rest quite while you are ahead so that you don't damage your reputation more than it is now literally hanging from a thin thread...

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Hi Doktordigital,

You have some good ideas. Some bad one's too! Seeing your new here it would be good to have some background to how you know all this stuff. By the way I'm a broadcast tech andfind all the info on here very valuable. The on the job experience and efforts of all the other members and administrators is very credible.

Anyway Welcome. )I think!)

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Thanks for your nice comments and warm welcome. It is a pleasure if I can be of some assistance...even though I can get a bit silly sometimes... :blink:

But I am very curious to know what the bad Ideas I have given...Could you please let me know what they are possibly???

It's all in good fun anyway...thanks

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At May 10 2006, 09:38 PM my reply explained the application of an amplifier before a filter. I assumed you could read!


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AHAHAHAHAHAHA...You're just a barrel of laughs...AHAHAHAHAHHA my stomach is hurting from laughing so much!!! :blink::P:P


It's a shame this forum doesn't display when, if and how many times the post has been edited...

Unfortunately for you I have a good memory and I can remember what you wrote at the start.

YOU HAVE CHANGED WHAT YOU ORIGINALLY WROTE QUITE EXTENSIVELY And that shows just how shifty you really are!

But, never the less, your explanation is still incorrect! and you are grappling at straws to save your reputation!

Why? Because you said...

Classically these are situations where there is very marginal wanted signal levels and yet C/N (carrier to noise ratio) is reasonable and at the same time some local significant signals are present. In this situation if the filter is placed before the amplifier, the filter's insertion loss eats into the already marginal signal.

then you said

By originally placing a very linear low gain (10-20dB) amplifier stage prior to the filter and careful positioning of the receive antenna, these losses are overcome and the resulting signal will present the receiver with a better C/N ratio without the intermods the uninitiated might expect. This is no theory, it's in use and delivers superior results in this application.

But if you knew anything about RF you would know that

A) Good quality filters can have a loss factor of less than 1dB (Sometimes .5dB) so by eliminating the interference BEFORE you amplify it, the amplifier will amplify ONLY the carriers that you wish it to amplify, not the interference.

B ) Each time you add an amplifier into the system you Add Noise. This is FACT AND A LAW OF PHYSICS Doesn't matter what type of transistors or technology you use, Hence your Carrier To Noise will be reduced because you are raising the noise floor by the amount of noise that the amplifier generates.

C) If you have an interference carrier that is high enough to overdrive the amplifier, doesn't matter what filtering you insert, the amplifier will be clipping/distorting because you have inserted the filter AFTER the amplifier AMPLIFIES the signal.

So in a nutshell, putting the filter AFTER the amplifier in YOUR example will achieve absolutely NOTHING!

Just think about points A, B & C very carefully tonight and tomorrow morning you should realise that you are wasting your time and better go back to school and learn more about RF before posting such rediculous and misleading information, which includes CHANGING what you had written initially...VERY SHIFTY !


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nearly 24 hours since james t has responded, he's on the ropes... or he's had a long week. herr doktor said it - back to school! that's what there is not enough of in oz, few are trained in rf for tv at all and some make a little knowledge dangerous. a good place to start would be an open mind. james t, where did you learn to be so rude?

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The next post after the one I quote occurs 8 minutes later, is it possible to edit a post after the next post is placed? I don't think so and I didn't. Your comprehension and understanding may be the issue here.

OK lets do it your way and have a play with your Questions A, B & C now because I'm beginning to think you don't believe what I write.

A. Firstly, yes filters can present an insertion loss below 1dB. So let me set the scene in the same way I have all along. You are operating in an area with very low receive levels and there is reasonable C/N ratio and there are some local significant signals, what can be done?

(1dB insertion loss filter before the amp) If we try this way the C/N ratio that is presented to the amplifier will be reduced by 1dB through the loss in the filter. If this amplifier has low gain and therefore it should be understood that this amp inherently would be low noise, at best the result would be the original C/N ratio minus 1dB, an admirable result.

(1dB insertion loss filter after the amp) This way the C/N ratio that is presented to the amplifier is maintained. Again by the use of a low gain inherently low noise amp two things become possible. Firstly the best C/N ratio possible would be close to the input C/N and secondly the low stage gain enables the other significant signals to be amplified but because of the low gain, there's no saturation (no clipping).

I am not the first to use this method effectively for television, I don't feel any concern over you not being aware of it. Not to mention the disastrous effect your theories would have on LNA's for satellites, GPS receivers and mobile phones. This application of filtering after amplification is everywhere and is heavily reliant on the characteristics of the receive antenna.

B. Can't argue with amplifiers introducing noise. However you are missing the point again, the level of noise introduced by an amplifier increases with gain. With low gain devices the opportunity presents itself to amplify signals with very little introduced noise.

C. The amplification of high level signals will not necessarily result in overdriving the amplifier if it is low gain. You must know what you are doing though to make this work.

Don't you know its wrong to cast nastertiums?


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Doctor Digital,

This is all a huge windup

The ACMA plans that the transmitters are separated physically to not give co-channel interference except in rare circumstances. The exception to this is Single Frequency Networks. These networks have identical frequencies and signal delays so that the signals match as much as possible. Where this is not achieved filtering will have no effect. The only alleviators are highly directional antennas of the correct polarisation.

As far as adjacent channel interference goes, the ACMA tested this problem prior to digital commencing. This is why the power of a digital transmitter is 1/4 of its adjacent analog PAL transmitter. If there is adjacent channel digital transmitters, the powers are identical within that band. Very high Q filters will work prior to any amplification but not afterwards where you have another signal "under" the other one.

Let us consider this matter closed.


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Actually the problem is that in James' example, there is no basis and no real example to prove that inserting a filter AFTER the amplifier is better. Just saying "It is better and has been in use for quite some time bla bla bla"... That just isn't good enough. There is no real factual evidence that was brought forward and no actual examples to prove his theory so until as such time as James properly explains this, I will always promote filters MUST be installed before amplifiers, In this particular example.

Sure there are other examples of filters being inserted after amplifiers but that is usually to remove channels in a particular section of a network, or something like that anyway.

But as far as interference is concerned...Why do we have single channel amplfiers/filters or channel processors? Because they only amplify the selected carrier and then distribute only that particular carrier throughout the network. And there is nothing anyone can say that will change this. The Single channel amplfiers are infact filters and they only pass the selected carrier or bandwidth. Hence, your final output will be nice and clean free from unwanted stuff. If you have ever connected a single channel amplifier headend together, you will understand that the incoming signal will arrive straight from your antenna. (And sure it might be from a mast head amplifier but even then you can have filtering). We can go deeply into this too.

Then the comment about LNAs (LNBs) is another classic example of filtering and when it was mentioned by james that my theory would have a disasterous effect on LNBs is another display of the lack of understanding about filtering.

Persons that read this forum that play with C-BAND satellite TV that live in Sydney area would know about filtering, especially when the word UN-WIRED is mentioned! Right? And that the problem with UN-WIRED is that they have a carrier, I think around the 3.4GHz Region, which is so high in power relative to the C-BAND LNBs (C-BAND, including Extended C-BAND is from 3.4GHz to 4.2GHz) that the LNB goes into overload and will either shut down or not work properly. You can solve this in two ways. Either purchase a filter, which connects between the flange of the LNB and the FEED (If you have the ADL type feedhorns with separate LNB) Therefore, remember that the satellite signal departs from the satellite, arrives at your dish, bounces off the dish into the focal point, through the feed horn and Into the LNB. The filtering here is done INBETWEEN the Feed and LNB.

Another solution can now be purchased from Gary Cratt which is a modified LNB that will FILTER out this UN-WIRED interference, BEFORE the amplfier kicks in... So all in all, another silly theory from james bites the dust yet again!

Then the last point C, where James says that "The amplification of high level signals will not necessarily result in overdriving the amplifier if it is low gain"...ABSOLUTELY REDICULOUS!

Well just remember the mathematical example given previously. Max Input = Max Output - Gain.

If an amplifier has output spec at 120dBuV and a Gain of 44dB and the calc = 120 - 44 = 76, then if you input more than 76dBuV (IN THIS PARTICULAR EXAMPLE) then you will ALWAYS overdrive the amplifier.

Sure if you have a 20dB Input attenuator you can use the input attenuator to reduce the input signal but then you will not have anymore a 20dB Attenuation (can also be referred to as Gain control).

But in the scheme of things, if the 20dB Attenuation is not used, you WILL ALWAYS overdrive the amplifierand that is for Input stage amplifiers. With Mid stage amplfiers the problem is even worse.

Even if the amplifier is Low gain, the same rule applies. If you have a Low gain amplfier with max output of 100dBuV and 20dB Gain then Max Input = 100 - 20 (80) so therefore if you input more than 80dBuV, you WILL ALWAYS OVERDRIVE THE AMPLFIER ! Simple like that! (Of course not taking into account the input attenuation control but I am sure you know what I mean).

To Finish off, the reason for the lower transmission power of the Digital Transmitions is because the Digital carrier actually occupies more surface area than an Analogue channel in a particular given bandwidth.

And before anyone corrects this, and YES there are many other factors here...You want me to go into them? Just let me know...But I am trying to keep it simple seeing this thread is getting a quite boring and past it's expiry date. A little like me I suppose :blink:

So if the Digital ADJACENT channel is at the same power as the ADJACENT Analogue carrier, it will cause digital interference with the analogue picture (ie. Very defined white dots etc...). The Digital Carrier must be around 10dB lower than the Analogue carrier. I can go into this in more detail if you like but I am sure you know already.

The SFN scenario is another issue altogether and not the correct place to debate this unless you wish to.

Look, I know I might seem nasty or very harsh, but I am really tired of reading alot of the crap that some so called experts, or really ignorant people, that profess to know the answer.

It's not fair to the guys who are just trying to learn and end up wasting precious time chasing dreams when they could have money in pocket and sitting back with beer in hand relaxing after a hard days work.

It seems to me that anyone that debates with factual evidence is not liked on this forum and it also seems that most people prefer to stick thier heads in the sand and talk BS because it sounds like a better solution instead of working through the problems rationally and logically. Therefore I will cease to post on this forum because what I have to say is not appreciated. Thanks for reading anyway...



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I am finding all this boring!! DoktorDigital I picture you as someone who walks down the beach and kicks sand in the little persons face and your reason why is because you can. Maybe next time people should think whether they should. Lets get back to the idea of this forum, to help people. Throughout life we meet people we disagree with, most of us accept this and move on. I suggest we all do that.

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there are 2 levels here. the simple 'gimme pictures' stuff and the 'why is it so' stuff. if herr doktors explanations are boring you - don't read them!

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You wouldn't know me from a bar of soap and yet you say that I would be someone who kicks sand in the little persons face because I can...hmmm well ACTUALLY it is NOT like that at all. If you knew me you wouldn't say that. I would never do anything like that. Guys like you just can't handle the truth thats all.

You would rather stick your head in the sand and listen to BS cos it sounds good and what you want to hear.

I know I can be a little on the RAW side but that is because I tell it how it is straight up with no nonsense and backed with factual evidence.

I am not going to whinge about it anymore. I just won't post anymore so you can be spared from your boredom...Sorry to bother you.

But remember, when you need REAL information backed up with factual evidence...don't wait for me to post because I will not anylonger.

Yeah, and have a nice day!

P.S. Dig2all, Thanks for your nice down to earth comment. It is a good idea and I appreciate it. Thanks.

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