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Discussion Starter #1
Been doing some research on swr in receiving antennas.

It's looks like swr is only important in transmitting antennas.

From my research, it's looks like signal to noise ratio is
way more important than swr.


Here is a link to stackexchange web page on this subject.

What is the relationship between SWR and receive performance?


What do you think?

Is swr important at all, in designing ota tv antennas?
 

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It's looks like swr is only important in transmitting antennas.

From my research, it's looks like signal to noise ratio is
way more important than swr.

Is swr important at all, in designing ota tv antennas?
High SWR will reduce the NET gain of an antenna design. Keeping the SWR below 2.7 will keep the gain loss below 1 dB. The gain loss is the same for a receiving antenna as a transmitting antenna because of the Antenna Reciprocity Theorem.
Formulae for Calculation of Net Gain
Formulae for Calculation of Net Gain

SNR is very important in reception, but it isn't a design factor in modeling.

The higher noise levels on VHF than on UHF reduce the SNR, making stronger signals necessary for sufficient SNR.



 

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He’s right; SWR is important for both. If you have a high SWR on a transmitter, you can destroy your radio. A reciever just gets a bad signal.
 

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Formulae for Calculation of Net Gain
Formulas do give more accurate results, but I get lost trying to figure them out. Following chart gives good enough results, to allow you to compare antenna results.


The chart allows you to see approximately how much SWR is affecting the gain of your antenna. Just find the SWR and subtract the db loss figure from 4nec2 raw gain, to get net gain, at that frequency. There's nothing magical about SWR=2.7, but it results in approximately 1db loss. Most people won't see a big difference, caused by 1db loss or less, so it's become a common design goal. SWR=5.8 creates 3db loss, or a loss of 1/2 of the signal. SWR=13.86 creates 6db loss or a loss of 3/4 of the signal. 4NEC2 gives raw gain figures, and you have to take SWR into account, to compare actual gain figures, between two antennas. High SWR figures are going to have an affect on your actual gain.

From my research, it's looks like signal to noise ratio is
way more important than swr.
The more important question is how does SWR affect SNR? High SWR numbers lower how much signal, as well as how much noise is received. Lowering the SWR will increase the reception of signal, as well as ambient noise. Since tuner noise isn't increased, the total SNR is improved by lower SWR figures. Ultimately your ability to watch a channel is dependent on a certain SNR. Improving the SWR will almost always improve the SNR, available to your receiver. There is always the possibility that an antenna, with a worse SWR, may have other features, that provide better actual gain, and as a result, better SNR.

Antenna reception pattern is usually more important to improvement in the SNR. Antennas that only pick up signal, from the direction of transmitter, will help by rejecting signals from other directions. Some ambient noise, such as electrical wires are very directional, but most "Ambient Noise" is coming in from all directions. Using a more directions antenna may allow you to pick up more signal and less ambient noise.

Matching the reception pattern, of an antenna, to your location is probably just as important as SWR and Gain. Some locations need a very directional antenna, and some locations work better with an Omni-directional antenna. I live in a valley, surrounded by very tall mountains. We have one transmitter, on the top of one of the mountains, about 6 miles away. I can use an omni-directional antenna where I live, even though the only signal, that I receive, is coming from one direction. I have neighbors, that are near sources of interference, so they have to use more directional antennas. This allows them to get more of the TV signal, and block out some of the noise(better SNR). I've tried a lot of antennas, at my location, and find that overly directional antennas are just harder to aim. I have some multipath issues, created by a row of trees, that block much of my direct view to the transmitter. Overly directional antennas have more problems with changes, caused by weather and seasonal changes, than a less directional antenna. I also find that I get better reception thru the 6" thick brick wall, than I get thru my one window, that faces the transmitter. Many people have transmitters all around them, and they want to get signals, that are coming from many different directions. This requires a different approach, than my situation.
All of this is to say, that your exact situation may make one antenna work for you, when it won't work for someone else. Many situations are very difficult to model, and require building a number of different antennas, and trying a number of different locations and directions, to see what works for you.
 

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What do you think?
Is swr important at all, in designing ota tv antennas?
receiving a signal is just the opposite of transmitting a signal. So, yes it is important...
 

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Discussion Starter #6 (Edited)
Been looking up net gain formula in digitalhome.ca forum, and found a very interesting post by holl_ands.

It about Swr in real life, and net gain.

holl_ands said:
...Mismatch Loss will ONLY occur at the exact points on the coax where the SWR Nulls occur..USER will only EXPERIENCE that amount of Loss if the NULL happens to occur on the END of the Cable...when the Total Length is an exact multiple of the half-wavelength for a particular channel....very rare that a User actually HAS this particular problem...on perhaps only one or two channels.....and can "cure" it by adding an extra length of Coax....

Link to holl_ands post on swr:

holl_ands post on Swr
 
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