Coaxial Cable

[loumaag]

...3.7 dB is a marginal difference, probably only worth it IF you have signals that today are a bit noisy and you want better.
If you don't hear it now, 3.7 dB is not going to "bring it in"!...

Well I was just going over new threads since yesterday and usually I would leave a technical one like this alone; however, the above statement needs to be addressed with facts (I think I got you this time N_Jay) :lol:

The rule is 3db of change in signal is a times 2 ratio. (+3db = 100% increase in signal)
That said a nifty online calculator I have used in the past says that the 3.7db increase you indicated is better than a 2 1/3 increase in the signal; I would hardly call that marginal. :wink:

Of course considering we are talking loss as opposed to gain the difference is more like this; by using N_Jay's numbers from above, 3.7 additional loss means 57% of the signal available at the end of the LMR-400 run will not be available at the end of the RG6 run, and I call that a significant difference.

 

[N_Jay]

Well I was just going over new threads since yesterday and usually I would leave a technical one like this alone; however, the above statement needs to be addressed with facts (I think I got you this time N_Jay) :lol:

The rule is 3db of change in signal is a times 2 ratio. (+3db = 100% increase in signal)
That said a nifty online calculator I have used in the past says that the 3.7db increase you indicated is better than a 2 1/3 increase in the signal; I would hardly call that marginal. :wink:

Of course considering we are talking loss as opposed to gain the difference is more like this; by using N_Jay's numbers from above, 3.7 additional loss means 57% of the signal available at the end of the LMR-400 run will not be available at the end of the RG6 run, and I call that a significant difference.

One "S unit" is 6 dB (if calibrated properly).
a bit over 1/2 and "S unit" is not going to make a hill of beans to most people.

Think about WHY an "S unit" is 6dB? Maybe because that is about the smallest amount that matters in a receiver system?

(No you did not get me, but you did allow me to provide additional ammunition for those who like trashing me for providing correct answers that are counter to their beliefs. :lol: )

 

[loumaag]

One "S unit" is 6 dB (if calibrated properly).
a bit over 1/2 and "S unit" is not going to make a hill of beans to most people.

Think about WHY an "S unit" is 6dB? Maybe because that is about the smallest amount that matters in a receiver system?

(No you did not get me, but you did allow me to provide additional ammunition for those who like trashing me for providing correct answers that are counter to their beliefs. :lol: )

Actually "S" units have no standing in my argument or yours (even calibrated properly) as they are not a linear relation; i.e., S1, S2, S3, S4, S5, etc. equals 1, 2, 4, 8, 16, etc. times the S1 signal.

I still contend that getting over twice the amount of signal by changing cable is significant. To use your numbers again, based on the 75' run and using 850 MHz, a person using LMR-400 gets just under 60% of the available signal from the antenna whereas the RG6 results in about 27%, still seems significant to me when you consider it that way.

 

[N_Jay]

Actually "S" units have no standing in my argument or yours (even calibrated properly) as they are not a linear relation; i.e., S1, S2, S3, S4, S5, etc. equals 1, 2, 4, 8, 16, etc. times the S1 signal.

I still contend that getting over twice the amount of signal by changing cable is significant. To use your numbers again, based on the 75' run and using 850 MHz, a person using LMR-400 gets just under 60% of the available signal from the antenna whereas the RG6 results in about 27%, still seems significant to me when you consider it that way.

This is going to take us down some fairly technical meanderings (just a warning)

Yes, 3 dB of loss is 1/2 the power, however, a 3 dB signal difference is very small to a receiver.

You should be looking at a 6 dB difference to represent 1/2 the voltage at the receiver, if you want to consider 1/2 as significant (Still an arbitrary definition).

There is probably a 20 to 30 dB range between a barely audible 25 kHz FM signal and fully quiet FM signal. Any improvement in signal level on a signal above this range is NOT noticeable at all.

In a mobile system the fading and flutter are typically about 20 dB. A change in the mean level of the signal by only 3 dB is literally "lost in the noise".

If you want to prove this out get a calibrated attenuator and start playing. Find a good, but just marginal signal and add loss to get a feel for what is significant and what is not.

I am fairly confident that you will quickly come to the realization that you can barley tell 3 dB, you need at lest 6 dB to make a truly noticeable difference and you need 10 to 20 dB to go from a "bad signal" to a 'good signal".

(Some might ask, why I am confident? BECAUSE I have done this on several occasions.)

 

[loumaag]

This is going to take us down some fairly technical meanderings (just a warning)

Thanks for the heads-up.

Yes, 3 dB of loss is 1/2 the power, however, a 3 dB signal difference is very small to a receiver.

I will admit that 3db is a small amount, I just disagree on what is significant in marginal conditions.

You should be looking at a 6 dB difference to represent 1/2 the voltage at the receiver, if you want to consider 1/2 as significant (Still an arbitrary definition).

Umm, when dealing with loss, 6db is 75% down not 50%. It is that pesky ratio thing that causes this problem.

There is probably a 20 to 30 dB range between a barely audible 25 kHz FM signal and fully quiet FM signal. Any improvement in signal level on a signal above this range is NOT noticeable at all.

No argument from me with this.

In a mobile system the fading and flutter are typically about 20 dB. A change in the mean level of the signal by only 3 dB is literally "lost in the noise".

Hmm, for a flutter to be noticeable it must attenuate the signal enough to actually cause loss of signal, I will agree that 20db would do it, I am not sure what that has to do with a 3db difference.

If you want to prove this out get a calibrated attenuator and start playing. Find a good, but just marginal signal and add loss to get a feel for what is significant and what is not.

I am fairly confident that you will quickly come to the realization that you can barley tell 3 dB, you need at lest 6 dB to make a truly noticeable difference and you need 10 to 20 dB to go from a "bad signal" to a 'good signal".

Many years ago I had one, no longer, I suspect it just got lost in one of my many moves and yes I will agree that to make a noticeable difference at least to the human ear, a 10db difference is needed; however...Let us take this example, a signal at approx 850 MHz is present at the base of the antenna, it is measured as ~ 0.7 µV. Using the previously mentioned runs the LMR-400 will present a signal of ~ 0.39 µV and the RG6 would give you a signal of ~ 0.19 µV. Using the Pro-96 as an example the LMR will give you over 12 dB SINAD and the RG6 will fall woefully short. I think that is significant, don't you. In this instance one is readable and the other is not. :wink:

(Some might ask, why I am confident? BECAUSE I have done this on several occasions.)

There is no argument that you have more current experience than I, but math is math. Aren't the rest of you just fascinated by this? :roll:

 

[richardc63]

What an interesting exchange of views- well done to both of you.

I'm going to risk splinters in the posterior by taking a mid position on this debate...

If you are trying to improve on a signal that is, say -80dBm, I would argue that trying to improve it by 3dB is a waste of time.

If you are designing a radio system (either trunk or conventional) and are concerned about a 3dB loss due to cable, I would argue that you haven't designed your radio system properly because you should NOT be expecting users to operate in areas where signal levels get down to -110 or less.

If you are trying to extract a reflection off the moon I would expect that 3dB could be a big deal.

So my view is that the significance depends on the circumstances...

Ouch, those splinters...

Regards,


Richard

Thanks for the heads-up.

I will admit that 3db is a small amount, I just disagree on what is significant in marginal conditions.

Umm, when dealing with loss, 6db is 75% down not 50%. It is that pesky ratio thing that causes this problem.

No argument from me with this.

Hmm, for a flutter to be noticeable it must attenuate the signal enough to actually cause loss of signal, I will agree that 20db would do it, I am not sure what that has to do with a 3db difference.

Many years ago I had one, no longer, I suspect it just got lost in one of my many moves and yes I will agree that to make a noticeable difference at least to the human ear, a 10db difference is needed; however...Let us take this example, a signal at approx 850 MHz is present at the base of the antenna, it is measured as ~ 0.7 µV. Using the previously mentioned runs the LMR-400 will present a signal of ~ 0.39 µV and the RG6 would give you a signal of ~ 0.19 µV. Using the Pro-96 as an example the LMR will give you over 12 dB SINAD and the RG6 will fall woefully short. I think that is significant, don't you. In this instance one is readable and the other is not. :wink:
There is no argument that you have more current experience than I, but math is math. Aren't the rest of you just fascinated by this? :roll:

 

[K9GTJ]

.... Aren't the rest of you just fascinated by this? :roll:

Yes, I am!

 

[N_Jay]

Thanks for the heads-up. . .
..Let us take this example, a signal at approx 850 MHz is present at the base of the antenna, it is measured as ~ 0.7 µV. Using the previously mentioned runs the LMR-400 will present a signal of ~ 0.39 µV and the RG6 would give you a signal of ~ 0.19 µV. Using the Pro-96 as an example the LMR will give you over 12 dB SINAD and the RG6 will fall woefully short. I think that is significant, don't you. In this instance one is readable and the other is not. :wink:
There is no argument that you have more current experience than I, but math is math. Aren't the rest of you just fascinated by this? :roll:

0.7 uV less 2 db is .56 uV
0.7 uV less 4.4 dB is .42 uV
(you have to keep your power vs. Voltage ratios from getting you so confused.)

Yes, you lose signal, but not as fast as you think.

Yes, it makes a difference at the margin, but so does a better antenna or better antenna placement.

 

[MB]

PS- I have bought many runs of LMR from this place. This is about as cheap as it gets when it comes to LMR-400

This place is cheaper:

http://www.therfc.com

 

[mastr]

This reminds me of the discone discussion a while back, where a number of us were tossing around the term "reasonable gain". I don't doubt for a minute that of the two cables under discussion, the LMR400 has less loss, however IMHO, the difference in performance between the two won't make a significant difference in reception perhaps 95 percent of the time. A typical FM reciever might go from an unreadable signal at .15uV to readable (but noisy) at say, .25uv with "full quieting" at around .50uv or so. (those figures are roughly what I recall from a Mastr 2 VHF station last week). Once you get to full quieting, the signal could be thousands of uV and your ear won't notice up until front end overload creates distortion products.

Many (if not most) casual scanner listeners are interested in agencies that serve the area they live in, and accordingly see signals well in to full quieting most of the time. In that case, 3, 6 or even 10db of extra loss isn't going to be noticed at all. If you are trying to listen to the local Wal-mart's simplex portable radios from 5 or 6 miles away, it might be worth while to change cable from rg6 to LMR 400, but a different antenna location or one with more gain (such as a yagi) would arguably make a more noticeable improvement.

W

 

[loumaag]

0.7 uV less 2 db is .56 uV
0.7 uV less 4.4 dB is .42 uV
(you have to keep your power vs. Voltage ratios from getting you so confused.)

Yes, you lose signal, but not as fast as you think.

Yikes!
Yes you are correct, my problem was applying a loss of signal percentage to the actual voltage.
For those of you who are interested, the following is the formula that gives this answer (for those of you not or are intimidated by math, move quickly on):
V2 = 10 ^ ((dB / 20) + Log V1) (based on reworking the standard voltage ratio formula: dB = 20 * Log(V1/V2) )
... so a -2 dB on 0.7 µV results in a signal of 0.5560 µV as follows
~ 0.5560 = 10 ^ ( ( -2 / 20 ) + (-0.25490196) ) where -0.25490196 = Log 0.7

So based on this the original numbers provided by you (a long time back) of relative losses of 2.9 and 6.6 (LMR-400 vs RG-6) would provide signals of 0.501 µV and 0.327 µV respectively. Both of which would satisfy the minimum for the radio I mentioned earlier to obtain a 12 dB SINAD. So I stand corrected on my example; however, if the base signal was lowered to 0.4 µV then the result would be a "hear and not hear" one.

I will admit that in normal conditions a difference of 3.7 dB would make little difference but I still contend in marginal conditions it is the difference between hearing and not hearing.

Yes, it makes a difference at the margin, but so does a better antenna or better antenna placement.

I am glad we agree on marginal conditions. As to the antenna or placement, no arguments, but the discussion at hand was not that, it was would the difference in coax be a factor.

Just for general information, when I was located in South Dakota, I was about 25 or 30 miles from an 800 MHz TRS which I monitored because the local tribal police used it. My best results were always obtained by using the RS 800 MHz rubber duck on the scanner. This includes using an outside omnidirectional antenna through admittedly poor cable or in mobile operation an exterior antenna vs the rubber duck inside the vehicle. Why, absolutely no loss between what is at the antenna and what is at the antenna port on the radio. My point is sometimes the easiest solution is the better one.

 

[N_Jay]

Yikes!
Yes you are correct, . . . .

So I stand corrected on my example; however, if the base signal was lowered to 0.4 µV then the result would be a "hear and not hear" one.

I will admit that in normal conditions a difference of 3.7 dB would make little difference but I still contend in marginal conditions it is the difference between hearing and not hearing.

I am glad we agree on marginal conditions. As to the antenna or placement, no arguments, but the discussion at hand was not that, it was would the difference in coax be a factor.

My point is "put your money where your mouth is", or more exactly, "Put your money where your ears are". Good cable is one place to spend $$, but it often not the only place nor always the best place.

Know what you are doing rather than following "blanket rules".

 

[Al42]

I am fairly confident that you will quickly come to the realization that you can barley tell 3 dB, you need at lest 6 dB to make a truly noticeable difference and you need 10 to 20 dB to go from a "bad signal" to a 'good signal".

Totally dependent on the capture ratio of the receiver. The better the capture ratio, the less signal change needed to go from "in the noise" to DFQ.

For Lou:

Actually "S" units have no standing in my argument or yours (even calibrated properly) as they are not a linear relation; i.e., S1, S2, S3, S4, S5, etc. equals 1, 2, 4, 8, 16, etc. times the S1 signal.

X times the signal voltage. A 6db change is twice the voltage, so it's 4 times the power. (But receiver sensitivity depends on voltage, so 2X is correct in this discussion. Just keeping you honest.)

 

[kb2vxa]

Hi all,

Re: N_Jay;
"3.7 dB is a marginal difference'''"
Since for every -3dB the signal halves I would hardly call it marginal.

"If you don't hear it now, 3.7 dB is not going to "bring it in!"
If the signal is marginal slightly more than doubling it surely WILL bring it in.

"At 50 feet it is 2.0 dB vs. 4.4 dB"
That's quite significant.

"2.4 dB is hardly worth worrying about on a receive system."
Maybe if you're sitting under the transmitting tower but if not EVERY dB counts, so the difference between readability and down in the mud surely IS worth "worrying about".

N_Jay, thanks for proving in your own way that what they're saying is right, replacing that RG-6U with LMR surely will make a significant difference.

EDIT:
Re: Al42;
"X times the signal voltage. A 6db change is twice the voltage, so it's 4 times the power. (But receiver sensitivity depends on voltage, so 2X is correct in this discussion. Just keeping you honest."
Just keeping YOU honest Mr. Engineer your figures are correct only when referenced to 0dB = 7.717VRMS across 600 ohms. Try your calcs again this time using 50 ohms and do it in dBW. Oh, if the meter reads S9 + 10dB what is the signal voltage at the antenna input terminals? (;->)

 

[N_Jay]

Hi all,

Re: N_Jay;
"3.7 dB is a marginal difference'''"
Since for every -3dB the signal halves I would hardly call it marginal.

"If you don't hear it now, 3.7 dB is not going to "bring it in!"
If the signal is marginal slightly more than doubling it surely WILL bring it in.

"At 50 feet it is 2.0 dB vs. 4.4 dB"
That's quite significant.

"2.4 dB is hardly worth worrying about on a receive system."
Maybe if you're sitting under the transmitting tower but if not EVERY dB counts, so the difference between readability and down in the mud surely IS worth "worrying about".

N_Jay, thanks for proving in your own way that what they're saying is right, replacing that RG-6U with LMR surely will make a significant difference.

EDIT:
Re: Al42;
"X times the signal voltage. A 6db change is twice the voltage, so it's 4 times the power. (But receiver sensitivity depends on voltage, so 2X is correct in this discussion. Just keeping you honest."
Just keeping YOU honest Mr. Engineer your figures are correct only when referenced to 0dB = 7.717VRMS across 600 ohms. Try your calcs again this time using 50 ohms and do it in dBW. Oh, if the meter reads S9 + 10dB what is the signal voltage at the antenna input terminals? (;->)

I really wish you would use the very nice quotation system to make your posts easier for everyone to read.
(It should not be too hard for such a smart person)

Hi all,

Re: N_Jay;
"3.7 dB is a marginal difference'''"
Since for every -3dB the signal halves I would hardly call it marginal.

For some reason you have attached significance to the fact hat the power is halved?
Why?

Why not attach significance when the voltage is halved (i.e. 6dB), or attach significant when the Readability is halved (some value determined only by experience).

Marginal is marginal, and significant is significant.
(I know a difficult concept for some to grasp.)

"If you don't hear it now, 3.7 dB is not going to "bring it in!"
If the signal is marginal slightly more than doubling it surely WILL bring it in.

Again, you are irrationally using the "doubling" as if it has some significance. What I meant by "If you don't hear it now" is if it does not break squelch, or it provides no usable voice. In this case an increase of even 3.7 dB will NOT bring it up to a usable signal.
(Again, understanding the difficult meaning of "significant"!

"At 50 feet it is 2.0 dB vs. 4.4 dB"
That's quite significant.

Why? You have provided NO information that says under what conditions 2.4 dB becomes significant. (Why oh Why is this SO hard for some people to grasp!)

"2.4 dB is hardly worth worrying about on a receive system."
Maybe if you're sitting under the transmitting tower but if not EVERY dB counts, so the difference between readability and down in the mud surely IS worth "worrying about".

Yes it is.
But it is only worth "worrying about" if it gets you from in the mud to out of the mud!
The difference between in the mud and readability is usually 10 to 20 dB, with a rule of thumb, that 6 dB is the minimum for a noticeable change.

N_Jay, thanks for proving in your own way that what they're saying is right, replacing that RG-6U with LMR surely will make a significant difference.

LOL, I like the way you think.:roll:
Proclaim you are correct without evidence and move on.:evil:

EDIT:
Re: Al42;
"X times the signal voltage. A 6db change is twice the voltage, so it's 4 times the power. (But receiver sensitivity depends on voltage, so 2X is correct in this discussion. Just keeping you honest."
Just keeping YOU honest Mr. Engineer your figures are correct only when referenced to 0dB = 7.717VRMS across 600 ohms. Try your calcs again this time using 50 ohms and do it in dBW. Oh, if the meter reads S9 + 10dB what is the signal voltage at the antenna input terminals? (;->)

Sorry sir, you (kb2vxa), are wrong.
The relationships between power and voltage changes are irrespective of impedance.

Now I am so glad you took the time to edit your post to add that little un-fact!:lol: :lol:

 

[loumaag]

...What I meant by "If you don't hear it now" is if it does not break squelch, or it provides no usable voice. In this case an increase of even 3.7 dB will NOT bring it up to a usable signal. ...

You know I think I have just struck the difference in what we (you and I) were talking about. You were assuming voice, I was assuming 800 MHz Control Channel data, as the topic way back when was UHF and 800 and lets face it, the majority of scanner-heads monitor trunked systems in the 800 MHz range and difference between the two types at typical UHF is really negligible.. I always assumed you could at least hear the voice, but static (and 3.7 dB might make a difference in that) on the CCh is problematic for monitoring. This is similar to the various arguments you and I agree 100% on, that is the non-value of a in-line amplifier for marginal signals, you just amplify the noise along with the signal; but increasing the actual signal, (even a small 3.7 dB) could make the difference between hearing a channel grant and not.

 

[N_Jay]

You know I think I have just struck the difference in what we (you and I) were talking about. You were assuming voice, I was assuming 800 MHz Control Channel data, as the topic way back when was UHF and 800 and lets face it, the majority of scanner-heads monitor trunked systems in the 800 MHz range and difference between the two types at typical UHF is really negligible.. I always assumed you could at least hear the voice, but static (and 3.7 dB might make a difference in that) on the CCh is problematic for monitoring. This is similar to the various arguments you and I agree 100% on, that is the non-value of a in-line amplifier for marginal signals, you just amplify the noise along with the signal; but increasing the actual signal, (even a small 3.7 dB) could make the difference between hearing a channel grant and not.

I don't know how well or poorly scanners decode the control channel.
If the signal is usable from a voice standpoint a radio will general decode the control channel without issue.

I am still not sure the 3.7 dB is significant.
I would like to find someone with a real data point. rather than assume either way.

 

[kb2vxa]

Hi again,

Re: N_jay;
"I really wish you would use the very nice quotation system to make your posts easier for everyone to read."
I'll use the quotation method my English teacher taught me in school thank you.

"(It should not be too hard for such a smart person)"
Reading the standard quote and reply taught in school "should not be too hard" for an educated person.

"Now I am so glad you took the time to edit your post to add that little un-fact!"
I'm glad that you're glad, I'm glad, I'm glad I'm glad (Cream) that you found my joke aimed at Al so amusing.

Looks like the last gasp of a dying argument boxed into a corner. No point beating it any more, the horse has expired by now.

 

[datainmotion]

Well folks, for those of you still awake, that about wraps up our "Battle of the Mega-posters". Join us next week when we debate belly button lint and its properties of electrical conductivity.

Good night.

 

[N_Jay]

Hi again,

Re: N_jay;
"I really wish you would use the very nice quotation system to make your posts easier for everyone to read."
I'll use the quotation method my English teacher taught me in school thank you.

"(It should not be too hard for such a smart person)"
Reading the standard quote and reply taught in school "should not be too hard" for an educated person.

LOL, you better recheck whatever "style Guide" you use.
For quoting in-line, you are correct, but for a block of quoted text and a response, you would typically use a different font, an inset paragraph, or a bulleted list with answers inset.:wink:

If you don't believe me ask an professional editor or technical writer.

Oh, well, that's just a matter of style (and that is why Style guides are published)

(Not that I like pointing out where you are wrong, but you make it so easy):lol: :lol: