DB Loss Question

[DewAddict]

Ok,

So I know my current antenna setup has some serious coax DB loss as I have run a total of about 140' of RG8X foam (Small stuff) and coupled that with some RG213 (thick stuff) as well. Now the reason for this is that I am testing an antenna up in a tree for my VHF setup. Its not permanent. I am happy with the signal I have so far but don't want to spend over $100 on new coax unless my signal would go up an "S-Unit" or two by installing the new coax. Which leads me to my actual question.

How many DB loss equals 1 "S-Unit" on my meter and would I notice a stronger receive signal if I make the proper run of LMR-400 of about 130' or so at the 146mhz frequency. I am figuring I have lost at least 6db or more right now.

Thanks for the help and hopefully I explained this well enough.

 

[RiceCake]

Based on limited internet resources it appears 1 S-Unit is in fact, 6db.
http://en.wikipedia.org/wiki/S_meter#IARU_Region_1_Technical_Recommendation_R.1

And rough graphing of loss characteristics is about 4.5 to 6 db at 100-150 MHz.
So, yeah, good cabling could handily double the power (3db) since '400 seems to drop 1.5.

* Danger. Not an expert. Wait for cross-confirmation...

 

[N_Jay]

Rice has it right!

Sounds like you have one or more connectors in line also, they add loss.

Go with good and fairly inexpensive RG-6.

 

[JayMojave]

Hello DewAddict:

The difference from RG8X to LMR400 Coax at 140 feet isn't a real big deal (a few dB) at VHF Frequencies. Here is a Coax Loss Chart at RF Parts.com
http://www.rfparts.com/commcoax.html

However stations on the edge of the scanners sensitivity will be a little scratchy with the loss. Probably again not a big deal.

I use 100 feet of RG6, Cable TV Coax feeding two scanners, and it works great.

The LMR 400 Coax would be more noticble increase at UHF frequencies, especially at the 900 MHz frequencies.

Good Luck.

Jay in the Mojave

 

[rfradioconsult]

Receiver sensitivity is measured in micovolts; a change of 6 dB equals a change of half. Transmit power is measured in Watts so 3 dB is half. That is why S meters are calibrated with 1 "S" unit equals 6 dB.

 

[N_Jay]

Receiver sensitivity is measured in micovolts; a change of 6 dB equals a change of half. Transmit power is measured in Watts so 3 dB is half. That is why S meters are calibrated with 1 "S" unit equals 6 dB.

Bels, and hence dB are typically power measurements, but can be used to refer to other logarithmic ratios.
3db is 1/2 the power,
A doubling of voltage provide a 4x increase in power.
Since a signal level meter is usually calibrated in units of voltage, a doubling in voltage (3 dB(V)) is a quadrupling 6dB in power.

Most "S" units are 6dB increase in receive power. This is also typically the smallest increment that a user can differentiate.

 

[prcguy]

Receiver S meters track RF power, not voltage like N_Jay mentioned. They are calibrated to a known voltage like 50uv = S9 for newer receivers and 100uv = S9 for older equipment, but after that they respond accurately to power changes. If you take an S meter reading on a signal and the transmitter doubles their power, the receiving S meter will read an increase of one half S unit. Increase the transmitter by 6dB and the receiver S meter will increase by one full S unit. Or, if you feed a newer receiver -73dBm the S meter should read S9 and if you up the signal to -63dBm (10dB increase in power) the S meter will read S9 + 10dB.
prcguy

Receiver sensitivity is measured in micovolts; a change of 6 dB equals a change of half. Transmit power is measured in Watts so 3 dB is half. That is why S meters are calibrated with 1 "S" unit equals 6 dB.

 

[N_Jay]

Receiver S meters track RF power, not voltage like N_Jay mentioned. They are calibrated to a known voltage like 50uv = S9 for newer receivers and 100uv = S9 for older equipment, but after that they respond accurately to power changes. If you take an S meter reading on a signal and the transmitter doubles their power, the receiving S meter will read an increase of one half S unit. Increase the transmitter by 6dB and the receiver S meter will increase by one full S unit. Or, if you feed a newer receiver -73dBm the S meter should read S9 and if you up the signal to -63dBm (10dB increase in power) the S meter will read S9 + 10dB.
prcguy

Power and voltage track together with a fixed impedance.

That's like saying that your speedometer does not track your speed, but only the speed of your wheel, or the axle, or the drive shaft, or the tech speedometer cable. All is true and mostly irrelevant to the point being made.

Point being they are 6dB Power units (as standardized, but many receivers are NOT standardized in this regard)

 

[DewAddict]

Awesome. Thanks guys for the help. Since I'm not really on the "fringe" of any signals, one "S unit" is not going to make or break me. I think I'll hold off on the LMR-400 and use some coax I already have. Pretty much all of what I listen to is at least half scale on the meter anyway.

Thanks again.

 

[kb2vxa]

Oh here we go again with the dB Show Off My Geek Wars! The bottom line is while technically speaking 6dB equals one S unit there is no such thing as a calibrated S meter! Once upon a time Hallicrafters had calibrated S meters, S9 equaled 50uV at the antenna input terminals but those boat anchors are so old all bets are off. Now for the geeks and your Cushmans their metering is calibrated in decibels, not S units, so THERE.

Having gotten that rant out of my system LMR400 cable is not the proper choice for more than roughly 100' at frequencies above 30MHz so my advice is look into better coax. Like the man said RG6-U quad shield (common in house CATV cable) is pretty amazing stuff and the price just can't be beat.

Now for a question of my own. How long before someone posts a link to this nonsense on Hamsexy? (;->)

 

[JayMojave]

Hello Dew Addict:

Rice Cake gave a great web site there showing the S-Meter values. The point driven home is that every 6 dB will change the S-Meter by one S-Unit. Since the critical S-Meter readings are in the lower end, remembering that 6 dB is equal in theory to one S-Unit.

But the web site showed another web site at the bottom of the page called:
Measurements of amateur radio transceiver S Meter calibrations

This showed the real world of amateur radio S-Meters. Ok so there not all the same or even accurate.

But 6 dB is a very important value to remember as it is one Range Octave. That is if a transmitter is S9 at 1 mile away it should be S8 at two miles away, and S7 at four miles away.

Or if a transmitter doubles its output voltage, it will double its output current resulting in 4 times the power output or a increase of 6 dB, or 1 S-Unit. Theoreticaly doubling its range.

This 6 dB worth of power can be very significant as it can be the difference of hearing or not hearing a station.

The Free Space Loss equation states the same thing. Fsl = 20 Log . Distance in NM. Freq in MHz +37.8

Example: Transmitter on 162.55 MHz, at 5 NM miles will have Free Space Loss of:

20. Log NM . Mhz + 37.8
20. Log 5 . 162.55 + 37.8
20. Log 812.75 + 37.8
20. 2.9 + 37.8
58+ 37.8
95.8 dB in free space loss.

At 10 NM miles its 102 dB free space loss. A 6 dB difference

So its possible the 6 dB per S-Unit may have meant a range octave. But receiver S-Meters failed us.

Jay in the Mojave

 

[WA1CRZ]

RG6-U quad shield ??
Is that the stuff thats thin like 58U? If so, I have 2 boxes of that stuff. Ive got a goldmine then.

 

[prcguy]

Good info. Free space loss cannot be used very accurately for terrestrial use due to ground bounce reflections, the results will can be anywhere from 6 to 20dB off. A NASA engineer forgot to include this in his calculations and they nearly lost one of the Mars rovers when they drove it out of radio range.
prcguy

Hello Dew Addict:

Rice Cake gave a great web site there showing the S-Meter values. The point driven home is that every 6 dB will change the S-Meter by one S-Unit. Since the critical S-Meter readings are in the lower end, remembering that 6 dB is equal in theory to one S-Unit.

But the web site showed another web site at the bottom of the page called:
Measurements of amateur radio transceiver S Meter calibrations

This showed the real world of amateur radio S-Meters. Ok so there not all the same or even accurate.

But 6 dB is a very important value to remember as it is one Range Octave. That is if a transmitter is S9 at 1 mile away it should be S8 at two miles away, and S7 at four miles away.

Or if a transmitter doubles its output voltage, it will double its output current resulting in 4 times the power output or a increase of 6 dB, or 1 S-Unit. Theoreticaly doubling its range.

This 6 dB worth of power can be very significant as it can be the difference of hearing or not hearing a station.

The Free Space Loss equation states the same thing. Fsl = 20 Log . Distance in NM. Freq in MHz +37.8

Example: Transmitter on 162.55 MHz, at 5 NM miles will have Free Space Loss of:

20. Log NM . Mhz + 37.8
20. Log 5 . 162.55 + 37.8
20. Log 812.75 + 37.8
20. 2.9 + 37.8
58+ 37.8
95.8 dB in free space loss.

At 10 NM miles its 102 dB free space loss. A 6 dB difference

So its possible the 6 dB per S-Unit may have meant a range octave. But receiver S-Meters failed us.

Jay in the Mojave

 

[N_Jay]

Your RG6 does not need to be quad shield, juts decent quality.

Never heard of "Range Octave" but it is good way to explain R-Squared loss to someone (I guess).