Call Search
     

New to Ham Radio?
My Profile

Community
Articles
Forums
News
Reviews
Friends Remembered
Strays
Survey Question

Operating
Contesting
DX Cluster Spots
Propagation

Resources
Calendar
Classifieds
Ham Exams
Ham Links
List Archives
News Articles
Product Reviews
QSL Managers

Site Info
eHam Help (FAQ)
Support the site
The eHam Team
Advertising Info
Vision Statement
About eHam.net

donate to eham
   Home   Help Search  
Pages: [1]   Go Down
  Print  
Author Topic: 1.5 dB/S-unit on VHF?  (Read 3169 times)
AK9S
Member

Posts: 7


WWW

Ignore
« on: September 16, 2003, 08:58:31 PM »

The ARRL handbook refers to an old standard of S9 calibrated at 50 microvolts RF input and 6 dB per S-unit down from that reference point.

However, after doing several field tests on 2-meters where I varied my power output by as much as 18 dB, and logging signal reports from local receiving stations, I found something in the order of 1.5 dB per S-unit.

It appears that the calibration standard is different for VHF transceivers.  Anyone have any thoughts on this one?  
Logged
WW1Z
Member

Posts: 35




Ignore
« Reply #1 on: September 17, 2003, 09:58:45 AM »

I don't think there is any real standard for S meters. Even different models from the same company may show variations, let alone band to band variations in the same rig.As I recall different manufacturers have had different standards.  Perhaps the commercial stuff does better. The early S meters just read 1 to 9 frome end to end before this db over S9 stuff came along. How can you be db over extremely (S9) strong? If you have access to a decent signal generator and adjustable attenuator (often built into the really good ones) you can calibrate your radio. If you know some one with the identical rig you could compare them, don't be surprised if there are some big differences! John
Logged
WB2WIK
Member

Posts: 21754




Ignore
« Reply #2 on: September 17, 2003, 02:33:57 PM »

My first thought is, "who cares?" but I must admit, I've seen the same thing with many VHF-UHF rigs...much more so with "FM" rigs, and this is totally understandable considering the nature of FM demodulation and that no stage in an FM receiver needs to be linear, until you get to the audio amplifier post-detection.

My Yaesu FT-736R reads S9 at 50uV +/- a few dB on each band it covers (50-144-220-432 MHz) when used on SSB or CW, but when switched to FM, then S9 = 2uV and the meter "pins" at only about 40uV.

I think many VHF-UHF rigs have so much receiving chain gain (preamp, another preamp, active mixer, multiple IF amps, etc) that they cannot possibly be linear beyond very weak signal inputs.  In the "weak signal" world, that probably doesn't matter for most of us, until we are operating on a mountaintop during a contest.  But of course, lack of predictable linearity absolutely causes S-meter inaccuracies, no way to avoid that.

WB2WIK/6
Logged
HB9PJT
Member

Posts: 355


WWW

Ignore
« Reply #3 on: September 18, 2003, 07:09:45 AM »

While there is not a real standard, normaly S9 is 50 uV on HF and 5 uV on VHF and higher. Most of the VHF rigs I have seen have about 5 uV for S9. But a S-unit often is 3 dB in SSB showing S1 for a S5 signal. The same rigs and all the FM only have very terrible S-Meter in FM mode, showing S1 when the signal is effective S7.

Logged
OZ2M
Member

Posts: 2


WWW

Ignore
« Reply #4 on: September 18, 2003, 08:15:09 AM »

I cannot comment on a S-meter standard outside IARU Region 1. But in the latter the following is the official recommendation:

S 9 level: HF = -73 dBm, VHF and up = -93 dBm
S-unit: 6 dB

So the question is not, in Region 1 though, about S-units but whether the rigs comply or not. With the micro processing capabilities present in modern radios these days much more accuracy is possible to make. But manufacturers are probably just lazy. Lab reports should address this as Compliant/Non-compliant.

Footnote: Lately it was discovered that no definition exists for 50 MHz or 70 MHz. However, this will be dealt with soon.

See more here (warning: PDF file!): http://home.hccnet.nl/a.dogterom/Handbook/6a.pdf

73
Bo, OZ2M
Logged
W3FHW
Member

Posts: 73




Ignore
« Reply #5 on: September 24, 2003, 10:21:16 AM »

Microvolts is the key word and VOLTS is the clue. One cannot confuse power (WATTS) with volts. The most common corrolation between WATTS output from a transceiver and the resulting "S" meter reading on a distant receiver is that the transmitter power must be raised to 4 times it's power to make one "S" unit increase on the distant receiver. In other words, the "S" unit difference on the receiver is only one s-unit when the transceiver output is changed from 100 watts to 400 watts. (a voltage db gain of 6 at the receiver).

The voltage db gain at the receiver is calculated differently than  the power db gain from the transceiver.

Voltage db is calculated: 20log of 10 v1 over v2.  
Power db is calculated: 10log of 10 p1 over p2.

I hope this helps,

Fred, W3FHW      
Logged
W3FHW
Member

Posts: 73




Ignore
« Reply #6 on: September 24, 2003, 10:37:21 AM »

Microvolts is the key word and VOLTS is the clue. One cannot confuse power (WATTS) with volts. The most common corrolation between WATTS output from a transceiver and the resulting "S" meter reading on a distant receiver is that the transmitter power must be raised to 4 times it's power to make one "S" unit increase on the distant receiver. In other words, the "S" unit difference on the receiver is only one s-unit when the transceiver output is changed from 100 watts to 400 watts. (a voltage db gain of 6 at the receiver).

The voltage db gain at the receiver is calculated differently than  the power db gain from the transceiver.

Voltage db is calculated: 20log of 10 v1 over v2.  
Power db is calculated: 10log of 10 p1 over p2.

I hope this helps,

Fred, W3FHW      
Logged
N2MG
Administrator

Posts: 10058



« Reply #7 on: October 01, 2003, 03:30:36 PM »

One of hamdom's most believed myths is that S-meters are somehow calibrated to be (even close to) 6dB per S-unit, or 50uV at S-9.  Perhaps some manfacturers, at one time or another used those specs as a goal, or published them somewhere, but most radios do not even come close.

Many HF transceivers are more like 1dB-2dB between S1-S4 and 3dB-4dB between S5 and S8 and maybe 5-6dB around S9.

I often hear guys who are REALLY loud on 10 meters who barely move my s-meter.  I never tell them they are S-zero!  ;-)

Now...your built-in attenutator is likely to be far more accurate.  Try using it to get an "eye-ball" on your rig's S-meter "calibration".

73 Mike N2MG
Logged
Pages: [1]   Go Up
  Print  
 
Jump to:  

Powered by MySQL Powered by PHP Powered by SMF 1.1.11 | SMF © 2006-2009, Simple Machines LLC Valid XHTML 1.0! Valid CSS!