This article is a follow up to Optimum receive system noise figure for given ambient noise – Flex 6600 using data published at (Farson 2014) to make similar estimates for the Flex 6700.
How to determine the amount of RF Preamp gain to apply for band conditions suggests that the 6700 figures might also apply to the 6500 and 6400(M).
Farson gives a table of MDS in 500Hz bandwidth figures for the 6700 on certain bandws, including MDS for 4 RF Gain configurations, 0, 10, 20, and 30dB.
Above is Farson’s data with my chosen RF Gain option (selected for SND<3dB) and calculated values in yellow and orange for: Continue reading Optimum receive system noise figure for given ambient noise – Flex 6700
Gerald Youngblood (K5SDR) of FlexRadio wrote of
optimal receiver noise figure relationship to antenna noise in a blog posting about SDR receivers.
This article discusses that posting in the context of linear receivers, ie effects of intermodulation distortion are not included.
His gives the following advice:
For optimal weak signal performance near the atmospheric (antenna) noise floor you want your receiver noise floor (sensitivity/MDS) to be 8 to 10 dB below the noise coming from the antenna. For strong signal reception, less sensitivity is almost always better.
The terminology is not industry standard, but that is quite usual for hams who have a need to redefine well known terms, and this is really loose with implied equivalence (eg sensitivity/MDS).
ITU-R P.372-14 speaks of natural noise as including
atmospheric noise due to lightning, and also speaks of man made noise.
It is likely Youngblood is actually talking about man made noise since he uses man made noise figures from an earlier revision of P.372.
Optimal is a compromise between weak signal performance (ie S/N degradation due to internal receiver noise) and handling of strong signals that might clip in the ADC of an SDR receiver.
He gives a table of measured MDS (minimum discernable signal, which actually is synonymous with Noisefloor) for recommended configurations of a Flex 6600 radio on several bands.
Above is Youngblood’s data with my calculated values in yellow and orange for: Continue reading Optimum receive system noise figure for given ambient noise – Flex 6600
Comments were received from some readers of the article S/N degradation is related to external noise level and receive system internal noise.
Essentially, two questions were asked:
- what is the minimum HF ambient noise level; and
- explain observation of lower HF ambient noise level.
What is the minimum ambient noise level?
Above is Fig 2 from ITU-R P.372-13 which shows some key components of total ambient noise. The solid line is entitled “minimum noise level expected”, and it is a combination of curves B, C and D. Above 0.7MHz, only curves C and D are at play. Continue reading Minimum ambient noise level – ITU-R P.372-13 guidance
(Franklin 1924) described a technique to cophase sections of a long antenna by “concentrating alternating half wave length portions of the wire within a small space, by winding such portions as inductance coils or by doubling such portions back on themselves so that there is practically no radiation from these portions”.
Let’s explore his second option, as unlike the first, it does work reliably.
Above is an NEC-4.2 model with current shown (magnitude and phase). The stubs conductors are all defined from top to bottom. Continue reading Franklin antenna – how does it work?
A reader of End Fed Half Wave matching transformer – 80-20m asked if a good transformer could be made with with a FT114-43 core.
The original transformer above comprised a 32t of 0.65mm enameled copper winding on a FT240-43 ferrite core, tapped at 4t to be used as an autotransformer to step down a load impedance of around 3300Ω to around 50Ω. Continue reading End Fed Half Wave matching transformer – 80-20m – LO1238 variant
A question that arises from time to time is what is the minimum receiver noise figure for a given application.
This discussion considers the question applied to linear receivers, ie receivers with zero intermodulation distortion (IMD) and other non ideal characteristics, other than their internal noise which can be described by their Noise Figure (NF).
By definition, NF is the amount by which the component or system degrades the NF, so in dB it is the difference in the S/N in to S/N out. Implicit in that definition is that it is based on source internal noise of 290K equivalent.
So for example lets say a receiver with quivalent noise bandwidth 2000Hz measures sensitivity of -125dBm for 10dB S/N out. We can calculate the noise in 2000Hz bandwidth from a 290K source to be -141dBm, and therefore the input S/N is -125 – -141 = 16dB. The ratio of the input S/N to output S/N is the difference in those in dB, 16-10=6dB. The NF is 6dB. We can also calculate an equivalent internal noise temperature of (10^(6/10)-1)*290=865K.
By convention, ambient noise (or external noise) is expressed in Kelvins, or dB wrt 290K. That does not imply that an antenna contributes exactly 290K. Continue reading S/N degradation is related to external noise level and receive system internal noise
A reader of End Fed Half Wave matching transformer – 80-20m asked if a better transformer could be made with a stack of 2 x FT240-43 cores and using half the turns.
The original transformer above comprised a 32t of 0.65mm enameled copper winding on a FT240-43 ferrite core, tapped at 4t to be used as an autotransformer to step down a load impedance of around 3300Ω to around 50Ω. Continue reading End Fed Half Wave matching transformer – 80-20m – 2xFT240-43 variant
I repaced the R134a refrigerant in my car aircon system with a hydrocarbon refrigerant, Hychill Minus 30 (HC-30).
Research had indicated that permeation of reduced barrier hoses was problem with HC refrigerants. The hoses in the car were Goodyear hoses with catalogue numbers, but I was unable to find data on them. Their diameter was comparable to standard barrier hoses, so I proceeded with the trial. Continue reading Post post implementation review R134a replaced with HyChill Minus 30
The Ferrite permeability interpolations calculator performs interpolations of tables of complex permeability data.
From manufacturer’s curves
Some of the data is derived from manufacturer’s published complex permeability curves. The plot above shows the Ferroxcube’s published curve for 3C81 material, and points at which it was digitised to extract a table of µ’ and µ”. Continue reading Online calculator of ferrite material permeability interpolations – more detail