Minimum ambient noise level – ITU-R P.372-13 guidance

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 antenna – how does it work?

(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?

End Fed Half Wave matching transformer – 80-20m – LO1238 variant

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 enamelled 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

S/N degradation is related to external noise level and receive system internal noise

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.

HF example

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

End Fed Half Wave matching transformer – 80-20m – 2xFT240-43 variant

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 enamelled 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

Post post implementation review R134a replaced with HyChill Minus 30

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

Online calculator of ferrite material permeability interpolations – more detail

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

Holux web site missing / offline, Holux ezTour crippled

I have had a couple of Holux GPS loggers for some time, a RCV3000 and GPS245+.

They come with a neat little application called Holux ezTour for Logger.

Over time, the application has been unreliable, mostly knock on effects of Google Maps licencing and Holux’s failure to obtain and distribute the appropriate licence keys. Continue reading Holux web site missing / offline, Holux ezTour crippled

OpenLog for TinyTrak – inline DE9 adapter

OpenLog for TinyTrak described a data logger for APRS NMEA data.

This article shows an innovative inline adapter for adding an Openlog to an existing APRS installation where the GPS is powered with 5V via the DE9 connector.

Above, the adapter comprises a DE9 male and DE9 female connector cabled together as a through connection, and the Openlog is wired into one of the backshells, wrapped in heat shrink tube, and velcro’d to the backshell. Continue reading OpenLog for TinyTrak – inline DE9 adapter