Category: Amateur Radio

Some thinking on ground wave as we enter another solar minimum

During low solar minima, the 40m band becomes very poor for reliable NVIS QSOs, but what of the prospects of ground wave QSOs?

Lets take two ITU-R recommendations for some insight:

  • ITU-R. Jul 2015. Recommendation ITU-R P.372-12 (7/2015) Radio noise.
  • ITU-R. Feb 2007. Recommendation ITU-R P.368-9 (2/2007) Ground-wave propagation curves for frequencies between 10 kHz and 30 MHz.

The first sets out expected median ambient noise in a range of precincts. It is based on measurements made with a short monopole, ie a vertically polarised antenna.

The second sets out the attenuation of ground waves at HF.

screenshot-24_10_16-22_00_39Whilst P.368-9 publishes a set of graphs like the one above for a limited set of grounds, they also publish a program to calculate values for the user’s choice of ground and that is what was used for this article. Continue reading Some thinking on ground wave as we enter another solar minimum

Last update: 31st October, 2016, 7:58 AM

Polarisation of man made noise – an 80m case

Polarisation of man made noise discussed an explanation for the common observation more ambient noise is captured by a vertically polarised antenna than for a horizontally polarised antenna.

This article documents an analysis of a case on 3.6MHz and is to be read in the context of Polarisation of man made noise.

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Remembering that P.368-9 publishes a set of graphs like the one above, and that they show that ground wave attenuation is dependent on distance, soil type and frequency.

Though ground wave attenuation is lower on 80m than 40m, the horizontal antenna used in the example is at a fixed height, so it is electrically lower on 80m which increases horizontal attenuation significantly. Continue reading Polarisation of man made noise – an 80m case

Last update: 28th October, 2016, 7:44 AM

Polarisation of man made noise

Ham lore has it that man made noise on lower HF is radiated predominantly vertically polarised, this is offered and accepted by hams without explanation.

It can be shown by simple observation that the ambient noise level on lower HF is quite different in business or commercial areas, residential areas, and rural areas (ITU-R P.372-12). Not only is there a significant difference, the change happens quite rapidly with distance which suggests there is a dominant component (man made noise) and that the propagation path is a very local one (ground wave).

If you look around a typical residential neighborhood where hams might establish stations, the most obvious conductors that might carry and radiate noise currents from noise generators like appliances, leaky insulators etc are aerial power lines… which are usually closer to horizontal orientation (with horizontal E field) than vertical which seems inconsistent with the common observation that vertically polarised receiving antennas tend to capture more man made noise power than horizontal ones.

This article proposes a mechanism that may explain the apparent inconsistency between noise radiators and noise receivers.

Though this explanation is based on experience, the quantitative analysis here depends on interpretation of Recommendation ITU-R P.368-9 (2/2007) Ground-wave propagation curves for frequencies between 10 kHz and 30 MHz.

screenshot-24_10_16-22_00_39Whilst P.368-9 publishes a set of graphs like the one above for a limited set of grounds, ITU-R also publishes the program (GRWAVE.EXE) which can be used to calculate values for the user’s choice of ground and that is what was used for this article. The graph above is for a vertical monopole over ground with 1000W radiated, the antenna has directivity of 3, and the dashed line (inverse distance curve) is the field strength for a lossless ground (PEC). This can be verified with a spot calculation at 1km. Continue reading Polarisation of man made noise

Last update: 27th October, 2016, 7:42 AM

Small signal diode characteristics

We often use diode detectors at microamp currents, and the question arises as to the type of diode best suited to sensitive detectors.

Setting aside zero bias Schottky diodes which are a topic in themselves, the choice is typically between commonly available germanium, silicon and Schottky signal diodes.

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Above is a plot of the I,V characteristic of four common signal diodes at currents up to 1mA. It can be seen that at currents below 600µA, the forward voltage drop of the humble 1N34A germanium diode is lower than the others. The 1N270 is an alternative if you really need its higher breakdown voltage. Both of these diodes are reasonably easy to obtain, and cheap at that.

References

 

Last update: 22nd October, 2016, 7:22 PM

A prototype small 4:1 broadband RF transformer using medium µ ferrite core for receiving use

Discussion at A method for design of small broadband RF transformers using medium µ ferrite core for receiving use was around a 9:1 transformer on a BN43-2402 core. In that design, 4t was proposed as a suitable winding for a nominal 50Ω primary.

This article describes a 4:1 transformer needed for a project and based on the same 4t primary design, and using a separate 8t secondary.

First, lets find the largest wire that will fit 12t in the core aperture.

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Ok, so allowing a bit or working room, lets use 0.25mm enamelled wire (~0.28mm dia). Continue reading A prototype small 4:1 broadband RF transformer using medium µ ferrite core for receiving use

Last update: 12th March, 2018, 10:11 AM

A method for design of small broadband RF transformers using medium µ ferrite core for receiving use

A simplified design for small broadband RF transformers using medium µ ferrite core for receiving use.

The characteristic of typical medium µ ferrite mixes, particularly NiZn, are well suited to this application.

This article continues with the design discussed at BN43-2402 balun example, but using a 4t primary and 12t secondary for a nominal 1:9 50:450Ω transformer.

Lets consider a couple of simple starting points for low end and high end rolloff.

Low end roll off

A simple model for these devices with low flux leakage is an ideal transformer with primary shunted by the magnetising impedance. To obtain low InsertionVSWR, we want the magnetising impedance in shunt with 50+j0Ω to have a low equivalent VSWR.

Typically complex permeability changes in-band, and although it tends to decrease, increasing frequency means that the critical point for magnetising impedance is the low end.

High end roll off

At the high end, transformation departs from ideal usually when the length of wire in a winding exceeds about 15°.

Going forward

A small core makes for short windings to obtain high frequency performance, and sufficient turns are needed for low end… but not too many as it restricts the high end.

There are lots of rules of thumb for minimum magnetising impedance, most treat the inductor as an ideal inductor and these ferrites are not that.

A quick analysis using the method in BN43-2402 balun example hints that a 4t primary is probably good enough down to 1.8MHz, depending on one’s limit for InsertionVSWR. We are not being too fussy here… this is not an application that demands InsertionVSWR < 1.2.

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Above is a plot of expected R and X for a 4t winding using my common mode choke design tool. Z at 1.8MHz is 49+j199Ω, or Y=0.00117-j0.00474S. (If your design tools are not giving you similar values, you might consider validating them.) Adding the shunt 50Ω (Y=0.02), we get Yt=0.02117-j0.00474S, and plugging that in to calculate VSWR… Continue reading A method for design of small broadband RF transformers using medium µ ferrite core for receiving use

Last update: 30th July, 2018, 11:36 AM

Turning 1kW into QRP

Effective Isotropically Radiated Power (EIRP) is one means of comparing the performance of a transmitting station.

An inefficient antenna can lead to very low EIRP, perhaps surprisingly low. Consider these four examples at 3.6MHz,

The following NEC-4.2 models give some insight.

QW vertical with 120 buried radials

Considered by so many experts to be the benchmark for a grounded monopole, here is a quarter wave vertical with 120 buried radials.

screenshot-05_10_16-00_08_36

Above, 120 buried radials: GAIN=-1.8dBi, radiation efficiency=20.7%.

At 1kW RF input, EIRP=661W. Continue reading Turning 1kW into QRP

Last update: 5th October, 2016, 1:04 AM

OCF short vertical dipole for HF – VY1GP

I wrote at OCF short vertical dipole for HF that some authors and some sellers wrote descriptions that might entice would be implementors.

George, VY1GP, made a really nice video production of his pitch.

His Youtube video seems to have been substituted with one by Portune, so I have removed the link.

…but do the claims stack up? Continue reading OCF short vertical dipole for HF – VY1GP

Last update: 31st December, 2018, 8:34 AM