Category: Antennas

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

TV upgrade

The restack of TV channels, and then the allocation of spectrum immediately adjacent to a 4G mobile site that is 1km away and directly in line with Knights Hill (30km) caused me to rethink our TV source and switched to Mt Gibraltar (5km) to escape the 4G interference.

For whatever reason, the signal from Mt Gibraltar has dropped in level and is intermittently very inconsistent.

So, it is back to Knight’s Hill with a LTE filter to try to alleviate the interference from the in-line 4G site.

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Above is the received channels at the TV set with a 6dB 75/50 pad inline. After the last restack, the five desired are now at least 90MHz lower than the edge of the 700MHz LTE (4G) allocation, and with an LTE filter in the masthead amplifier, it seems interference is not noticeable. Signal quality reported by the TVs is consistently 100%.

The spectrum analyser plot underestimates RF S/N due to the system noise floor.

The channels used are 35 (ABC),36 (WIN),37 (CTC),38 (CBN),39 (SBS), all 250kW. Ch 35 and 39 are on the BA tower, the others on the WIN tower.

If only there was something worth watching!

Last update: 9th November, 2016, 8:42 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

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.

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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

OCF short vertical dipole for HF

The OCF short vertical dipole for HF has become popular, particularly disguised as a flag pole for low impact installations and encouraged by claims of outstanding performance. The antenna was described in QST and a commercial version was available at time of writing.

The rationale for the design is that it is a short dipole, not requiring radials, and feed point offset downwards by 30% as an optimal value for performance (driven by often unsound assessments of coax loss).

Claims include:

Off-Center Fed Vertical Dipole design means no radials, 90% efficient or better across 80m – 10m

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Above is the promising gain plot for one of the commercial implementations, it is only one S point (6dB) behind a quarter wave vertical with 4 buried radials. Continue reading OCF short vertical dipole for HF

Last update: 25th November, 2018, 7:03 AM

Cadweld Plus Control Unit

cadwelpluscu

The CADWELDPLUSCU is the igniter for the Cadweld Plus line of electrically initiated thermite weld capsules. The plug seen on the end of the cable slides over the flat connection strip to the capsule.

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The environment around the weld crucible is subject to a sudden ejection of hot gases and smoke, much of which is emitted out of the slot in a Oneshot crucible intended for the connection strip, and this is only a 45g capsule. There is probably less gas emitted towards the connector on reusable moulds as the gap for the connection strip is very narrow.

cadweldpluscuplug01

Despite dressing the plug away from the stream of hot gasses emitted, it can be seen above that the plug has suffered heat damage. Of particular concern is deformation of the parts at the cable clamp rendering it less effective. The plugs come on a new lead set that is relatively expensive.

Since one wants to be able to yank the cord away from the crucible as soon as possible to minimise heat damage, the cord grip needs to be effective.

This article describes two measures taken to protect the plug and ensure the cord grip works. Continue reading Cadweld Plus Control Unit

Last update: 3rd October, 2016, 5:52 PM

MFJ-993B internal balun review

The MFJ-993B auto antenna tuner includes an internal balun, this article is a review of that balun.

screenshot-29_09_16-10_02_24The schematic shows the balun as a Guanella 4:1 balun with the usual external link to one of the coaxial antenna sockets. (The label “Z balanced” is misleading, clearly one of the terminals is grounded and this is the unbalanced connection to the coax connector via a link. The antenna connects to the left hand terminals.)

Unlike almost all ATUs with an internal balun, this is a current balun (to their credit), but a 4:1 balun.

There are two aspects of balun behaviour that are of particular interest:

  • choking or common mode impedance; and
  • impedance transformation.

Continue reading MFJ-993B internal balun review

Last update: 11th August, 2018, 9:10 AM