At TV upgrade I reported a change in TV antenna pointing to a different and distant transmitter, and gave a Spectrum Analyser plot at the main TV receiver.
At that time, I adjusted the antenna accurately (within 1°) based on compass heading, but antennas are not perfect and two significant path obstructions may have bearing on best signal.
I could have run up and down the ladder making small adjustments and observing amplitude or better, RF S/N on the Spectrum Analyser but that is tedious and suboptimal so I purchased a DVB signal analyser.
Importantly, a good DVB analyser gives measurement of not just signal strength, but carrier to noise (C/N) ratio (which is actually RF S/N), Bit Error Rate (BER) and Modulation Error Ratio (MER), the last two very important statistics in optimisation and validation and not available on an ordinary Spectrum Analyser.
MER is calculated as the sum of the squares of the magnitudes of the ideal symbol vectors is divided by the sum of the squares of the magnitudes of the symbol error vectors. The result, expressed as a power ratio in dB, is defined as the Modulation Error Ratio (MER).
MER is a good overall single statistic for quality, but BER is more sensitive to occasional errors, so they are both important.
Above, the DVB analyser (the red device) enables a view of measurements whilst adjusting the antenna. The analyser here is connected to the masthead amp output and of course powers the masthead amp. Continue reading TV upgrade – MER optimisation
Walt Maxwell (W2DU) made much of conjugate matching in antenna systems, he wrote of his volume in the preface to (Maxwell 2001 24.5):
It explains in great detail how the antenna tuner at the input terminals of the feed line provides a conjugate match at the antenna terminals, and tunes a non-resonant antenna to resonance while also providing an impedance match for the output of the transceiver.
Walt Maxwell made much of conjugate matching, and wrote often of it as though at some optimal adjustment of an ATU there was a system wide state of conjugate match conferred, that at each and every point in an antenna system the impedance looking towards the source was the conjugate of the impedance looking towards the load.
This is popularly held to be some nirvana, a heavenly state where transmitters are “happy” and all is good. Happiness of transmitters is often given in online discussion by hams as the raison d’être for ATUs . Continue reading Walter Maxwell’s teachings on system wide conjugate matching
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.
Whilst 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
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.
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
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.
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!
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.
Whilst 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
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.
Above, 120 buried radials: GAIN=-1.8dBi, radiation efficiency=20.7%.
At 1kW RF input, EIRP=661W. Continue reading Turning 1kW into QRP
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
The term Effective Radiated Power or ERP appears frequently in ham discussions, antenna literature and regulatory documents. Continue reading Effective Radiated Power (ERP)
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).
Off-Center Fed Vertical Dipole design means no radials, 90% efficient or better across 80m – 10m
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