A recent online posting asserted that an antenna is optimal when itself resonant, and fed with a resonant feed line length so delivering a purely resistive load to a source, and further that implementors needed to be careful that a shorter dipole could be offset to some extent by a longer feed line but it would be inferior because:
no short antenna is more efficient than a resonant-length antenna
… but does that stand scrutiny?
An NEC experiment
Lets walk though an experiment using NEC-4.2 models of a dipole of 2mm copper wire at 10m height at 7.1MHz over average ground (0.005,13).
- source has a Thevenin equivalent source impedance of 50+j0Ω;
- feed line is lossless.
The results are sensitive to the model assumptions.
We will calculate the ratio of radiated power to the power delivered by the transmitter to a matched load, let us call it TransmitEfficiency for the purposes of this article. Continue reading “No short antenna is more efficient than a resonant-length antenna”
A ham recently posted a graph on QRZ to educate his fellow hams on the behaviour of transmission lines under mismatch.
Above is one of his graphs (the red arrow is my annotation). It plots Impedance variation along a mismatched 75Ω transmission line. The curves look graceful, but are they science or just pretty artwork? Continue reading Failure to treat impedance as a complex quantity leads to…
An online expert recently reported:
I tried to make an antenna loop for longwave with cat 5 and after it did no good I realized the twisted wires canceled each other out.
Or did they really cancel?
I constructed a loop of one Cat 5 pair and measured its inductance when both wires are bonded at the ends.
The conductors are 0.5mm diameter and spaced 0.9mm. To estimate the inductance we use the geometric mean radius (GMR) as the equivalent radius of the pair. GMR=(0.5*0.9)^0.5=0.67, diameter=1.34mm. So let’s calculate the inductance of a single turn circular loop of 0.8m perimeter and round conductor of 1.34mm diameter.
The estimate above is 850nH.
Above is the measurement, the screen is not readable, but it is 852nH, very close to the estimated 850nH. Continue reading Inductance of a loop of CAT5 pair
Messi & Paoloni Ultraflex 7 coax cable compared M&P UF7 with RG-213. This article does a similar comparison between M&P Ultraflex 10 and LMR400UF.
Both cables are of similar size, ~10mm overall, stranded centre conductor and foil+braid outer conductor. The shield stranding is different and the foil is copper in the UF10, aluminium in the LMR400UF.
Let’s take the loss factors calculated for TLLC and de-construct the conductor and dielectric loss for each line type.
Above is a comparison of the cables. Continue reading Messi & Paoloni Ultraflex 10 coax cable
A correspondent wrote about trying to reconcile by G/T worksheet with EME Calc.
Many times I have tried to validate it and run into problems. At one time I reported them to the author, but they were never acknowledged, much less fixed.
The specific problem on this occasion relates to the receiver performance tab.
Above is a screenshot (with my annotations) where I have basically stripped the configuration down to a receiver attached to a noiseless antenna with lossless line. Continue reading EME Calc v11.11 reconciliation issues
Richard (G3CWI) published an interesting blog article Comparison of groundwave performance of Small Transmitting Loop and Quarterwave GP summarising a recent WSPR test on 40m over 20km distance.
This article is a walk through of the expected WSPR receive S/N for the case of the 20mW tx on a quarter wave vertical.
100% efficient tx and rx antenna systems
Ground wave suffers attenuation due to two key components:
- dispersion of energy as the wave spreads out from the source; and
- absorption of energy in heating the soil.
Item (1) is simply inverse square law effect, and Norton provides us with several approximations for estimating (2) from Sommerfields work.
Calculate efficiency of vertically polarised antenna from far field strength uses Norton’s f5 approximation for ground wave attenuation.
Above is a calculation for a 100% efficient transmitter. (The trick to getting this is to leave the measured field strength field empty and the calculator will insert the value that gives 100% efficiency.)
So the next question is what ambient noise level might we expect in a rural setting on 40m. Continue reading G3CWI’s ground wave tests Jul 2017 using WSPRlite
Resolving the sign of reactance – a method – Smith chart detail
Exploiting your antenna analyser #28 gave an example of use of one method to resolve the sign of reactance comparing measurements made with a slightly longer known transmission line.
One way to predict the input impedance to the longer line is using a Smith chart. This article presents a Smith chart prediction of the expected input impedance of a 8′ section of RG8 at 14.17Mhz (vf=0.66, length=0.175λ) for the cases of Zload being 60.3+j26.9Ω and 60.3-j26.9Ω.
The impedance is normalised to 50Ω and plotted on the Smith chart, point 1 above. A radial from the centre through point 1 is drawn to the edge of the chart. Another radial is drawn a distance towards the generator of 0.175λ and using a pair of dividers or ruler, point 2 is plotted on that radial at the same distance from the centre (same VSWR) as point 1.
These points are on a constant VSWR arc but the arc has not been draw because the two arcs would overlap and might be confusing to some readers. Continue reading Exploiting your antenna analyser #29
Chinese sellers offer low cost 50Ω SMA terminations mostly without specs, but some sellers specify VSWR<1.2 to 3GHZ.
Above is the internals of one, it is a 51Ω 5% metal film resistor.
They often fail a DC test and tapping them gives erratic resistance readings up to hundreds of ohms, and of course they can be unreliable at RF.
They rely upon the resistor pigtail to make a spring contact with the inside of the barrel, and give that the pigtail is soft copper with little spring the contact is not very reliable. Continue reading Cheap and nasty 50Ω SMA terminations
Resolving the sign of reactance – a method
Many analysers do not measure the sign of reactance, and display the magnitude of reactance, and likewise for magnitude of phase and magnitude of impedance… though they are often incorrectly and misleadingly labelled otherwise.
The article The sign of reactance explains the problem and dismisses common recipes for resolving the sign of reactance as not general and not reliable.
This article gives an example of one method that may be useful for resolving the sign of reactance.
My correspondent has measured VSWR=1.68 and |Z|=66 and needs to know R and X. From those values we can calculate R=60.3 and |X|=26.9.
The method involves adding a short series section of known line, short enough to provide a measurement difference in R, and that R would be different for the case of =ve and -ve X, all of these measured at the same frequency. Continue reading Exploiting your antenna analyser #28
The MFJ-225 is a recent antenna analyser from MFJ.
An important attribute of such devices is their ability or not to measure the sign of reactance. Continue reading MFJ-225 – sign of reactance