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
The 5/8λ ground plane is regarded by hams widely as a superb antenna for DX, and since the main reason for modern ham radio is DX, it is an antenna of interest.
The idea behind the 5/8λ ground plane popularity is that claim that it has higher gain at low angles than a simple 1/4λ ground plane.
The 5/8λ ground plane is not resonant, and the feed point impedance is hardly suited to direct coax feed.
The chart above is for a 5/8λ ground plane elevated to 5m height above average ground (σ=0.005, εr=13). The feed point impedance in this case at 5/8λ radiator height (14.2MHz) is about 110-j485Ω. Continue reading Matching a 5/8λ ground plane
Hams embrace the UHF series connectors like no one else, including for its use on test equipment where its performance is lacking.
This is the likely reason why it is so hard to find low VSWR 50Ω terminations with UHF series plug. It is rare to find something with VSWR quoted in specifications, and nigh on impossible to find one at a reasonably low price.
On the other hand, SMA terminations start at about $2 each (posted), and it is not too hard to find ones specified with VSWR<1.2 to several GHz.
Above is a low cost, low quality solution. It is a SMA termination selected from a bunch using a high accuracy DMM (selected, R is 49.86Ω) and a SMA(F)-UHF(M) adapter, total cost $7 (posted) (but you might be advised to buy 5 loads to select the best one). Despite the specification, they are probably only good to 100MHz, and can be unreliable. Continue reading A check load for antenna analysers with UHF series socket
An online expert helped recently helped his Small Transmitting Loop (STL) disciples with:
Also remember that the bandwidth given by the calculators is the half power point. That’s equivalent to an SWR of about 4.3 at the ends.
Most STL, and lots of other resonant antenna systems exhibit a classic VSWR curve being that of a approximatly constant resistance in series with an ideal capacitor and inductor.
Above is that classic VSWR curve. Continue reading Extrapolating VSWR of a simple series resonant antenna
There are applications for estimating the inductance of the outside of LDF4-50A at radio frequencies.
For the purpose of calculating the inductance, the geometric mean radius is appropriate. This article offers two methods for estimating the geometric mean diameter (GMD) of the conductor.
Above a section of LDF4-50A.
Above is a magnified view of the profile, it is corrugated copper outer conductor with a shallow but not quite symmetric profile. Continue reading Finding the inductance of the outside of LDF4-50A
As the popularity of low cost, low end antenna analysers increases, client software appears to enhance the capability of the analyser.
The SARC-100 is one of these low end analysers, it and its many close derivatives are marketed under various model names.
The sign of reactance discusses a major weakness of these and many other low end instruments in that they do not ‘measure’ the sign of reactance, displaying the magnitude of reactance and leaving it to the user to solve the sign problem.
SM6WHY is one of the many who have produced software for the SARC-100 that purports to solve the sign of reactance problem. He gives this graphic on his website to demonstrate the capability of his software used with a SARC-100 (which does not sense the sign of reactance).
Above is part of the graphic he offers. Though the image is poor quality, the VSWR plot appears smooth and quite typical of that which might be obtained by measuring an antenna system near its VSWR minimum.
However the accompanying Smith chart plot which has points plotted with both negative and positive reactance is inconsistent with the VSWR plot and appears flawed. Continue reading The sign of reactance – SM6WHY’s take