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.)
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.
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
From time to time one sees ‘traditional wisdom’ that inclining the radials of a VHF ground plane to raise its feed point resistance degrades it performance significantly.
I have constructed NEC-4.2 models of a 52MHz ground plane with four 45° inclined radials at 10m height above ‘average ground’ (σ=0.005, εr=13) on and connected to a conductive support pole which is bonded to ground at the lower end, and one with horizontal radials.
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
Walt Maxwell (W2DU) described a simple common mode choke or 1:1 current balun using ferrite sleeves slipped over a coaxial cable.
Maxwell gives the choking impedance of two of his recommended chokes in Fig 21-3 from (Maxwell 2001). He does not give any detail of how he arrived at the curves, and in correspondence declined to give any detail.
This article focusses on a linear design for HF using 50 x FB-73-2401 (2673002402) ferrite sleeves.