I was working recently on an antenna and a visitor was intrigued by a device I was using.
Above, the device is for tensioning a wire span, commonly called a come-along though that term gets applied to all manner for appliances for broadly similar purpose. A significant difference is that this was designed for smooth hard drawn copper wires, and has smooth flat jaws (70x6mm) and does not put a kink in the wire (as do most wire grips for more general fencing and FSWR use). This one has had the strap replaced, it came with a 1.25″ canvas reinforced rubber transmission belt which became hard over time. Continue reading Come-along from the past
This article continues on from Implementation of G5RV inverted V using high strength aluminium MIG wire documenting impedance measurements and voltage calculations.
Impedance was measured with an AA-600 looking into 500mm of RG400 then the Guanella 1:1 balun, then 9m of fabricated transmission line as described in earlier articles in the series. The balun is located at the antenna entrance panel and the coax shield is grounded via that panel (ie the normal operating common mode current path exists).
Above is the impedance measurement plotted on a Smith chart. This is more useful and very meaningful as an interactive display in Antscope where are you move the cursor, the frequency and key data are displayed. Continue reading Implementation of G5RV inverted V using high strength aluminium MIG wire – impedance measurements
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
This article continues on from Workup of G5RV inverted V using high strength aluminium MIG wire and describes the implementation.
Above is a view of the steel mast with the Inverted V G5RV rigged from the top of the 11m mast using a halyard though a purchase on a small gibbet to offset the antenna and feed line from the mast. There are lateral guys at 7m height, and the left hand one is non-conductive synthetic fibre rope. Atop the mast is a 2m/70cm vertical. Continue reading Implementation of G5RV inverted V using high strength aluminium MIG wire
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
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.
Comparing the patterns at low angles shows there is not much in it, but below 32° elevation which tends to be of greater interest at VHF, the winner is actually the inclined radials though the difference is less than 1 dB. Continue reading Ground plane ham myth – inclined radials
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