Some of us use a resistor as a load for testing a transmitter or other RF source. In this application they are often rated for quite high power and commonly called a dummy load. In that role, they usually do not need to be of highly accurate impedance, and commercial dummy loads will often be specified to have maximum VSWR in the range 1.1 to 1.5 (Return Loss (RL) from 26 to 14dB) over a specified frequency range.
We also use a known value resistor for measurement purposes, and often relatively low power rating but higher impedance accuracy. They are commonly caused terminations, and will often be specified to have maximum VSWR in the range 1.01 to 1.1 (RL from 46 to 26dB) over a specified frequency range.
It is more logical to discuss this subject in terms of Return Loss rather than VSWR.
The EFHW can be deployed in a miriad of topologies, this article goes on to explore three popular practical means of feeding such a dipole.
The models are of the antenna system over average ground, and do not include conductive support structures (eg towers / masts), other conductors (power lines, antennas, conductors on or in buildings). Note that the model results apply to the exact scenarios, and extrapolation to other scenarios may introduce significant error.
End Fed Zepp with current drive
A very old end fed antenna system is the End Fed Zepp. In this example, a half wave dipole at λ/4 height is driven with a λ/4 600Ω vertical feed line driven by a balanced current source (ie an effective current balun).
Above is a plot of the current magnitudes. The currents on the feed line conductor are almost exactly antiphase, and the plot of magnitude shows that they are equal at the bottom but not so at the top. The difference between the currents is the total common mode current, and it is maximum at the top and tapers down to zero at the bottom. Icm at the top is about one third of the current at the middle of the dipole. Continue reading EFHW exploration – Part 2: practical examples of EFHW
The so-called End Fed Half Wave (EFHW) has become very fashionable amongst hams. The idea of end feeding a half wave antenna is hardly new, and there is widespread use of the broad concept… but from online ham discussion, it can be observed that the things are not well understood and indeed, there is magic about them.
A simple model of a simple antenna
This article presents some NEC-4.2 model results for a 7MHz λ/2 horizontal 2mm copper wire at height of λ/4 above average ground.
The model is impractical in a sense that it does not include unavoidable by-products of a practical way to supply RF power to the antenna, but it is useful in providing insight into the basic antenna.
The NEC model has 200 segments, and varying the feed segment gives insight to what happens to feed point impedance.
QST publishes a design by W0SJ for a nominal 50:450Ω EFLW matching device from 160-10m using the following circuit.
The article is ‘in-brief’ as technical stuff that will not interest most hams is published privately on a members-only page. This article is based on the information in the QST article alone (ie not on the private members only supplementary information).
The core has a modest price in North America, but shipping to other parts of the world may make it very expensive… IOW unobtainium to most parts of the world.
AIM915a was recently pulled from the distribution site and replaced by a new release, AIM916.
AIM916 chokes on some calibration files created with earlier versions, so again historical scan data is rendered worthless. Note the illogical diagnostic message… typical AIM quality.
I cannot recall ever finding a new release that did not have significant defects, commonly inconsistency between displayed values. In the common theme of one step forward, two steps backwards, this version has defects that were not present in AIM910B.
This problem existed in AIM915a, it persists in AIM916.
Let’s review the internal consistency of this part of the display screen.
Before trusting measurements made with any instrument, its behaviour should be validated, and this article documents issues discovered in one thread of tests. The developer does not like the term “defects”, he prefers “issues”, a soft denial of “problems”.
So, the test scenario is the VA5 measuring the impedance looking into a 35m length of RG6 coax with an open circuit at the far end. The VA5 has been SOL calibrated with the higher quality loads sold by SDR-kits, and the test is a 3.5MHz. The firmware is the latest, v1.08 (about 3 months old).
The screenshots are taken with a camera, there does not seem to be a method of uploading screenshots to a PC.
Whilst preparing A first test of the FA-VA5 antenna analyser, issues were noticed with the user interface design / implementation. I stated in a later article that The matter of the clumsy / unproductive user interface will be explored more at a later time.
This article introduces a short video demonstration of the frustrating / unreliable user interface (UI) in firmware v1.08 where buttons do not seem to operate intuitively and consistently.
This article started off as a video demonstration of measuring the Matched Line Loss (MLL) of a 6m length of old / budget grade RG58CU for comparison with the datasheet.
Using the instrument was such a frustration due to the user interface design / implementation, but more time was devoted to trying to understand it and experimenting with button press timing etc… but I must admit, to no avail. I persevered and made the measurements which are reported here, the matter of the interface issues will be dealt with separately.
So, the interpolated datasheet MLL for quality cable, Belden 8262 (RG58C/U), is 0.319dB.
The measurement technique is the measure the ReturnLoss of the DUT with o/c and s/c terminations, and estimate MLL=(RLo+RLs)/4.
The analyser is a low cost kit (~A$265 including high accuracy cal kit and postage), the SM components are already fitted to the PCB, but the other components like switches, display connector etc need to be fitted. Whilst these parts are hand soldered, some pins are quite close to other components and require a fine soldering tip and steady hand. It is probably an hour’s work to complete the assembly.
Above is the completed FA-VA5. As can be seen, it has just three buttons which are used to navigate a menu system and to perform data entry, both of which can be a bit tedious but that is the trade off for a simple user interface.