Lets use a simple test circuit to review the meaning of some oft misused terms associated with VNA and antenna analyser measurements.
Above, the test circuit is a nominally 220pF COG capacitor connected between tx and rx ports of a two port VNA. An extra 1Ω series resistance is included to model the likely effect of capacitor ESR. Continue reading Ham grade analysers and VNAs often use unconventional meanings for well known terms
I am not a SimSmith user, and with upgrade of my desktop computer, I have lost access to the Smith chart application I have used for 20+ years. That has given me reason to evaluate various Smith chart applications for a replacement.
Smith charts are about modelling problems in transmission line terms, and what better test than a simple transmission line problem.
Above, a model of Belden 8216 (an RG-174 type cable) picked from SimSmith’s library of transmission line data (source KN5L). The model is at 1MHz and essentially indicates the Matched Line Loss of 100m by deducting the left hand dBW figure from the next one to the right, -5.88228e-3–2.33357=2.33dB. (Duh, I could not copy and paste these values, they had to be read and typed in by hand which is not only laborious but more importantly gives scope for error.)
Lets check the Manufacturer’s data sheet. Continue reading Checkout of SimSmith v16.3 – spot check of transmission line database
At Surecom SW-102 VSWR meter review I wrote a review of a meter which I had purchased a little over a year ago, it was at v4.5.
One of the many problems identified was inconsistency of displayed values.
Surecom’s versions are confusing, the highest number is not necessarily the latest version. It appears a partial version history from their current page advertising the SW-102 is:
OLD VERSION : V3.3 ,V3.8 ,V4.5,V4.9 ,V5.0,V5.1
2017-8 NEW VERSION : V2.02 ,V2.03
The following image is from Surecom’s current page advertising the SW-102, and I assume that the version shown here (v2.6) is the latest at time of writing.
The image captures the outputs of two tests with poor and good dummy loads.
Let’s check the displayed values for internal consistency. Continue reading Surecom SW-102 VSWR meter review – v2.6
This article is a tutorial in use of Velocity factor solver to find the velocity factor of a sample coaxial transmission line using an antenna analyser.
we have two lengths of H&S RG223 terminated in identical BNC connectors at both ends. Let’s connect each in turn to a Youkits FG-01 antenna analyser and find the quarter wave resonance of each (ie the lowest frequency at which measured X passes through zero).
Above, the line sections are connected to the Youkits, and the length overall is measured from the case of the analyser to the of the cable.
Continue reading Finding velocity factor of coaxial transmission line using the velocity factor solver
An upcoming article works through an approach to finding the velocity factor of a sample of coaxial cable using an antenna analyser.
As a precursor, this article poses a challenge that will identify the issues relevant to the problem.
A Rigexpert has been used to measure the first quarter wave resonance of a length of ‘unknown’ semi air dielectric RG6.
The length of RG6 Dual Shield is terminated in an F connectors at one end, the other end cut cleanly square. It is connected via N(M)-BNC(F) and BNC(M)-F(F) adapters to a Rigexpert AA-600 antenna analyser and the quarter wave resonance noted (ie the lowest frequency at which measured X passes through zero).
Above, the line section is connected to the Rigexpert via adapters, and the length overall is measured from the case of the AA-600 to the of the cable. The measured length is 1.077m, make any adjustment to that length that you think is justified on the information presented here.
Continue reading Finding velocity factor of coaxial transmission line – a challenge
In another long running discussion on QRZ about End Fed Antennas, WA7ARK offered a contribution:
(1) Back in post #30 I showed that with a halfwave wire fed close to its end works just like the same wire fed in the center; the only difference being the feed point impedance. I let EzNec figure this out; I didn’t have to explain it with any mysterious “displacement” currents. Shown as (1) in the attached.
Since, in the model, the source is a constant current source, that forces the current on either side of the source to be equal, and the radiation pattern predicted by EzNec reflects that, because the patterns for the end-fed and center-fed match… (go back and look at post #30)
His post #30 is of a 67′ dipole at 66′ above poor ground @ 7.18MHz, fed at one end.
Above is the current distribution of my approximate re-creation of his model in NEC-4.2. It reconciles with his published graphs. Continue reading Discussion of WA7ARK’s contribution to a QRZ thread on an End Fed Dipole
An online expert recently expounded on detailed design of a balun, this is an excerpt about wire sizing.
The wire gauge used limits the current handling capacity of the wire, run too thin a wire and it will heat up. Run much too thin of a wire for the power in use and it will fuse open. Current carrying capacity of wire is typically rated for either power transmission applications or chassis wiring applications. The latter, and higher, current capacity for a wire is relevant to designing a balun. How much current your 50 watt signal generates depends on the impedance its looking into. If you’re talking about a 50 ohm system, with a perfect match you’ll deliver one amp through your balun wires when driving 50 watts into it. Allowing for say a 4:1 SWR the worst case current(@12.5 ohms) is 2 amps. If you’re using this as a tuner balun, perhaps to drive a multi-band doublet then the impedance can vary widely so over sizing the wires is easy insurance. Here’s a table of wire current carrying capability: https://www.powerstream.com/Wire_Size.htm
For convenience, the relevant part of the table linked above is quoted for discussion.
So, the poster recommends wire with chassis wiring rating of 2A for 50W with reserve capacity for worst case VSWR=4. Continue reading Baluns – wire size insanity
I often receive emails from folk trying to validate continued performance of an installed antenna system using their analyser.
With foresight they have swept the antenna system from the tx end and saved the data to serve as a baseline.
The following are example sweeps from one of my own antennas, a Diamond X50N with 10m of LDF4-50A feed line.
Now I have plotted Return Loss rather than VSWR for several reasons:
Now a hazard in working with Return Loss is that many authors of articles and software don’t use the industry standard meaning.
Lets just remind ourselves of the meaning of the term Return Loss. (IEEE 1988) defines Return Loss as:
(1) (data transmission) (A) At a discontinuity in a transmission system the difference between the power incident upon the discontinuity. (B) The ratio in decibels of the power incident upon the discontinuity to the power reflected from the discontinuity. Note: This ratio is also the square of the reciprocal to the magnitude of the reflection coefficient. (C) More broadly, the return loss is a measure of the dissimilarity between two impedances, being equal to the number of decibels that corresponds to the scalar value of the reciprocal of the reflection coefficient, and hence being expressed by the following formula:
20*log10|(Z1+Z2)/(Z1-Z2)| decibel
where Z1 and Z2 = the two impedances.
(2) (or gain) (waveguide). The ratio of incident to reflected power at a reference plane of a network.
Return Loss expressed in dB will ALWAYS be a positive number in passive networks.
The relationship between ReturnLoss in dB and VSWR is given by the equations:
So now that we are on the same page about Return Loss, lets look at my 2m plot.
The X50N does not have VSWR or Return Loss specs, but we might expect that at the antenna itself, VSWR<1.5 which implies Return Loss>25dB. Measuring into feed line, you can add twice the matched line loss to the Return Loss target (see why Return Loss is a better measure).
Continue reading Baselining an antenna system with an analyser
A recent online discussion on common mode feed line current was directed to Thompson’s article with the recommendation that is ALL basically needed to discuss the common mode current.
Above is Thompson’s diagram of currents in a feed coax, and it contains two significant errors that could / would lead to formation of the wrong concepts in a learner’s mind. Continue reading Thompson’s coax common mode explanation
This article describes a prototype RF choke (RFC) for use in a power injector for 50Ω coax over range 1.8-30MHz. Power injector / extractors are often used to connect power and / or signalling on a shared common RF coax feed line to accessories such as remote antenna switches and ATUs.
Design criteria are:
The core chose is a LF1260 ferrite suppression bead from Jaycar. It is a medium / high µ core readily available in Australia at $7.50 / 6.
Above is the prototype RFC wound with data cable wire for the purpose of measurement. In application it could be wound with 1mm enamelled copper or PTFE insulated wire (Curie point is lowish at 120°+, but it still benefits from higher temperature insulation). Continue reading An RF choke for a 1.8-30MHz coax power injector – LF1260 core