In Aug 2018 QST John Portune (W6NBC) described an improvement he made to ordinary windowed ladder line.
Above is his improved line, it is ordinary windowed ladder line fitted into black foam water pipe lagging.
Continue reading Portune’s magic windowed ladder line
Last update: 17th August, 2018, 7:45 AM
A correspondent wrote asking about the design of a matching network for a Half Square antenna for 80m, voltage fed at one end.
Above is the current distribution on the half square voltage fed. It is essentially two in-phase vertical quarter waves separated a half wavelength, a broadside array.
Feed point impedance at resonance is very high 5700Ω, and being a high Q antenna, they are very sensitive to dimensions, nearby clutter etc. Note that this is calculated for an antenna in the clear, it will be different where trees or conductive mast exist nearby. Continue reading 80m voltage fed Half Square matching workup
Last update: 11th November, 2018, 7:06 AM
The article Checkout of SimSmith v16.3 – spot check of transmission line database raises an issue with SimSmith’s modelling of transmission lines.
The case chosen was Belden 8216, a RG174 type line with silver clad steel stranded inner conductor.
Fully developed skin effect
Most practical transmission lines used for HF and above have fully developed skin effect above some frequency, and are well represented by the loss model MLL=k1*f^0.5+k2*f. For an RLGC model, the R is given by the first term and with fully developed skin effect, it is proportional to square root of f. The loss of good dielectrics is usually simply proportional to f and indicated by the second term.
Under this model, L and C are independent of frequency.
Many calculators use this model, and it works fine where skin effect is fully, or even well developed. The model coefficients are commonly discovered by performing a regression on measured matched loss at a range of frequencies, and the quality of the regression fit is a good indicator of the quality of the model for that particular line. Continue reading Checkout of SimSmith v16.3 – spot check of transmission line database – further discussion
Last update: 27th June, 2019, 5:11 PM
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
Last update: 6th May, 2018, 12:00 PM
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
Last update: 6th May, 2018, 11:40 AM
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
Last update: 2nd February, 2019, 8:48 PM
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.
Example 1: Youkits FG-01
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
Last update: 23rd March, 2018, 2:08 PM
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
Last update: 22nd March, 2018, 8:35 PM
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
Last update: 6th March, 2018, 11:50 AM
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
Last update: 23rd February, 2018, 8:49 AM