Finding velocity factor of coaxial transmission line – a challenge

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.

Case 1:

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

4NEC2 plots of STL VSWR III

Conintuing from 4NEC2 plots of STL VSWR II, this article is a tutorial in using 4NEC2 to determine the Half Power Bandwidth of a simple model of the main loop.

The model is drawn from AA5TB’s calculator’s initial values.

The model is in NEC-4.2, and is a 20 segment helix in free space, and tuned for resonance at 7.000MHz. (If you repeat this using NEC-2, you may need fewer segments to avoid violating NEC-2’s segment limits.)
Continue reading 4NEC2 plots of STL VSWR III

4NEC2 plots of STL VSWR II

At 4NEC2 plots of STL VSWR I explained a method of working around a limitation of 4NEC2 values for Zo that can be applied using the Settings menu.

I asked the developer to consider a change, but I gathered that he regarded 4NEC2 to be at End Of Life.

It appears that 4NEC2 enforces a requirement that Zo>=0.1, so having discovered that by trial and error, one wondered if it was possible to change that threshold by hacking the exe file.

The IEEE754 Double representation of 0.1 is 0x3FB999999999999A, and of course it would be stored backwords in the exe file. Searching for 0x9A99999999999FB3F found only one occurrence, offset 0x1490. That was changed to 0x8DEDB5A0F7C6B03E (the backwords representation of 0.000001) and the exe tested. (It might be tempting to set it so zero, but that would permit entering zero which may cause run time errors). Continue reading 4NEC2 plots of STL VSWR II

Riding the RF Gain control – part 5

Every so often one sees advice from experts on how to operate a communications receiver or transceiver for SSB reception on the HF bands.

Very often that advice is to adjust AF Gain to max, and adjust RF Gain for a comfortable listening level. This is argued today to deliver the best S/N ratio, partly due to delivering the lowest distortion due to IMD in the receiver front end.

This is the last of a series of articles exploring and discussing the wisdom of that traditional advice. The preceding parts have examined a range of receiver types identifying their susceptibility to overload in one form or another, means of minimising the risk of overload, and effects of S/N ratio.

Most recommendations to intervene lack quantitative evidence to support the claimed benefits.

Let us quantitatively explore the advice on a modern receiver.

A quantitative example

In this test, a modern budget priced receiver, an IC-7300, is used to evaluate SINAD (similar to S/N) on a steady signal off-air, trying initially the ‘sensible’ basic automatic setting to suit the 40m band, and then various preamp, attenuator and RFGAIN settings to try to win an improvement in SINAD.

Above is a screenshot from SpectrumLab of a SINAD measurement on the IC-7300 setup normally for 7MHz (PREAMP OFF, ATTENUATOR OFF, RFGAIN MAX). Without signal, the S meter indicates around S4, with signal the S meter readings is around S7 and SINAD is around 16dB (it dances around a few tenths of a dB due to the combination of FFT bin size and integration interval). Continue reading Riding the RF Gain control – part 5

Discussion of WA7ARK’s contribution to a QRZ thread on an End Fed Dipole

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

Wooly thinking on the nature of feed line common mode current

An online expert somewhat exasperated that the audience hasn’t absorbed his wisdom elaborated in apparently many previous posts said:

We’ve been round and round on this discussion but in a current mode balun aka a common mode choke the losses due to the windings and core are common mode not differential mode losses. You DO NOT dissipate your transmitted and received signals, which are carried as differential mode signals, as choke losses.

I know you’ve been reminded of this many times and don’t expect you to accept it now but that’s how common mode chokes work.

Now there is a sense in ham radio forums that repetition transforms assertions to fact, but setting that aside, let’s look at the assertion from an energy conservation point of view. Continue reading Wooly thinking on the nature of feed line common mode current

Hobbyking 2500mAh 18650 LiIon cells (9210000181-0) initial capacity test

This article is documentation of a capacity test of 5 x Hobbyking 2500mAh 18650 LiIon cells (9210000181-0).

The cells were purchased on 26/02/2018 (~$7 + shipping) and received at about 30% charge. They were each charged in a XTAR VC2 Plus charger at 0.5A until charged.

The cells are 65mm long, and do not claim to contain protection modules which are prudent in some applications.

Each cell was then discharged at 1A (0.4C) to 2.8V, the discharge was captured.
Continue reading Hobbyking 2500mAh 18650 LiIon cells (9210000181-0) initial capacity test

Aluminium ground system suitability for ham radio station

I have been asked a few times about my article Implementation of G5RV inverted V using high strength aluminium MIG wire, and conversations ran to the suitability of the wire to a radial system on Marconi type antennas.

Firstly, a progress report on the antenna, no news to report and that is good news, there have been no issues so far. Inspection of connections without disassembly has not shown signs of corrosion or fatigue. Continue reading Aluminium ground system suitability for ham radio station

Baluns – wire size insanity

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:

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

ARRL guidance on design of ferrite cored inductors

The ARRL handbook for radio communications (Ward 2011) gives guidance on designing with ferrite cored inductors:

Ferrite cores are often unpainted, unlike powdered-iron toroids. Ferrite toroids and rods often have sharp edges, while powdered-iron toroids usually have rounded edges.
Because of their higher permeabilities, the formulas for calculating inductance and turns require slight modification. Manufacturers list ferrite AL values in mH per 1000 turnssquared. Thus, to calculate inductance, the formula is



L = the inductance in mH
AL = the inductance index in mH per 1000 turns-squared, and
N = the number of turns.

Example: What is the inductance of a 60-turn inductor on a core with an AL of 523? (See the chapter Component Data and References for more detailed data on the range of available cores.)


Lets follow the example through. Continue reading ARRL guidance on design of ferrite cored inductors