Month: March 2018

Effect of shorting turns on a tapped air cored solenoid at RF #2

Effect of shorting turns on a tapped air cored solenoid at RF offers a simple model for estimating the effect of shorting turns on inductance (L) and Q.

A correspondent sent me a set of measurements he made of an air cored solenoid using a Q meter.

The coil was a 22t air solenoid of length 99.5mm and radius 31.56mm. Q of the whole coil (L12) was measured at 6MHz to be 475.

L and Q were estimated and measured with three different tapping points at one end of the coil.

Whilst the method described in the reference article does not attempt to estimate the effect of tapping where the unused turns are left open circuit, we might expect than when the unused section is a small part of the coil, that the effect is similar to that if the unused turns were not there.

A model as described in the reference article was constructed.

The notation is L1 is the used part of the coil, L2 is the unused part, L12 is the whole coil with no taps, Lms is measured L unused shorted, Lmo is measured L unused open, like wise for the Q subscripts.

Above, the model results There is quite good reconciliation with the predicted behaviour. Continue reading Effect of shorting turns on a tapped air cored solenoid at RF #2

Last update: 29th March, 2018, 11:53 AM

Finding velocity factor of coaxial transmission line using the velocity factor solver

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

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

Last update: 22nd March, 2018, 8:35 PM

IoT water tank telemetry project – part 3

Battery trials

The project continues, albeit slowly.

Some inexpensive DC-DC boost converters have been very slow from China, though multiply source, they have not yet arrived.

An older module which was on hand has enabled progress of reliability and battery trials.

Above is the current prototype. The module on the white plug in cable is a 4-20mA simulator set to 20mA for maximum drain during battery trials. The module at upper right of the pic is a TP4056 batter charger and 1S protection board for the 2000mAh LiPo. The PV array (partially obscured) is capable of 80mA of charge current in full sunlight. The prototype includes a red LED drawing 1mA, an additional 24mAh load per day. Continue reading IoT water tank telemetry project – part 3

Last update: 18th October, 2018, 8:07 AM

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

Last update: 17th March, 2018, 8:34 PM

4NEC2 plots of STL VSWR II (v5.8.16)

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 0x9A9999999999B93F found only one occurrence, offset 0x1490. That was changed to 0xfca0f1d24d62503f (the backwords representation of 0.001) 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 (v5.8.16)

Last update: 7th March, 2022, 4:14 AM

Balancing a ceiling fan

I have a ceiling fan which has poor balance and is quite annoying on its high speed.

To solve the problem, I attached a flight controller board which I had on hand for this sort of purpose to the stationary spindle extension, and I have the associated configuration software installed for flying machines

Above, the OmnibusF4 v1 flight controller. Not a good flight controller for flying machines because of the silly pinout, but cheap (for that reason), about $20 on eBay. The flight controller contains a 3 axis gyro and accelerometer, the latter will be used here. Continue reading Balancing a ceiling fan

Last update: 11th March, 2018, 5:51 PM

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

Last update: 9th March, 2018, 7:54 AM

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

Last update: 6th March, 2018, 11:50 AM

woolly 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 woolly thinking on the nature of feed line common mode current

Last update: 22nd May, 2020, 12:34 AM