Category: Amateur Radio

Exploiting your antenna analyser #27

An Insertion VSWR test gone wrong

We often learn more from failures than successes, this exercise is one of those opportunities.

An online poster tried to validate his newly purchased MFJ-918 by measuring Insertion VSWR.

That is done preferably by measuring a good termination (dummy load) to validate that it has a very low VSWR, then inserting the Device Under Test (DUT) and measuring the VSWR as a result of insertion of the DUT.

The poster did not mention measurement of the dummy load alone, and it is a type that warrants validation.

Above is the poster’s test setup, his Rigexpert AA-170 is connected to the balun’s input jack using a M-M adapter. The output wires on the balun form a rough circle of about 550mm perimeter by eye. Continue reading Exploiting your antenna analyser #27

Last update: 24th June, 2017, 1:44 PM

Loss of windowed ladder line at MF/HF/VHF

A ham in the need of help recently asked for advice on eHam about the use of Wireman Ladder Line and the like.

After a fairly well considered, detailed and lengthy answer from on online expert, another online expert stepped in to confuse the matter with conflicting advice:

Wire resistance (loss due to current ) is not a factor with higher voltages typically seen in high impedance antenna feed applications. Attenuation loss is a factor depending on dielectric properties in VHF and UHF frequencies. Their is little skin effect below 50 MHz in wire antennas and feeds.

This comes down to line strength. I would go with the solid Copperweld for HF antenna work.

Let’s examine the above quote. Continue reading Loss of windowed ladder line at MF/HF/VHF

Last update: 7th March, 2017, 8:26 AM

Messi & Paoloni Ultraflex 7 coax cable

Messi & Paoloni Ultraflex 7 coax cable is being marketed as similar to RG-213 in performance, but 7mm overall (against 10mm).

There is lots of comment by online experts questioning the claims, and critical of things like the braid coverage, copper foil etc, but without real evidence that it does not live up to specification.

Accepting the specifications for Ultraflex 7 and Belden 8267 (B8267, RG-213) for a moment, how do they compare.

Let’s take the loss factors calculated for TLLC and de-construct the conductor and dielectric loss for each line type.

Above is a comparison of the cables. Continue reading Messi & Paoloni Ultraflex 7 coax cable

Last update: 5th March, 2017, 10:09 AM

WSPR checkout on new workstation

I have been building a new workstation and a simple test of its reliability for logging signals via the IC-7300 sound card is to run WSPR.

So, WSJT-X was configured for WSPR on 40m and run for 24h using the low G5RV inverted V dipole with tuned feeders (designed primarily as an NVIS antenna for local contacts).

Above is a map of the spots involving VK2OMD over the 24h survey. Continue reading WSPR checkout on new workstation

Last update: 6th March, 2017, 12:43 PM

NH7RO 7-foot diameter QRO STL for 40M

NH7RO describes his loop project at Building a 7-foot diameter QRO STL for 40M in my HOA backyard.

The loop appears to be made from 7/8″ copper tube, and is 7′ in diameter. He estimates its efficiency to be 66% and initially reports I’ve got it less than 4 feet above ground yet it tunes flat to 1.1>1 with roughly 10kHz bandwidth.. Curiously, 10kHz is the result calculated by AA5TB’s spreadsheet, though I have written elsewhere it is deeply flawed (Small transmitting loop calculators – a comparison).

Let us assume that these figures are correctly reported, and that the unqualified bandwidth means the half power bandwidth of a matched loop.

We can estimate the efficiency of a Small Transmitting Loop (STL) in free space.

Before getting excited about the results, let us question the validity of the model. There are three important factors that question the validity of the model:

  • bandwidth;
  • size of the loop; and
  • proximity to ground.

Continue reading NH7RO 7-foot diameter QRO STL for 40M

Last update: 18th July, 2017, 7:42 AM

Check / calibrate frequency accuracy of IC-7300

The IC-7300 is a transceiver where all heterodyning oscillators are derived from a single master oscillator.

This type of radio makes for very easy checking and calibration of frequency accuracy.

The video below demonstrates the technique.

The video used a local GPS disciplined source at 50.1MHz. The frequency was chosen to provide the greater resolution in setting the oscillator, though setting it to within 1 part in 50,000,000 or 0.02ppm is better than the stability of the oscillator (specification is 0.5ppm or 5Hz at 10MHz).

Any accurate known reference can be used, it could be WWV or the like, or even a MW broadcast station, though an accurate signal at 10MHz or higher is better.

The technique can be applied to the much older IC-7000, and many transceivers released since then, of various brands. The important thing is that ALL oscillators are derived from a single master oscillator.

 

Last update: 9th February, 2017, 9:21 AM

Surecom SW-102 VSWR meter review

I recently purchased a Surecom SW-102 VSWR meter. It looked a little like a supercharged RedDot copy.

sw102-02

Above the Surecom SW-102 VSWR meter with backlight and photographed under normal interior lighting. The display lacks contrast, and overall is difficult to read due to size of text, fonts used, and lack of contrast. (The pic is taken with a screen protector installed, but the evaluation is based on the bare meter with original protective film removed as it degraded readability.) Continue reading Surecom SW-102 VSWR meter review

Last update: 18th May, 2018, 12:25 PM

Does common mode current flow inside coax?

The term “common mode current” applied to coaxial transmission lines is bandied about with abandon these days in online fora, awareness of its existence has increased if not understanding.

A simplistic analysis is that in TEM mode, ONLY differential current is supported inside a coaxial line, ie that at any point the current on the outer surface of the inner conductor is exactly equal to a current in the opposite direction on the inner surface of the outer conductor.

But, lets look at the wider context of the meaning of common mode current when a uniform coaxial line is connected to an antenna system. Whilst an antenna might have an obvious two terminal connection to the feed line, in the presence of ground, the current in those two terminals are not necessarily equal and opposite. Continue reading Does common mode current flow inside coax?

Last update: 12th June, 2017, 1:24 AM

Exploiting your antenna analyser #26

Find coax cable velocity factor using a very basic analyser

A common task is to measure the velocity factor of a sample of coaxial transmission line using an instrument that lacks facility to backout cable sections or measure SOL calibration (as discussed in other articles in this series). The older models and newer budget models often fall into this category.

The manuals for such instruments often explain how to measure coaxial cable velocity factor, and the method assumes there is zero offset at the measurement terminals (whether they be the built-in terminals or some fixture / adapters). In fact even the connectors are a source of error, especially UHF series connectors.

It is the failure to read exactly Z=0+j0Ω with a S/C applied to the measurement terminals that adversely impacts efforts to measure resonant frequency of a test line section.

The method described here approximately nulls out offsets in the instrument, measurement fixture, and even in the connectors used and for that reason may sometimes be of use with more sophisticated analysers.
Continue reading Exploiting your antenna analyser #26

Last update: 30th January, 2017, 7:41 AM