On testing two wire line loss with an analyser / VNA – part 1

This article series shows how to measure matched line loss (MLL) of a section of two wire line using an analyser or VNA. The examples use the nanoVNA, a low end inexpensive VNA, but the technique is equally applicable to a good vector based antenna analyser of sufficient accuracy.

Above is a short piece of the line to be measured. It is nominal 300Ω windowed TV ribbon. It has copper conductors, 7/0.25, spaced 7.5mm, though as can be seen the spacing is not perfectly uniform. The dielectric is assumed to be polyethylene… but later measurements suggest is has slightly higher loss than polyethylene. The test section length is 4.07m. Continue reading On testing two wire line loss with an analyser / VNA – part 1

Last update: 18th February, 2022, 5:15 PM

The mower appears affected by Ethanol

I have a Toro ride-on mower which ran out of fuel when it was just a couple of years old, perhaps 50 hours on it. Okay, so that is going to happen with failure to check fuel level before starting… but the problem was that when the tank was filled, it would not start, the petrol pump would not self prime even though the suction head was very small with a full fuel tank.

Mindful that long cranking is very hard on batteries, I used the starter sparingly in short bursts with cooling off time. Nevertheless, the battery went open circuit.

So I replaced the battery and primed the pump by hand… and all has gone well for some years… until last week, I ran it out of fuel, duh!

Same problem, when the tank was filled, the pump would not self prime.

The machine has a problem that needs to be fixed.

The inline filter was checked, and fuel flowed though it quite well by gravity. This time, I primed it by hand early in the process and got on with mowing. This was probably not a vapor lock problem, everything was quite cool by the time I fueled the machine. Continue reading The mower appears affected by Ethanol

Last update: 21st September, 2021, 6:34 AM

The mulcher appears affected by Ethanol

I have a chipper / shredder that has a Briggs and Stratton 6.5hp (~5kW) Intek I/C 206cc engine on it.

On a recent routing maintenance inspection, I dropped the carburettor bowl to look for signs of Ethanol damage (mainly corrosion). During that inspection I noted that the bowl gasket was hardened even though the engine is only 13 years old, and that a stain on the mulcher body hinted a fuel leak even though the bowl exterior seemed dry on test..

The machine is usually stored with the petrol cock off, so the opportunity for a needle / seat leak would be during a day’s yard work where the petrol cock is on for hours but the machine is run for a lesser time.

A pressure test might provide a firm diagnosis.

A piece of silicone tube was connected to the carburettor in place of the normal fuel hose, a little petrol added to the hose with a pipette, and then a sphygmamometer used to pressurise the hose.

Above, the test in progress. The fuel level in the hose can be seen about 30mm from the end. The pressure is leaking down quite slowly, the leak is not fast enough to upset engine operation (by flooding or even an over rich mixture) but it might well result in dilution of the crankcase oil when the engine is left standing. It needs repair. Continue reading The mulcher appears affected by Ethanol

Last update: 31st July, 2023, 9:15 AM

Do I ‘need’ a masthead preamp to work satellites on 2m? – G/T vs G/Ta

A reader of Do I ‘need’ a masthead preamp to work satellites on 2m? – space noise scenario has written to say he does not like my comments on the hammy adaptation of G/T.

Above is an archived extract of a spreadsheet that was very popular in the ham community, both with antenna designers and sellers and end users (buyers / constructors). It shows a column entitled G/T which is actually the hammy calculation. The meaning possibly derives from (Bertelsmeier 1987), he used G/Ta.

Ta is commonly interpreted by hams to be Temperature – antenna. It is true that antennas have an intrinsic equivalent noise temperature, it relates to their loss and physical temperature and is typically a very small number. But as Bertelsmeier uses it, it is Temperature – ambient (or external), and that is how it is used in this article.

Let’s calculate the G/Ta statistic for the three scenarios in Do I ‘need’ a masthead preamp to work satellites on 2m? – space noise scenario.

Base scenario

Above is a calculation of the base scenario, G/T=-29.74dB/K.

Also shown in this screenshot is G/Ta=-23.98dB/K. Continue reading Do I ‘need’ a masthead preamp to work satellites on 2m? – G/T vs G/Ta

Last update: 12th October, 2021, 5:15 PM

QEX on SWR dependence on output impedance #2

(Wright 2021) sets  out to prove a dependence of VSWR on source impedance, a common ham assertion.

Wright gives the schematic of the minimal VSWR detector he simulates in SPICE.

The schematic is sparse, it does not show where the forward and reflected signals are measured. Continue reading QEX on SWR dependence on output impedance #2

Last update: 18th September, 2021, 11:21 AM

Chinese chainsaw review – SX25 #1

When we moved to this place in 2009, I could see the utility of a ‘aborist chainsaw’, or ‘top handle chainsaw’.

I bought a cheap Chinese 25cc jobbie, in fact it was so good I bought a second for spares.

SX25#1

This saw had seen off 6 chains by 2021 and forced by some reliability issues, I decided to replace it with the spare and repair it at my leisure (parts from China are real slow). It has had a good life, usually warmed up before being gunned up to WOT to avoid cold seizure. Continue reading Chinese chainsaw review – SX25 #1

Last update: 16th September, 2021, 9:21 AM

QEX on SWR dependence on output impedance

(Wright 2021) sets  out to prove a dependence of VSWR on source impedance, a common ham assertion.

Wright gives the schematic of the minimal VSWR detector he simulates in SPICE.

The schematic is sparse, it does not show where the forward and reflected signals are measured. Continue reading QEX on SWR dependence on output impedance

Last update: 18th September, 2021, 11:21 AM

Measurement of one of those ham lore components – figure 8 twinline

A long time ago when I first ‘got on the air’, a mentor suggested I build the famous figure 8 dipole (or zip line dipole in North America). The advice was that it had characteristic impedance very close to 70Ω, ideally suited to a half wave dipole and could be made with a clever not obviating the need for a centre insulator. A ham rite of passage?

Above, the dipole from the ARRL Antenna Book. Continue reading Measurement of one of those ham lore components – figure 8 twinline

Last update: 24th September, 2021, 7:37 AM

Do I ‘need’ a masthead preamp to work satellites on 2m? – terrestrial noise scenario

Do I ‘need’ a masthead preamp to work satellites on 2m? – space noise explored a scenario for a high gain antenna pointed skywards. This article explores the case of a omni antenna which basically captures ‘terrestrial’ noise.

Base scenario is a low end satellite ground station:

  • 144MHz;
  • terrestrial noise (satellite with omni antenna);
  • IC-9700, assume NF=4.8dB;
  • omni antenna;
  • 10m of LMR-400.

Continue reading Do I ‘need’ a masthead preamp to work satellites on 2m? – terrestrial noise scenario

Last update: 13th September, 2021, 6:12 AM