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

A question asked online recently provides an interesting and common case to explore.

Base scenario is a low end satellite ground station:

• 144MHz;
• satellite;
• IC-9700, assume NF=4.8dB;
• high gain (narrow beamwidth antenna);
• 10m of LMR-400.

## MFJ-261 – review of review

A recent review of the MFJ-261 (Bogard 2021) was interesting.

From MFJ’s web site listing:

Connects directly to the transmitter with PL-259 connector. No patch cable used, reduces SWR. Finned aluminum, air-cooled heatsink. Handles 100 Watts peak, 15 Watts average. 50 Ohms. Covers DC to 500 MHz with less than 1.15:1 SWR. 1 ⅝” round by 3″ long.

That is pretty stunning for a device with a UHF connector, more on that later. Continue reading MFJ-261 – review of review

## On testing coax cable loss with an analyser / VNA – part 4

On testing coax cable loss with an analyser / VNA – part 3 drilled down on a better method of approximating the matched line loss (MLL) of a section of transmission line based on measurements of ReturnLoss with the section terminated in both an open circuit and short circuit. This article goes a little further using the saved measurement files to answer a reader’s questions.

## Measurement vs TLLC & TLDetails

TLLC and TLDetails are two line loss calculators, and they use quite different predictive models.

Above is the calculation results from TLLC. for the 10m section of Belden 8267 (RG213) with short circuit termination. Note the calculated loss model coefficients k1 and k2 which will be used in a later graph. Continue reading On testing coax cable loss with an analyser / VNA – part 4

## On testing coax cable loss with an analyser / VNA – part 3

On testing coax cable loss with an analyser / VNA – part 2 gave a method of approximating the matched line loss (MLL) of a section of transmission line based on measurements of ReturnLoss with the section terminated in both an open circuit and short circuit. The article demonstrated the method using TLLC to provide expected measurement values.

So, does it work in practice?

Let’s measure a 10m length of Belden 8267 (RG-213) fitted with N connectors using a Rigexpert AA-600 and an instrument grade N(F) short circuit.

ReturnLoss @ 3.5MHz is 0.15dB. Continue reading On testing coax cable loss with an analyser / VNA – part 3

## On testing coax cable loss with an analyser / VNA – part 2

On testing coax cable loss with an analyser / VNA – part 1 questioned a common method of measuring Matched Line Loss (MLL) of a section of an open circuit transmission line section, posing the questions:

The example gives MLL’ (based on half ReturnLoss) of about two thirds cable MLL.

• Why is that?

• What does it say for the measurement technique?

## On testing coax cable loss with an analyser / VNA – part 1

A recent online video provides instruction on how to measure loss of a section of coax cable, loss to mean Matched Line Loss, $$MLL=\frac{P_{in}}{P_{out}}$$ when the cable is terminated in its characteristic impedance Zo, and which can be expressed in db as $$MLL=10 log_{10} \frac{P_{in}}{P_{out}}$$. Note that MLL in dB is ALWAYS a +ve value for a passive DUT such as this.

There is nothing new in the method, it appears in lots of analyser user manuals, and has a built in assist in many analysers.

The video deals with the case of an antenna analyser that has a ‘measure cable loss’ function and using a VNA. Lets use the VNA graphic as it shows more detail of what is happening.

Above is the video’s graphic for the case. The narration says to use the dB magnitude of s11 or ReturnLoss as equivalents. They aren’t equivalent (a hammy Sammy thing), $$ReturnLoss=-20 log_{10}|s11|$$ or $$ReturnLoss_{dB}=-s11mag_{dB}$$ (both wrt the VNA reference impedance). Continue reading On testing coax cable loss with an analyser / VNA – part 1

## Review of inexpensive Chinese thermostat – DST1020

The DST1020 targets the market for inexpensive digital thermostats, the most popular being the STC1000. There are two novel features to the DST1020:

• uses DS1820B digital temperature sensor (well probably a Chinese clone);
• two line display shows PV and SV simultaneously.

The DS1820B should be considerably more accurate and overcome the significant error in the conversion of NTC resistance to temperature in the other thermostats (experience is that the approximation used for the NTC characteristic is simple and inaccurate).

It is sold with brief and inadequate / incorrect user instructions.

Above, the front of the DST1020. Continue reading Review of inexpensive Chinese thermostat – DST1020

## Small 2-stroke engines and popoff pressure

This article explores the physics of fuel metering in a typical small diaphragm carburettor (carburetor to some) as used on small 2-stroke chainsaws, leaf blowers, brushcutters etc.

The discussion following is in terms of absolute pressure, and it is assumed that atmospheric pressure is 100kPa. ALL pressures are absolute unless stated otherwise, to find gauge pressure, subtract 100kPa.

Above from Zama is a cross section view of the metering chamber of a typical butterfly carburettor. The metering lever pivots on an axle (1), the distance from the axle to the needle (2) is 3.5mm, to the spring (3) is 3.5mm, and to the contact to the metering diaphragm (4) is 8mm. The needle seat is 0.55mm diameter. Continue reading Small 2-stroke engines and popoff pressure

## Small engines and green fraud

Recent weeks have seen some catch up work on maintenance of small engine yard equipment. There are 22 engines in all, 6 4-stroke and 16 2-stroke.

For some years, ‘green’ measures implemented by government meant that ordinary unleaded petrol (ULP) was not available retail, one had to use E10 (ULP with 10% Ethanol).

Greens claimed that at such low Ethanol, that the fuel was compatible with all existing and new engine equipment.

## Diaphragm carburettors

Most of my 2-stroke small engines use so called diaphragm carburettors. These ‘all position’ carburettors are common on yard equipment like brushcutters, leaf blowers, chainsaws etc.

Above is a Chinese after market clone of a Zama ‘butterfly’ type (note the throttle butterfly) diaphragm carburettor that suits a Stihl BG85 and similar leaf blower. (This cost \$15 inc shipping on Aliexpress.) Continue reading Small engines and green fraud