Category: Transmission lines

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.

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: 18th September, 2021, 8:16 AM

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

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: 14th September, 2021, 8:57 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

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.

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

Last update: 15th September, 2021, 7:04 PM

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

Last update: 8th September, 2021, 7:45 AM

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

Last update: 8th September, 2021, 6:31 AM

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

Last update: 7th September, 2021, 7:16 AM

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?

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

Last update: 6th September, 2021, 8:58 AM