(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
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
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:
A question asked online recently provides an interesting and common case to explore.
Base scenario is a low end satellite ground station:
Continue reading Do I ‘need’ a masthead preamp to work satellites on 2m? – space noise scenario
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 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.
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 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 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
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
A question was asked recently online:
I am about to measure a 1/4 wave of 450 ohm windowed twinlead for the 2m band using my NanoVNA. My question is, since I will be making an unbalanced to balanced connection, should I use a common mode choke, balun or add ferrites to the coax side to make the connection, or does it really matter at 2m frequencies? The coax lead from my VNA to the twinlead will be about 6″ to 12″ long. I will probably terminate the coax in two short wires to connect to the twinlead.
It is a common enough question and includes some related issues that are worthy of discussion. Continue reading Measuring a 1/4 wave balanced line – nanoVNA