SND implications of variations on the untuned small loop at MF/lowHF

Three recent articles developed an explanation of the YouLoop-2T at MF/lowHF:

The first and third articles explained the concept of signal/noise degradation (SND) statistic, and gave graphs of the behavior of the subject antennas.

This article draws together those SND plots for two antennas, and some variations to the configurations.

Configurations

Simple loop with transformer

Above, the “simple loop” with 0.5:1 ideal transformer. It could be implemented as a shielded loop (with transformer) with similar behavior (but improved common mode suppression). Continue reading SND implications of variations on the untuned small loop at MF/lowHF

Towards understanding the YouLoop-2T at MF/lowHF

Small untuned loop for receiving – simple model with transformer gave a simple model for analysing a loop. If you haven’t already read it, you should. It provides a step towards understanding the YouLoop-2T at frequencies where is is a small loop (perimeter<λ/10).

Above is the Airspy Youloup-2T. Try to put the two turns thing out of your mind, it is misleading, panders to some common misunderstanding, and so does not help understanding.

This is somewhat similar to the simple loop, but now the transformer primary is connected to the loop gap terminals by two parallel sections of 50Ω transmission line, the combination being effectively a 100Ω with similar parameters to the component coax sections. Because of the series connection at the transformer and parallel connection at the loop gap, there is a 1:4 impedance transformation additional to that of the coax sections themselves. Continue reading Towards understanding the YouLoop-2T at MF/lowHF

A transmission line 1:4 impedance transformer

This article explains the operation of a simple nominally 1:4 impedance transformer using transmission line (TL) elements.

Above is a diagram of the device. The currents shown are differential currents in the coax (ie wholly inside the coax), the current on the outside of the shield is not shown on the diagram.

At very low frequencies it may be intuitive that \(V_1\approx V_2\) and \(I_1\approx I_2\), but as frequency increases, a more exact solution is needed. Continue reading A transmission line 1:4 impedance transformer

Small untuned loop for receiving – simple model with transformer

I have written several articles on untuned loops for receiving, as have others. A diversity of opinions abounds over several aspects, probably none more than the idea of an optimal load impedance for the loop.

This article analyses a simple untuned / unmatched loop in the context of a linear receive system (ie no IMD) of known Noise Figure. Continue reading Small untuned loop for receiving – simple model with transformer

nanoVNA-H – Port 1 attenuator for improved what???

At nanoVNA-H – Port 2 attenuator for improved Return Loss I explained the reasons for essentially permanent attachment of a 10dB attenuator to Port 2 (Ch 1 in nanoVNA speak).

 

Above, the 10dB attenuator is semi permanently attached to Port 2 principally to improve the Return Loss (or impedance match) of Port 2, a parameter that becomes quite important when testing some types of networks than depend on proper termination (eg many filters). I should remind readers that the improvement in Port 2 Return Loss comes at a cost, the dynamic range of Port 2 is reduced by 10dB. Continue reading nanoVNA-H – Port 1 attenuator for improved what???

nanoVNA-H – woolly thinking on MLL measurement

There is little doubt that the nanoVNA has made VNAs very popular in the ham community, possibly more so that any other device.

Eager owners are trying to apply them to solve lots of problems, often without sufficient knowledge or experience to properly inform the measurements.

An example that has a appeared a few times on online forums in the last weeks is measuring the matched line loss (MLL) of a section of RG6 coax… to inform a decision to discard it or keep it.

The common approach is to use a measurement of |s11| and to calculate Return Loss and infer the MLL.

DUT

For discussion, lets consider an example of 30′ of Belden 1694A RG6 solved in Simsmith. We should note that unlike most RG6 in the market today, this uses a solid copper centre conductor.

Short circuit termination

Some authors insist that the half return loss method is to be performed using a short circuit test section. Bird does this in their Bird 43 manual.

Above is a plot of calculated |s11| (-ReturnLoss) from 1 to 20MHz for the test section. The three plots are of |s11| wrt 50Ω, 75Ω and frequency dependent actual Zo (as calculated for the model). The cursor shows that the actual |s11| is -0.37474dB (ReturnLoss=0.37474dB). Using the half return loss method MLL=ReturnLoss/2=0.37474=0.187dB/m. Continue reading nanoVNA-H – woolly thinking on MLL measurement

nanoVNA-H – thinking laterally

A question was asked in an online forum specific to nanoVNA as to how the use the nanoVNA to check the attenuation loss in some old & weathered RG-6 (75 ohm) cables for the TV signal frequencies. Excuse the term attenuation loss, lets assume the poster is asking for matched line loss (MLL).

The assembled experts are offering solutions to transform the ports to 75Ω and make a measurement, deducting the loss of the transformation (minimum loss pads were suggested).

There is a very simple solution that should be quite practical for the scenario described. Let’s work through two examples using 35.5m of unbranded quad shield RG6 with CCS centre conductor (of unknown quality) for the DUT. Continue reading nanoVNA-H – thinking laterally

MFJ ATU hand effects on capacitor knobs

The problem

Users of some ATUs may have noticed particular sensitivity to hands on the capacitor adjustment knobs. It is a common problem with cheap implementations of the T match as the capacitor rotor is usually at high RF voltage and if that shaft is extended to the adjustment knob, under certain circumstances tuning becomes very sensitive to hands on the knobs.

In some of these implementations, if the users hand touches the metal grub screw in the knob, or the metal panel bushing behind the knob they may get a significant RF burn.

The cause

Let’s use the MFJ-949E as a discussion example. It is a T match, and the metal capacitor shafts in the knobs and panel bushings carry RF voltages.

So why is this only sometimes a problem?

The RF voltage across the coil, and impressed on the capacitor shafts can be extremely high when using loads with small resistance and large negative reactance, more so on the lower bands. Continue reading MFJ ATU hand effects on capacitor knobs

nanoVNA-H – de-embedding the feed line in remote measurement

There are often times when it would be useful to transform measurements made looking into a feed line to the other end of the feed line.

Ham lore

Common advice given by online ham experts include:

  1. it just cannot be done, the best (only) point to measure an antenna is at the feed point;
  2. it can be done, but only with an integral number of half waves of feed line;
  3. use the port extension facility in your software;
  4. use software package x;
  5. do an OSL cal with the feed line being part of the fixture.

Continue reading nanoVNA-H – de-embedding the feed line in remote measurement