Differences in two similar simple untuned small loop configurations

A correspondent asked about the difference between two small untune loops mentioned in two of my articles, this article explains.

Firstly lets set the context, a small loop means less than λ/10 perimeter, and untuned is to mean that the loop is loaded directly, in this case by a receiver which we will assume has an input impedance of 50+j0Ω.

Let’s look at the two cases. The key difference is in the connection at the gap:

  • the first has a short circuit coaxial stub of half the perimeter between the inner conductor at the right side of the gap and the outer surface of the outer conductor at the right side of the gap; and
  • The second directly connects the inner conductor at the right side of the gap and the outer surface of the outer conductor at the right side of the gap.

Small single turn un-tuned shielded loop

Above is a diagram of the loop. Continue reading Differences in two similar simple untuned small loop configurations

Shorting winding sections of a ferrite cored EFHW transformer

A chap recently posted some advice on construction of a dual ratio transformer for EFFHW antennas, advice with an informative pic, but without measurement evidence that it works well.

Pictured is a dual UnUn. I made this for experimenting. It’s both a 49 and 64 to 1 UnUn.

The 49 to 1 tap uses the SS eye bolt for the feed through electrical connection and the SS machine screw on the top is the 64 to 1 connection. If I want to use the 49 to 1 ratio, there’s a jumper on the eye bolt that connects to the top machine screw where the antenna wire is attached. The jumper shorts out the last two turns of the UnUn. Disconnect the jumper from the top connection and now you have a 64 to 1 ratio.

Continue reading Shorting winding sections of a ferrite cored EFHW transformer

N6THN’s novel balun – flux leakage

N6THN’s novel balun presented measurement of the Insertion VSWR of the subject balun, and N6THN’s novel balun – an explanation gave explanation that included mention of flux leakage as a contributor to the quite high inductance per unit length of the transmission line formed by the two windings.

A correspondent suggested that with a ferrite core, flux leakage is insignificant. This article calculates the coupled coils scenario.

The balun as described

Above is the ‘schematic’ of the balun. Note the entire path from rig to dipole. Continue reading N6THN’s novel balun – flux leakage

N6THN’s novel balun

One sees lots of articles and videos on how to make a current balun suited to a low VSWR antenna. This one was recommended in an online discussion on QRZ.com. N6THN might not have invented this balun, but he made a video of it.

In this case, it is described in the referenced video as part of a half wave dipole antenna where you might expect the minimum feed point VSWR to be less than 2.

Apologies for the images, some are taken from the video and they are not good… but bear with me.

The balun as described

Above is the ‘schematic’ of the balun.Note the entire path from rig to dipole. Continue reading N6THN’s novel balun

Comment on KN5L on balun CMRR – series through impedance fixture

In recent articles, I flagged that on some of John’s VNWA plots he showed flawed impedance calculations using VNWA’s t2s inbuilt function.

The function t2s is documented in the VNWA help.

t2s is a VNWA built in function intended to solve the so-called s21 series through fixture for impedance measurement of two terminal Zx connected between Port 1 and Port 2.

None of John’s test fixtures were equivalent to the circuit above required for valid t2s transformation. Continue reading Comment on KN5L on balun CMRR – series through impedance fixture

Comment on KN5L on balun CMRR – two wire line example

The article Comment on KN5L on balun CMRR dealt with model and measurement of John’s coaxial choke in fixture, dealt with first because it is a simpler model. This article builds on that and models the balun wound with a pair of wires.

Above is the subject balun in fixture.

John’s schematic shows the balun as coupled coils, but that does not capture the transmission line transformation that occurs in the actual device. Again the test fixture is used without explanation. Continue reading Comment on KN5L on balun CMRR – two wire line example

Comment on KN5L on balun CMRR – coax example

One of the ham fashions of proposed solutions to characterising a balun is to find the Common Mode Rejection Ratio (a term carried over from other applications, eg voltage driven operational amplifiers).

(Anaren 2005) explains a method of finding balun CMRR. Anaren gives a definition of CMRR:

Common Mode Rejection Ratio is defined and the ratio between the differential mode insertion loss/gain versus the common mode signal loss or gain.

Note that in a passive system, CMRR (or CMR) in dB will usually be positive, and the larger the better. You might even think that the plain English meaning of the words Common Mode Rejection Ratio would suggest that a large ratio (or high +ve dB value) would mean most rejection, goodness. Such a meaning would be quite consistent with that of CMRR (CMR) applied to operation amplifiers (for a very long time).

Anaren does not mention applying the CMRR statistic to antenna systems. I have commented elsewhere on the lack of utility of CMRR in analysing common antenna systems.

Then immediately after the above definition, they give a formula which implies the inverse:

\(CMRR=\frac{S_{1c}}{S_{1d}}\) and goodness would be a tiny fractional value, or a small (-ve) dB value.

John, KN5L, has published his own solution to balun characterisation in some online forums. Continue reading Comment on KN5L on balun CMRR – coax example

Transmission line filter for a field day station – implementation

Transmission line filter for a field day station – designs laid out some designs for a transmission line filter for harmonic reduction of a field day station on 7MHz. This article describes Bruce’s, VK4MQ, implementation of the “two stubs are better than one” option. Huber+Suhner RG214 coax was used.

Firstly two quarter wavelengths OC stubs were tuned to 14.2MHz by iterative cut and measure. The coax was 20mm longer than prediction, I am not convinced that the transmission line models in Simsmith are better than that. Then the tees were made up and the connecting section and tuned by cut and measure for minimum |s11| at 7.1MHz.

Above is the VNA sweep for the completed filter. Rejection around 14.2MHz exceeds 50dB with bandwidth of over 0.6MHz. Continue reading Transmission line filter for a field day station – implementation