Signal to noise degradation (SND) concept

The nature of radio signals received off-air is that they are accompanied by undesired noise.

A key measure of the ability to decode a radio signal is its Signal to Noise ratio (S/N) at the demodulator (or referred to some common point).

We can speak of think of an external S/N figure as \(S/N_{ext}=10 log\frac{S_{ext}}{N_{ext}}\) in dB.

Receiver systems are not perfect, and one of the imperfections is that they contribute undesired noise. Continue reading Signal to noise degradation (SND) concept

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

A common scheme for narrow band match of an end fed high Z antenna – surely it is a 1:9 transformer?

A reader of A common scheme for narrow band match of an end fed high Z antenna commented:

…if the coil is tapped at 1/3, surely then the coil is a 1:3^2 or 1:9 transformer and the capacitor simply ‘tunes out’ the coil reactance, what is the input impedance when it has a 450+j0Ω load?

That is very easy to calculate in the existing Simsmith model.

Above, with load of 450+j0Ω, the input impedance at 50MHz is 8.78+j34.36Ω (VSWR(50)=8.4), nothing like 50+j0Ω. Continue reading A common scheme for narrow band match of an end fed high Z antenna – surely it is a 1:9 transformer?

A common scheme for narrow band match of an end fed high Z antenna

This article discusses the kind of matching network in the following figure.

A common variant shows not capacitor… but for most loads, the capacitance is essential to its operation, even if it is incidental to the inductor or as often the case, supplied by the mounting arrangement of a vertical radiator tube to the mast. Continue reading A common scheme for narrow band match of an end fed high Z antenna

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