## Differential and common mode components of current in a two wire transmission line

A pair of conductors in proximity of some other conductors or conducting surface (such as the natural ground) can operate in two modes simultaneously, differential mode and common mode.

Differential mode is where energy is transferred due to fields between the two conductors forming the pair, and common mode is where energy is transferred due to fields between the two conductors forming the pair together and another conductor or conducting surface.

The currents flowing in the two conductors at any point can be decomposed into the differential and common mode currents.

Differential current Id is the component that is equal but opposite in direction, it is half the difference in the two complex line currents I1 and I2.

Common mode current Ic is the component of the line currents common to both conductors, it is half the sum of I1 and I2.

• Id=(I1-I2)/2
• Ic=(I1+I2)/2
• I1=Ic+Id
• I2=Ic-Id

So, for example, if I1=2A and I2=-1A, Id=(2–1)/2=1.5A, Ic=(2-1)/2=0.5A.

A line that is operating with perfect current balance has only differential current, ie common mode current is zero. It is unlikely that a feed line in a practical antenna system is perfectly balanced, but with due care, it can have very low common mode current, 20dB or more less than the differential component.

## Folding back the ends of a wire dipole

A correspondent asked about the effect of folding back the ends of a wire dipole. Above, a diagram of the scenario discussed in this article. The dipole of length L1 has its ends turned back by a length of L2.
Continue reading Folding back the ends of a wire dipole

## Using the AIM to measure matched line loss

A correspondent wrote seeking explanation of difficulty he was having measuring line loss using the advice given in the AIM manual using a scan with either O/C or S/C termination:

Note the one-way cable loss is numerically equal to one-half of the return loss. The return loss is the loss that the signal experiences in two passes, down and back along the open cable.

Because my correspondent was using one of the versions of AIM that I know to be unreliable, I have repeated the measurements on a cable at hand using AIM_900B to demonstrate the situation.

The test cable I have used is 10m of RG58C/U which I expect should have matched line loss (MLL) of 0.26dB, but I expect this to be a little worse as it is a budget grade cable that I have measured worse in the past. Continue reading Using the AIM to measure matched line loss

## Antenna span tensioner using a counterweighted halyard

Correspondents have asked about application of the technique used in Antenna span spring tensioner using Antenna wire catenary calculator to a span tensioned with a counterweighted halyard.

The scenarios bear a lot of similarity, the main difference being that the tension from the counterweight is constant up to the point that the counterweight travel reaches its limit.

The tension applied is the weight force of the counterweight with a little increase to allow for friction in the sheave block.

So, lets say the scenario is a 42m wire plus halyard that adds 1m to the span under no wind conditions and can pay out a further 1m at which it reaches its limit. Lets say the counterweight is 5kg weight so 49.0N tension. Continue reading Antenna span tensioner using a counterweighted halyard

## Antenna span spring tensioner

A correspondent asked about application of the Antenna wire catenary calculator to a scenario with a spring tensioner in a simple span.

His proposed tensioner has a maximum length of 2.6m at a tension of 445N, and the intention is to tension the span with no wind loading to 178N at which the tensioner is 1.92m long. Minimum GBS of the tensioner is 1560N, WLL=456N (ie the tensioner is specified that with safety factor 3.5, it reaches its working load limit at full extension).

The following is a simple analysis that assumes the fixed supports are equal height, the tensioner has the same m/l as the wire, and 2mm (#12) 30% Copperweld is used for the wire which is 42m long, so the distance between supports is taken to be 45m. Continue reading Antenna span spring tensioner

There are applications where you might want to make the tuning of a wire dipole adjustable.

Adjusting the length is often not convenient, especially in-service tuning which might be triggered by changing vegetation, ground moisture etc.

This article shows some simple means of attachment of a small capacitive load to deploy near the high charge points, and to adjust their effect by moving them to or fro on the main dipole wire. Continue reading Capacitive loading device for fine tuning wire dipoles

## VK3IL’s 3m circumference LDF4-50B loop on 20m

David, VK3IL, describes a small transmitting loop (STL) at Portable magnetic loop antenna.

At VK3IL’s 3m circumference LDF4-50B loop on 40m. I reviewed his loop behaviour on 40m, and its efficiency was quite low… though typical of a loop of that size at that frequency.

Radiation resistance of a STL is proportional to the fourth power of frequency, and since it is often dwarfed by loss resistance, we should expect that doubling frequency will dramatically improve performance.

As far as I can glean from the article, it is made from a 3m length of LDF4-50B Heliax, and uses a Patterson match to tune it. David offered measurement of VSWR around centre frequency for the loop approximately matched (VSWR=1.24) on 20m. He has measured the VSWR=2.86 bandwidth shown between markers 2 and 3 to be 45kHz. Continue reading VK3IL’s 3m circumference LDF4-50B loop on 20m

## Enhancement of Calculate small transmitting loop gain from bandwidth measurement

At Efficiency and gain of Small Transmitting Loops (STL) I explained an approach to assessing the gain the efficiency of STL, and provided a link to a calculator to perform the calcs.

This expands on application of the concepts and introduces an enhanced calculator to perform the calculations. Firstly, this technique applies to antennas where the VSWR characteristic is consistent with a feed point or virtual feed point where around the frequency of minimum VSWR, X varies with frequency much more than R. The simplified analysis assumes that R is constant, and change in X is the reason for the VSWR characteristic. See VSWR curve of a simple series resonant antenna for more information. Continue reading Enhancement of Calculate small transmitting loop gain from bandwidth measurement

## VK3IL’s 3m circumference LDF4-50B loop on 40m

David, VK3IL, describes a small transmitting loop (STL) at Portable magnetic loop antenna.

As far as I can glean from the article, it is made from a 3m length of LDF4-50B Heliax, and uses a Patterson match to tune it. David offered measurement of VSWR around centre frequency for the loop matched on 40m. He has measured the VSWR=2.6 (the half power) bandwidth shown between markers 2 and 3 to be 22kHz. Continue reading VK3IL’s 3m circumference LDF4-50B loop on 40m

## VK3YE’s 3m circumference copper tube loop with RG213 stub tuning

Peter, VK3YE, describes a small transmitting loop (STL) in his video at https://www.youtube.com/watch?v=Cv_RnLpZ9gw. As far as I can glean from the video, it is made from a 3m length of copper tube 19mm diameter, and uses about 1.8m of RG213 to tune it, and appears to have its centre 0.7m above ‘ground’ .

Let us firstly look at a free space model of the antenna using Reg Edwards’ RJELoop1 tool. This model has its limitations, but the calculated inductance is of interest. We can calculate the inductive reactance to be 118Ω. The capacitive stub of RG213 will need around 107Ω reactance, and solving for RG213, we find that 1.94m gives 0.19-j107Ω. The resistive component is important as it is ignored by the above model. The stub resistance is a loss resistance, and we need to recalculate the efficiency. Efficiency=Rrad/Rloss=0.005/(0.19+0.0351+0.005)=2.17% (-16.6dB). We can also calculate the Q as 107/(0.19+0.0351+0.005)=465 and half power bandwidth as 7100/497=15.3kHz. Continue reading VK3YE’s 3m circumference copper tube loop with RG213 stub tuning