Design / build project: Guanella 1:1 ‘tuner balun for HF’ – #5

Fifth part in the series documenting the design and build of a Guanella 1:1 (current) balun for use on HF with wire antennas and an ATU.

Installation / testing

AtuBalun201

The balun packaged in a non-conductive housing was designed to have minimal stray capacitance to ground to minimise common mode current with asymmetric loads.

AEP01

Above, the balun is attached to the exterior side of the antenna feed entrance panel using a male to male N adapter, done up very tight. The feed line connections are liberally coated with marine grease to prevent ingress of water and oxygen, a measure to reduce corrosion. Continue reading Design / build project: Guanella 1:1 ‘tuner balun for HF’ – #5

Improved cooling for the MFJ-949E

vt_00032

At A look at internal losses in a typical ATU I demonstrated that it is quite easy to raise the temperature of the coil in the MFJ-949E to an unsafe level, even with quite modest power.

The most heat sensitive component in this ATU is the coil, specifically the coil supports which are probably polystyrene, and the glass transition temperature of polystyrene is around 100°.

This article documents modification of my MFJ-949E to reduce the risk of damage under some operating conditions. Continue reading Improved cooling for the MFJ-949E

SPICE model of Zcm of a Guanella 9:1 current balun

At Why the preference for Guanella 1:1 current baluns for HF wire antennas I compared a 1:1 Guanella balun with a 4:1 using the same component baluns.

Broadly, the findings were that the common mode impedance of the 4:1 balun was around a quarter of that of the component baluns, and mediocre at that.

This article extends the reference, documenting the SPICE model extended to a 9:1 balun.

Screenshot - 07_03_16 , 11_17_27

Above, the same component baluns were interconnected to make a 9:1 balun. Note that Zo of the TL section has been increased to 150Ω to suit the notional nominal 50 to 450Ω broadband transformer. Continue reading SPICE model of Zcm of a Guanella 9:1 current balun

Rigexpert’s Antscope takes a step backwards

AT Measuring balun common mode impedance – #1 I gave an example of the use of a Rigexpert AA-600 to measure the common mode impedance of a current balun.

Screenshot - 31_01_2015 , 06_08_57

Above is a plot from that article. I cannot be sure what version of Antscope was used to create the graph, but it was no later than v4.2.57, as one of the ‘improvements’ of v4.2.62 and v4.2.63 was to reduce zooming of the Z scales to a maximum of 600Ω. Continue reading Rigexpert’s Antscope takes a step backwards

Design / build project: Guanella 1:1 ‘tuner balun for HF’ – #4

Fourth part in the series documenting the design and build of a Guanella 1:1 (current) balun for use on HF with wire antennas and an ATU.

Packaging

The prototype fits in a range of standard electrical boxes. The one featured here has a gasket seal (a weep hole would be advisable in a permanent outdoor installation).

AtuBalun201

Above, the exterior of the package with M4 brass screw terminals each side for the open wire feed line, and an N(F) connector for the coax connection. N type is chosen as it is waterproof when mated.

AtuBalun203

The interior shows the layout. The wires use XLPE high temperature, high voltage withstand, moderate RF loss insulation. Two short pieces of 25mm electrical conduit serve to position the balun core against the opposite side of the box, and a piece of resilent packing between lid and core holds the assembly in place.

AtuBalun202

Differently to the example shown in the earlier articles, this prototype uses twisted PTFE insulated wires which have voltage breakdown higher than the XLPE shown earlier.

Clip 124

The self resonant frequency of the built balun was measured as 7.4MHz and the predictive model above calibrated. The balun has high choking impedance on the lower half of HF.

Low power Guanella 1:1 tuner balun using a pair of Jaycar LF1260 suppression sleeves

The article describes a current balun intended for use with an ATU at modest power levels. It is lightweight and well suited to portable operations, and can be made with materials readily available in Australia (LF1260 cores are a little over $1 each in packs of six.) Continue reading Low power Guanella 1:1 tuner balun using a pair of Jaycar LF1260 suppression sleeves

Reconciling my #52 choke design tool with G3TXQ’s measurements

A correspondent wrote with concern of the apparent difference between graphs produced by my #52 choke design tool with a graph published by G3TXQ of his measurement of 11t on a pair of stacked FT240-52 cores.

I published a note earlier about my concerns with a similar graph by G3TXQ compared to the Fairrite datasheet, and he reviewed the data, found the error and published a corrected graph.

FT240-52x2-11t

The corrected graph above might at first glance appear different to my model’s graphs, and the first obvious difference is that G3TXQ uses a log Y scale (which is less common). The effect of the log scale is to compress the variation and give the illusion perhaps that in comparison with other plots, this balun has a broader response.

Screenshot - 09_02_16 , 18_29_42

To compare the two, I have roughly digitised G3TXQ’s graph above and plotted the data over that from my own model (with linear Y scale). Continue reading Reconciling my #52 choke design tool with G3TXQ’s measurements

Low Insertion VSWR HF Guanella 1:1 balun for instrumentation

The article describes a current balun intended for measurement use with low power instrumentation. It is lightweight and can be made with materials readily available in Australia (LF1260 cores are a little over $1 each in packs of six). The target application is for a 600mm square small loop for field strength measurement below 15MHz.

Design

The design is an implementation of (Duffy 2007) which used RG174 coax for the choke to give low Insertion VSWR.

The balun is intended for low power measurements and will withstand dissipation of a few watts.

The LF1260 cores are made from a medium µ ferrite and have an ID of 7.8mm.

Screenshot - 12_01_2015 , 20_08_34

Above, the cores will accommodate four round conductors of diameter 3.2mm, so they will comfortable accommodate the four passes of RG174 (2.5mm dia). (For the mathematically minded, the minimum enclosing circle diameter for four equal circles is 1+√2 times the diameter of the smaller circles.)

Implementation

LF1260Balun101Above, the balun cores  are housed in a small Jiffy box with a BNC-F flange mount connector at one end and a pair of M4 screw terminals at the other. Small brass tabs were made as non-rotating terminal tags and the M4 brass screws soldered to them. The cores are attached to each other with a piece of double-sided foam tape to prevent them shattering, and two pieces of the same used to secure the cores to the box. A packing between the cores and lid helps to hold them in place.

As discussed at Baluns – show me the numbers, implementation details without quantitative models or measurement are of little value. Continue reading Low Insertion VSWR HF Guanella 1:1 balun for instrumentation

DK7ZB’s balun

(Steyer nd) describes the DK7ZB balun / match for VHF and UHF Yagis.

unsymm_engl

To understand how the “DK7ZB-Match” works look at the left picture. Inside the coax cable we have two currents I1 and I2 with the same amount but with a phase shift of 180°.

No. At any point along the coaxial line, a current I on the outer surface of the inner conductor causes an equal current in the opposite direction on the inner surface of the outer conductor.

As the currents are shown with the designated directions, I2=I1, not I2=I1<180.

A consequent simplification is that I4=I2-I3=I1-I3.

There is an issue with the current arrow I3 in the lower right of the diagram. It might imply that the only current in the conductors is I3, but the current between the nearby node and lower end of the shield is I3-I1.

If the structure was much much shorter than the wavelength, there would be negligible phase change in currents along the structure, so I1 would be uniform along the centre conductor, I2 uniform along the inside surface of the outer conductor, and I3 uniform along the outer surface of the outer conductor.

The diagram notation does show that I3 (which is equal to the dipole drive imbalance) is uniform along the structure, and that I3 flows to ground.

DK7ZB goes on:

If we connect a dipole or the radiator of a Yagi direct to the coax, a part of I2 is not running to the arm 2 but down the outer part of the coax shield. Therefore I1 and I4 are not in balance and the dipole is fed asymmetric.

But how can we suppress the common-mode current I3? A simple solution is to ground the outer shield in a distance of lambda/4 at the peak of the current.

So, the length of the structure is in fact a quarter wavelength electrically, or close to it to achieve the choking effect. I3 will be in the form of a standing wave with current maximum at the lower (‘grounded’) end, and current minimum at the upper end.

It happens also that his usual configuration of this balun is that there is a standing wave on the inside of the coax, and so I1 and I2 are not uniform along the conductor, and whilst it is relevant to the designed impedance transformation, it is inconsequential to reduction of dipole current imbalance.

DK7ZB continues with the development of his variation of a Pawsey balun:

But now we get a new interesting problem: For the transformation 28/50 Ohm we need a quarterwave piece of coax with an impedance of 37,5 Ohm (2×75 Ohm parallel). The velocity of the wave inside the coax is lower than outside (VF = 0,667 for PE).

The outside of the shield has air (and a litle bit of insulation) in the surrounding and VF = 0,97. For grounding the common mode currents this piece should have a length of 50 cm, with a VF = 0,667 and a length of 34,5 cm this piece of coax is to short. By making a loop of this two cables as shown in the picture down we get an additional inductivity and we come closer to an electrical length of lambda/4. Ideal is coax cable with foam-PE and a VF = 0,82

schleifeAbove is DK7ZB’s implementation of his balun with the loop and additional inductivity.

I copied the above implementation and measured the common mode impedance Zcm.

Dk7zbBalun144

Above is the Zcm measurement. There is a quite narrow self resonance where Zcm is quite high for about 10MHz bandwidth centred on 125MHz, but at 144MHz Zcm=83-j260Ω which is too low to qualify as a good common mode choke.

Like all narrowband / tuned common mode chokes, tuning to the desired frequency band is essential to their effective operation.

Like most published balun designs, this one is published without measurements to demonstrate its operation or effectiveness.

Links

Differential flux leakage in a Guanella 1:1 balun – an experiment

The article reports a simple experiment on the balun described at Low power Guanella 1:1 tuner balun using a pair of Jaycar LF1260 suppression sleeves to assess the loss with near zero common mode current.

This test would not subject dielectrics to high electric field strength.

LF1260x2-01

The balun above had the two wires at one end connected together, and a current of 1.41A at 7MHz passed between the terminals of the device at the other end.

The device so configured looks like a s/c transmission line stub and we would expect that the input impedance would be a very small resistance and small inductive reactance. Continue reading Differential flux leakage in a Guanella 1:1 balun – an experiment