Transmission lines – Page 20

A low Insertion VSWR high Zcm Guanella 1:1 balun for HF – more detail #3

This article expands on the detail behind A low Insertion VSWR high Zcm Guanella 1:1 balun for HF with focus on InsertionVSWR and possible compensation schemes.

A low Insertion VSWR high Zcm Guanella 1:1 balun for HF – more detail #2 discussed the imperfection caused by the quite short pigtails, and although small, it is measurable.

Chris, NX0E, related experience with Dr E M T Jones at TCI where they made, among other things, TCI’s HF baluns. These baluns were compensated using capacitors, and we see that very occasionally in ham grade baluns.

The pigtails can be seen as a short transmission line of higher Zo, and although not uniform, it provides a model for understanding their effect.

Above is a Simsmith model that treats the pigtails as short sections of 300Ω line, the lengths adjusted to calibrate the model to the observed impedance at 30MHz.
Continue reading A low Insertion VSWR high Zcm Guanella 1:1 balun for HF – more detail #3

Last update: 8th March, 2019, 7:07 AM

A low Insertion VSWR high Zcm Guanella 1:1 balun for HF – more detail #2

This article expands on the detail behind A low Insertion VSWR high Zcm Guanella 1:1 balun for HF with focus on InsertionVSWR.

Insertion VSWR is the VSWR looking into the balun with a matched load (termination) on its output, it is a measure of imperfection of the balun. It ought to be a specification item for low Insertion VSWR baluns, but it rarely given.

What is not mentioned in the above definition is the symmetry or balance of the load.

Above is a Smith chart plot of input Z of the balun with an isolated load of 50+j0Ω. Isolated to mean that there is no direct path from either load terminal to ground, it could be seen as a symmetric load with extremely high common mode impedance. All of the external connections use N type connectors with Zo=50Ω.
Continue reading A low Insertion VSWR high Zcm Guanella 1:1 balun for HF – more detail #2

Last update: 15th April, 2021, 7:04 PM

Online expert on coax loss

An online expert opined:

Whether your antenna is a perfect 1:1 or a 10:1, a 50 foot length of coax will have HALF the loss of the exact same coax on the exact same antenna system as measured with the 100 foot piece.

Is it true? Can we learn from it?

Let’s take a worked example of Belden 8259 (RG58A/U) with a load of 5+j0Ω at 146MHz. VSWR is approximately 10. Continue reading Online expert on coax loss

Last update: 24th February, 2019, 4:49 AM

Voltage symmetry of practical Ruthroff 1:1 baluns

Well, I guess Voltage symmetry of practical Ruthroff 4:1 baluns begs the question, what about Ruthroff 1:1 voltage baluns?

The Ruthroff 1:1 voltage balun can be seen as two back to back Ruthroff 4:1 voltage baluns with the redundant winding removed… and that prompts the thinking that the cascade of two baluns back to front might cancel the phase delay.

Let’s measure a popular Ruthroff 1:1 voltage balun.

RAK BL-50A

Above, the RAK BL-50A was a quite popular balun, and probably the balun of choice for half wave dipoles… well until the message about current baluns escaped. Continue reading Voltage symmetry of practical Ruthroff 1:1 baluns

Last update: 21st February, 2019, 6:12 PM

Voltage symmetry of practical Ruthroff 4:1 baluns

Much is written about antenna system balance, this article looks at balance issues with the very common ATU configuration that uses a Ruthroff 4:1 voltage balun to adapt coax transmitter output to two wire open feed line. This type of balun is employed in most ham market ATUs that contain an integral balun.

Above is Ruthroff’s equivalent circuit, Fig 3 from his paper (Ruthroff 1959).

If one looks carefully at the transmission line form, there is effectively a two wire line wound into a helix (usually on a magnetic core) and connected from the unbalanced source to one half of the load inverting the connection for the necessary phase reversal.

Ideally, Vout of this line is equal to Vin, ie Vout/Vin should be 1∠0°. That is unlikely as it implies a zero length transmission line which provides the decoupling of the phase inverting line.

This article looks at the Ruthroff 4:1 balun balance using the very popular MFJ-949E as an example.

Above is a pic of the MFJ-949E Ruthroff 4:1 balun. The transmission line is not uniform, but let’s make an approximation to predict its behavior with a centre tapped 100Ω load, the centre of which is connected to the ground terminal. Continue reading Voltage symmetry of practical Ruthroff 4:1 baluns

Last update: 9th March, 2019, 6:52 AM

Equivalent circuit of an antenna system

Common practice is to treat antenna systems as a two terminal device in free space.

Pickup most handbooks, and even text books, and antennas and often antenna systems are described in this way.

That model is quite inadequate for many or most antenna systems installed in proximity of natural ground. For example, a two terminal dipole and feed line system representation cannot have feed line common mode current, and it follows that thinking in terms of two terminal models denies a full understanding of the antenna system.

A three terminal model of an antenna system

(Schmidt nd) sets out a three terminal model of an antenna system in presence of ground using quite conventional linear circuit theory.

Above is Schmidt’s Y network based on values of three intermediate impedances, ZD, ZU, and ZC. These are found from measured values Za, Zb and ZC as explained by Schmidt: Continue reading Equivalent circuit of an antenna system

Last update: 9th May, 2019, 8:45 AM

AIM 916 produces internally inconsistent results

AIM915a was recently pulled from the distribution site and replaced by a new release, AIM916.

AIM916 chokes on some calibration files created with earlier versions, so again historical scan data is rendered worthless. Note the illogical diagnostic message… typical AIM quality.

I cannot recall ever finding a new release that did not have significant defects, commonly inconsistency between displayed values. In the common theme of one step forward, two steps backwards, this version has defects that were not present in AIM910B.

This problem existed in AIM915a, it persists in AIM916.

Let’s review the internal consistency of this part of the display screen.

Most of the values given above are calculated from a single measurement value, and should be internally consistent. That measurement value is translated to different quantities, many based on the stated Zref (50Ω in this case). Continue reading AIM 916 produces internally inconsistent results

Last update: 17th February, 2019, 12:49 AM

Radcom Feb 2019 “cable balun” – a deeper look

This article is a continuing discussion of Radcom Feb 2019 “cable balun”.

The article Baluns in antenna systems explores some different dipole and feed line configurations and the effectiveness of common mode chokes at various locations on the feed line.

Models 1, 2 and 3 particularly show the effect of a quarter wave vertical common mode conductor grounded and isolated, and a half wave vertical common mode conductor grounded.

These illustrate that those common mode conductors can be viewed to some extent as a ‘single wire’ transmission line, and the impedance presented at the dipole feed point is low or high in keeping with simple transmission line analysis of a shorted or open line of quarter or half wave length.

The question then arises with the Radcom “cable balun”, does it behave similarly, to what extent does the folding of the conductor affect its quarter wave resonance.

NEC models

One way to explore this is to construct an NEC model of the structure and a reflection of itself.

Three quarter wavelength a side folded

Above is the serpentine structure of three quarter wavelength folded, and below it, a reflection of itself. The whole structure is fed in the middle and the impedance vs frequency charted. Continue reading Radcom Feb 2019 “cable balun” – a deeper look

Last update: 25th January, 2019, 5:10 AM

Designing a Gamma Match – confirmation of as-built antennas

Much is written about the virtue of the Gamma Match, and near as much about how they work, and the difficulty in design and implementation.

Designing a Gamma match using a Smith chart showed a design method for a simple Gamma Match using a Smith chart as the design tool.

This article visits the implementation on a pair of antennas that I built 50 years ago, and are still in use today (albeit with some small preventative maintenance once during that interval). The basic antenna is a four element Yagi for 144MHz copied from an ARRL handbook of the time, probably based on NBS 688. It was designed to deliver a split dipole feed point impedance of 50+j0Ω.

I built them using a Gamma Match, partly to get some familiarity, but mostly to implement a Gamma Match that was reliable, weatherproof and lasting… features that are alien to most implementations I had seen at that point.

Both antennas were constructed and the Gamma Match adjusted for VSWR<1.1 using a Bird 43 directional wattmeter. The dimensions of each (including the key gamma dimensions) are the same, not surprising, but a confirmation of repeatability. See Novel Gamma Match Construction for more discussion.

Above is a dimensioned drawing of the construction. Continue reading Designing a Gamma Match – confirmation of as-built antennas

Last update: 18th January, 2019, 12:15 PM

Radcom Feb 2019 “cable balun”

A correspondent asked for an explanation of a novel balun described in Radcom Feb 2019 by K3MT.

Above is a diagram of the so-called “cable balun”.

To evaluate it, I have inserted it into one of the NEC models used for the article Baluns in antenna systems, the model used for Model 3 which I will repeat here for convenience. Continue reading Radcom Feb 2019 “cable balun”

Last update: 13th February, 2020, 6:13 AM