Category: Transmission lines

An improved simple Simsmith model for exploration of a common EFHW transformer designs (v1.03)

The article A simple Simsmith model for exploration of a common EFHW transformer design – 2t:14t proposed a simple model.

The previous proposal

Above is the equivalent circuit used to model the transformer. The transformer is replaced with an ideal 1:n transformer, and all secondary side values are referred to the primary side.

The model works quite well for low leakage inductance / low ratio transformers but falls down for the higher leakage inductance / higher ratio transformers.

An improved model

The improved model is similar, but Cse in the model above is distributed to the outer sides of the lumped constant model.

Above is the equivalent circuit used to model the transformer. The transformer is replaced with an ideal 1:n transformer, and all secondary side values are referred to the primary side. Continue reading An improved simple Simsmith model for exploration of a common EFHW transformer designs (v1.03)

Last update: 13th November, 2022, 3:18 PM

Modelling an antenna as a simple two terminal resistance is often naive

in the article A simple transformer model of the Guanella 1:4 balun – some further observations I stated:

Note that a two terminal impedance is a naive representation of many if not most antennas, popular, but a naive over simplification that does not facilitate evaluation of current balance.

An example was a recent posting above that used the model to make assertions about the behaviour of a Guanella 1:4 balun.

This article reports results of two experiments with NEC to model an ‘imperfect’ half wave dipole. It is not exactly resonant, but the main issue is that it is tilted from one end to the other, it is not parallel to the ground surface. Continue reading Modelling an antenna as a simple two terminal resistance is often naive

Last update: 12th November, 2022, 7:52 AM

A simple transformer model of the Guanella 1:4 balun – some further observations

A simple transformer model of the Guanella 1:4 balun discussed a simple model for the operation of the device, but a model that is too simple for most RF baluns. Notwithstanding that, it does expose some interesting issues that are not only valid at lower frequencies, but will also be manifest in an RF balun.

Isolated load

Consider the effect of breaking the connection at the red X, so that we now have  what is often referred to as an “isolated load”. Continue reading A simple transformer model of the Guanella 1:4 balun – some further observations

Last update: 8th November, 2022, 4:50 AM

A simple transformer model of the Guanella 1:4 balun

(Guanella 1944) described a 1:4 balun, of a type often known as a current balun.

From Definition: Current Balun, Voltage Balun:

An ideal current balun delivers currents that are equal in magnitude and opposite in phase.

A good current balun will approach the ideal condition. It will deliver approximately equal currents with approximately opposite phase, irrespective of the load impedance (including symmetry).

Common mode current will be small.

If the load impedance is not symmetric, then the voltages at each output terminal will not be equal in magnitude and opposite in phase. (Note that for a truly ‘isolated’ load, one well represented as a two terminal load, the currents MUST be equal in magnitude and opposite in phase, but the voltages may not be equal in magnitude and opposite in phase.)

A simplified model

 

Above is a schematic of the Guanella 1:4 balun as often presented, this is an edited graphic from the ARRL manual, so may be familiar to readers. Continue reading A simple transformer model of the Guanella 1:4 balun

Last update: 7th November, 2022, 1:31 PM

Some wooly thinking on Antenna Factor online

Antenna Factor is often given / used as a parameter for an antenna (system).

An antenna with (nearly) constant AF can be quite convenient to simple field strength measurement where the AF value establishes a simple relationship between antenna terminal voltage and the external electric field strength.

Antenna Factor (AF) is the ratio of field strength to antenna terminal voltage for an antenna, dimensionally \({AF}=\frac{E}{V}=\frac{V/m}{V}=1/m\), AF units are 1/m or can be expressed in dB as \(AF_{dB}=20 \log_{10} AF \text{ dB/m}\).

It is lazy practice (though not uncommon) to simply express AF in dB, but wrong.  Continue reading Some wooly thinking on Antenna Factor online

Last update: 28th October, 2022, 8:29 AM

Diagnosis of a 9:1 transformer from NanoVNA plot – part 2

Diagnosis of a 9:1 transformer from NanoVNA plot discussed an example measurement of a 9:1 transformer on a binocular ferrite core. These are often recommended for use with Beverage antennas on 160 and 80m bands, and this was the maker’s application. In that article, I hinted that the core might not be #73 as the maker thought, or wished.

This article reports measurements of a 9:1 transformer wound on a Fair-rite 2873000202 (#73) binocular core. The pic above shows the test fixture. Continue reading Diagnosis of a 9:1 transformer from NanoVNA plot – part 2

Last update: 13th October, 2022, 8:23 AM

Diagnosis of a 9:1 transformer from NanoVNA plot

A chap recently posted online a question:

I have added two 1:9 transformer (2T/6T) back to back (high side together) and measured with the nanovna – 2 port measurement, as the binocular core I am not confident BN73 or not.

Also I swiped with one port S11, with one transformer where the high side is terminated with a 470ohm resistor load.

Please advise if it can be used for beverage antenna for 160/80m.

Let’s focus on the second test, and assume that the measurements are valid (and that is often an issue), that the 470Ω resistor is close enough to 450+j0Ω and the connections are short.

Above is his s11 sweep from 1 .5-7MHz.

I suspect this is actually #43 material. Continue reading Diagnosis of a 9:1 transformer from NanoVNA plot

Last update: 11th October, 2022, 4:26 PM

NanoVNA source mismatch error

One of the popular ideas online is that the correction process in the NanoVNA does not correct errors in mismatch at Port 1 and Port 2. This article deals with the first case ONLY, Port 1 mismatch.

An experiment with source VSWR nominally 2:1

A NanoVNA was configured with a SMA tee connected to Port 1 and a good 50Ω termination connected to the branch port, see the pic below. The left hand side of the tee becomes the new Port 1 interface, and by virtue of the additional 50Ω shunt termination, if the native Port 1 was indeed well represented by a Thevenin equivalent circuit with Zs close to 50+j0, the Thevenin source impedance is now closer to half that, Zs close to 25+j0.

Some would calculate this mismatch as causing a mismatch loss of 0.512dB that is additional loss in the s21 path.

Above is the test setup. The NanoVNA was SOLT calibrated with cal parts attached to the left hand side of the tee and the 200mm coax jumper from that point to Port 2. Continue reading NanoVNA source mismatch error

Last update: 6th July, 2022, 9:15 AM

NanoVNA-H – modification of v3.3 PCB to start the bootloader from the jog switch

Later NanoVNA-H* hardware allows the device to start in bootloader mode by holding the jog switch in whilst powering on. It is a very convenient facility for firmware update, much more convenient than taking the case apart to jumper BOOT0 to VDD. (Some later firmwares provide a menu option to start the bootloader… but of course that is only useful if the firmware is running properly and may not be useful in the event of a failed firmware update.)

This was a mod I devised prior to the v3.4 hardware change, it is not identical to that change as it preceded it, but it works fine on v3.3 hardware and may work on earlier versions.

Boot switch

The mod calls for replacing R5 with a 1k (1402) and running a short jumper from the T terminal of the jog switch to the un-grounded end of R6.

To use it, hold the jog switch in and turn the nanoVNA on.

Above a pic of the mod. It is a simple mod, but very fine soldering so it might not be within everyone’s capability.

Last update: 19th June, 2022, 7:40 AM