Antenna Tuners (ATU) – owenduffy.net

4:1 voltage balun vs 1:1 current balun on the MFJ-949E @ 1.8MHz

A question of often asked is whether the ‘best’ balun on a T match ATU is:

a Ruthroff 4:1 voltage balun (as it often fitted internally);
a Guanella 1:1 current balun; or
a Guanella 4:1 current balun.

The subtext of the question is often that the OM has switched from (a) to (b) in pursuit of improved common mode current rejection, and found they cannot match their existing antenna which matches ‘just fine’ using the (a).

This article explores (a) and (b) using a configuration similar to the popular MFJ-949E @ 1.8MHz, and comments on (c).

Let’s declare that although the MFJ-949E is specified for 1.8MHz, it is disadvantaged… which will exaggerate the problem to some extent… but in so doing make the problem easier to see.

We will use a SimNEC model of the ATU and a purely resistive load, as large as the ATU will match. The Smith charts shown are normalised to Zref=400+j0Ω as it is a better context for visualising loads on 400Ω transmission line typical of the use with two wire lines.

1. The bare MFJ-949E T match to a coax output jack

Above is the model. The coloured squares a sample points in a sweep of values of the matching components C1, L1, and C2. Green is loss less than 3dB, Yellow 3-6dB, and red greater than 3dB. Continue reading 4:1 voltage balun vs 1:1 current balun on the MFJ-949E @ 1.8MHz

Last update: 17th March, 2024, 1:40 AM

My ATU is soooo good, it will give a perfect match on anything, even…

I was reading the mail on a net on 40m one day many years ago, and one of the participants asked people for their recommendation on his intended ATU purchase. This was pretty technical for this net where the qualification to join is that you tell all what you are going to have for dinner.

So, one of the guys tells ’em “mine will tune the proverbial wet string.”

Gotta do better than that? Next guy says “mine is so good… I tuned up the other day for a perfect match then thought the band was a bit dead until I realised I did not have an antenna connected!”

Let’s work a real example

MFJ claimed that the MFJ-949E is the most popular tuner purchased… so lets use it as an example.

We will use 160m as I measured the magnetising impedance of the internal balun a few days ago and used in the a recent article. This article uses the same basic model… but no antenna connected.

Above is the test setup, NanoVNA connected to the transmitter jack, mode switch set to Tuned / Balanced… and there is no antenna on the balanced line terminals (the balun link is in place). Continue reading My ATU is soooo good, it will give a perfect match on anything, even…

Last update: 13th March, 2024, 6:45 AM

Antenna assessment using NanoVNA – learning from a user example

A recent online posting provides content for learning. K3EUI posted a NanoVNA-Saver screenshot of his antenna described as:

Set out a horizontal loop wire antenna for possible NVIS paths
Wire is about 140 ft length with an outside CLC tuner, fed with 50 ft RG213. …

Here were the Nano VNA graphs of this new loop antenna, measured from inside the shack (50 ft RG213)…

Can we learn something from this? Continue reading Antenna assessment using NanoVNA – learning from a user example

Last update: 19th December, 2023, 8:04 AM

IC-7300 VSWR protection

A ham consulting the experts on QRZ asked:

On 30 meters, my SWR reads 3:1 to my antenna (an EndFed 53 feet long wire up about 25 feet). Reading a chart I have, I see that at 80 watts output, my reflected power should be 20 watts. I verified this by looking at my Diamond SX-200 Meter which also indicates the reflected power is 20 watts. My questions are these: does the 20 watts reduce my 80 watts output to 60 watts at the antenna? I have a choke on my feed line in my shack (near my transceiver) & the SX-200 Meter is between the choke & transceiver….

The OP later explained that the transceiver is a IC-7300 and it appears that the internal tuner is in use above… so let’s proceed on that basis.

Analysing the OP’s report, his SX-200 indicates VSWR=3 Pf=80, therefore Pr=20, and P=60W. Note that \(P=P_f-P_r\) is valid because Zref is real, so the answer to his question about power to the antenna is 60W, he is quite correct.

He went on to ask where the 20W reflected goes to… but I will leave that to Walt Maxwell devotees to discuss energy sloshing around and re-re-re reflections… the stuff of ham lore.

Understanding the IC-7300

As an example of what might be expected of the IC-7300 with a mismatched load, I did a series of measurements at 7MHz with a sample variably mismatched load.

Above is a plot of power output vs VSWR for a sample mismatched load. Also plotted is the measured reflected power and the calculated power output based on the ham lore \(P=P_f (1-\rho^2)\). Continue reading IC-7300 VSWR protection

Last update: 3rd October, 2023, 7:12 AM

Balanced ATUs – the Holy Grail?

It seems that the Holy Grail of many ham HF enthusiasts is a “true balanced ATU.”

The word “true” in there bodes poorly!

It seems that while there are plenty of online experts who have very strong opinions on common mode current, baluns and ATUs, it is very rare that we see quantitative evidence of their assertions, measurements even.

Less commonly does a “true balanced ATU” description include valid measurement of common mode current as evidence of operation.

A “true balanced ATU” project by LY1O in unusual in that it contains a probe purported to measure and display current balance.

Above is a schematic of LY1O’s measurement system, it has a pair of current transformers each with half wave diode detectors in each leg of ATU output. It is important to note that the detectors convert the RF AC wave into a DC value close to the peak value of the AC wave… so they respond to the magnitude only of the current in each leg. Continue reading Balanced ATUs – the Holy Grail?

Last update: 21st January, 2023, 5:09 PM

Matching a centre loaded 80m vertical – a shunt match tutorial

This article describes a method of measurement and adjustment using an antenna analyser or VNA to quickly set up a shunt match, a narrow band match (ie for one band, or even only part of the band).

The article uses Rigexpert’s Antscope as the measurement / analysis application, the techniques will work with other good application software.

To demonstrate the technique for matching such an antenna, let’s use NEC-4.2 to create 80m feed point impedance data for a 12m high vertical with 8 buried radials (100mm) and centre loading coil resonating the antenna in the 80m band for simulation of measurement data.

An s1p file was exported from 4NEC2 for import into Antscope, to simulate measurement of an example real antenna.

Analysing the ‘measured’ data

Step 1

Above is the VSWR curve displayed in Antscope. Note that the actual response is dependent on soil types, antenna length and loading etc, but this is a good example for discussion. It is not real bad, another example might be better or worse. Continue reading Matching a centre loaded 80m vertical – a shunt match tutorial

Last update: 20th December, 2021, 11:13 AM

Low power Guanella 1:1 balun with low Insertion VSWR using a Fair-rite 2843009902 binocular – measurement of Zcm

The article Low power Guanella 1:1 balun with low Insertion VSWR using a Fair-rite 2843009902 binocular – design workup describes a current balun with low Insertion VSWR for operation at modest power levels. The design was based on a low cost Fair-rite 2843009902 binocular core (BN43-7051).

This article documents measurement of the complex common mode impedance Zcm, and calibration of the predictive model.

Zcm is a most useful quantity, it can be used in NEC models of an antenna system. Continue reading Low power Guanella 1:1 balun with low Insertion VSWR using a Fair-rite 2843009902 binocular – measurement of Zcm

Last update: 4th December, 2021, 3:04 PM