The MFJ-993B auto antenna tuner includes an internal balun, this article is a review of that balun.
The schematic shows the balun as a Guanella 4:1 balun with the usual external link to one of the coaxial antenna sockets. (The label “Z balanced” is misleading, clearly one of the terminals is grounded and this is the unbalanced connection to the coax connector via a link. The antenna connects to the left hand terminals.)
Unlike almost all ATUs with an internal balun, this is a current balun (to their credit), but a 4:1 balun.
There are two aspects of balun behaviour that are of particular interest:
- choking or common mode impedance; and
- impedance transformation.
Continue reading MFJ-993B internal balun review
At MFJ-993B on my G5RV with tuned feeder I discussed first impressions of the replacement ATU.
This article documents the physical layout.
The antenna is a G5RV with tuned feeders (Varney 1958). The tuned feeder is home made two wire line using 2mm diameter copper spaced 50mm.
Above at the right, the open wire line terminates on a home made balun on the feed line entrance panel, see
Design / build project: Guanella 1:1 ‘tuner balun for HF’ – #4 for details of the balun. This is all under the building eaves, but it is waterproof… the area is regularly jetted with high pressure water to clean insects away. Continue reading Shack entry / ATU configuration for my G5RV with tuned feeder
EA2BQH described an adapter to use a JRC NFG-170 / NFG-230 ATU.
The description is in Spanish, and a Google translation doesn’t help me much, his published HEX file for a PIC12C508A helps more.
Building the device and observing the output, it seems to have two input pins and when one is high OR the other is low, it sends a ASYNC command string at 1200Bd on its output pin. The command string is repeated every 2.5s if the input condition remains. This string appears to command the ATU to review / retune.
The 12C508A is a very old chip, still available, but in low cost form, an OTP.
I have written some code for a PIC12F510 from the ground up to do a similar thing as far as I can see, and built an adapter for testing by VK1EA.
My redesign uses different pins to the original to better cater for ISCP and to utilise weak pullup as much as feasible. IN1 is pin3 (GP4), IN2 is pin4 (GP3), TX is pin5 (GP2). IN1 OR /IN2 causes the adapter to send the configuration command. The output (TX) is open collector.
Above, a view of the adapter from chip side encapsulated in heatshrink and a patch of double sided adhesive foam to fix it in the ATU. Pin3 is wired to ground, Pin4 (IN2) is green, TX is yellow, ground is black and VDD (5V) is red. The TX pin is wired to pin 6 of the opto isolatorC next to the input terminal block TB1 on the ATU. (It may be possible to cut a track and insert the module after the opto isolator. Continue reading Command adapter for JRC NFG-170 NFG-230 ATU
This article is an analysis of why my recently acquired MFJ-993B will not match my multiband antenna system on most bands above 20m. The MFJ-993B replaces an Ameritron ATR-30 which was capable of matching the antenna system on all HF amateur bands.
A detailed analysis is performed 18.15MHz on the first problem band.
The antenna system uses a tune feeder configuration.
The alternative tuned feeder arrangement described at (Varney 1958).
In this case, the open wire line is 9m of home made 450Ω line (2mm copper wires spaced 50mm air insulated), a 1:1 current balun and 0.5m of RG400 tail to the ATU.
A 470K 1W metal film resistor and a 2095-100 gas discharge tube in parallel are connected in parallel with the ATU antenna terminals to reduced static build up and modest transient protection.
Impedance was measured looking with a Rigexpert AA-600 into the cable end that plugs onto the ATU, at 18.15MHz is is 4.7-j69.5Ω.
Continue reading MFJ-993B on my G5RV with tuned feeder
Designs appearing in the ham literature and online articles tend to espouse relatively large diameter conductors, conductors that can be challenging to wind onto the toroidal cores often used.
This article analyses the copper losses in a practical Guanella 1:1 balun where a fabricated twisted pair line is used.
Total losses comprise core losses and transmission line losses. Continue reading On copper loss in transmitting baluns
I have published a number of transmitting balun designs, and none of them use enamelled wire. I am sometimes asked why is that so, but more often advised that it is a better solution than the wires that I have used.
enamelled wire depends on an insulating coating, and its breakdown voltage depends on the wire diameter, polymer used, the minimum thickness applied, coating cure / bake processes, temperature, humidity etc.
Whilst I have seen specifications promising breakdown voltage of a single round enamelled wire in the regions of 5-10kV, and you might hope for nearly double that between a pair of twisted wires, unless you have sourced specific product, new performance may be closer to 2kV. Continue reading On use of enamelled wire in transmitting baluns
The article describes a current balun with low Insertion VSWR for operation 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 balun with low Insertion VSWR using a pair of Jaycar LF1260 suppression sleeves
At HC-500 I showed some VNA plots of the HC-500 matching a 50+j0Ω load at 3.5MHz.
The following commentary is on a single load scenario, a 50+j0Ω load at 3.5MHz, and while the results are not simply extensible to other loads and frequencies, it does provide some interesting insight into the devices.
THP HC-500 (Ultimate Transmatch (McCoy 1970))
Above is the behaviour of the unmodified HC-500 (an Ultimate Transmatch).
Losses at match are 12% of input power. At its rated 500W maximum power, that is 60W (which might seem high but heat tolerant insulation materials are used). On modification to a T match, losses at match were reduced to 8% or 40W at rated maximum power.
Continue reading A tale of three tuners
In the early 1970s I purchased a Tokyo High Power Labs HC-500 ATU based on recommendation of other hams and the seller’s representations (Dick Smith Electronics) that it was a T match with 200pF capacitors.
The circuit configuration is of the so-called Ultimate Transmatch, an invention of (McCoy 1970) that claimed a bunch of advantages over the ordinary T match.
The HC-2500 would appear to use the same circuit.
It wasn’t long before several authors waded into the Ultimate Transmatch over its poorer efficiency. With an ambitious name like Ultimate Transmatch, it had a lot to live up to… but it failed.
Within months, a reconfigured topology appeared entitles the SPC Transmatch, but it also had issues.
The reality is that none of these designs is ‘ultimate’, they all have advantages and disadvantages and are mostly used in ignorance of those.
So, I have had this HC-500 which worked well enough I suppose, but was quite difficult to tune on some loads that ordinary T matches handled with ease. It has always been my intention to reconfigure it to a T match be rewiring the grounded stator of the input cap to parallel it with the other stator… a minimal modification to get rid of the shunt capacitor and use it to help to keep coil voltage down on some loads.
Before performing the modification, I measured transmission loss when matched to a 50+j0Ω load at 3.5MHz using a two port VNA.
Above, transmission loss is 0.54dB, efficiency is 88.3%. Continue reading HC-500
Much is written about ATU efficiency, about the need for them or not, and often in subjective terms like “lossy ATU”, and most of it lacking quantitative detail.
The little quantitative detail is almost entirely for purely resistive loads… as if that is typical of real life conditions.
The most common configuration used today is the ‘high pass T match’, but a range of other configurations are seen as being superior… though usually without quantitative evidence.
More Hams use MFJ-949s than any other antenna tuner in the world! Why? Because the worlds leading antenna tuner has earned a worldwide reputation for being able to match just about anything.
… so let’s make some measurements with a reactive load on a MFJ-949E. Capacitive loads tend to be very common for antenna systems at lower HF, so let’s choose a load of 50Ω with a 100pF silver mica cap in series at 3.6MHz. The reactance of the cap is -442Ω, so the load is 50-j442Ω, and the 50Ω part is a RF power meter (RFPM1).
The test setup then is:
- a standard signal generator (SSG) on 3.6MHz with 20dB precision attenuator so that we are confident that Zs=50Ω (important to the adjustment of the ATU for maximum power as indication of 50Ω match);
- 100pF silver mica capacitor (low loss);
The SSG was adjusted for -10dBm out directly into the RFPM1, then the ATU+cap inserted and ATU adjusted for maximum power indication. Power indicated was 1.4dB lower, so InsertionLoss and TransmissionLoss are both 1.4dB.
Above is a simulation of the T network in RFSim99, component values are adjusted for a match and inductor Q is calibrated to the measured loss of 1.4dB. Continue reading ATU efficiency