JC-4 AUTOMATIC ANTENNA TUNER
10-Sep-2016 Another balun tested.
This article is under construction.
SCHEMATICS: JC-4(1) , JC-4(2) , JC-4(3)
Recently I temporary got a JC-4 automatic antenna tuner for end-fed wire or rod antennas. A welcome opportunity for me to "play" and compare it with my home made ATU and an ICOM AT-120. In terms of price a JC-4 is an attractive alternative to similar but more expensive couplers from other manufacturers. It may be up to a difference of 50% in price:
JC-4 1000W/PEP about EUR 500, SGC-235 500W/PEP about er EURO 1000.
JC-4 has 11 coils and SGC-235 8 coils available to obtain resonance.
Most comparative tests were done with my antenna (fig») as shown in the picture on the right. The antenna was symmetrical fed with a current balun or one of the feeder lines was disconnected to use the other as L-antenna.
The most important features
1. 1kW PEP (SSB 1.6 - 30 MHz) with a 25 m wire antenna.
2. Remote control with ICOM, ALINCO en KENWOOD transceivers.
3. Tuning steps: inductance 0.08 µH, capacitance input 25 pF and output 6 pF.
4. Automatic selection of CL, LC of Pi-circuit.
5. Choise of 2 antennas (A en B) both with 50 memories.
6. Switching to A or B without retuning.
7. Not used antenna is grounded.
8. Dipole or open feedline can be connected across antenna outputs A and B.
9. Memory remains when supply switched off.
10. Memory not stored if SWR > 1.3.
11. Protection against static discharge during operation.
12. Both antenna outputs (wing nuts) grounded when powered off.
Other than the specified equipment does not apply. For example YAESU devices needs the control box equipped with a 4-pin microphone connector and a 4 core cable. The operation is as follows:
If one has a G5RV or ZS6BKW, 1 kW PEP is possible when not using 160 m, because the antenna is too short for that big power.
The coax and control connectors (fig») are not watertight is an often comment in reviews, but that is not true, because the relevant feedthrough components are glued. The galvanized fastening strips should be of stainless steel, but maybe it was a method to reduce production costs.
JC-4 wire is thicker than AT-120 wire.
MATCHING RANDOM ANTENNA's
Generally the tuner is able to match random rods and wire antennas, even if no antenna is connected. Not always with SWR = 1 but often with SWR < 2. It will be clear that with a short antenna the bulk of the energy is absorbed in the tuner itself. To protect the device one have to reduce the transmitter power significantly or use recommend length of antenna systems as stated in the documentation. The image («fig) shows that two antennas permanently can be connected to ouputs A and B, so you have a choice of two systems, each with 50 memories. For example very convenient during a contest to select the strongest signal.
When using an end-fed antenna it is important tho connect a counterpoise as drawn in the image. Another possibility is a single insulated wire lying on the ground. The size is about 60% shorter than antenna due to the velocity factor caused by the earth. A counterpoise of 15 - 17 meters is than suited for a "horizontal wire 25 - 28 meters" as recommended in documentation.
RF INTERFERENCE (FEED BACK)
SYMMETRICAL FEED WITH CURRENT BALUN
There were and are still discussions going on about the (best) place of a (choke, current) balun with an asymmetrical tuner. Suppose a giant ferrite bead exists that we can put over a tuner. Then there are three options which are shown in the image: (1) between TX and ATU, (2) over the ATU and (3) between ATU and antenna. In terms of operation it does not matter where the balun is placed. So there is no real difference whether it is before or after the ATU! However, at (1) and (2) the tuner is part of the antenna system with all the attendant consequences such as RF voltage at PCB or cabinet or interference in own equipment. It need not be so bad to happen but it is possible. The best place of a balun is as close as possible to the antenna system, so between the antenna tuner and antenna system. This is my conclusion and on Internet you can find more like-minded comments.
If the balun is getting hot on a particular band it is due to high RF voltage at that point of the supply line.
There are three solutions:
1. Lengthen or shorten the balanced feeder for maximum antenna current at the balun connection.
2. Take a large ferrite toroid or more toroids so that power is distributed.
3. Use a 1 ÷ 4 balun for that band.
EXPERIMENTS WITH JC-4
Many types of baluns tested with FC-4.
The tuner is designed for end-fed antennas, bu toften hams use a doublet with open feedline or ribbon. Connecting a symmetrical system directly at antenna outputs A and B (2 wing nuts) works, however it is better to do it differently. Use a current balun to couple unbalanced output of the tuner to that antenna system.
A symmetrical antenna system coupled with current balun.
For experiment, I made (fig») an experimental current balun with TEFLON insulated wire and a junkbox toroid of unknown ferrite material. It proved to be suitable for testing. Better is to use a Amidon/Micro Metals toroidal type FT-240-K, AL = 4.9μH/turns², so one can achieve a high μ with few turns.
A toroidal core has the property that a large choke reactance is obtained with a small size, so that common mode currents remain low. The currents in the wires are almost equal and opposite, ultimately the flux is also small. If one ensures that the reactance of the windings is much larger than that of the antenna system, the core will have less chance to become saturated.
Example with FT-240-K toroid: 10 turns = 10² × 4.9 = 490 µH = 5700 Ohm on 160 m!
Heathing of the core is more due to the RF warming up current through (too thin) wires of the balun. With the differential-mode current, the respective flux is virtually zero, so that the ferrite does not have any influence on the characteristic of the transmission line. That is the theory because in practice, performance depends on construction, insulation of the windings and their mutual distance.
ANTENNA OUTPUTS A en B
A dipole or ladder line can be connected across antenna terminals A and B, but there is a difference of tuning when A or B is grounded..
Schematically (f«ig) A or B are grounded in like manner, but on some bands more antenna current occurs at only one switch position. That's probably because A and B are not grounded via a similar path.
The image .shows (fig») that most of the difference arises on the 160 m band.
So it pays to try in which position of the switch most antenna current is obtained.
To expose the difference between tuner without balun and with balun a test was done in accordance with the («fig) image. JC-4 was connected to a 2 m long ladder line followed by a "standard" voltage 1 ÷ 1 balun and then passed to a professional 50 ohm dummy load. Incidentally the test is was also done with a voltage 4 ÷ 1 balun with almost the same results in terms of difference in balance or imbalance. The test shows clear that a current balun contributes a lot to equal currents in both conductors of the feeder. Furthermoer one can observe that the higher the frequency the more the imbalance. Other JC-4 owners report to me that RF feedback in their own equipment was reduced or even was solved with the current balun.
The aforementioned experimental balun made with an unknown type ferrite toroid works great but is an uncertain element to readers of this article. Because FT240-43 toroids are well known and be more available, I used them for further experimentation. Teflon insulated wire is not easy to obtain in contrast to 50 ohm PTFE coaxial cable and that was used for the baluns. The short coaxial cables have negligible loss and any small impedance transformation caused by the current balun is irrelevant because the tuner will compensate. Two cables in parallel have an impedance of 2 × 50 = 100 Ohm. The windings do not need of any specific characteristic impedance but this greater value is more favorable for a ladderline connected to a multi-band system. The feeder almost never presents a uniform load to the balun. As the operating frequency changes, the balun load impedance can change from several thousand ohms to a few ohms. The balun will pass the impedance through and 1 ÷ 1 baluns are generally much efficient and have a wide operating impedance range. They provide better balance, often have lower loss, tolerate load impedance and imbalance variations much better than voltage baluns. Without exaggeration, one can say that with a random length of the ladder line a 4 ÷ 1 balun has twice as many losses as a 1 ÷ 1 balun. In case of low impedance much current flows through the relatively thin inner core of the Teflon cable and it can be pretty hot.
Upper left a 1 ÷ 1 current balun which usually works well with any ladder line antenna system. If no match can be found, one may use an additional switch to select a 1 ÷ 4 balun. Remember: 1 ÷ 1 baluns are generally much more efficient and have a wide operating impedance range! The inductance of ten turns is about 200 µH. That might be inadequate for some antenna systems, but at present I do not have (yet) other ferrite material of a high μ and suitable for high power output. For the time being I use for each branch 3 × stacked FT240-43 toroids.
If one chooses for "A" or "B" of the controlbox, mostly there is a difference of maximum antenna current or no equal current in both conductors of the feeder.
Smaller ferrite toroids can be used for assembling pipes to obtain more inductance with less turns. One pipe of the test model has just over 650 µH with 7 turns. The wires are Teflon isolated for arcing protection. The transmission lines are connected in series to increase the impedance. The system is a 1 ÷ 1 balun of random impedance to feed a random ladderline with a random dipole.
This type balun was so far the best. Unfortunately I have no information about toroid's type, because the ready-made construction came from a flea market and was at home dismantled for experiments.
In fact the principle is the same as previous construction with two toroids and coax cable and also with the following one toroid balun.
With one toroid the second coaxial cable or the second set windings should be wound in opposite direction as was for example done («fig) with a (1 : 4) coaxbalun on one PVCc pipe. However in magnetic terms, the wires on the toroidal are installed in a correct direction.
COMPARISON OF AT-120, JC-4, S-Match
Set-up used to determine the mutual difference.
AT-120, JC-4, and S-Match are compared with my antenna («fig) by measuring the antenna current at the same point in the two conductors of the ladder line. Due to folding back a part of the antenna, the current in the feedlines are not always equal. The higher the frequency the greater the difference is. In contrast to the AT 120, JC-4 was able to match for SWR = 1.
Efficiency of JC-4 is better than an AT-120, but the difference is greater in the higher frequency range. On 30 m the AT-120 could not find any match, JC-4 had less difficulty, but the S-Match with his roller inductor did clearly better.
Not much difference is between JC-4 with current balun and symmetric S-Match. An exception is 160 m; the antenna for that band is too short and efficiency of the JC-4 is clearly less. The automatic tuner works with steps and my S-Match is continuously adjustable. Further the problem is caused by the 6 Ohm input impedance of the feedline line on 160 m. With such impedance many ATU's have a low efficiency. Each antenna tuner has the property that at a certain frequency and a certain antenna system, adjustment fails or efficiency is disappointing. It is due to the components used, the matching range, or way of construction.
JC-4, S-Match. The difference between JC-4 and S-Match was reduced when the ribbon line was lengthened or shortened. Apparently for both antenna couplers the input of my 10 m feedline was an unfavorable impedance on 160 m.
JC-4, AT-120. If the ribbon line was lengthenend or shortened JC-4 reached almost always SWR = 1 while AT-120 often mathes to SWR = 1.5 - 1.7 and repeatedly the efficiencey was less than JC-4.
G5RV, ZS6BKW. My antenna behaves the same as G5RV/ZS6BKW and a JC-4 will match both types without any problem.
ATTENUATOR 10dB, 100 W/50 Ohm
Earlier it was reported that the transmitter temporarily is loaded with a highly fluctuating impedance during tuning. In addition, one has to reduce the drive for 10 - 20 W by turning a potmeter and the setting accuracy for that power is often different per band.
TO BE CONTINUED