The issues arising with AutoT 6.25 precluded prolonged FT8 operation at full (100 W) power. And performance (in terms of power loss) was a reason for concern. Time to switch to the backup setup.
AutoT 6.25 was replaced by the ATAS-25 style sliding coil. This move generated new (well, they were known) issues. When radiator length is much longer than one quarter wavelength, there is no way to tune it adding a coil. When it is too short, it is inefficient.
For a given radiator length, maximum operation frequency is limited. The coil can be adjusted for operation below that frequency. Until certain limit.
Wire length in the coil (plus its own height) is around 7.5 m. Plus 5 m of radiator, makes 12.5 m. A full wavelength would be 50 m, or 6 MHz. Possibly a bit lower. But not more than 1-1.5 MHz below]. This would be the minimum tuneable frequency. Not low enough to operate in 3.5 MHz band. In practical terms, 7 MHz would be the limit. Operation in 3.5 or 1.8 MHz bands would require additional coil(s).
3, 4 & 5 m radiators were evaluated. 5 m radiator was the one described in section “2024-06-26. Testing at summer QTH garden.”. To obtain a 4 m radiator, the upper section (10 mm diameter) was removed. In 3 m radiator, the two uppermost sections (12 & 10 mm diameter) were removed.
Maximum operating frequency as a function of radiator length (including coil’s length with 0 turns, 0.34 m) is summarized in Table 6. For operation between 7 and 14 MHz, 5 m rod is the most convenient radiator. For frequencies above 14 MHz, 3 m rod is the only practical possibility.
3 m aluminium rod seems a sweet spot combo. Increasing length by 35 cm impeded autotuner operation in 21, 25 and 28 MHz bands. Using a thinner radiator (8 mm diameter) decreased bandwidth by a small amount, but enough to hinder autotuner operation in 28 MHz band.
Indeed, 3 m rod is an all-round solution. It allows operation from 7 to 51 MHz. Although at the expense of reduced efficiency in the lower part of HF spectrum.
Table 7: Summary of operation settings for sliding tuning coil.
VNA graph. 3 m x 20-16-12 mm Aluminium radiator, sliding coil 0 turns.
VNA graph. 3 m x 8 mm brass radiator, sliding coil 0 turns.
VNA graph. Al and brass radiators comparison, sliding coil 0 turns.
By the end of June, the family moved to the summer QTH at IN52 Maidenhead square. A small garden was available, and the antenna setup was adapted to the new situation. The AutoT 6.25 was installed on a tripod. 3 x 10 m, 2.5 mm2, wires were used as counterpoise. On top of it, a 5 m Aluminium radiator was mounted. It was composed of 5 x 1 m sections. Diameters (bottom to top) are 20 mm, 16 mm, 12 mm, 12 mm and 10 mm. Each section end was provided with brass inserts (male at bottom, female at top), to avoid galling and wear. Base section has a 7075 alloy insert, taped to M6, instead of the male one.
Summer QTH setup. Left: antenna. Right: “Shack”.
Initial SWR measurements were satisfactory. Tuning was possible from 7 MHz to 50 MHz. A few QSO were made in all bands. PSK reporter reception reports were good. Who could ask for more?
2024-06-30 ¿Funciona? ¿Sí? ¡No lo toques! (or, if it ain’t broke, don’t fix it!)… o quizá sí
Out of sheer curiosity [on the effect of parasitic inductance], it was decided to remove the ferrite bead inserted in the wire from UNUN output to the antenna stud (see section “2024-05-21 update: Adding parasitic inductance.”).
This was a bad move. SWR raised in 7 MHz, and FT-710 autotuner was unable to find a match. In this situation, it was considered interesting to get some insight on the result of adding some inductance (and not-negligible coiled wire length, too) between the output of the AutoT and the radiator.
For this purpose, it was used a home-built version of Yaesu’s ATAS-25 loading coil, made a few years ago. It has 40 mm diameter and 2.5 mm pitch. For increased reliability and easier construction, the coil was built with 1.5 mm2 (1.38 mm diameter) solid copper wire. The device allows inserting easily any number of turns between 0 and 58, and has a basic length of 337,5 mm for 0 turns. Equivalent length for 58 turns is 7.48 m, allowing tuning down to 7 MHz band when used with a 3 to 5 m radiator.
Inductance calculated with “Coil32” was 1.34 μH for 5 turns, 2.253 μH for 7 turns and 3.826μH for 10 turns. Equivalent added length were 0.953 m, 1.200 m and 1.569 m respectively. It must be noted that effects on SWR are a combination of both inductance and added length.
In this case, measurements are presented in tabular form (Table 6). Line graphs had too many entangled lines, making difficult the interpretation of results. Colours were graded according to SWR readings, green for low values and red for high ones, to highlight SWR differences according to each setup, and to allow identifying easily the best one.
Table 6: summary of SWR measurements by band and loading coil settings.
The two fist columns show measurements made at home QTH with a 3.88 m radiator, before (SWR-Base) and after (SWR Ind_Par) adding the extra parasitic inductance.
Column marked “SWR PA Base noIP” corresponds to measurements made with a 5.0 m radiator, no parasitic inductance added, no loading coil added.
Column marked “SWR PA Pir Base” corresponds to measurements made with a 5.0 m radiator, with the loading coil installed, all turns short-circuited.
Column marked “SWR PA Pir 5e” corresponds to measurements made with a 5.0 m radiator, and 5 turns in the loading coil.
Column marked “SWR PA Pir 7e” corresponds to measurementsmade with a 5.0 m radiator, and 7 turns in the loading coil.
Column marked “SWR PA Pir 10e” corresponds to measurementsmade with a 5.0 m radiator, and 10 turns in the loading coil.
Column marked “SWR PA PInd Int” corresponds to measurements made with a 5.0 m radiator, and the ferrite bead (BN-43) back in place, wire threaded through two apertures. Loading coil was removed.
It can be noticed that most favourable values were 5 and 7 turns, with around 2 uH inductance and 1.0 m added length. No surprises!
2024-07-04: Final fireworks
After the above tests, the ferrite bead was reinstalled, and operation from 7 to 50 MHz was again possible. Operating in FT8 at full power (100 W) went smoothly for some days, with short periods of activity at the beginning and the end of the day, sometimes after lunch. No big issue was detected. The AutoT 6.25 felt warm at the end of each session, but not too much.
However, after a long FT8 session in the evening of the 4th of July, the antenna started to tilt, and it felt down to the ground. At first, it was thought that the cause was the wind (the antenna was only held by the tripod, with not too ample base).
But a closer scrutiny brought a different picture. AutoT 6.25 felt warmer than usual. So it was open for inspection. What was found was a very hot core. So hot it was that it was fused to the wall of the enclosure, charring it.
Ferrite core fused to enclosure’s wall.
When pulled, a chip was left adhered to the wall. These symptoms alone would mean temperatures higher than 150 ºC.
Charred wall with ferrite chip adhered.
But Temperature was higher. There were found symptoms of solder melting, soaking the borders of foam packing pieces. Solder was an almost eutectic SN60-Pb40 alloy, so melting point was around 185 ºC. A respectable temperature. And core material should have been hotter.
Melted solder, soaking packing foam.
Indeed, metallic parts were so hot that they softened PVC in contact with them. Among them were the spacers that held together the assembly and the grower washer in M6 stud.
Imprints by hot spacers and Grower washer.
This caused a progressive (and accelerated) tilt in the 5 m tall aluminium radiator rod. To the point that the centre of gravity of the antenna moved outside of the legs’ enclosing triangle. Then it felt of the ground.
That the cores could reach such temperatures would provide an explanation for the SWR rising when using a BN-43 core (see section “Update 2024-06-19. BN-43 overheating.” in a previous post). In their datasheets, material 43 Curie temperature is declared as TC > 130 ºC, while for material 61 is TC > 300 ºC. If material 43 temperature rises above 140-150 ºC, its magnetic properties would change significantly, even to the point of a very reduced permeability. The issue should not arise with material 61. Soldered joints would fail before the core could reach its Curie temperature.
AutoT 6.25 was reassembled, including trueing spacers and bolts seats. It looks fine again, but now its limits are clearer. No more than 50 W when operating in FT8 for prolonged periods!
So, case closed (by now)! And back to the drawing board and the workshop when returning home!
“On the air” tests allowed additional insights on the design and operation principle of Alpha and Chameleon’s “Matching devices”.
It is possible broadband (3.5-50 MHz) operation with no adjustments and the help of any modern rig autotuner.
However, it seems that the UNUN used in them has [possibly intentionally] high core loss. A significant portion of RF power would be dissipated in the core. Low SWR would not be the result of a correct matching, but of the equivalent loss resistance. Just like placing a resistor in parallel with the antenna. Only a fraction of RF power would be placed on the air.
On tests with AutoT 6.25:
Material 43 gives broader bandwidth but has higher core loss. And more important, lower Curie Temperature. It is adequate only for phone or CW operation at moderate power levels (100 W PEP). When operating FT8 for extended periods, QRP. Power should be lower than 25 W.
61-material can sustain elevated temperatures, but initial permeability and bandwidth are lower. It would be wise to try to keep it as cold as possible. If this material is used, a design including a heat diffuser and some kind of heatsink would be desirable.
If using 61-material core, adding 7-8 turns, 40 mm diameter, 2.5 mm pitch coil in series with the antenna connection could improve bandwidth.
PVC is not the best material for the enclosure. Its 80 ºC melting point is too low. A technical plastic with higher melting point should be used. Minimum Delrin, which melts at 165 ºC. Teflon melts at 327 °C but tends to be softer. PEEK, which melts at 343 °C, would be optimal.
On the other hand, these devices make a convenient solution for casual broadband operation (preferably QRP, in view of their power ratings), with no adjustments needed in the antenna. But be ready to pay the toll in terms of low efficiency.
Finally, the experience of building a duplicate of the whole system (Matching device, vertical rod, counterpoises) was time-consuming and needed machine tools and tooling not readily available at any ham workshop. At least a lathe and a vertical drill. If it is considered desirable operating such system, it would be advisable purchasing it from any reputable manufacturer (my personal advice would be Chameleon, if you can afford it).
With the 5 m radiator, from VNA readings, it wasn’t expected a tuning solution below 7 MHz. But in 3.5 MHz band, as with the original Alpha, it was found. As in all other bands.
Then, a few FT-8 contacts were made in 14, 18 and 21 MHz. PSK reporter showed reception reports from all Europe and the East coast of the USA.
PSK reporter monitoring graph. 2024-06-17, 5 m radiator.
On the next day, it was tested the 3.88 m radiator. Again, the autotuner did its job without issue from 3.5 MHz to 6 m. A few FT8 contacts were made in 10 and 14 MHz, although propagation conditions were not too good. PSK reporter did show reception reports only in Europe.
PSK reporter monitoring graph. 2024-06-18, BN43 core, 3.88 m radiator.
Update 2024-06-19. BN-43 overheating
While testing the antenna in FT8, at 100 W output, it was noticed that SWR started to rise slowly. This was a bad symptom. After an FT 8 session, with some contacts, the AutoT felt warm, so it was open for inspection. The binocular core was very, very hot. Worse symptom. That meant that the core (and the device) is a lossy one. Too lossy. Indeed, this can explain the low FM/digital modes rating of Alpha and Chameleon “Matching Devices”.
SWR rise can be explained by the low Curie Temperature (Tc) of material 43. In its datasheet, it is specified that Tc > 130 ºC. Above this temperature, the magnetic properties would change, including a substantial decrease in permeability. Core temperature wasn’t measured, but it is likely that it was above 150 ºC (more on this in the post on summer QTH tests).
2024-06-20 update. Change to BN-61 core
In view of the issues with BN-43-7051 core, it was decided to replace it by a BN-61-002, and retest SWR and operation.
SWR readings weren’t too good. When using the 3.88 m aluminium rod, SWR and lower cutoff frequency were higher than when using BN-43 core.
PSK reporter monitoring graph. 2024-06-19, BN61 core, 3.88 m radiator.
FT-710 autotuner was capable to find a match only for and above 10 MHz. Not on 7 MHz and below. On the other hand, reception reports by pskreporter improved. In the absence of field strength measurements, this was a positive indicator.
It wasn’t clear why performance below 10 MHz was not too good. Analysis of inductance readings suggested that possibly it could be due to too low inductances, including parasitic inductance. To test this hypothesis, a small BN-43-202 core was slipped in the output wire of AutoT 6.25. Output parasitic inductance raised from 0.55 to 2.0 μH.
SWR readings improved slightly (from 5.6 to 4.4). However, SWR improvement was not sufficient to allow rig’s autotuner to find a match.
SWR before (base) and after (Ind_Par) adding parasitic inductance.
SWR before (base) and after (Ind_Par) adding parasitic inductance. Detail 0-20 MHz.
On the other hand, operation at 10 MHz and above was satisfactory. Operation tests were continued in the morning (around 07:30 UTC, 09:30 CEST). PSK reporter reception monitoring had good aspect. Especially encouraging was that it was possible to make a FT8 QSO with Costa Rica in 14 MHz.
WSJT-X screenshot. 2024-06-21, BN61 core, 3.88 m radiator.
PSK reporter screenshot. 2024-06-21, BN61 core, 3.88 m radiator.
To check if it was possible to shift down the cutoff frequency, some additional tests were carried out. First, using a longer counterpoise (4.8 + 2 m = 6.8 m). A short wire stub (0.45 m) was attached to the base of the radiator, in anticipation to the next test.
BN61 core, 3.88 m radiator + 0.45 m stub, 6.8 m counterpoise.
VNA graph. BN61 core, 3.88 m radiator + 0.45 m stub, 6.8 m counterpoise.
The longer counterpoise didn’t change significantly the cutoff frequency. SWR @ 7 MHz was still outside the matching range of FTDX10 autotuner.
Therefore, we proceeded to the next test. The counterpoise was reverted to its original length. The 2 m wire section was attached to the 0.45 m stub, making a hanging 2.45 m wire from the base of the Aluminium rod.
BN61 core, 3.88 m radiator + 2.45 m hanging wire, 4.8 m counterpoise.
This alteration improved significantly SWR in 7 MHz band (2 vs 4), to the point of allowing the autotuner to do its job. SWR was higher from 9.5 to 14.5 MHz; but was still in autotuner’s matching range. Some FT8 QSOs were made, but not too many. At 16:45 local time (14:45 UTC), propagation wasn’t excellent. Or there were not many stations on the air. Most reception records came from Europe, and USA East coast. Nevertheless, the station was heard in Japan on 21 MHz!
On the other hand, although BN-61 core losses were lower than BN-43’s, heating of UNUN’s core was still perceptible. However, never to the point of having effect on its magnetic properties. Or at least, not to the point of having a noticeable effect on SWR.
Tests with 2.45 m hanging wire. PSK reporter screenshot.
It seems that, in despite of the similarities in the matching devices, the intended use arrangement is different for Alpha and Chameleon. At least in the manuals.
Alpha calls for installing the NVIS radiator horizontally, one end attached to the base of the whip (and to the output of matching device), and the other held by a stake. Therefore, the antenna is more an off-centre-fed radiator than a proper vertical. The counterpoise is the real ground, electrically connected through the grounding rod and attached wire.
Chameleon instructions include operation as a genuine end-fed vertical. Depending on the operation bands, it could be necessary to add the extender. Recommended counterpoise is a set of one to four wires, 6.350 m long, its end held in place by stakes.
However, for comparing performance, it would be necessary to define a fixed (constant) test layout. Moreover, a layout like the one I would use would be a convenient election. For this reason, a balcony-mounted setup with a 4.8 m counterpoise (Same length than Carlos’ Alpha) was selected.
AutoT was attached to the balcony using an intermediate Aluminium alloy flat bar. In some tests, balcony railings were connected to ground as additional counterpoise, and some interesting results were obtained.
The antennas were measured from the shack. Cable to the antenna was RG-58, 5 m long in Alpha’s initial measurements. Later, it was replaced by RG-142, with a short length of RG-316 for passing a window.
Measurements were taken with a Mini VNA Pro by Mini Radio Solutions and BlueVNA software for Android. The VNA was calibrated directly on the DUT port (Intentionally, no cable compensation was made, as the purpose was to measure impedance and SWR as seen by the transceiver). Therefore, readings included impedance changes caused by the coaxial cable. Data was exported in csv format. Screenshots were taken for the sake of a quick analysis.
Tests were started on Alpha antenna system. In a first stage, only the AutoMatch and counterpoise were installed. It must be noted that electrical contact with balcony railing was not guaranteed.
Alpha AutoMatch with counterpoise, without whip.
In these conditions, a high, but not too high -SWR between 3 and 5- was observed above 40 m band. Also, periodic resonances due to the coaxial cable can be observed.
Alpha AutoMatch with counterpoise, without whip. SWR readings.
Then the whip was installed.
Alpha AutoMatch with whip.
Initial measurements didn’t look promising in the lower bands (1.8 and 3.5 MHz): SWR was above 3, the maximum theoretical range of FTDX10 autotuner. However, it could find a tuning solution, reaching less than 1.3 SWR in both bands. In higher bands, up to 70 MHz, there was no issue.
Alpha Antenna System. MiniVNA Pro screenshot.
FT-8 reception was good, but no contacts were made. Then, the antenna was packed carefully, to be returned to Carlos at the earliest opportunity.
A few months later, it was started the construction of the prototype of AutoT 6.25 UNUN. When completed, it was tested with the counterpoise alone and then with four radiators: A 1.8 m telescopic. An aluminium tubing one, 2 m long. A brass tubing one, 3 m long. And an aluminium tubing one, 3.88 m long. In this case, a good connection between electric ground and balcony railing was established.
Counterpoise alone results were similar to Alpha’s (when comparing graphs, note the difference in frequency span).
AutoT 6.25. Counterpoise only readings.
Alpha and AutoT 6.25 comparison. Counterpoise only readings.
As it could be expected, for the 3.88 m radiator, results were similar to Alpha’s. For shorter radiators, the cutoff frequency (defined as that for which SWR = 3) was higher. The shorter the length, the higher the cutoff frequency. No surprises!
AutoT 6.25 with 3.88 m radiator. MiniVNA Pro screenshot.
Data was imported into an Excel spreadsheet, which allowed preparing comparative graphs.
AutoT 6.25. SWR graphs comparison for several radiators.
The only apparent anomaly was the rise in Alpha’s SWR between 7 and 12.5 MHz. It could be explained by the bad connection from ground to balcony railing when testing it.
To test this hypothesis, ground was disconnected from the railing. The SWR graph changed, becoming more similar to that of the original Alpha (although in general better, with lower SWR!).
SWR graphs comparison. Left: Ground to railing. Right: Isolated railing.
Ground-railing connection made the graph smoother, and gave less SWR in 50 MHz, at the cost of higher SWR in 7 and 28 MHz and higher cutoff frequency. Not an interesting trade-off!
Some additional tests were carried out using a 5 m long Aluminium radiator. Up to 20 MHz results were slightly better than for 3.88 m radiator. Between 20 and 32 MHz, it was the opposite. And above 32 MHz, there was a continuous crossing, with the 5 m radiator performing better in 50 and 70 MHz bands. Overall performance in HF bands -measured in terms of SWR readings- was slightly better for the shorter radiator.
When looking for an antenna appropriate for activities like SOTA, POTA, etc., short verticals always appear in the list. They also appear when looking for “reduced space” or “difficult ubications (AKA locations)” antennas. Like balcony-installed antennas when living in a Condominium.
Among them, a couple of products, sold as “Tactical antennas” look attractive, due to their presumptive ruggedness and easy deployment: Alpha’s “6-80M HF MIL 2.0 FMJ antenna + Mount” and Chameleon’s “CHA MPAS (Modular Portable Antenna System) 2.0”.
While Chameleon is honest, and states prominently that “This item is NOT suitable for military use. More ruggedized military versions of the products are available….”, Alpha isn’t so much, by including “MIL” in product’s name, military theme pictures, etc.
Alpha Antenna (left) and Chameleon (right) “Tactical” Antennas
Both Alpha and Chameleon offer a foldable whip, in 7 sections, with a basic length around 2.850 m (Alpha: 2.845 m, Chameleon: 2.870 m).
In the case of Chameleon, it can be supplemented by an extender, 2.68 m long, for a total length of 5.55 m.
In the case of Alpha, it seems that longer models were available in the past. A fellow ham, Carlos – EA4HET, was so kind to lend a complete system for testing, which has a 10 section, 3.975 m long whip (Thanks Carlos!). Otherwise, it looks the same than the current model. Moreover, a length of wire (7.620 m), sold as “NVIS (Near Vertical Incidence Skywave) radiator” can be attached to the base of the whip (more on this later).
It is simply a piece of wire, a grounding wire, or the body of a car.
In the case of Alpha, the counterpoise is supplied as a “Grounding kit”, composed of a 2.44 m long piece of wire and a ground rod. Curiously, in the case of Carlos’ antenna, it was supplied a 4.80 m long counterpoise. A stake for holding its end was included.
In the case of Chameleon, one to four wires, 6.35 m long, can be used as counterpoise, also with stakes at their ends.
Wire counterpoises are not used when whips are mounted on a car.
These antennas, as any short vertical, need some kind of “matching device” to make them appear with an impedance in the operating range (usually SWR < 3) of the autotuners included in modern Ham transceivers.
“The AutoMatch enables touch free tuning on 10 through 80 meters when used with properly designed antennas. As an added benefit, an antenna tuner can even still be used, especially if your antenna is installed over a poor ground or near objects. This is especially useful if you have a disability such as being mobility impaired“
Can be used as a replacement match for properly designed antennas that include the MIL 2.0, FMJ, among others from Alpha including the HexTenna.
For example, when the AutoMatch is used with the HexTenna it enables touch free tuning on 10-80 meters in two easy steps shown here.”
Alpha Antenna “AutoMatch”
Specifications:
Female 3/8 24 Hex Nut is on top and a 3/8 34 (sic) Bolt is on the bottom.
Power Rating: 250W PEP SSB, 125W CW, and 25W Digital
“The CHA HYBRID SERIES Base is designed to enhance the capabilities of the common HF radio application by allowing faster tuning operation across the HF bands including MARS/CAP frequencies. This antenna base has an integral broadband impedance matching network (transformer 5:1) allowing broadband antenna tuning. An external antenna tuner is required to provide a low VSWR. The unit is water resistant.
Can be used with the CHA MIL 2.0, CHA MIL EXT 2.0, CHA CAP-HAT and CHA TACHAT…“
Chameleon’s CHA HYBRID MINI
Specifications:
Frequency: 6M – 160M
Power: 500W SSB PHONE, 100W CW, 100W HIGH DUTY CYCLE DIGITAL MODES
He started by Chameleon’s. The housing is built from CNC-machined materials (Aluminium alloy for top and bottom sections, technical polymer-reportedly Delrin- for the central body). Inside, the “matching device” reduces to a simple UNUN. He didn’t make any measurement, but some insights can be extracted from the video. Most important are:
1.- The UNUN is simply an autotransformer, with one-turn primary and total 2.5 turns. The primary is made of copper tubing with a shorting plate in the side opposite to transceiver’s connections. Inside it is coiled a… 1.5 turn! winding. This “feat” is accomplished by threading the last turn back in the same aperture by which the wire came.
Chameleon’s CHA HYBRID MINI transformer (left) and winding diagram (right)
2.- The UNUN is built on a large binocular core. Most probably from FairRite. Cores of such large size are offered only in material 43 (μi = 800), ref. BN-43-7051 and 61 (μi = 125), ref. BN-61-002. Both are used in output transformers of HF Power Amplifiers delivering 100-120 Wats CW. So, why not in an UNUN? Regarding the material used in Chameleon’s UNUN, in the absence of inductance measurements, only an educated guess can be made. From the shine of the core, it looks more like a BN-43-7051. A higher permeability core would perform better in the lower bands, in despite of higher core loss at high frequencies. Finally, higher core losses would improve SWR…. apparently (more on this later).
The schematics is simple:
Chameleon’s CHA HYBRID MINI schematic diagram
With:
1 to 2: 1 turn
2 to 3: 1.5 turns
1 to 3: 2.5 turns
This arrangement has some interesting features:
Turns (voltage) ratio is 2.5:1.
Impedance transformation ratio is 6.25:1.
The forth and back arrangement of the last turn adds some stray/parasitic inductance. It seems that this “feature” is intentional. It may help to compensate for the capacitive behaviour of electrically short antennas (more on this later).
However, none of the above values correspond to the advertised transformation ratio value (transformer 5:1). Why? Erratum or intentional diversion? This question will remain unanswered. There is not enough data to elucidate!
Alpha: Electrically, it looks identical to Chameleon’s: It is an UNUN, with the same turns’ ratio and schematic. However, construction is sloppier, using ferrite toroidal cores threaded in copper tubing, closing the first turn with thick solid wire. The arrangement of secondary wiring is the same than that of Chameleon’s.
Alpha Antenna AutoMatch transformer
The enclosure is made from cheap USA-standard PVC plumbing fittings. The description includes sentences as “Alpha Match constructed in a polyvinyl chloride housing” and “The AutoMatch is computer designed for proper balance. It contains a transformer that is enclosed in a rugged housing. The match and elements are what give the AutoMatch both the inductive and capacitive balance. Once the elements are installed, their capacitive characteristics also enables RF to radiate.” These assertions look… emmm… somehow like… mountebank speech?
Finally, the power ratings of Alpha are in general lower than Chameleon’s:
Table 1: Alpha Antenna AutoMatch and Chameleon’s Cha Hybrid Mini power ratings.
SSB (W PEP)
CW (W)
FM/Digital (W)
Alpha
250
125
25
Chameleon
500
100
50
Especially interesting is the FM and digital modes rating. Alpha’s 25 W figure looks too low. However, in view of the overheating detected in later transmission tests, it could be the right rating.
Now, some measurements. To check the hypothesis about turns ratio, inductance measurements, open circuit, were taken on Carlos’ “Alpha Match” or “AutoMatch”. An L/C meter IIB from AADE (Almost All Digital Electronics, RIP-SK) was used. It is not as accurate as a professional instrument, but it is sufficient for the intended purpose. Results are summarized in Table 2.
Table 2: Inductance of Alpha Antenna AutoMatch. Raw measurements.
Measuring points
Inductance (μH)
In (1-2)
4,048
Out (2-3)
10,35
Total (1-3)
26,06
From college physics, it could be expected that inductance would be proportional to the square of the number of turns. However, this is not exactly the case. Nevertheless, results can be explained by the presence of stray inductances. A possible model of the device is:
Where L’0 and L’1 represent the stray inductances. With the above measurements, and assuming the hypothesis of a 2.5:1 turns ratio, it is possible to compute the values of actual inductance of the windings and of stray inductances. Results are given in Table 3.
Table 3: Inductances of Alpha Antenna AutoMatch. Estimation of stray inductances.
(turns ratios were computed as the square root of actual inductance ratios)
For the sake of comparison, a couple on UNUN to the above design were constructed. One was wound on a BN-43-7051 core, the other on a BN-61-002.
UNUN built on a BN-43-7051 core
Results (measured inductance) were comparable to Alpha’s, after accounting for the different permeabilities of the ferrites used.
Measuring inductance on the UNUN built on a BN-43-7051 core
Table 4: Inductances of the UNUN built on a BN-61-002 core. Estimation of stray inductances.
UNUN on BN-61-002 core
Stray Inductance
Measuring points
Measured Inductance (μH)
Actual inductance (μH)
Turns Ratio
L’0
L’1
In (1-2)
1,075
1,055
0,02
Out (2-3)
2,94
2,37
1,50
0,02
0,55
Total (1-3)
7,14
6,59
2,50
0,55
Table 5: Inductances of the UNUN built on a BN-43-7051 core. Estimation of stray inductances.
UNUN on BN-43-7051 core
Stray Inductance
Measuring points
Measured Inductance (μH)
Actual inductance (μH)
Turns Ratio
L’0
L’1
In (1-2)
9,638
9,488
0,15
Out (2-3)
24,37
21,32
1,50
0,15
2,9
Total (1-3)
62,16
59,26
2,50
2,9
It must be noted that in the case of Alpha Antenna AutoMatch, ferrite’s permeability is an unknown. However, from the inductance values, it should be between those of material 61 and material 43. Curiously, this intermediate value is found in many unbranded Chinese ferrite cores supplied with HF Power Amplifier kits.
The above results are totally compatible with the proposed hypothesis about “Matching devices” structure.
We can conclude that both Alpha and Chameleon matching devices reduce to an UNUN with 2.5:1 turns ratio, designed to add some stray inductance to the antenna circuit.
Such UNUM will make 50 Ω in the transmitter side to appear as 312.5 Ω in the antenna side.
Final note: Palomar Engineers offer a similar device under “The Bullet” trademark. It is intended for long wire antennas, and useful information is provided on their website (https://palomar-engineers.com/tech-support/tech-topics/best-hf-end-fed-antenna/). Externally, it is similar to Alpha’s: Constructed from USA-standard PVC pipe fittings and hardware available at any ironmonger’s shop. Internally, it seems to contain a 3:1 turns ratio UNUN (9:1 Z ratio).
Esta entrada se ha escrito con el único propósito de registrar cuando se activó este sitio. Que fue el 6 de octubre del 2018, nada más obtener el indicativo EA4HCN. Ha estado inactivo durante varios años, fundamentalmente debido a los comentarios basura a las entradas en la bitácora, y a tener cosas más interesantes (o urgentes) que hacer.
Hasta julio de 2024, cuando hubo el tiempo y ganas suficientes para emprender la [no trivial] tarea de renovar el certificado SSL del sitio y empezar a añadir contenidos.
This post was written only to record when this site was started. 2018-10-06, just after getting my EA4HCN Callsign. It has not been active for years, because of spam comments to posts, and to having more interesting (or urgent) things to do.
Until July 2024, when I had some time (and will) for undertaking the [nontrivial] task of renewing site’s SSL certificate and adding contents.
