The United States and a few other countries have an astounding array of homeowners’ associations...Vibroplex, which first manufactured and sold its signature Morse code “bug” key back in 1904, is the longest continually operating ham radio equipment provider on the planet, but that doesn’t mean the venerable company doesn’t have more than a few new tricks up its well-worn, rolled-up sleeves.
DX Engineering’s Rod Ehrhart, K8RR, caught up with Scott Robbins, W4PA, Vibroplex’s owner since 2009, at Dayton Hamvention 2024 to discuss the mAT-TUNER line of automatic antenna tuners, which the Knoxville, Tennessee-based company has distributed in the U.S. since 2019. Watch the video below:
Scott, W4PA, highlights two new mAT-TUNER offerings—mAT-50 Automatic Antenna Tuner and Coupler Packages and the 1500PRO Automatic Antenna Tuner, both available along with the entire mAT-TUNER lineup at DXEngineering.com.
The multipurpose, 1.8-54 MHz mAT-50 Automatic Antenna Tuner comes with either an Icom, Yaesu, or Kenwood coupler and corresponding control cable. This remote tuner has the capability of matching a variety of impedances and antennas including long-wires, dipoles, verticals, Yagis, whips, and virtually any coax-fed antenna. It features a maximum input power of 120W SSB/60W Digital.
Install the sturdy, weather-resistant tuner outside near the antenna. Then install the coupler inside near the transceiver to provide power for the tuner and control its operation. The tuner does not require a control cable between the remote tuner and coupler, making installation simple. The only required connection between the antenna tuner and coupler is one 50-ohm coaxial cable. Advanced technology employed in the tuner delivers stable performance and excellent anti-interference capabilities for convenient one-key tuning.
The mAT-50 can also be used as a universal tuner for all transceivers if a mAT-50-Y or mAT-50-M coupler is connected to an external DC power supply (not included) and a control cable is not installed. You can then operate the tuner manually.
The mAT-1500PRO Automatic Antenna Tuner is an upgraded version of the original mAT-1500 tuner. Improvements include reduced weight, high- quality housing, and additional interfaces for Yaesu, Icom, and Kenwood transceivers and their corresponding operating modes. This allows the tuner to be controlled directly from the transceiver, reducing the need for manual operation.
In addition, two antennas can be connected simultaneously, and the corresponding buttons for switching (in receive mode) are placed on the front panel for easy access. Other features include:
See all mAT-TUNER automatic antenna tuners and accessories at DXEngineering.com, including the MAT-Y200 for Yaesu transceivers, the MAT-705PLUS for the Icom IC-705, and the MAT-180H for select Icom and Kenwood transceivers.
The post New Products Spotlight: mAT-TUNER mAT-50 and 1500PRO Automatic Antenna Tuners (video) appeared first on OnAllBands.
The Frankentenna consists of parts from three different antennas to create a 25 foot tall vertical non resonant radiator that when combined with a remote antenna tune results in amazing performance on the bands. Join me at the Plover River State Fishery and Wildlife Area (POTA US-10054) in North Central Wisconsin as I put this park on the air.
Chameleon URT-1 Remote Antenna Tuner: https://chameleonantenna.com/shop-here/ols/products/cha-urt-1
Chameleon SS17 17 foot stainless whip https://chameleonantenna.com/shop-here/ols/products/cha-ss17
Chameleon Mil-Ext 105 inch extension: https://chameleonantenna.com/shop-here/ols/products/cha-mil-ext-20
Feather Flag Base: https://amzn.to/3O6O7al
Antenna mirror mount jaw clamp: https://amzn.to/48yzRPL
3/8×24 SO-239 stud mount: https://amzn.to/3Sjre69
Faraday Fabric, 108×44 inches (3 yards): https://amzn.to/3RTjDv7
As a bonus, patrons can view the full, unedited phone contacts for this Parks on the Air activation. Visit my page on Patreon for details: https://www.patreon.com/kb9vbrantennas
I do return QSL, if you made a contact with me and would like a QSL, please send me one. Return postage not necessary, but always appreciated. As they say, KB9VBR is ‘good in the book.
Links may be affiliate links. As an Amazon Associate, I earn from qualifying purchases. This does not affect the price you pay.
The post Build the Frankentenna: the ultimate portable ham radio vertical antenna appeared first on KB9VBR Antennas.
The Chameleon URT-1 is a remote outdoor antenna tuner for almost any type of antenna or model of transceiver. This wide range tuner matches resonant and non-resonant wires, verticals, and long wire antennas with its range of 5 to 1500 ohms of impedance. The URT-1 has a 50 ohm coaxial port and a wire beehive connector for added versatility. And the unit is weather proof so it can be mounted outdoors near the feed point of the antenna.
Chameleon CHA URT-1 https://chameleonantenna.com/shop-here/ols/products/cha-urt-1
The purpose of an antenna tuner is not to ‘tune’ your antenna, but instead to provide a proper impedance match to your transceiver. Modern transceivers require a 50 ohm impedance and if there is a mismatch, the transceiver will respond, at the very least, by reducing output power, and at the worst, damaging the final amplifier components. So in order to deliver all available power to your antenna, a proper impedance match is required. This is a simplistic explanation, but sufficient for today.
Usually the tuner, either manual or automatic is placed near the transceiver. This is fine in most cases. If your feed line run is short, it won’t make a difference. But coaxial cable does introduce losses into the antenna system and if your antenna has a high impedance at the feed point, the mismatch will create standing waves, and the elevated SWR can be characterized as power lost in the feed line. Again that’s a simplistic explanation.
So how do we increase the overall efficiency of our antenna system? One method is to move the antenna tuner from the transceiver over to the antenna feedpoint. Situating the tuner at the antenna allows it to deliver the proper impedance match, which will be 50 ohms, to the feed line. Since impedance is matched at the coax, we can run longer pieces of cable without fear of losing energy due to high SWR on the cable.
Now remote tuners are typically used with non resonant antennas, either long wires or verticals. A resonant antenna, like a dipole or end fed half wave should have close to a 50 ohm impedance at the feed point. The tuner located at your transceiver will be used to fix slight mismatches or to extend the bandwidth of your antenna. Remote tuners would be overkill in these situations.
But with, say a 43 foot vertical antenna, the impedance may be between 400 – 900 Ohms. A 9:1 transformer could help knock that down, but using a remote tuner instead, will take whatever impedance the antenna is and deliver a consistent 50 ohms to the feed line, reducing overall system losses.
The URT-1 covers 1.8 to 54 Mhz and has 16,000 memories for quick recall when tuning. It can match any antenna with an impedance of 5 to 1,500 Ohms. That’s like a 30:1 match. And it can handle up to 125 watts sideband or CW and 60 watts on the digital modes.
Opening up the box, things may look a little familiar. This tuner is custom manufactured by Mat Tuner for Chameleon. It does look very similar to their MAT 40 remote tuner, but there are some key differences.
The tuner comes in two parts, the first is the coupler box. This connects to the transceiver and to a 12 volt power source. Then your coax cable runs all the way out to the tuner box which sits at the antenna feed point. You will notice that there are not separate control cables for the tuner. Power and tuner control are fed through the coax cable by way of a Bias-T circuit. The benefit to that is that you don’t have to run a second cable to the tuner unit, but the downside is that you need to initiate tunes by pressing the tuning button on the coupler.
The tuner unit is constructed out a aluminum alloy and it weather proof. On the top of the unit is a beehive connector for feeding wire or vertical antennas. On the bottom is a counter poise and ground connector along with two UHF female connectors. One UHF connector connects to your coax run coming from the coupler and the second is for feeding antennas that have a similar SO-239 connection point.
The tuner comes with a set of rails so that you can mount it to a post or board using a pair of U-Bolts. An option 12 volt AC adapter is also available.
So what’s different between the URT-1 and the MAT 40 tuner? Namely the addition of a 50 ohm coaxial output port and the removal of the brand specific control cables. The URT-1 is a bit more universal in that the coupler unit will work with just about any brand or model of transceiver.
Using the URT-1 is pretty simple. We’ll first connect the coupler to our transceiver. My main antenna, a G5RV, is connected to my LDG auto tuner, so going to put this one onto the 2nd antenna port on my transceiver. A coax jumper goes from the radio to the coupler. The antenna coax is then connected to the other port on the coupler. Finally connect the power. The green power light should glow. If you see the red error light, that means the there is a short circuit somewhere in the coax connection between the coupler and the tuner.
When connecting the tuner, you can not have any devices like switches, diplexers, or meters in the path between the coupler and the tuner. These could cause a short circuit, potentially damaging the tuner or your device.
To initiate a tune, put your transceiver into a constant carrier mode like RTTY. Set the power level to 15 watts or less, briefly hit the tune button, and immediately key the transmitter. Watch the transceiver SWR meter and it will show the resulting SWR when the tuning cycle is complete. Unkey the transmitter. A complete tuning cycle will take five seconds or less.
At this point you can transmit normally. When changing bands, you will reinitiate the tune process. The tuner has 16,000 memories, so once the unit finds a good match, it will remember it for faster subsequent tunes.
Now let’s head outdoors and I’ll show you a couple of ways you can use the remote tuner in your portable amateur radio operations.
What are my thoughts on the Chameleon URT-1 remote antenna tuner? First off, I must say that this unit is well constructed. The tuning unit consists of an aluminum alloy body that has a certain amount of heft to it. It wins points on that item alone. Taking this out into the field was a breeze as I didn’t have to run a separate control cable to the tuner for power. Operationally, it tunes fast and had no problem finding a match that was 1.5:1 or less. I think the only time I had issues with it getting a good match was with my 25 foot Franken-tenna on the 15 meter band. In that instance it gave up at about 1.8:1. But with the Frankentenna it did perform quite well on 10, 15, 20, and 40 meters despite the bands not being in the best condition. I made 210 contacts on those bands activating the Mountain Bay state trail, with the bulk of them on the 20 meter band.
With the end fed random wire antenna, I connected the tuner up to UHF connection on the antenna. The addition of the 9:1 transformer with the tuner made for super fast tune times. Every time it found a match at rocket speed. For that activation of Ackley Wildlife Area I ended up with 176 contacts on 10, 12, 15, 17, and 20 meters. The great thing about non resonant antennas is their agility. To switch bands, all I had to do was hit the tune button and I was good to go.
I had the same experience back here at home with the 71 foot non resonant wire. Once it learned the antenna, it would re-tune almost instantly. I was able to get matches with this wire from 10 meters all the way down to 80 meters. Operationally, the antenna performed just as well, maybe a bit better than with my other tuner located at the feed point. This antenna is being fed with 75 feet of RG-8X, so the better match at the feed point does make a difference.
As for things I don’t quite like about the tuner, First off these UHF ports are not labeled, so you need to look at the instructions to determine which goes to the coupler and which goes to the antenna. Also the ports didn’t come with covers. If you are using the bee hive connector, having a cover on the unused UHF port would be nice. I’ve got a friend that 3D printed some, so i did have something that worked. Finally, and probably the biggest, is that you have to press the tune button on the coupler to initiate a tune. If your SWR changes or you change bands, the tuner won’t automatically retune, it needs to be activated. That involves switching to a carrier mode like RTTTY, pressing the tune button, and then transmitting a carrier. Not the worst thing in the world, but also not fully automatic like some brands of remote tuners. But those require a separate control and power cable and up side of this tuner is that you don’t have to run a separate power and control cable to the tuner. Your power runs through the coax, which makes installation a breeze.
But, final words, I’ve been looking for a remote tuner at the hamfests. Everything I’ve seen is overpriced or in bad shape. It’s a bit serendipitous for Chameleon to be sending me this as it opens up the door to a bunch of different antenna configurations that I’ll be able to demonstrate. I’ve got some great ideas to use this tuner with, so you’ll want to stick around for that.
Build a random wire antenna and make amazing contacts: https://youtu.be/D_-aNzrIXWs
Yaesu FT-891 transceiver: https://amzn.to/3RSg4DO
213 inch whip antenna: https://amzn.to/3O3Bvkm
Feather Flag Base: https://amzn.to/3O6O7al
Antenna mirror mount jaw clamp: https://amzn.to/48yzRPL
3/8×24 SO-239 stud mount: https://amzn.to/3Sjre69
As a bonus, patrons can view the full, unedited phone contacts for this Parks on the Air activation. Visit my page on Patreon for details: https://www.patreon.com/kb9vbrantennas
I do return QSL, if you made a contact with me and would like a QSL, please send me one. Return postage not necessary, but always appreciated. As they say, KB9VBR is ‘good in the book.
Links may be affiliate links. As an Amazon Associate, I earn from qualifying purchases. This does not affect the price you pay.
The post Maximize Your Signal with the Chameleon URT-1 Remote Antenna Tuner appeared first on KB9VBR Antennas.
… The Choice had to be made…
Field Day 2024 will be starting in 3 hours. In previous posts I had mentioned that I was either going to be operating as a 1B station or a 1D stations depending on the weather. “Mother Nature” helped me make the choice and its not nice (or smart) to argue with Mother.
The choice is “1D” and if you read the following from The Weather Network you will see why.
Special Weather Statement
Issued at Sat 8:59 AM Jun. 22
Issued by: Environment and Climate Change Canada
Description:
Significant rainfall possible this afternoon into Sunday.
Hazard:
Rainfall amounts of 30 to 50 mm. Locally higher amounts possible.
Timing:
This afternoon into Sunday.
Discussion:
The potential exists for multiple rounds of showers and thunderstorms this afternoon into Sunday. Local amounts exceeding 50 mm are possible. A rainfall warning may be required for some areas.
For information concerning flooding, please consult your local Conservation Authority or Ontario Ministry of Natural Resources and Forestry office. Visit Ontario.ca/floods for the latest details.
Heavy downpours can cause flash floods and water pooling on roads.
Watch for possible washouts near rivers, creeks and culverts.
###
So there you have it….
Setting up in a Park to operate 1B in a Monsoon is one way to ruin my almost new FT891 seeing that as a general rule they don’t like water.
And to be honest it would not be fun for me either.
I still will be participating and hope to give out the rare exchange of 1D ONE to as many stations I can contact during the event. At home I have the chance to operate VHF, 6m, and the HF Bands that the Field Day rules allow.
So have fun, stay safe and make lots of contacts
73bob
That was even after I had replaced the two polyvaricon capacitors. The two screws for mounting each of the variable capacitors were too long. The result was that they interfered with the moving plates of the capacitor and I ruined both of them. There was no warning of this in the manual, so beware anyone who buys this kit.
Despite fixing this, it didn't really tune anything. I later learned that it probably works for low bands like 80 m as it is, but I didn't notice that. I thought maybe the toroid core of the inductor was of inferior quality, but never really investigated it, so it sat unused on the shelf for 7-8 years.
Recently, however I learnt that there is an error in the instructions which makes all the difference: The order of the taps in the inductor should be reversed. In this way, one can get the small inductance value consisting of only a single turn at position A of the rotary switch. Prior to this fix, the inductor starts with 10 turns at position A, and this is too much except for the lower bands like 80m.
This is explained in this video by KP4MD, Carol, from 7 July 2021 starting from about 10 minutes. The particular frame that shows this is reproduced here also.
What I did was to desolder all 12 or so connections from the inductor to the rotary switch - be careful not to melt the plastic around the switch contacts. I actually did that and had to disassemble it in order to get it back in working order. Then I unwound the wire from the toroid core and rewound it in the opposite direction. And then resoldered it back.
That made the trick, now it tunes 20 m also!
By the way, the odd name for this tuner seems to be a mistranslation from Chinese. It should have been the more prosaic "Manual antenna tuner kit".
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| The TS-570D's front panel |
Asking about it, he said that it worked fine, but that the "tuner wouldn't stop", so it had to be used with the antenna tuner bypassed. Visually inspecting it, it looked to be in "good nick" (a 4 out of 5) so I shut up and gave him the money.
After digging out from underneath a few other projects, I finally took a look at it and sure enough, pressing the AT TUNE button started a bout of furious clicking that didn't stop for about 30 seconds with the radio beeping an error. I couldn't help but notice, however, that there was no SWR or power output indication while the tuner was doing its thing - but if I bypassed the tuner, both of these were true.
Going into the menu (#11 - "Antenna tuner operation while receiving") I set that to "on" and noticed that the receiver went mostly dead - a sure sign that something was amiss with the signal path through the tuner. Popping the covers, I whacked on the relays with the handle of a screwdriver while the radio was connected to an antenna and could hear signals come and go. This attempt at "percussive repair" quickly narrowed the culprit to relay K1, the relay that switches the antenna tuner in and out of the signal path.
A few weeks later, after having ordered and receive a new relay, I cleared enough space on the workbench to accommodate the radio and commenced a repair.
The repair:
The antenna tuner is on the same, large circuit board as the final and low-pass filter, which meant that not only were there a zillion screws to take out, but I also had to remove the white thermal heat-sink compound from several devices, un-clip the back panel connectors and un-plug a few signal cables. Using my trusty Hakko DFR-300 desoldering gun, I was able to cleanly remove both K1 and - because I had two relays, and they were identical - K3 as well, soldering in the replacement.
When I'd pulled the board, I also noticed that components "D10" - which is a glass discharge tube across Antenna connector #2 - had some internal discoloration, possibly indicating that it had seen some sort of stress, so I rummaged about and found two 350 volt Bourns gas discharge tubes and replaced both "D10" and "D11" - the unit on the Antenna #1 connector. Unlike the originals - which are glass - these are metal and ceramic, requiring that I put a piece of polyamide (a.k.a. Kapton) tape on the board to insulate them from the traces underneath. The leads of these new devices were also much heavier and would not fit through the board (drilling larger would remove through-plating!) so I soldered short lengths of #24 tinned wire through the holes and used these to attach the straight leads of the new discharge tubes.
After cleaning the board of flux with denatured alcohol and an old toothbrush, I put an appropriately sparse amount of heat sink compound on the required devices, loosely started all of the screws and with everything fitting, I snugged them all down, finishing with the RF output transistors - and then re-checking everything again to make sure that I didn't miss anything.
After plugging the connecting cables back in I noted that the receiver now worked through the tuner and pressed the AT Tune button and was greeted with lots of clicking and varying VSWR - but still, it continued and eventually errored out.
Figuring that the radio's computer may have been messed up, I did a complete CPU reset, but to no avail. Because the SWR and power indication were working correctly, I knew that this wasn't likely to be a component failure like the reverse power detection circuit, so it had to be something amiss with the configuration, so I referred to the service manual's section about the "Service Adjustment Mode".
Going through the Service Adjustment Mode Menu:
Like most modern radios, this one has a "Service Menu" where electronic calibration and adjustments are performed and to get to it, I inserted a wire between pins 8 and 9 of the ACC2 jack and powered up the radio while holding the N.R. and LSB USB keys and having done this, a new menu appeared. On a hunch, I quickly moved to menu #18 - the adjustment for the 100 watt power level.
What is supposed to happen is that if you key the radio, it will transmit a 100 watt carrier on 14.2 MHz, but instead, I got about 60 watts, and checking the related settings for 50, 25, 10 and 5 watts, I got very low power levels for each of those as well. To rule out an amplifier failure, I went back to the 100 watt set-up and pressed the DOWN button, eventually getting over 135 watts of output power, indicating that there was nothing wrong with the finals, but rather that the entire "soft calibration" procedure would have to be followed.
Starting at the beginning of the procedure which begins with receiver calibration, I found everything to be "wrong" in the software calibration, indicating that either it was improperly done, or the original calibration had somehow been lost and replaced with default values. I checked a few of the hardware adjustments, but found them to be spot on - the exception being the main reference oscillator, which was about 20 Hz off at 10 MHz, which I dialed back in, chalking this up with aging of the crystal.
During the procedure, I was reminded by a few peculiarities - and noticed some likely errors, and here they are in no particular order:
The result:
As mentioned, the "hardware" calibration seemed to be fine and only the "soft" calibration was off and after following this procedure, the tuner worked exactly as it should. What I suspect was occurring was a combination of the the output power being too low to calculate an SWR (e.g. setting the radio to "5 watts" yielded less then 2) and that the SWR meter calibration itself was incorrect and that this combination of factors prevented the tuner from being able to find a match.
Since the repair, the TS-570 has been used several times per week and it is working just as it should!
This post stolen from ka7oei.blogspot.com
[End]
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| Figure 1: The completed absorptive VSWR bridge. |
A bit about the Wheatstone bridge:
The Wheatsone bridge is one of the oldest-known types of electrical circuits, first having been originated around 1833 - but popularized about a decade later by Mr. Wheatstone itself. Used for detecting electrical balance between the halves of the circuit, it is useful for indirectly measuring all three components represented by Ohm's law - resistance, current and voltage.
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| Figure 2: Wheatstone bridge (Wikipedia) |
Figure 2 shows the classic implementation of a Wheatstone bridge. In this circuit, balance of the two legs (R1/R2 and R3/Rx) results in zero voltage across the center, represented by "Vg" which can only occur when the ratio between R1 and R2 is the same as the ratio between R3 and Rx. For operation, that actual values of these resistors is not particularly important as long as the ratios are preserved.
If you think of this is a pair of voltage dividers (R1/R2 and R3/Rx) its operation makes sense - particularly if you consider the simplest case where all four values are equal. In this case, the voltage between the negative lead (point "C") and point "D" and points "C" and "B" will be half that of the battery voltage - which means the voltage between points "D" and "B" will be zero since they must be at the same voltage.
Putting it in an RF circuit:
Useful at DC, there's no reason why it couldn't be used at AC - or RF - as well. What, for example, would happen if we made R1, R2, and R3 the same value (let's say, 50 ohms), instead of using a battery, substituted a transmitter - and for the "unknown" value (Rx) connected our antenna?
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| Figure 3: The bridge, used in an antenna circuit. |
This describes a typical RF bridge - known when placed between the transmitter and antenna as the "Tayloe" bridge, the simplified diagram of which being represented in Figure 3.
Clearly, if we used, as a stand-in for our antenna, a 50 ohm load, the RF Sensor will detect nothing at all as the bridge would be balanced, so it would make sense that a perfectly-matched 50 ohm antenna would be indistinguishable from a 50 ohm load. If the "antenna" were open or shorted, voltage would appear across the RF sensor and be detected - so you would be correct in presuming that this circuit could be used to tell when the antenna itself is matched. Further extending this idea, if your "Unknown antenna" were to include an antenna tuner, looking for the output of the RF sensor to go to zero would indicate that the antenna itself was properly matched.
At this point it's worth noting that this simple circuit cannot directly indicate the magnitude of mismatch (e.g. VSWR - but it can tell you when the antenna is matched: It is possible to do this with additional circuitry (as is done with many antenna analyzers) but for this simplest case, all we really care about is finding when our antenna is matched. (A somewhat similar circuit to that depicted in Figure 3 has been at the heart of many antenna analyzers for decades.)
Antenna match indication and radio protection:
An examination of the circuit of Figure 3 also reveals another interesting property of this circuit used in this manner: The transmitter itself can never see an infinite VSWR. For example, if the antenna is very low resistance, we will present about 33 ohms to the transmitter (e.g. the two 50 ohm resistors on the left side will be in parallel with the 50 ohm resistor on the right side) - which represents a VSWR of about 1.5:1. If you were to forget to connect an antenna at all, we end up with only the two resistors on the left being in series (100 ohms) so our worst-case VSWR would, in theory, be 2:1.
In context, any modern, well-designed transmitter will be able to tolerate even a 2.5:1 VSWR (probably higher) so this means that no matter what happens on the "antenna" side, the rig will never see a really high VSWR.
If modern rigs are supposed to have built-in VSWR protection, why does this matter?
One of the first places that the implementation of the "Tayloe" bridge was popularized was in the QRP (low power) community where transmitters have traditionally been very simple and lightweight - but that also means that they may lack any sophisticated protection circuit. Building a simple circuit like this into a small antenna tuner handily solves three problems: Tuning the antenna, being able to tell when the antenna is matched, and protecting the transmitter from high VSWR during the tuning process.
Even in a more modern radio with SWR protection there is good reason to do this. While one is supposed to turn down the transmitter's power when tuning an antenna, if you have an external, wide-range tuner and are quickly setting things up in the field, it would be easy to forget to do so. The way that most modern transmitter's SWR protection circuits work is by detecting the reflected power, and when it exceeds a certain value, it reduced the output power - but this measurement is not instantaneous: By the time you detect excess reflected power, the transmitter has already been exposed - if only for a fraction of a second - to a high VSWR, and it may be that that brief instant was enough to damage an output transistor.
In the "old" days of manual antenna tuners with variable capacitors and roller inductors, this may have not been as big a deal: In this case, the VSWR seen by the transmitter might not be able to change too quickly (assuming that the inductor and capacitors didn't have intermittent connections) but consider a modern, automatic antenna tuner full of relays: Each time the internal tuner configuration is changed to determine the match, these "hot-switched" relays will inevitably "glitch" the VSWR seen by the radio, and with modern tuners, this can occur many times a second - far faster than the internal VSWR protection can occur meaning that it can go from being low, with the transmitter at high power, to suddenly high VSWR before the power can be reduced, something that is potentially damaging to a radio's final amplifier.
While this may seem to be an unlikely situation, it's one that I have personally experienced in a moment of carelessness - and it put an abrupt end to the remote operation using that radio - but fortunately, another rig was at hand.
A high-power Tayloe bridge:
It can be argued that these days, the world is lousy with Tayloe bridges as they are seemingly found everywhere - particularly in the QRP world, but there are fewer of them that are intended to be used with a typical 100 watt mobile radio - but one such example may be seen below:
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| Figure 4: As-built high-power Tayloe bridge with a more sensible bypass switch arrangement! This diagram was updated to include a second LED to visually indicate extreme mismatches and provide another clue as to when one is approaching a match. |
In this case, the 50 ohm resistors are thick-film, 50 watt units (about $3 each) which means that between the three of them, they are capable of handling the full power of the radio for at least a brief period. Suitable resistors may be found at the usual suppliers (Digi-Key, Mouser Electronics) and the devices that I used were Johanson P/N RHXH2Q050R0F4 (A link to the Mouser Electronics page is here) - but there is nothing special about these particular devices: Any 50-100 watt, TO-220 package, 50 ohm thick-film resistor with a tolerance of 5% or better could have been used, provided that its tab is insulated from the internal resistor itself (most are).
How it works:
Knowing the general theory behind the Wheatstone bridge, the main point of interest is the indicator, which is, in this case, an LED circuit placed across the middle of the bridge in lieu of the meter shown in Figure 1. Because RF is present across these two points - and because neither side of this indicator is ground-referenced, this circuit must "float" with respect to ground.
If we presume that there will be 25 volts across the circuit - which would be in the ballpark of 25 watts into a 2:1 VSWR - we see that the current through 2k could not exceed 25 mA - a reasonable current to light an LED. To rectify it, a 1N4148 diode - which is both cheap and suitably fast to rectify RF (a garden-variety 1N4000 series diodes is not recommended) along with a capacitor across the LED. An extra 2k LED is present to reduce the magnitude of the reverse voltage across the diode: Probably not necessary, bit I used it, anyway. QRP versions of this circuit often include a transformer to step up the low RF voltage to a level that is high enough to reliably drive the LED, but with 5-10 watts, minimum, this is simply not an issue.
Because the voltage across the bridge goes to zero when the source and load impedance are matched (or the switch is set to "bypass" mode) there is no need to switch the detector out of circuit but note that the LED and associated components are "hot" at RF when in "Measure" position which means that you should keep the leads for this circuit quite short and avoid the temptation to run long wires from one end of a large enclosure (like an antenna tuner) to the other as excess stray reactance can affect the operation of the circuit.
Note: See the end of this article for an updated/modified version with a second LED .
A more sensible bypass switch configuration:
While there are many examples of this sort of circuit - all of them with DPDT switches to bypass the circuit - every one that I saw wired the switch in such a way that if one were to be inadvertently transmitting while the switch was operated, there would be a brief instant when the transmitter was disconnected (presuming that the switch itself is a typical "break-before-make" - and almost all of them are!) that could expose the transmitter to a brief high VSWR transient. In Figure 3, this switch is wired differently:
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| Figure 5: Inside the bridge, before the 2nd LED was added |
An as-built example:
I built my circuit into a small die-cast aluminum box as shown in Figure 5. Inside the box, the 50 ohm resistors are bolted to the box itself using countersunk screws and heat-sink paste for thermal transfer. To accommodate the small size of the box, single-hole UHF connectors were used and the circuit itself was point-to-point wired within the box.
For the "bypass" switch (see Figure 6) I rescued a 120/240 volt DPDT switch from an old PC power supply, choosing it because it has a flat profile with a recessed handle with a slot: By filing a bevel around the square hole (which, itself was produced using the "drill-then-file" method) one may use a fingernail to switch the position. I chose the "flush handle" type of switch to reduce the probability of it accidentally being switched, but also to prevent the switch itself from being broken when it inevitably ends at the bottom of a box of other gear. | |
| Figure 6: The "switch" side of the bridge. |
As on might posit from the description, the operation of this bridge is as follows:
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| Figure 7: The "enhanced" version with TWO LEDs. |
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| Figure 8: It has two LEDs now! |
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