Simple to build, even simpler to use, here is a device that will allow you to tune your antenna system without transmitting a detectable signal. Not only is it stealthy but it is polite: No more annoying tones and squeals. You can optimize your antenna tuner settings while listening to the QSO without missing a word. With just a few parts that you probably have in your junk box this makes a nice one-evening project.
A single momentary-action toggle switch mounted on the front panel of the tuner puts some background noise in the receiver if the antenna is not exactly 50 ohms. Null the noise with the tuner, release the switch, and you are ready to transmit. You can do it with your eyes closed in the dark. Unlike normal tuner operation, you can switch switches and adjust roller inductors while tuning without arcing (you aren't transmitting). So tuning is faster. No stresses on your transmitter, either.
The momentary switch operates a Double Pole Double Throw relay. When in the idle position the transceiver is connected to the tuner circuitry but when the switch is depressed the relay inserts the noise bridge. Power for the relay and the new circuitry can be provided from the tuner's power if it uses DC for operation. If not, a battery or external power can be used.
Why the relay? Two reasons: I had several taking up room in a parts drawer just looking for a good use and I wanted to keep the changes to the tuners minimal. I feel much more comfortable with all additional leads as short as possible and placing the circuitry right next to the connector seemed the best choice. Having modified eight tuners (so far) this strategy has paid off. No change in tuner performance is evident in any of them.
A noise generator with a single stage amplifier provides a wide band signal that covers the entire HF range and feeds a balanced transformer bridge. The bridge uses a 50 ohm resistor as a reference. When the antenna feed impedance appears to be equal to the reference resistance then current through the two windings cancel and the voltage at their junction is zero. The station receiver is connected to that junction. When the bridge is inserted into the antenna system then the received signal is just added to the error voltage at the balanced point and appears in the receiver. Any imbalance just adds noise to that received signal. Under normal conditions this circuit can detect a better than 40 dB null, more than sufficient to satisfy even the most critical operation.
An NPN bipolar transistor is the heart of the noise generator. By reverse biasing the emitter-base junction into avalanche mode and limiting it to just a few microamps there is no damage to the transistor and it generates shot noise at a much higher level than most diodes. This noise is then coupled to a high gain amplifier stage which boosts it to nearly half a volt RMS. Much stronger noise levels would overload the receiver and make the null difficult to hear. This circuitry is powered only when the switch is operated.
Construction
This project is much too simple to bother with an etched printed circuit board. I used glued pads for the first tests but quickly reverted to surface mount parts on knife-carved boards for my later versions. I like the idea of just gluing the board to one surface of the relay then using a piece of double-sided tape to mount the device in the tuner.
My first prototypes were done with “standard” through-hole type components mounted in “stand-up” fashion for easy modification. I usually use this construction mode to test my initial concept since I can then rip and re-build in minutes. I don't limit myself to just a few pad sizes. I use larger pads for multiple connections and strips for some power busses. I keep long strips of .1” pads on hand and cut off lengths to accommodate DIP components and the like.
For those projects that I intend to keep and use I prefer surface-mount construction. It is more durable, cheaper, and doesn't take up as much room. I will often make up a few boards, like this noise generator or audio amplifiers, and keep them in a little parts drawer. If I build them on small boards, most 1⁄2 or 3⁄4 inch square, I can just grab one and wire it in to a new project in minutes. No brain work. These boards are just made with a sharp knife on bare PC board stock. By not etching the board with chemicals I not only save time but am able to refine the layout with each new board.
Nothing in the circuit is especially critical but Q1, the noise generator, needs to be an NPN silicon switching transistor. A 2N2222, PN2222, 2N3904, or 2N4401 transistor will work well. Particularly the plastic packaged ones. My favorite is the little SOT-23 packaged units that I purchased as PN2222a and are marked as “1B”. They produce shot noise at levels 20 dB higher than a zener diode and are effective from low audio to over 30 MHz, saving an additional stage of amplification.
The collector of the reverse-biased transistor is left disconnected although I solder it to an isolated pad to make it sturdy. Noise signal is passed through a capacitor to block the DC component and feeds a single stage amplifier. The amplifier is configured for maximum simplicity and no concern for linearity is addressed. I tried it and it made no difference. Actually, a noisy and distorting amplifier would be perfect! Once again all values are non-critical. Use what you have.
The amplifier then feeds a hybrid transformer impedance bridge. Mine were all trifilar wound using 28 AWG magnet wire with about 10 twists per inch. I have used from 6 turns to 10 turns on FT37-43 toroids with insignificant performance difference. Polarity of the primary winding makes no difference but the builder needs to observe the polarity of the secondary (balanced) windings. Keep the leads as short as possible to maintain maximum balance.
I usually keep a few rolls of bifilar, trifilar, and quadrifilar wire handy. When I am in the mood I'll make some up by getting out a variable speed drill, laying out some wire, grabbing one end of the bundle in the shop vise and stretching the wire into an orderly bundle. The other end of the bundle is then clamped in the chuck of the drill. Keeping tension I slowly put a nice even twist on the wire. The twist keeps it uniform and easy to wind toroids. When I need some then I can grab it off the shelf.
For those not used to multi-conductor toroid winding here is a quick explanation. If you don't have individual colors on the conductors don't worry. Just wind the toroid like you would a single conductor, strip the ends, then use an ohmmeter (better yet, a beeper continuity tester) to sort out which ends belong to each other. Just reference the leads on one side of the core to those on the other side. In the case of the first pair showing continuity, call that your primary and solder that lead to the output of C3 and its mate to the ground pad near the emitter of Q2. That leaves two sets of conductors. Referencing a conductor on one side, find the conductor on the other side that does not have continuity to it, then solder these two together. This will be the center tap that feeds the normally open lug that feeds the receiver. You should now have continuity between the remaining two leads. Solder one (doesn't matter which) to the pad at the ungrounded end of R4, the other to the normally open lug of the relay side that looks toward the rest of the tuner. Describing it is much harder than doing it!
R4 should be as close to 50 ohms as possible. This is your impedance standard. If it is 52 ohms then you will be tuning your antenna to 52 ohms. This is not critical since I am sure that your equipment would still be happy with 75 ohms (a 1.5:1 VSWR). Just keep it in mind. I go through my stock and hand-pick the closest possible value to 50 ohms.
Choose a relay with contacts that will handle your power level. This is not difficult since it will always be operating at 50 ohms. The coil of the relay is more critical since this is your primary power consumer. If you use a 9 volt battery mounted on the rear of the tuner then you need a relay that operates from 9 volts and draws as little current as possible for long battery life. If you use the power from within a tuner, like that used to power dial lamps or metering circuit, then you need a relay that will use that source of power. This gadget only uses power for the short period that the switch is pressed: Half a second to check match, just a few seconds to re-tune. So the battery lasts a long time.
Place the bridge as close to the connector facing the rig as possible. The lead to the center conductor of that connector is removed and connected to the relay pin that I have labeled “to rest of tuner”. A lead from the relay that is labeled “Toward Rig” is then connected to the previously vacated connector on the tuner. Keep the ground connection to the new circuit as short and direct as possible. Usually the coax connector is a good choice. If it already has a ground lug then just solder it there. If not, then you can usually add one on one of the mounting screws. Remember that this is the RF return as well as the DC path.
Feed the DC power through a small momentary switch. I prefer a sub miniature toggle mounted on the front panel of the tuner. It is much easier to press down on the toggle to operate it rather than pressing a button because most tuners are light weight and prone to moving if pushed. Do not use a non-momentary switch. If you forget to switch it off before transmitting then you will destroy this thing instantly. The toggle feels nice and requires the operator to use one hand. A great reminder.
Cost of the project? The relay and the switch are probably the most expensive parts. The rest is much less than $1 if you use surface mount. Building your own transceiver? Consider building the noise bridge there. There is power available and you could easily include an inhibiting function to absolutely prevent transmitting while measuring. That would also permit fine-tuning a magnetic loop antenna where you cannot use a tuner. In this case a 3PDT switch could be used instead of a relay.
You may find that the existing VSWR bridges in the tuner or transmitter are not perfectly calibrated. Surprised?
Now you can tune in confidence. Silently, too! Nearby hams will never know that you are tuning up. How nice is that!
de ND6T