Complete (except for antenna and matching) 630 and 2200 meter
transmit station. The IF radio (a Yaesu FT-817) is tuned to
the frequency for 2200 meter WSPR operation -
136.0 kHz (+ WSPR audio offset)
Click on the image for a larger version.
There have been some attempts to use commercial amateur transceivers to produce transmit RF at these frequencies but due to the 630 meter band being less than 1/3rd the frequency of 160 meters, the filtering and circuitry within simply isn't designed for this - and that's if you can even get around the radio's inhibition to transmit outside its designed frequency range!
For example, here are a few different radios and their attempts to be used at these frequencies:
- Flex 6xxx: Several amateurs successfully use radios in the Flex 6000 series for receive and transmit on the 630 and 2200 meter bands - but with a caveat: A low level transmit signal on these bands is available only from the transverter port and an external power amplifier and filtering is required. I don't know what other Flex models may support this as well.
- Icom IC-7300: Several have reported that the IC-7300 will "go" down to 630 meters. This radio may seem to "tune" down here - and the wattmeter may even show power, but analysis has snown that not only is output at this frequency loaded with harmonics, but that attempted operation at this frequency may well stress other components. Receive performance is reportedly rather poor, requiring strong band-pass filtering for 630 meters and (possibly) some receive signal amplification.
- Drake TR-7/A: The TR-7 - a solid-state all band HF transceiver from the late 1970-early 1980s - has an "LF Input" pin on a rear panel connector which allows, with some external circuitry (amplifier, filtering) reception down to almost DC. A slight modification of the radio can permit a transmit signal to be produced on this pin as well down to a few 10s of kHz. Because this radio normally uses an analog VFO, a stable, outboard digital VFO is required to obtain the stability necessary for the narrow-band digital modes often used on these band.
- Icom IC-735: Some have reported the ability to "transmit" at 630 meters, but like the IC-7300 there is very little output at the desired frequency and there is the possibility of stressing components in the attempt. Reception requires strong filtering and some amplification.
- Elecraft K3/K3S: The K3S can reportedly produce low power (approx. 1 mW) at 630 meters on its transverter port. It would appear that doing similar for 2200 meters is not possible and that most K3S owners that operate 630/2200 meters use transverters. I do not know about the receive performance on these bands. For more information about using the K3 at 630 meters, read this app note from Elecraft.
- Even if the radio can be made to go into transmit mode at a frequency below 500 kHz, it is likely that it is producing very little power at these frequencies and is stressing transmit components: The radios' power amplifiers simply cannot be used as-is. In many radios, if they allow transmitting at all, the desired 630/2200 meter signal may be among harmonics and spurious signals, requiring good filtering if it is to be at all usable.
- Many receivers are somewhat "deaf" at these frequencies - particularly at 2200 meters. Strong band-pass filtering for the band of interest is usually warranted along with appropriate amplification.
- A practical 500 kHz low-pass filter is described here: Low-Pass filter for LF/MF (2200 meter and 630 meter) reception. This filter works well for preventing AM broadcast station overload to a receiver and it may also be used for low-pass filtering in low-power (<1 watt) transmit circuits.
Off-the-shelf 630 and 2200 meter converters:
What all of the above means is that some sort of transmit converter may be warranted. There are a number of transmit-capable converters out there designed for operation on one or both of these bands,
I'll leave it up to the reader to find these as I have not used any of them and cannot, in good faith, offer any recommendations, but a homebrew transmit converter is not particularly difficult to construct.
A practical transmit converter:
As the name implies, a transmit converter takes another frequency - produced by a conventional HF transceiver - and converts it. In my case I use an FT-817 - a low-power (5 watt) all-mode, all-band transceiver that is a favorite for VHF, UHF and microwave enthusiasts that use transverters. Because of its small size, feature set and already-low output power, it is a natural to be used in this application.
If you don't have an FT-817 (or FT-818) on hand that can be modified to transmit "everywhere" you may have an HF transceiver that has a transverter output port that can produce1-10 milliwats. If your transceiver doesn't have a low-power transverter output you will either need to modify the transceiver to have one or use a 100 watt dummy load in conjunction with a 20 dB tap (or a 20 dB pad capable of handling 100 watts) to drop the power to a "safe" level.
I constructed my transverter from parts that were on-hand, but these parts are readily available: A schematic diagram of the circuit may be seen below.
The local oscillator frequency chosen for this converter is 5 MHz - a frequency band available on many HF transceivers that have been "opened up" to allow operation on the 60 meter amateur frequencies. The choice of this frequency was also influenced by the convenience being able to use a readily-available 10 MHz oscillator, which could be a 10 MHz TCXO, an "ovenized" oscillator or 10 MHz from an available in-shack reference such as a GPSDO. The use of a "low-ish" IF frequency like 5 MHz can also enhance the stability: With many modern transceivers, a single, internal reference sets its frequency stability and accuracy and the lower the frequency, the greater the stability and I took advantage of the available of an inexpensive (<$20) EvilBay TCXO for my FT-817 to give it an overall stability that is better than one part per million over a wide temperature range.
The output of the 10 MHz oscillator is amplified/buffered if necessary and then divided-by-two by U102 - a 74HC(T)7474 - a chip that is still readily available as a DIP part. This divide-by-two step is necessary as the mixer requires a 50% duty cycle for best balance and efficiency.
Inexpensive, stable TCXOs are readily available with 1ppm ratings or better: One such a part is the Taiten TXETALSANF-10.000000 (Digi-Key 1664-1262-1-ND) which, at the time of writing, costs $2.92 in single quantities and has a rated stability of 0.5 ppm. This is a tiny 3.3 volt surface-mount device, but it can be easily adapted for this circuit: The use of a device like this - with an output that is too low to drive TTL directly - would utilize the single-transistor converter seen in the upper-right corner of Figure 2.
For an example of using small SMD TCXO like the Taiten device mentioned above, see the 20 February, 2018 entry of this blog - Better frequency stability for the QRL Labs ProgRock synthesizer - link.
The heart of the converter is U201, a 74HC4066 quad bilateral switch, used as a commutating switching mixer. While the popular FST3251 (or similar) could have been used, that chip is available only in a surface-mount package while the 74HC4066 is available in DIP - and works at least as well in this application.
On the input and output ports of this mixer are simple transformers used to assure a balanced signal in and out and these are trifilar-wound on small ferrite toroids. For my version I used some FT37-43B toroids because they were on-hand, but the more-common FT37-43 or FT50-43 could have been used instead with equal results. The exact number of turns is not particularly important, but a general rule of thumb is for such a transformer's winding to have at least three times the inductive reactance as the operating circuit (more is better - within reason).
Because our lowest intended frequency will be 136 kHz, we would calculate the inductance thusly, designing for an inductance that yields at least 3 times the operating impedance at the lowest frequency (e.g. 3x 50 = 150 ohms):
Z = 2*Pi*F*L
Z = Inductive reactance in ohms
F = Frequency in Hz
L = Inductance in Henries
2*Pi = approximately 6.28
To get inductance we rearrange the equation as:
L = Z/(2*Pi*F)
So, for 137kHz and an assumed "Z" of 150 ohms (3x 50 ohms input/output), L =
150 / (6.28 * 136000) = 175uH
Let is now refer to a handy online toroid caculator - toroids.info. If we have some FT-50-43 cores on-hand we can find this particular toroid, enter the desired inductance and we'll need 20 turns to get 175uH.
Although a bit difficult to tell from the schematic, the "inside" windings of T201 and T202 are really two of the trifilar windings in series and this is used to double the impedance seen by the switch U201 and minimize losses. Practically speaking, this is probably unnecessary in this application, but it's easy to do.
In some cases, builders have been known to apply a mid-voltage DC bias (2.5 volts in this case) to the center of the input/output windings on such a mixer, but that was not done here as testing showed that it didn't seem to make a measurable difference in performance as either a transmit or receive conversion mixer. If you don't use bias, make sure that these windings can "float" with respect to DC and the local ground.
On the input side may be seen a high-pass filter that nominally blocks signals below 5 MHz. Perhaps this is overkill, but this was included to eliminate any signals below 5 MHz that might enter the mixer - specifically any local AM broadcast stations that might have strong enough signals to ingress the cable between transceiver and the converter - not to mention the (possibly) very strong MF/LF signal from our own transmitter!
Preceding the mixer is a simple 20dB attenuator pad that is used to reduce the nominal 1 watt from an FT-817 to about 10 milliwatts. This attenuator was designed to be able to withstand the full 5 watts from the '817 in the event full power was accidentally used. As noted in the text, a 5-watt 62 ohms non-inductive resistor is ideal, but I didn't have one so I used the resistor combination shown, which is more than "good enough".
Following the mixer is a low-pass filter that removes signals above approximately 500 kHz - which includes the mixer images in the 10 MHz area and included in this circuit are R206 and C205 which form a crude diplexer to terminate those image frequencies while minimally affecting the desired LF/MF signals.
By this time the reader may have noticed that J201 and J202 are labeled as both inputs and outputs. When this circuit was first built, I envisioned making it usable as both a transmit and receive mixer - and this is possible because the signal path is completely passive. In other words, if one connected a receiver tuned to the 5 MHz area to J201 and LF/MF signals to J202, it would function as a high-performance receive converter as well.
The only caveat with its use as dual transmit-receive mixer is the presence of the 20dB attenuator - but this isn't as much of a problem as one might think: Receive antennas at LF/MF are typically amplified and the sub-microvolt sensitivity of modern HF receivers means that, in many cases, this additional 20dB of attenuation will not put the LF/MF noise floor below the receiver's noise floor. Practically speaking, a relay inserted at this point, keyed by the transmitter to insert the attenuator, would eliminate this loss but I chose to omit this circuit as it would have been inconvenient to wire this to the transmitter as well - plus it is likely that I would have accidentally transmitted into the mixer when it was in "receive" mode (e.g. no attenuator) and destroyed U201!
Ultimately, I decided to use other receive gear for 630 and 2200 meter reception: An RFSpace SDR-14 is used on 630 meters and a SoftRock Ensemble II (the LF/MF version) is used for 2200 and 1750 meter reception, each sharing a connection from an low-pass filtered, active E-field whip.
This converter will produce a few 10s of milliwatts of linear RF so some "help" is needed for driving an external amplifier, so a suitable driver amplifier is depicted in the schematic below:
Transmit driver amplifier. This amplifier is linear up to about 200 milliwatts.
Click on the image for a larger version.
As noted in the diagram, the transistor should be heat-sinked as is it is running in the linear range and is sinking a fair amount of current when idle.
The output of this amplifier is intended to be passed along to a high-power amplifier. On 630 and 2200 meters, amplifiers are typically not linear because most of the communications uses modes that transmit only single tones (e.g. CW, JT-9, WSPR) and linearity is usually traded for the higher power efficiency of a class D or E power amplifier.
An example power amplifier:
In the figure below, a typical single-ended FET-type power amplifier is depicted schematically:
The drive signal is applied via J401, which is capacitively coupled to the gate of Q401, a high-power N-channel switching FET. Transformer T401 transformers the lower impedance of the drain (10-15 ohms) to 50 ohms and this is coupled to the output via capacitors C410 and C411. Capacitors C406-C408 together form a low-impedance RF bypassing network to remove RF from the power supply lead.
An optional bias supply is shown below the main circuit, using a 78L05 5 volt regulator as a stable voltage reference which is then made adjustable via potentiometer R402. The bias is applied to the gate of Q401 via resistor R401, a 100 ohms, 2 watt resistor, the "cold" (non-RF) end of which is RF-grounded by C402 and C403. If the bias supply is omitted, the "bottom" of R401 would be connected directly to ground.
Transistor Q401 is a high-power N-channel FET of the sort found in mains-powered switching power supplies and as such, it should have a voltage rating of at least 200 volts (a higher voltage rating like 400 volts is better!) and a current rating of at least 15 amps. This transistor should be well heat-sinked: The body of the Hammond 1590D enclosure has proven adequate for continuous duty operation with the amplifer operating into a reasonably-well matched load.
A power supply voltage of 12-14 volts will produce RF power output in the 15-20 watt range while a 30 volt supply will yield 60-90 watts of RF power.
Particularly at this higher supply/output end of this range, a higher-voltage (>=400 volt) power FET is recommended to be able to withstand mismatch conditions that could occur if the antenna system is detuned. It is strongly recommended that a current-limited power supply be used with its threshold current set just above the maximum current pulled by the amplifier when driven to full output into the intended load: If a poor antenna match occurs, the transistor is somewhat protected and if there is a transistor failure, the damage to other components will be minimized.
In typical "non-linear" use the bias is either set to zero volts (R402's wiper to ground or R401 grounded) or increased such that there is only a few milliamps of FET no-signal idle current: This latter condition slightly reduces the RF drive requirement.
It is an unfortunate fact that while inexpensive power FETs can be used as inexpensive, high-power amplifiers that they are also easy to blow up. As noted in the caption of Figure 7, several components were added to improve overall stability.
This amplifier is powered from a current-limited adjustable bench-top supply (a Tenma 72-6628) that can produce 34 volts at about 3.25 amps. Having strict current limiting goes a long way toward protecting the amplifier under fault conditions (mistuned, open or shorted antenna) and has likely prevented the need to replace the transistor as often.
On the air, the amplifier has been quite reliable - never having failed while in service unless something we amiss with the antenna like the arcing-over of a loading coil or insulator - but if "proper" reverse power protection had been included, it's likely that I'd still be running the original transistor.
Linear operation at reduced power:
If linear operation is desired it is strongly recommended that the power supply voltage be limited to 18 volts as the amplifier circuit can become unstable at higher voltages (30 volts) when biased into the linear range, instantly destroying the gate-source junction of the FET. Using a 18 volt supply, approximately 25 watts PEP of RF was produced with the transistor biased at about 200mA: On the air, the audio report during a 630 meter SSB QSO was good and the observed spectra using a waterfall display appeared to be clean.
Similar RF amplifier circuits may be found at the "472kHz.org" Useful Links web page - see the "transmitting" section, near the bottom of that page.
The low-pass filter:
Figure 6, above, also depicts a low-pass filter that adequately removes the 2nd and higher harmonics and this filter is shown in the figure below.
Low-pass filter for 630 meters using 17 AWG wire wound on PVC forms.
Click on the image for a larger version.
Additional harmonic suppression will occur in any practical antenna matching network (e.g. series loading coil) of reasonable "Q".
Other low-pass filter designs suitable for high-power 630 and 2200 meter transmitting are described by W1VD at his web site:
Other entries on related topics found at this site:
- KA7OEI now QRV on 630 and 2200 meters - This page describes the actual use of this transmit converter as well as a practical matching network and antenna system.
- A (semi)-typical suburban E-field whip receive system for the 630 and 2200 meter bands - link. This page describes various techniques to effectively use an E-field whip in a noisy environment for LF/MF reception.
- Low-pass filter for LF/MF reception - link. Many HF rigs, while capable of reception at 630 meters (and possibly 2200 meters) are overloaded by signals from local AM broadcast stations. This filter can help reduce this problem.
- Completely containing switching power supply RFI - link. Switching power supplies are the bane of the LF/MF listener and this page - and the pages linked to it - contain information about how these devices may be quieted.
This list is by no means comprehensive. Peruse the "links" sections on the sites below for even more information.
- NJD Technologies - link - This web page has a wealth of information related to 630 meter operation, propagation and reports of activity, plus lists of known-active operators on both 630 and 2200 meters. This web site also has many links to others that have credible information on LF and MF band topics.
- W1TAG's web site - link - John, W1TAG, has long been an experimenter and operator on the MF and LF bands. This site has details on equipment both for operating and measuring performance at these frequencies.
- W1VD's web site - link - Jay, W1VD, has long been an experimenter on the LF/MF bands and this page offers a lot of information on equipment for transmitting and receiving on these bands.
- Antennas by N6LF - link - The callsign gives you the clue that this guy likes LF/MF operation. This page includes detailed information on LF/MF antennas and how to characterize/improve them.