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Recent discussion on the RSGB LF Group reflector about high-powered LF / MF amplifiers brought an interesting response from Tom, DK1IS, and his unique solution.

It's no secret that a Class D / E amplifier using switching MOSFETs is a popular and reasonably inexpensive method of generating some serious RF on the LF and MF bands. Equally well-known is their propensity to gobble-up FETs should the amplifiers encounter much reactance in their output load. Most builders include some form of protection for sudden over-current or unwanted SWR excursions which will shut down the amplifier before any FETs can self-destruct. Those that don't usually end up replacing FETs.




I would venture to guess that over 90% of the transmitters now being employed on LF or MF are using switching MOSFETs in a Class D / E design but there are some amateurs using vacuum tubes to do their heavy-lifting ... and with good results.

DK1IS's beautiful homebrew amplifier is shown below. Tom provided the following description:

Hi Wolf and group,

nice to hear that someone else is thinking about this approach! I´m
content with my homemade tube PA for LF and MF which has provided
reliable service since nearly 4 years now. Only some thoughts about this
concept - I hope not to bore all those hams who are happy with their
semiconductor PAs:

Years ago I had a MOSFET PA for LF, Class B push-pull with 250 W RF. It
worked well at constant conditions, but when I had to retune the antenna
due to larger QSY or made antenna experiments there always was the
danger of blowing up these nervous semiconductors. After 4 or 5 times
changing the MOSFETS I decided to build a new PA - with tubes! Looking a
little bit anachronistic this PA is absolutely good-natured. Designed for
broadband service on LF and MF it makes no problems when changing the
antenna coarse tuning from one band to the other even when the fine
tuning isn't done yet. With my former MOSFET-PA this would have been
impossible.

I wanted to have a linear PA - this usually means class B. You have to
decide between narrow band and broad band (like an audio-amp) design.
For narrow band you can use a single-ended PA but you have to add a
resonance circuit. For broad band you should use a push-pull PA and have
to build a suitable output transformer. I opted for broad band design
because it is usable for LF and MF without changes at the PA. With this
design and sin-driving I reach a total harmonic distortion of about 5 %
at 700 W RF on a pure resistive dummy load. With the usual narrow,
narrow band antennas on LF and MF you don´t need additional filters!

Concerning the tubes: If you take the common TX tubes with plate
voltages of several kV all output circuits have rather high impedances,
that means large coils for the resonance circuits resp. large
transformer windings and very high voltages - potentially a construction
problem. This led me to the choice of 2x 4x PL519 in push-pull, a rugged
colour TV line output tube with low plate voltage and high plate
current. In this way I came down to a plate-to-plate resistance of about
1 kOhm at 600 V DC plate voltage, where you easily can build a ferrite
broad band output transformer down to 50 Ohms. A disadvantage of this
concept is that you have to give individual bias to each tube, that
means for the first start-up you have to align 8 potentiometers
carefully to nearly equal cathode currents for all the tubes. But
according to my experience this alignment remains stable over a long
time. I have inserted 1-Ohm-resistors in each cathode line and have
brought the voltage drops to 8 cinch connectors, where I can monitor the
DC component (with external filtering) as well as the real time current.
With 4 tubes in parallel per branch of course you have to take care for
self oscillations. The extensive use of bypass capacitors, ferrite beads
and parasitic chokes in the plate lines is mandatory as well as good
grounding concepts are. The tubes don´t pull control grid current (this
would even be true in class C!) but you need 3 or 4 W RF input power due
to all the ohmic loads at the tube´s control grids caused by the
individual bias paths. On the other hand this certainly helps to avoid
oscillations. You can see some pictures of this PA at https://www.qrz.com/db/DK1IS

By the way: why not to try these tubes at class D? With DC plate
voltages of perhaps 1200 V you should get a nice QRO-PA ...

Wolf, you are right: building such a PA from scratch is a time consuming
enterprise. I didn´t count the working hours but according to my lab log
the whole project took about 9 months - an adequate time for a new baby!
It was a great experience anyway.

Good luck and 73,
Tom, DK1IS

2x 4x PL519 Push-Pull

TX, power supply, RX, exciter

As 'day one' for U.S. amateurs gaining their new LF / MF bands draws closer, many are seriously thinking about building or modifying rigs to get on the air. I'm sure the use of switching MOSFET amplifiers will still reign supreme when it comes to decision-making time but with some feeling more comfortable with tube technology, especially when used for power amplifiers, I suspect we will see some interesting tube transmitters being employed as well. I'm sure that some will even seek to modify older tube rigs, knowing that a pair of 6146's can muster enough power to have plenty of fun on the new band as well.

DK1IS has provided an inspiring example of what can be done using vacuum tubes ... they certainly should not be discounted as a viable method of generating your hard-earned LF / MF ERP.

Recent discussion on the RSGB LF Group reflector about high-powered LF / MF amplifiers brought an interesting response from Tom, DK1IS, and his unique solution.

It's no secret that a Class D / E amplifier using switching MOSFETs is a popular and reasonably inexpensive method of generating some serious RF on the LF and MF bands. Equally well-known is their propensity to gobble-up FETs should the amplifiers encounter much reactance in their output load. Most builders include some form of protection for sudden over-current or unwanted SWR excursions which will shut down the amplifier before any FETs can self-destruct. Those that don't usually end up replacing FETs.




I would venture to guess that over 90% of the transmitters now being employed on LF or MF are using switching MOSFETs in a Class D / E design but there are some amateurs using vacuum tubes to do their heavy-lifting ... and with good results.

DK1IS's beautiful homebrew amplifier is shown below. Tom provided the following description:

Hi Wolf and group,

nice to hear that someone else is thinking about this approach! I´m
content with my homemade tube PA for LF and MF which has provided
reliable service since nearly 4 years now. Only some thoughts about this
concept - I hope not to bore all those hams who are happy with their
semiconductor PAs:

Years ago I had a MOSFET PA for LF, Class B push-pull with 250 W RF. It
worked well at constant conditions, but when I had to retune the antenna
due to larger QSY or made antenna experiments there always was the
danger of blowing up these nervous semiconductors. After 4 or 5 times
changing the MOSFETS I decided to build a new PA - with tubes! Looking a
little bit anachronistic this PA is absolutely good-natured. Designed for
broadband service on LF and MF it makes no problems when changing the
antenna coarse tuning from one band to the other even when the fine
tuning isn't done yet. With my former MOSFET-PA this would have been
impossible.

I wanted to have a linear PA - this usually means class B. You have to
decide between narrow band and broad band (like an audio-amp) design.
For narrow band you can use a single-ended PA but you have to add a
resonance circuit. For broad band you should use a push-pull PA and have
to build a suitable output transformer. I opted for broad band design
because it is usable for LF and MF without changes at the PA. With this
design and sin-driving I reach a total harmonic distortion of about 5 %
at 700 W RF on a pure resistive dummy load. With the usual narrow,
narrow band antennas on LF and MF you don´t need additional filters!

Concerning the tubes: If you take the common TX tubes with plate
voltages of several kV all output circuits have rather high impedances,
that means large coils for the resonance circuits resp. large
transformer windings and very high voltages - potentially a construction
problem. This led me to the choice of 2x 4x PL519 in push-pull, a rugged
colour TV line output tube with low plate voltage and high plate
current. In this way I came down to a plate-to-plate resistance of about
1 kOhm at 600 V DC plate voltage, where you easily can build a ferrite
broad band output transformer down to 50 Ohms. A disadvantage of this
concept is that you have to give individual bias to each tube, that
means for the first start-up you have to align 8 potentiometers
carefully to nearly equal cathode currents for all the tubes. But
according to my experience this alignment remains stable over a long
time. I have inserted 1-Ohm-resistors in each cathode line and have
brought the voltage drops to 8 cinch connectors, where I can monitor the
DC component (with external filtering) as well as the real time current.
With 4 tubes in parallel per branch of course you have to take care for
self oscillations. The extensive use of bypass capacitors, ferrite beads
and parasitic chokes in the plate lines is mandatory as well as good
grounding concepts are. The tubes don´t pull control grid current (this
would even be true in class C!) but you need 3 or 4 W RF input power due
to all the ohmic loads at the tube´s control grids caused by the
individual bias paths. On the other hand this certainly helps to avoid
oscillations. You can see some pictures of this PA at https://www.qrz.com/db/DK1IS

By the way: why not to try these tubes at class D? With DC plate
voltages of perhaps 1200 V you should get a nice QRO-PA ...

Wolf, you are right: building such a PA from scratch is a time consuming
enterprise. I didn´t count the working hours but according to my lab log
the whole project took about 9 months - an adequate time for a new baby!
It was a great experience anyway.

Good luck and 73,
Tom, DK1IS

2x 4x PL519 Push-Pull

TX, power supply, RX, exciter

As 'day one' for U.S. amateurs gaining their new LF / MF bands draws closer, many are seriously thinking about building or modifying rigs to get on the air. I'm sure the use of switching MOSFET amplifiers will still reign supreme when it comes to decision-making time but with some feeling more comfortable with tube technology, especially when used for power amplifiers, I suspect we will see some interesting tube transmitters being employed as well. I'm sure that some will even seek to modify older tube rigs, knowing that a pair of 6146's can muster enough power to have plenty of fun on the new band as well.

DK1IS has provided an inspiring example of what can be done using vacuum tubes ... they certainly should not be discounted as a viable method of generating your hard-earned LF / MF ERP.

5. Antenna height will be limited:

... we will require that the antennas used to transmit in these bands not exceed 60 meters in height above ground level (AGL), as ARRL proposed.

6. Regarding transmission modes, no bandwidths have been specified in order to encourage experimentation:

Consistent with our proposal in the WRC-12 NPRM,  and with the existing rules in Section 97.305 for the frequency bands below 30 MHz, we authorize amateur stations to transmit the following emission types throughout the new amateur bands: CW (international Morse code telegraphy), RTTY (narrow-band direct-printing telegraphy), data, phone, and image emissions.   These emission types provide amateur operators with maximum flexibility, and we find that additional restrictions would needlessly hinder experimentation.

7. Experimental stations appear to 'still be in business' but are encouraged to transition to the 'amateur' service:

Finally, we decline to permit previously licensed experimental stations – some of which have been authorized with significantly more radiated power than the adopted EIRP limits for these new amateur service bands – to communicate with amateur stations operating in these bands.  Amateur operations in these bands currently authorized under experimental licenses should transition their operations in accordance with the adopted rules and not circumvent such rules by use of experimental licenses.

My understanding of the R&O document is that participating parties may still file a 'Petition For Reconsideration' notification within 30 days of the R&O's publication in the Federal Register. Once these (if any) are dealt with, there are no other roadblocks preventing immediate implementation.

The document contains additional details not discussed here and makes fascinating reading for amateurs that might be looking forward to the new allocations.

This is the news that many U.S. amateurs have been waiting many years to hear! It is also good news for Canadian's operating on these bands to know that they may soon see a large increase in activity south of the border. Let's hope things continue to transpire favorably and that we will finally see the new bands become a reality.

Get those soldering irons out guys and gals!

5. Antenna height will be limited:

... we will require that the antennas used to transmit in these bands not exceed 60 meters in height above ground level (AGL), as ARRL proposed.

6. Regarding transmission modes, no bandwidths have been specified in order to encourage experimentation:

Consistent with our proposal in the WRC-12 NPRM,  and with the existing rules in Section 97.305 for the frequency bands below 30 MHz, we authorize amateur stations to transmit the following emission types throughout the new amateur bands: CW (international Morse code telegraphy), RTTY (narrow-band direct-printing telegraphy), data, phone, and image emissions.   These emission types provide amateur operators with maximum flexibility, and we find that additional restrictions would needlessly hinder experimentation.

7. Experimental stations appear to 'still be in business' but are encouraged to transition to the 'amateur' service:

Finally, we decline to permit previously licensed experimental stations – some of which have been authorized with significantly more radiated power than the adopted EIRP limits for these new amateur service bands – to communicate with amateur stations operating in these bands.  Amateur operations in these bands currently authorized under experimental licenses should transition their operations in accordance with the adopted rules and not circumvent such rules by use of experimental licenses.

My understanding of the R&O document is that participating parties may still file a 'Petition For Reconsideration' notification within 30 days of the R&O's publication in the Federal Register. Once these (if any) are dealt with, there are no other roadblocks preventing immediate implementation.

The document contains additional details not discussed here and makes fascinating reading for amateurs that might be looking forward to the new allocations.

This is the news that many U.S. amateurs have been waiting many years to hear! It is also good news for Canadian's operating on these bands to know that they may soon see a large increase in activity south of the border. Let's hope things continue to transpire favorably and that we will finally see the new bands become a reality.

Get those soldering irons out guys and gals!

Several QSL cards have arrived after the last 630m 'crossband' event ... including one from ZF1EJ in the Cayman Islands confirming our 630m QSO in January.



 

The contact was made on JT-9, the 'WSPR QSO' mode, and represents DXCC country #3 for me on 630m ... only 97 more to go! ZF1EJ was running just 32 watts output when we had our 630m JT-9 contact but has since cranked his output to around 60 watts. Eden is beaconing most nights on WSPR and puts out a well-heard signal. He is very interested in two-way JT-9 work with other VE stations as well as any Europeans and down-under stations.

From what I can tell, it looks like JT-9 (similar to JT-65 but a much narrower bandwidth of 15.6Hz) is establishing itself as the go-to mode for weak signal two-way work on 630m. It has a couple of things going for it that makes it very attractive for this band ... it can dig way down into the noise (-25 db approximately) and communicate with very weak signals and, it does not require amateurs to know CW, a growing trend with newer operators and a real hindrance to two-way CW work. I suspect, and hope, that there will be much more CW activity on 630m once amateurs in the U.S.A. get the band as the amount of information that can be exchanged per transmission on JT-9 is limited ... time will tell.

In the meantime, here is a request for more two-way 'crossband' CW activity with amateurs in all parts of North America. I have recently totally revised the 'CQ Crossband' page on my website, 'The VE7SL Radio Notebook'. Please note that my web address for well over a decade, is no longer valid and everything has been moved to this new location. If you have the old one bookmarked or are linking to it from your own site, please be aware that previous links will now be dead.

The crossband concept allows amateurs not yet on 630m to still participate in this exciting part of the spectrum ... and to check out their ability to hear anything on MF. If we were to make a schedule for a crossband contact, I would be transmitting on 630m at full ERP while you would be answering on one of the HF bands ... usually 160, 80 or 40m.

I am very much interested in setting up crossband schedules for 630m at any time and can very likely enlist several other VE7s to be there as well so that you can work more than one station. I have full details on my updated 'CQ Crossband' web page but please do not hesitate to give crossband a try!


Roger, VE7VV in Victoria, B.C., recently became the 8th VE7 to muster RF on 630m, with power limited to 1 watt at present. Our contact was on CW while he worked stations in Vancouver on JT-9. Hopefully he will continue to build his station and become more active on the band.

Crossband continues to be a subject of much interest both here and with many U.S. stations that are waiting for the band. Recent cards from Colorado and California, shown below, are the latest to arrive.




K6YK gave me an RST of '519' but explained the reason for this was because he was receiving on his 3 el HF tri-bander which provided the best signal-to-noise value! This is often the case on 630m so try what you have. Many times a 'non-resonant' antenna will pick up less noise and yield the best signal readability.

If you would like to try a crossband QSO, please contact me at VE7SL (at) shaw.ca ... I'll keep the rig warmed up!

Several QSL cards have arrived after the last 630m 'crossband' event ... including one from ZF1EJ in the Cayman Islands confirming our 630m QSO in January.



 

The contact was made on JT-9, the 'WSPR QSO' mode, and represents DXCC country #3 for me on 630m ... only 97 more to go! ZF1EJ was running just 32 watts output when we had our 630m JT-9 contact but has since cranked his output to around 60 watts. Eden is beaconing most nights on WSPR and puts out a well-heard signal. He is very interested in two-way JT-9 work with other VE stations as well as any Europeans and down-under stations.

From what I can tell, it looks like JT-9 (similar to JT-65 but a much narrower bandwidth of 15.6Hz) is establishing itself as the go-to mode for weak signal two-way work on 630m. It has a couple of things going for it that makes it very attractive for this band ... it can dig way down into the noise (-25 db approximately) and communicate with very weak signals and, it does not require amateurs to know CW, a growing trend with newer operators and a real hindrance to two-way CW work. I suspect, and hope, that there will be much more CW activity on 630m once amateurs in the U.S.A. get the band as the amount of information that can be exchanged per transmission on JT-9 is limited ... time will tell.

In the meantime, here is a request for more two-way 'crossband' CW activity with amateurs in all parts of North America. I have recently totally revised the 'CQ Crossband' page on my website, 'The VE7SL Radio Notebook'. Please note that my web address for well over a decade, is no longer valid and everything has been moved to this new location. If you have the old one bookmarked or are linking to it from your own site, please be aware that previous links will now be dead.

The crossband concept allows amateurs not yet on 630m to still participate in this exciting part of the spectrum ... and to check out their ability to hear anything on MF. If we were to make a schedule for a crossband contact, I would be transmitting on 630m at full ERP while you would be answering on one of the HF bands ... usually 160, 80 or 40m.

I am very much interested in setting up crossband schedules for 630m at any time and can very likely enlist several other VE7s to be there as well so that you can work more than one station. I have full details on my updated 'CQ Crossband' web page but please do not hesitate to give crossband a try!


Roger, VE7VV in Victoria, B.C., recently became the 8th VE7 to muster RF on 630m, with power limited to 1 watt at present. Our contact was on CW while he worked stations in Vancouver on JT-9. Hopefully he will continue to build his station and become more active on the band.

Crossband continues to be a subject of much interest both here and with many U.S. stations that are waiting for the band. Recent cards from Colorado and California, shown below, are the latest to arrive.




K6YK gave me an RST of '519' but explained the reason for this was because he was receiving on his 3 el HF tri-bander which provided the best signal-to-noise value! This is often the case on 630m so try what you have. Many times a 'non-resonant' antenna will pick up less noise and yield the best signal readability.

If you would like to try a crossband QSO, please contact me at VE7SL (at) shaw.ca ... I'll keep the rig warmed up!

Last weekend's 630m Midwinter Activity Event appeared to bring out a lot of new listeners to the band as well as to the crossband activity.


John, KB5NJD, reports in his daily 630m summary, that numbers were higher than previous events, indicating much new interest in what might eventually become the new 'Topband'. John has a very detailed timeline of events for the night including extensive coverage of experimental station reports.

Unfortunately, as is often the case, geomagnetic conditions were still suffering the effects of a week long coronal hole stream bombardment, particularly geo-effective in VE7 and the PNW, which always seems to tickle the southern elongated tail of the auroral oval further north. Stations to the south reported better, but quickly shifting propagation paths, while VE3OT in Ontario seemed to have no difficulty in working his numerous QSX callers.

Murphy's Law in action. The yellow disturbance coincides exactly with the event!

The path from VE7 was predominantly north-south, with the east-west path almost non-existent ... often the case when K indices are higher than 0 or 1. Several of the crossband stations reported heavy QRM on their HF QSX frequencies, which was expected. There were a number of CW events, including the NA CW and the FOC parties, as well as an international RTTY contest in full swing. I found my QSX of 3526 kHz to be busy but manageable as stations did not seem to stay too long before moving to another frequency. My 40m QSX of 7115 kHz was clear all night but most callers chose to use 80m.

Eventually, if and when the U.S. gets the 630m band, crossband work will no longer be needed. With all of the loud VE7 and Washington state activity on 630m, it will be an interesting challenge to work within the band itself ... but what great fun it will eventually be to hear 630m sounding like 160m during a winter CW contest!

Here is a rundown on the Canadian crossband action:

Joe, VO1NA out on the rock, used 80m as his talkback frequency while running 50W to a large inverted-L.

• PE5T              
• VO1DI             
• PAØO             
• K1PX          

Additional 'heard reports' were received from LA6LU, VE2PEP, DL4HG and PAØRDT.

Moving further west, Mitch, VE3OT, had a busy night with his 250 watts and 340' rectangular loop pointing east-west:

• VA3DN---ON
• W3TS---PA
• K1PX---CT
• W8PI---MI
• WB3AVN---MD
• K3PA---KS
• K3CCR---MD
• AC9S---IN
• WA8ZZ---MI
• W3WH---PA
• WA9ETW---WI
• AB4KJ---IL
• NS8S---MI
• N9SE---IN
• WA3TTS---PA
• W2JEK---NJ
• VE3GRO---ON
• WØBV---CO
• K2PI---VA
• K1HTV---VA
• N2MS---NJ
• KB5NJD---TX
• NO3M---PA
• NA5DX---MS
• K9RT---IN
• WØJW---IA

Mitch adds:

"Good conditions here - and similar frequency choice as last year….all but 2 QSOs on 3.5Mhz. Lost 3 possible QSOs - just too weak - at the noise level, but they obviously were copying me on 477….interesting.
Thinking about band condx - I think I should have stayed another hour or so and see i the band finally opened further West than Colorado.
It was interesting to see the East slowly fade away and the Mid-West and Western stations started calling. A good exercise - and lots of compliments and thanks from the U.S. operators."

Mitch is working on a special QSL for those stations that worked him.

Out on the west coast, things were busy as well but other than a couple of brief periods, there seemed to be a Faraday shield not too far east of the Rockies ... mostly a north-south affair.

John, VE7BDQ, reports:

• W7FI---WA
• K7WA---WA
• K6YK---CA
• VE6XH---AB
• VA7JX---BC
• VE7BGJ---BC
• K7CW---WA
• AH6EX/W7---WA
• CF7MM---BC
• K6IR---WA
• K7SS---WA
• W9PL---WA
• CF7MM---BC
• CG7CNF---BC
• VE7SL---BC

From Toby, VE7CNF:

• AH6EZ/W7---WA
• K7CW---WA
• CF7MM---BC
• K7SS---WA
• W9PL---WA
• N7BYD---MT
• VE7BDQ---BC
• W7FI---WA
• W6TOD---CA
• VE7KW---BC
• VE6XH---AB
• VE7BGJ---BC
• VA7JX---BC
• K6YK---CA
• KB5NJD---TX

From Mark, VA7MM:

• W7FI---WA
• K7CW---WA
• W6RKC---CA
• W6TOD---CA
• AH6EZ/W7---WA
• VE6XH---AB
• VE7KW---BC
• K6YK---CA
• VE7BGJ---BC
• VA7JX---BC
• K7SS---WA
• CG7CNF---BC
• VE7BDQ---BC

630m top-loaded 'T' (and multiband HF dipole) at VA7MM...100' vertical x 50' tophat.

Besides being just a lot of fun, these events always provide some interesting 'takeaways'.

It's clear that there is a lot of interest in this band and it continues to grow ... reporting levels have never been higher. One crossbander in Washington state indicated that he has a station already to go, once the U.S. gets the band.

Activities such as this continue to demonstrate that stations running something less than the maximum allowable 5 watts eirp can produce impressive signal levels, allowing solid aural contacts over considerable distances via skywave ... even under the marginal conditions just experienced.

Considering the amount of RF being generated nightly for several years by high erp experimental stations as well as during numerous frenzied 630m activity nights, there should be little doubt that interference to hydro switching systems is a non-issue. Sadly, this argument by power authority lobbyists still appears to be the main obstacle for the FCC's foot-dragging of 630m implementation in the U.S.A.

It was great to see participation and interest from VE6 land! Hopefully more Canadian amateurs will take up the challenges offered by 630m ... both in operating and in building a station. You need not have anything more than a suburban backyard to enjoy transcontinental work and like so many activities ... the more, the merrier!