Top band season will soon wrap up for me. While giving a tour of my station to a visiting contester the snow and ice was sufficiently thawed that it was once again possible to trip on the 160 meter antenna radials. That means hay growing season is approaching and soon I'll have to roll up the radials until late summer. Until I come up with a permanent antenna this mode of operation is necessary. It is therefore a good time to reflect on the season that was.
Still no QRO
For my second year with an effective 160 meter antenna my accomplishments are incremental rather than spectacular. Surprisingly my DXCC count went up very little: it now sits at 108, only 11 more than after last season. I'd be substantially higher were I running QRO since the opportunities were there. I continue to make do with 200 watts, and less in contests to stay within the low power category. In the Stew Perry TBDC I operated 5 watts once again to hand out QRP point multipliers.
TBDC brought out lots of activity around the world making it possible to try to do the impossible with QRP. One was a new country: 3V8SF. Since he was running low power we each earned 90 points for the QSO. Perusing the interim results this appears to be the highest point QSO in the contest overall. My other success was surpassing 8000 km distance by working RW7K. It took awhile but he somehow managed to tease my call and grid out of the noise. Kudos to him, and to the horde who stood by to let it happen. My challenge was find novel ways to communicate the information to help him as best I could.
Aside from the successes there was far more DX that I failed to work with low power. Of particular note were the many African DXpeditions: 5V, 5X, 7P, FH, 3B8 and more. The inland operations were especially difficult despite being within reach because they found it a challenge to put out an exceptional. Looking west, island operations such as VP6D (Ducie) and T31EU (Central Kiribati) I worked quite easily since they had great signals by using the Pacific Ocean as a ground plane.
Typically I can hear the DX. The reverse is often not true due to asymmetry of power and noise. To corrupt a common saying: you can't work them if they can't hear you. More DXpeditions than ever are taking amplifiers, helping them to be heard with their modest antennas and giving me a false sense of hope. Tropical QRN even with directional receive antennas on their end are frequently not enough to get my low power signal into their logs.
It is possible to work the world on 160 meters with 100 watts if you are willing to wait for the occasional exceptional opening, but a kilowatt is necessary to routinely work the DX. It is increasingly likely that I will make the jump in time for the 2019-2020 season. However I will continue with low power or QRP in select contests.
Waiting for exceptional conditions
Despite many differences there are commonalities between 6 and 160 meters. Without propagation enhancement it is possible to work out to some distance any day of the week. Enhanced propagation requires patience, and hoping that the DX happens to be there when it happens. This can happen any day on 160 meters, so in that respect it is less frantic than 6 meters when missing an opening can mean waiting until next year.
But when conditions are hot they can be very good indeed, and especially so for those of us running less than a kilowatt. A good example was one weekday night this winter when I noticed a few European stations coming in with signals several S-units stronger than usual. I picked a frequency and called CQ. Over the next 40 minutes I worked two dozen European stations at or a little before their local sunrise. It made 20 meters seem boring! Unfortunately that is the exception. To do it with regularity requires more power (to be heard) and good receiving antennas to pull callers out of the noise.
Exceptional conditions can occur during DXpeditions. Some luck is required since it may come just once during their limited duration. The recent V84SAA operation is a good example. Several times I heard them very weakly at my sunset or their sunrise; the same was true of XX9D whose operation overlapped theirs.
There was one but only one brief period that V84SAA was a true S9. It was astonishing to hear, not requiring a directional receive antenna to copy them perfectly. Ten minutes later they were gone. Needless to say I didn't work them. K1ZM described the operational challenges they had with working this part of North America. I didn't feel too bad since even the top band big guns had little luck.
Not being a morning person I find it difficult to wake up to try my luck at sunrise enhancements. I find it a chore even midwinter when sunrise occurs at a more human time. But enhancements can be elusive, requiring frequent attempts before getting lucky. If I happened to wake up early by chance I would give top band a try. Other than that I only made the effort during contests and DXpeditions. My dedication to top band is clearly not what it could be. I have yet to work VK and ZL.
Contests
When it comes to contests there is no possibility of waiting for good conditions. A contest has a fixed time period during which you play the hand you're dealt. So does everyone else; we all suffer equally. But without QRO it can be slow going. That happened to me this year in the ARRL 160 and CQ 160 CW contests. It was discouraging enough that I didn't put it a full effort. For a contester this is a poor attitude and yet I let it determine my activity level.
When the slow going gets even slower my interest quickly wanes. Sitting on a frequency with the computer robot sending endless CQs while I browse the internet is not terribly attractive. Knowing that I am still competitive despite the tedium does little to boost my morale. It isn't so bad during multi-band contests such as CQ WW since I only need to occasionally pop down to 160 to pick up a few multipliers before returning to higher rate low bands.
During the CQ 160 SSB contest I surprised myself by working some DX. Although it was only Caribbean and one European I was impressed that I could do it with low power. As usual I heard far more DX than the DX heard me.
West Beverage
The temporary west Beverage did very well despite being a mediocre performer on top band. A length of 89 meters is barely adequate. I cannot definitively claim that it enabled more QSOs. All I can say for certain is that it sure made operating more comfortable. Noisy signals were now comfortably copied.
Very soon the antenna will have to be removed, well before the vertical's radials need to be rolled up. The RG6 wandering around the hay field and the house yard is a hazard now that the snow is almost gone. It won't be a great loss since there will be few opportunities to make good use of it over the next few weeks that the transmit antenna is still up.
Next year I'll have to get serious about completing my receiving antenna system. Running QRO on top band is unwise without the ability to copy the weak callers a big signal will attract. I don't want to become an alligator: all mouth, no ears.
Still enjoying it
Despite my long list of challenges and slow progress this season I do enjoy operating 160 meters. That may not have come across. Challenges spur me to try harder, including planning better antennas and improving my operating habits.
The similarity to 6 meters is striking despite the obvious differences: short propagation windows, weak signals most of the time, a global community of enthusiasts and the need for excellent antennas. I am not surprised that FT8 has become very popular on top band, just like on 6 meters. With effort I'll continue to do progressively better.
I suspect the only substantial station improvements next season will be QRO and progress on my Beverage system. That should be enough to make noticable improvements in my contest and DX results, and that will increase my enjoyment of 160 meters. I might even give FT8 a whirl.
Tuesday, March 26, 2019
Tuesday, March 19, 2019
Reacting to Honest Feedback
In my first years as ham, oh so long ago, I had no money and could not afford decent equipment. Those days I used a Hammarlund HQ129X receiver and a bizarre mix of hacked equipment for a transmitter that I picked up at a flea market for a few dollars.
The foundation of the transmitter was originally a crystal controlled multi-band mobile AM transmitter with an three 807 tubes: one for the final and two for the modulator. In combination with a home brew AC power supply it had been converted to an HF CW only transmitter that put out perhaps 30 watts. The modulator tubes were still lit due to the peculiar way in which the filaments were wired to a 12 VAC transformer, and I was too green to attempt changing that. A VFO was bolted on somehow.
As you might expect this was not the cleanest transmitter. The chirp wasn't too bad although it did drift quite a bit for the first hour after being turned on. Harmonics were almost certainly there even though I couldn't test for that and instead relied on the dipole resonance to filter those out. I received two OO (official observer) notices with this setup.
It is a rare transmitter indeed nowadays that has drift or chirp. We have more sophisticated flaws to deal with than the simple ones of an earlier day. Now we have phase noise, harmonic distortion, key clicks and more. Some are inherent in the commercial gear we buy while others, with a measure of knowledge and bravery, can be fixed. Too many hams are blithely unaware of what their transmitters are doing.
Yaesu, for one, is notorious for key clicks which exist to a greater or lesser degree over several generations of transceiver. They were particularly bad in the FTdx1000, which I dealt with soon after purchasing one. Happily they are old enough that fewer and fewer of these are heard on the bands since not many make the effort to fix the problem.
The FTdx5000 I now use is not innocent. For some odd reason they (and other manufacturers) include an option to vary the amount of key clicks the transmitter generates rather than fix the problem when it was designed. The menu option is 064 "A1A SHAPE" with a selectable rise/fall time of 1 to 6 milliseconds. The default value is 4 ms.
I was aware that the default value was problematic. I somehow failed to remember to raise it to 6 ms. That is, until I learned about it in the worst way possible.
The report
During a contest earlier this winter the stations were tightly packed on one of the low bands. With the benefit of modern receiver technology we could successfully operate when spaced just 300 to 400 Hz apart. Like most I was looping CQs to attract any new stations that might appear. There were stations doing the same both below and above me. All seemed well.
In the midst of this one of those operators, a well known contester with a competitive station, dropped down to complain that I was causing him grief due to my rigs's key clicks. He asked me to QSY a small amount. I don't know what others would do, but I hurriedly agreed and sidled a little closer to the station below me. I really did not want to hurt anyone else's prospects in the contest, especially one whose signal was so clean despite running a kilowatt.
I kept operating that evening as I normally would, other than being more self-conscious than usual about how close I got to others. That there were many others splattering the spectrum with key clicks did not make me feel any better.
Resolution
The next morning during off time I plunged into the FTdx5000 manual and those K9YC technical reports I linked to above. There were two things to considers: firmware version and keying rise/fall time. As already mentioned I was surprised to find that the keying parameter was set to the default 4 ms.
I connect the FTdx5000 to a dummy load and tuned my second rig, with no antenna connected, to the same frequency. Cycling through the parameter settings I monitored how widely the FTdx5000 key clicks could be heard. The difference between settings was large. Only at the maximum 6 ms setting was the signal narrow enough to satisfy me. I locked in that value.
There was a firmware update released by Yaesu in 2014 that improved CW keying bandwidth; there is at least one later firmware update but it addressed other issues. This was only measured and reported on by K9YC (link above). The previous owner of the rig told me that he kept the firmware at the latest version. It was only now that I confirmed that he had indeed applied the 2014 firmware update. That was reassuring.
With further testing I concluded that the key clicks were at an acceptably low level though not as good as the best rigs on the market. I might be possible to do better by modifying it, however I am reluctant to attempt that without guidance from someone with relevant expertise. It is too easy to do it wrong and make the problem worse. More than keying rise/fall time is involved, including ensuring linearity all along the amplifier chain. That's a tall order.
Path forward
In my previous article I spoke briefly about changes I plan within the shack this year. One of those is to sell a couple of transceivers -- KX3 and FT950 -- and use the funds to purchase a second high end rig. Most likely it will be is an Elecraft K3 or K3S, but not a Yaesu or any other manufacturer with a poor track record of transmitter cleanliness.
I have tended to favour Yaesu rigs out of both habit and comfort with the control panel and feature access from the back panel. My first Yaesu rig was a FT101B purchased new in 1974 (one of only two HF rigs bought this way), and along the way I had an FT101E, FT102 (a favourite of mine at the time), FT726R, FTdx1000 and more recently a FT950. It is now possible that this is the end of the line for Yaesu rigs in my shack.
The foundation of the transmitter was originally a crystal controlled multi-band mobile AM transmitter with an three 807 tubes: one for the final and two for the modulator. In combination with a home brew AC power supply it had been converted to an HF CW only transmitter that put out perhaps 30 watts. The modulator tubes were still lit due to the peculiar way in which the filaments were wired to a 12 VAC transformer, and I was too green to attempt changing that. A VFO was bolted on somehow.
As you might expect this was not the cleanest transmitter. The chirp wasn't too bad although it did drift quite a bit for the first hour after being turned on. Harmonics were almost certainly there even though I couldn't test for that and instead relied on the dipole resonance to filter those out. I received two OO (official observer) notices with this setup.
It is a rare transmitter indeed nowadays that has drift or chirp. We have more sophisticated flaws to deal with than the simple ones of an earlier day. Now we have phase noise, harmonic distortion, key clicks and more. Some are inherent in the commercial gear we buy while others, with a measure of knowledge and bravery, can be fixed. Too many hams are blithely unaware of what their transmitters are doing.
Yaesu, for one, is notorious for key clicks which exist to a greater or lesser degree over several generations of transceiver. They were particularly bad in the FTdx1000, which I dealt with soon after purchasing one. Happily they are old enough that fewer and fewer of these are heard on the bands since not many make the effort to fix the problem.
The FTdx5000 I now use is not innocent. For some odd reason they (and other manufacturers) include an option to vary the amount of key clicks the transmitter generates rather than fix the problem when it was designed. The menu option is 064 "A1A SHAPE" with a selectable rise/fall time of 1 to 6 milliseconds. The default value is 4 ms.
I was aware that the default value was problematic. I somehow failed to remember to raise it to 6 ms. That is, until I learned about it in the worst way possible.
The report
During a contest earlier this winter the stations were tightly packed on one of the low bands. With the benefit of modern receiver technology we could successfully operate when spaced just 300 to 400 Hz apart. Like most I was looping CQs to attract any new stations that might appear. There were stations doing the same both below and above me. All seemed well.
In the midst of this one of those operators, a well known contester with a competitive station, dropped down to complain that I was causing him grief due to my rigs's key clicks. He asked me to QSY a small amount. I don't know what others would do, but I hurriedly agreed and sidled a little closer to the station below me. I really did not want to hurt anyone else's prospects in the contest, especially one whose signal was so clean despite running a kilowatt.
I kept operating that evening as I normally would, other than being more self-conscious than usual about how close I got to others. That there were many others splattering the spectrum with key clicks did not make me feel any better.
Resolution
The next morning during off time I plunged into the FTdx5000 manual and those K9YC technical reports I linked to above. There were two things to considers: firmware version and keying rise/fall time. As already mentioned I was surprised to find that the keying parameter was set to the default 4 ms.
I connect the FTdx5000 to a dummy load and tuned my second rig, with no antenna connected, to the same frequency. Cycling through the parameter settings I monitored how widely the FTdx5000 key clicks could be heard. The difference between settings was large. Only at the maximum 6 ms setting was the signal narrow enough to satisfy me. I locked in that value.
There was a firmware update released by Yaesu in 2014 that improved CW keying bandwidth; there is at least one later firmware update but it addressed other issues. This was only measured and reported on by K9YC (link above). The previous owner of the rig told me that he kept the firmware at the latest version. It was only now that I confirmed that he had indeed applied the 2014 firmware update. That was reassuring.
With further testing I concluded that the key clicks were at an acceptably low level though not as good as the best rigs on the market. I might be possible to do better by modifying it, however I am reluctant to attempt that without guidance from someone with relevant expertise. It is too easy to do it wrong and make the problem worse. More than keying rise/fall time is involved, including ensuring linearity all along the amplifier chain. That's a tall order.
Path forward
In my previous article I spoke briefly about changes I plan within the shack this year. One of those is to sell a couple of transceivers -- KX3 and FT950 -- and use the funds to purchase a second high end rig. Most likely it will be is an Elecraft K3 or K3S, but not a Yaesu or any other manufacturer with a poor track record of transmitter cleanliness.
I have tended to favour Yaesu rigs out of both habit and comfort with the control panel and feature access from the back panel. My first Yaesu rig was a FT101B purchased new in 1974 (one of only two HF rigs bought this way), and along the way I had an FT101E, FT102 (a favourite of mine at the time), FT726R, FTdx1000 and more recently a FT950. It is now possible that this is the end of the line for Yaesu rigs in my shack.
Friday, March 15, 2019
My Matching Network is a Filter
A very long time ago I had an 80' length of ancient RG58 so bad that a penniless teenager like myself was prepared to throw it out. A friend I mentioned this to insisted that I give it to him. I was surprised because he was a UHF enthusiast. Coax that is no good at HF is surely useless at 432 MHz.
He explained that he uses old coax for dummy loads, provided the characteristic impedance is still reasonably close to 50 Ω, since there is near infinite return loss. Also a poor student, he couldn't afford a dummy load that would work well at 70 cm. He got the coax and I never thought of coax loss quite the same way again.
Well, that was a fun story despite being only tangentially related to this article. The relevance is that I determined that an exceptionally long (100,000 meter) transmission line with low attenuation is a good way to emulate a resistive load with EZNEC. I needed it to demonstrate behaviour of L-networks at the feed point of a real antenna. The transmission line is put across the source on a single segment wire that is much too short (0.5 meters) to function as an antenna at 3.5 MHz.
To explore the characteristics of L-network topologies (there are 4 if you care to count them) I combined the aforementioned dummy load with an L-network, putting the source at the 50 Ω port. A virtual wire connects the source to the L-network. For the purpose of this exercise the dummy load is 25 Ω. The L-network was designed using TLW. Wire and L-network components are made zero loss to improve clarity in the calculations; that is, extraneous variables are removed so that we can focus on what is important.
Testing with EZNEC confirmed that the load impedance of the lengthy coax is broadband with a reactance term that is a small fraction of an ohm. Now we can move on to the L-network, an EZNEC example of which is shown above.
TLW supports several topologies of matching networks (tuners), picking the one that meets the stated objectives. Here we have a design frequency of 3.5 MHz using a "low pass" L-network to transform between 50 + j0 Ω and 25 + j0 Ω. Typically the coax is connected to the 50 Ω port and the antenna to the other port. If you want the reverse you simply swap ports. TLW does that automatically for you when you set the source and load impedances accordingly.
Now watch what happens when we plot the SWR from 1 to 15 MHz.
As expected the impedance is a perfect match at 3.5 MHz. The match is not broadband despite the load being a constant 25 Ω; the impedance transformation is frequency dependent. In the plot I chose to highlight the impedance at 14 MHz, as seen at the L-network's input (50 Ω) port. The impedance is very low. But is this consistent with the selected "low pass" topology? Further, what does this mean?
It is no accident that matching networks and filters look alike. Both have two ports and are constructed of inductors and capacitors, often with the same L or Pi (π) topology. The only difference is the design objective, the choice of which determines the result. Which brings us to a fundamental question: what happens to the RF energy that a filter is designed to reject? Physics tells us that energy is conserved so it much go somewhere. There are only a few possibilities:
What does happen, as demonstrated in the above example, is that the filter presents an impedance (high SWR) to the generator that reflects unwanted frequencies. The R component of the impedance of that matching network starts to decrease a little above the design frequency and keeps falling until it is very low indeed. Impedance mismatch is responsible for the filtering function.
As to what happens below the design frequency, well, it's the same as what happens above the design frequency when we choose the high pass topology for the L-network. In the top EZNEC screen capture the L-network is the high pass topology designed with TLW. Here is the SWR plot.
Again the 14 MHz impedance is marked. It's quite different than for the low pass network. As the frequency increases the impedance tends towards that of the dummy load alone. That is, the L-network effectively disappears, and the RF passing through with no difficulty other than a small amount of attenuation. As expected for a high pass filter the impedance declines below 3.5 MHz, reflecting RF back to the generator.
If an L-network is a filter what is happening at the design frequency? The corner frequency of the filter is where the transition between pass-through and blocking occurs. The R and X components of the impedance seen at both ports swings rapidly in the vicinity of this frequency. For a matching network we choose L and C values that give us what we want at the design frequency, often without regard to what happens far below and above that frequency.
In comparison to the dummy load we're using in the example it is never so simple for a real antenna since the load impedance varies with the frequency. Indeed, the antenna is also a species of network that is frequency sensitive! Out of band energy may be reflected while in band energy is matched to the impedance of free space (377 Ω) and radiated.
Sometimes it is possible to design a network that achieves a broader match by designed for frequency sensitive behaviour that in part compensates for the antenna's changing impedance. But hams rarely do this since it is difficult and is very situation specific.
For a receiver filter the impedance swing at the network's corner frequency is responsible for the ringing we hear in our receivers. This is largely due to variable phase shift (reactance change) over a small frequency range. Filter designers try to control for this. Eliminating ringing is more tractable with software DSP than less cooperative analogue filters.
How good a filter is it?
We all want to avoid transmitting harmonics. It is good practice and it keeps us compliant with our country's regulations. For contesters even properly attenuated harmonics are a serious nuisance at operating positions on higher bands (SO2R or multi-op). The question is whether the supplemental filtering function of a L-network can be helpful.
The answer: maybe. In the low pass example above the return loss at 14 MHz, the fourth harmonic, is 0.2 db. This ~14 to 15 db reduction. At 7 MHz, the second harmonic, the reduction is only ~3 db. Clearly we cannot rely on feed point L-networks alone to deal with the problem of harmonic interference. In combination with purpose-built band pass filters the additional harmonic attenuation may be helpful.
Our antennas can also help by being non-resonant on harmonics, which is one reason, although not the most important one, why many contesters look unkindly upon multi-band antennas such as tri-band yagis.
A simple L-network at the feed point of a 20 meter yagi is unlikely to be helpful since the only HF band being protected is 10 meters and then only minimally, ~3 db, since this is the second harmonic. There is no compelling reason to substitute one for a hairpin (beta) match, which is a form of high pass filter (shunt C and parallel L). The hairpin may be better by attenuating the received fundamental energy of adjacent transmitters on lower bands.
If filtering is a really wanted in a matching network it is worth considering a Pi-network which with its additional filter pole (one more shunt component) is a better filter. TLW will happily design for you either high pass or low versions of this more complex network. Pi-networks have been employed in power amplifier circuits for decades, transforming impedance and effectively filtering harmonics. Many tuners also employ the pi topology.
For better filtering a multi-pole filter is required. Although these networks can also transform impedance it is not usually done since optimizing one can come at the expense of the other.
Other reasons to choose an L-network topology
Filtering is not the only secondary criterion when it comes to choosing a matching network for an antenna. When I designed the L-network for my 160 meter antenna I chose the low pass topology since the L and C values were more easily achievable with parts on hand. Both low and high values for coils and capacitors can present difficulties, so picking the topology that best avoids them is attractive. Filtering was a unplanned benefit.
For the same reason I am choosing a low pass L-networks for my new 80 meter vertical yagi. As I write this the matching network is designed but not built. I'll describe it when the project is complete, including the benefits of the selected topology. Once complication I encountered is that at least two L-networks are needed -- one for the yagi (directional) mode and one for the omni-directional mode -- and by careful design I was able to simplify switching between modes.
Unfortunately the choice of a low pass design defeats a benefit. The 80 meter yagi works well as an omni-directional vertical on 30 meters, the third harmonic. It is currently my only effective antenna for that band. Adding a low pass L-network may put an end to that. For multi-band antennas it may be better to select a high pass topology even if you need to switch network elements on one or more bands.
Conclusion
The takeaway is that every network transforms impedance and does so in a manner that is frequency dependent. What we call a "tuner" or matching network is a network that transforms impedance to a desired value at a desired frequency. A filter is a network that passes or blocks desired and undesired frequencies, respectively. Filters and tuners are essentially the same but with parameters adjusted to suit the application. With forethought it can be possible to achieve both.
Simple as they are, L-networks are versatile. There is more to be said about them, some of which I may touch on in the future. None of this will surprise hams who have long experience designing filters and matching networks. For the rest of us there is always more to learn.
The content of this article nibbles at the edges of network design, only highlighting a few interesting points. Hopefully I haven't simplified too much for knowledgable readers. For those of you surprised (or horrified) that I used EZNEC as a network analyzer my excuse is that I stuck with what I know rather than fooling around with more suitable software tools I know less well. EZNEC works when one is careful. With additional effort it'll even handle more complex networks, by chaining L-networks port to port.
He explained that he uses old coax for dummy loads, provided the characteristic impedance is still reasonably close to 50 Ω, since there is near infinite return loss. Also a poor student, he couldn't afford a dummy load that would work well at 70 cm. He got the coax and I never thought of coax loss quite the same way again.
Well, that was a fun story despite being only tangentially related to this article. The relevance is that I determined that an exceptionally long (100,000 meter) transmission line with low attenuation is a good way to emulate a resistive load with EZNEC. I needed it to demonstrate behaviour of L-networks at the feed point of a real antenna. The transmission line is put across the source on a single segment wire that is much too short (0.5 meters) to function as an antenna at 3.5 MHz.
To explore the characteristics of L-network topologies (there are 4 if you care to count them) I combined the aforementioned dummy load with an L-network, putting the source at the 50 Ω port. A virtual wire connects the source to the L-network. For the purpose of this exercise the dummy load is 25 Ω. The L-network was designed using TLW. Wire and L-network components are made zero loss to improve clarity in the calculations; that is, extraneous variables are removed so that we can focus on what is important.
Testing with EZNEC confirmed that the load impedance of the lengthy coax is broadband with a reactance term that is a small fraction of an ohm. Now we can move on to the L-network, an EZNEC example of which is shown above.
TLW supports several topologies of matching networks (tuners), picking the one that meets the stated objectives. Here we have a design frequency of 3.5 MHz using a "low pass" L-network to transform between 50 + j0 Ω and 25 + j0 Ω. Typically the coax is connected to the 50 Ω port and the antenna to the other port. If you want the reverse you simply swap ports. TLW does that automatically for you when you set the source and load impedances accordingly.
Now watch what happens when we plot the SWR from 1 to 15 MHz.
As expected the impedance is a perfect match at 3.5 MHz. The match is not broadband despite the load being a constant 25 Ω; the impedance transformation is frequency dependent. In the plot I chose to highlight the impedance at 14 MHz, as seen at the L-network's input (50 Ω) port. The impedance is very low. But is this consistent with the selected "low pass" topology? Further, what does this mean?
It is no accident that matching networks and filters look alike. Both have two ports and are constructed of inductors and capacitors, often with the same L or Pi (π) topology. The only difference is the design objective, the choice of which determines the result. Which brings us to a fundamental question: what happens to the RF energy that a filter is designed to reject? Physics tells us that energy is conserved so it much go somewhere. There are only a few possibilities:
- It is dissipated in the filter.
- The filter reflects energy back toward the generator.
What does happen, as demonstrated in the above example, is that the filter presents an impedance (high SWR) to the generator that reflects unwanted frequencies. The R component of the impedance of that matching network starts to decrease a little above the design frequency and keeps falling until it is very low indeed. Impedance mismatch is responsible for the filtering function.
As to what happens below the design frequency, well, it's the same as what happens above the design frequency when we choose the high pass topology for the L-network. In the top EZNEC screen capture the L-network is the high pass topology designed with TLW. Here is the SWR plot.
Again the 14 MHz impedance is marked. It's quite different than for the low pass network. As the frequency increases the impedance tends towards that of the dummy load alone. That is, the L-network effectively disappears, and the RF passing through with no difficulty other than a small amount of attenuation. As expected for a high pass filter the impedance declines below 3.5 MHz, reflecting RF back to the generator.
If an L-network is a filter what is happening at the design frequency? The corner frequency of the filter is where the transition between pass-through and blocking occurs. The R and X components of the impedance seen at both ports swings rapidly in the vicinity of this frequency. For a matching network we choose L and C values that give us what we want at the design frequency, often without regard to what happens far below and above that frequency.
In comparison to the dummy load we're using in the example it is never so simple for a real antenna since the load impedance varies with the frequency. Indeed, the antenna is also a species of network that is frequency sensitive! Out of band energy may be reflected while in band energy is matched to the impedance of free space (377 Ω) and radiated.
Sometimes it is possible to design a network that achieves a broader match by designed for frequency sensitive behaviour that in part compensates for the antenna's changing impedance. But hams rarely do this since it is difficult and is very situation specific.
For a receiver filter the impedance swing at the network's corner frequency is responsible for the ringing we hear in our receivers. This is largely due to variable phase shift (reactance change) over a small frequency range. Filter designers try to control for this. Eliminating ringing is more tractable with software DSP than less cooperative analogue filters.
How good a filter is it?
We all want to avoid transmitting harmonics. It is good practice and it keeps us compliant with our country's regulations. For contesters even properly attenuated harmonics are a serious nuisance at operating positions on higher bands (SO2R or multi-op). The question is whether the supplemental filtering function of a L-network can be helpful.
The answer: maybe. In the low pass example above the return loss at 14 MHz, the fourth harmonic, is 0.2 db. This ~14 to 15 db reduction. At 7 MHz, the second harmonic, the reduction is only ~3 db. Clearly we cannot rely on feed point L-networks alone to deal with the problem of harmonic interference. In combination with purpose-built band pass filters the additional harmonic attenuation may be helpful.
Our antennas can also help by being non-resonant on harmonics, which is one reason, although not the most important one, why many contesters look unkindly upon multi-band antennas such as tri-band yagis.
A simple L-network at the feed point of a 20 meter yagi is unlikely to be helpful since the only HF band being protected is 10 meters and then only minimally, ~3 db, since this is the second harmonic. There is no compelling reason to substitute one for a hairpin (beta) match, which is a form of high pass filter (shunt C and parallel L). The hairpin may be better by attenuating the received fundamental energy of adjacent transmitters on lower bands.
If filtering is a really wanted in a matching network it is worth considering a Pi-network which with its additional filter pole (one more shunt component) is a better filter. TLW will happily design for you either high pass or low versions of this more complex network. Pi-networks have been employed in power amplifier circuits for decades, transforming impedance and effectively filtering harmonics. Many tuners also employ the pi topology.
For better filtering a multi-pole filter is required. Although these networks can also transform impedance it is not usually done since optimizing one can come at the expense of the other.
Other reasons to choose an L-network topology
Filtering is not the only secondary criterion when it comes to choosing a matching network for an antenna. When I designed the L-network for my 160 meter antenna I chose the low pass topology since the L and C values were more easily achievable with parts on hand. Both low and high values for coils and capacitors can present difficulties, so picking the topology that best avoids them is attractive. Filtering was a unplanned benefit.
For the same reason I am choosing a low pass L-networks for my new 80 meter vertical yagi. As I write this the matching network is designed but not built. I'll describe it when the project is complete, including the benefits of the selected topology. Once complication I encountered is that at least two L-networks are needed -- one for the yagi (directional) mode and one for the omni-directional mode -- and by careful design I was able to simplify switching between modes.
Unfortunately the choice of a low pass design defeats a benefit. The 80 meter yagi works well as an omni-directional vertical on 30 meters, the third harmonic. It is currently my only effective antenna for that band. Adding a low pass L-network may put an end to that. For multi-band antennas it may be better to select a high pass topology even if you need to switch network elements on one or more bands.
Conclusion
The takeaway is that every network transforms impedance and does so in a manner that is frequency dependent. What we call a "tuner" or matching network is a network that transforms impedance to a desired value at a desired frequency. A filter is a network that passes or blocks desired and undesired frequencies, respectively. Filters and tuners are essentially the same but with parameters adjusted to suit the application. With forethought it can be possible to achieve both.
Simple as they are, L-networks are versatile. There is more to be said about them, some of which I may touch on in the future. None of this will surprise hams who have long experience designing filters and matching networks. For the rest of us there is always more to learn.
The content of this article nibbles at the edges of network design, only highlighting a few interesting points. Hopefully I haven't simplified too much for knowledgable readers. For those of you surprised (or horrified) that I used EZNEC as a network analyzer my excuse is that I stuck with what I know rather than fooling around with more suitable software tools I know less well. EZNEC works when one is careful. With additional effort it'll even handle more complex networks, by chaining L-networks port to port.
Wednesday, March 6, 2019
Dreadful Contest Conditions: Version 2.0
When I was first licensed in 1972 conditions were on a rapid slide downward from a mediocre solar maximum into a long, deep trough. Adding to the woe was that I lived in the radio black hole known as Manitoba (VE4). But we were young and didn't know better. We were happy working what little DX we could. Soon I got into contests and like all my friends focused on domestic (North American) events such as ARRL Sweepstakes in which propagation was less of a factor.
Then came 1979. Not only was that a very good solar maximum it was also when I moved to Ottawa and became a VE3. One of the first things I did after moving into my apartment was to turn on my FT101E and attach a 10' wire on the back for an antenna. It did it to listen around since I missed being on the air.
I clicked over to 40 meters since it was evening and I was astounded. The band was packed with European stations. With a low inverted vee in VE4 during a solar minimum it took patience to work any Europe at all. In my excitement I phoned a friend back home and turned up the audio so he could hear what it was like out east. VE3 is hardly an ideal location for DXing but it shows that your location can change your perspective on the hobby.
Which brings me to this past weekend and the ARRL DX SSB contest. Conditions were dreadful and, making it worse, we are deep within a solar minimum. Although everyone suffered we are far enough north that I could only sigh as I listened to stations south of me working DX that I could barely hear or not hear at all.
Despite what I once wrote about contesting in dreadful conditions I did not stick with it. Although I've been happy to do SSB contests with QRP or low power in the past I had little enthusiasm to persevere this time. That decision was made before I discovered how bad conditions were in the wake of a geomagnetic storm, in that I planned a part time effort. Instead I operated for brief spurts and abandonned the contest entirely early Sunday morning with only 170 QSOs in the log.
That everyone else was suffering didn't comfort me this time around. Operators further west had it much worse, almost cut off from Europe on all bands due to absorption in the auroral zone.
Looking through the claimed scores on 3830 it appears that had I stuck with it I might have been competitive. Unlike the CW weekend I was fortunate that all my antennas worked, including the intermittent ones like the 40 meter yagi. It would have been an enthusiasm sapping slog, moderately rich in multipliers (band-countries) and poor in QSOs (rate). I'll never know for sure how I would have placed.
When I contested with QRP I went into each contest knowing my QSO rate and score would be low, even if competitive within the category. As my antennas and power improved my expectations increased. I am no longer as accepting of a low score, no matter my competitiveness. There is nothing wrong with this. Entering contests against your feelings can reduce your interest in the activity for all future events. Of course those feelings can turn more positive once you roll up your sleeves and get busy working stations, digging out all the QSOs and multipliers your station and skill permits.
There is no easy answer, just be aware of the dilemma when propagation falters. It can be worth the experiment to jump into a contest when conditions are poor to learn how to deal with it. That experience can guide what you do in future. You may surprise yourself.
Then came 1979. Not only was that a very good solar maximum it was also when I moved to Ottawa and became a VE3. One of the first things I did after moving into my apartment was to turn on my FT101E and attach a 10' wire on the back for an antenna. It did it to listen around since I missed being on the air.
I clicked over to 40 meters since it was evening and I was astounded. The band was packed with European stations. With a low inverted vee in VE4 during a solar minimum it took patience to work any Europe at all. In my excitement I phoned a friend back home and turned up the audio so he could hear what it was like out east. VE3 is hardly an ideal location for DXing but it shows that your location can change your perspective on the hobby.
 |
| From WM7D's solar indices site |
Despite what I once wrote about contesting in dreadful conditions I did not stick with it. Although I've been happy to do SSB contests with QRP or low power in the past I had little enthusiasm to persevere this time. That decision was made before I discovered how bad conditions were in the wake of a geomagnetic storm, in that I planned a part time effort. Instead I operated for brief spurts and abandonned the contest entirely early Sunday morning with only 170 QSOs in the log.
That everyone else was suffering didn't comfort me this time around. Operators further west had it much worse, almost cut off from Europe on all bands due to absorption in the auroral zone.
Looking through the claimed scores on 3830 it appears that had I stuck with it I might have been competitive. Unlike the CW weekend I was fortunate that all my antennas worked, including the intermittent ones like the 40 meter yagi. It would have been an enthusiasm sapping slog, moderately rich in multipliers (band-countries) and poor in QSOs (rate). I'll never know for sure how I would have placed.
When I contested with QRP I went into each contest knowing my QSO rate and score would be low, even if competitive within the category. As my antennas and power improved my expectations increased. I am no longer as accepting of a low score, no matter my competitiveness. There is nothing wrong with this. Entering contests against your feelings can reduce your interest in the activity for all future events. Of course those feelings can turn more positive once you roll up your sleeves and get busy working stations, digging out all the QSOs and multipliers your station and skill permits.
There is no easy answer, just be aware of the dilemma when propagation falters. It can be worth the experiment to jump into a contest when conditions are poor to learn how to deal with it. That experience can guide what you do in future. You may surprise yourself.
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