QRM from a transmitter to receivers on lower bands
[Go to the end of this article for an "After-Field-Day follow-up" about this filter]
A friend of mine belongs to a club in a town north of me and he was describing an issue that they've been having for the past several years during ARRL Field Day: A station on an upper band (e.g. 15 or 10 meters) degrading reception on 20 or even 40 meters when transmitting. What was needed was something that could be used on both 15 and 10 meters and protect the lower bands (e.g. 20, 40 and 80) meters - and this protection would go the other way, preventing the 15/10 meter station's receiver from being overloaded by transmissions on the lower bands.
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| Figure 1: Exterior of the 15 Meter high-pass filter, built into a die-cast aluminum box with single-hole UHF connectors on the sides. Click on the image for a larger version. |
First, a bit of background.
The ARRL Field Day event is held on the fourth (but not last) full weekend every June. During this event thousands of clubs and individuals go forth into the wilds to set up and operate an event where they attempt to contact as many stations as they can in a 24 (or 27) hour period. In the case of club stations - or where multiple individuals are involved - it's very common to have more than one transmitter at a given site.
As the Field Day rules stipulate that all antennas/radios be within a 1000 foot (305 meter) circle it isn't possible to provide much geographical separation between different transmitters. This separation is important because a transmitter produces a very strong signal and the received signals are very weak by comparison: Receivers can be easily overloaded by these nearby strong signal sources and transmitters can produce low-level signals on frequencies other than those on which they are operation - ones that are too weak to cause problems under normal situations but when placed in close proximity to a receiver these weak emissions can block out/interfere with other receivers - even on different frequency bands.
Other-band signals can cause problems
The degree to which a transmitter radiates these low-level spurious signals - and that to which a receiver is able to tolerate a very strong signal - depends considerably on the transmitter/receiver itself. Some high end makes of radios (e.g. Elecraft, Flex Footnote 1) can be very clean in terms of transmitted spectra and higher-end receivers of all makes may be capable of tolerating a very strong signals - perhaps even in the same band - and this strategy works as long as the potentially-interfering transmitter itself is clean: If that "other" transmitter is producing noise at any frequency of reception, there's nothing that can be done at that receiver to fix the problem other than to quiet or clean up the errant transmitter. Meanwhile, even a "good" radio - such as an Icom IC-706MK2G or IC-7300 Footnote 2 or a Yaesu FT-757 - which works well by itself - may not "play nice with others" when immersed in an environment with multiple transmitters and receivers in very close quarters for reasons largely related to their design architecture.
What this means is that if one uses directional antennas (e.g. Yagis or beams) they are often placed north-south of each other and pointed parallel Footnote 3 so that there is some isolation off to the sides of these antennas - and it goes without saying that antennas of any sort are separated as far as the rules - or the operating space (e.g. park, yard, forest clearing) allows.
Sometimes, this isn't enough: Interference can result despite the precautions (e.g. transmit/receiver separation) so additional filtering may be necessary.
The use of Band-Pass filters
Barring the ability to separate antennas or place them in each others' nulls, there are other options: From a number of manufacturers Footnote 4 there are available band-pass filters that - as the name implies - are designed to pass one specific amateur HF band with low attenuation (loss) while offering significant rejection of other bands above and below. By placing one of these filters inline with the radio and the antenna, it not only will reduce the probability that a very strong signal from another band might overload the receiver (a particular problem with a radio like the Icom IC-7300 and certain models of other radios from other manufacturers) but it also attenuates the broad-band noise Footnote 5 that almost all HF radios produce that can encompass frequencies other than the band on which they are operating.
This low-level interference - often in the form of a white noise (or hiss) is produced by the amplifier stages in the transmitter itself. Most modern HF transceivers - while equipped with low-pass filters that attenuate harmonics at multiples of the transmitted signal and generally prevent this noise from being emitted on the next-higher non-WARC band - do NOT have an equivalent high-pass filter in them that prevents low-level spurious signals or broadband noise from being output to the antenna on frequencies below that on which it is operating. What this means is that a transmitter operating on, say, 15 meters, can produce a "hiss" that may degrade reception on 20, 40 or even 80 meters whenever it is keyed up - in this example, depending on how well that 15 meter antenna can radiate such signals and how close the two antennas are to each other. (I discussed this very problem in an early article of this blog: Getting the rigs ready for Field Day - Link).
For this reason it is often preferable to use a band-pass filter on every transmitter that is used, for the specific band on which it will be operated: This will not only protect that receiver from the other bands' signals but also prevent the low-level energy from being emitted on bands other than that on which it is being used.
Note: If you are experiencing interference from another transmitter that is actually producing noise on your receive frequency (this, given the presumption that your receiver isn't being overloaded) the addition of a filter on the station receiving interference will do no good at all since it cannot possibly filter out interference that is already there, on-frequency. In this case, the only appropriate remedy would be to filter the offending transmitter.
As an aside: In some cases simply enabling the radio's built-in antenna tuner - or using an external tuner - may significantly reduce the amount of out-of-band energy that the transmitter emits as well as adding to the attenuation from "other-band" signals during receive. Footnote 6
A high-pass filter
While band-specific filters are preferred, my friend presented a case where a high-pass filter (one that blocks signals below a certain frequency) may be appropriate. In his Field Day environment there has always been a station operating on 20 meters and usually another operating on 40 meters as well - but a third station was available to operate on 15 or 10 meters - depending on propagation conditions. The problem was that when this third station transmitted, 20 and 40 meters were often degraded - likely by the broadband noise mentioned earlier.
While it would be possible to obtain separate 15 and 10 meter band-pass filters at some expense, I decided on a different approach: A 15 meter high-pass filter. This filter - which could be made to strongly attenuate frequencies on the non-WARC amateur bands below 15 meters (e.g. 20, 40 and 80 meters) - it would have the advantage of also being usable on both 15 and 10 meters. Since it was unlikely that they would have stations on both 15 and 10 meters this strategy seemed sound for their application.
Using the ELSIE program (from Tonne software - link), I first calculated an "N=5" pro-forma high-pass filter using the "Elliptical" (e.g. "Cauer") circuit topology observing that I could get low attenuation at 15 meters and above while achieving more than 40dB on 20 meters and below. Using the ability of the ELSIE program to do Monte-Carlo type optimizations, I then tweaked the filter topology from a pure Elliptical filter to a hybrid one and this resulted in even better attenuation at 40 meters than the original: The schematic diagram of this filter is shown below:
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| Figure 2: The 15 meter high-pass filter as iterated by the ELSIE program. The circuit topology - originally Cauer (Ecliptic) was modified by using simple inductors in sections 1 and 5 and a capacitor at position 6 and then re-iterated to optimize performance. Click on the image for a larger version. |
As can be seen from the diagram, there are three capacitors in series with the signal path with two inductors directly to ground: The center inductor is in series with another capacitor, forming one of the "notches" typical of the Elliptical filter topology - and it so-happens that it's possible to tweak the filter so that this notch just happens to land in the middle of the 20 meter band to maximize attenuation there.
Rummaging around in my junk box I found several 500 volt silver-mica capacitors: For some reason I have a lot of 160pF units, so that was placed at section #2 and three of them were put in parallel for the series capacitor in section #3. I found a 200pF capacitor for section #6 and I paralleled a 120pF silver mica and an NP0 disc ceramic for that in section #4.
Many people doing homebrew construction seem to intensely dislike toroidal inductors - but while they would be more compact, there is no need to use them here, so large-ish air-core inductors were used. As the inductors are all "about" the same value (in the 225-300nH range) I wound 7 turns of 17 AWG (but anything 14-18 AWG would do) wire on a 13/32" drill bit for each of them, the precise value being unimportant as their turns would be stretched/compressed while using a VNA to "dial in" the filter response. As mentioned earlier, I'd added one more inductor from the initial design because the inductors were the cheapest of all of the components (they are just wire!) and they are very adjustable - simply by compressing/spreading the turns which meant that by picking capacitor values that were just "pretty close" to those called out by ELSIE, the coils could be used to tweak the filter's response. Note in Figure 3 that the inductors that are close-ish to each other are placed at right-angles, or in parallel with each other: Avoid placing two adjacent coils "end to end" with each other to minimize coupling between them.
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| Figure 3: Inside the 15 meter high-pass filter. Copper-clad PC board material is used as the backplane (ground) with small pieces used as "islands" for connection and support points. The added capacitor and inductor are those on the right-hand side. Click on the image for a larger version. |
The filter was built on a piece of copper-clad PC board material as a back-plane and ground and small pieces of that circuit board material were cut out to form "islands" - the so-called "Manhattan" construction: These islands would allow the junctions of the various circuit components to be connected together and with these islands glued to the back-plane and mechanically support the components soldered to them.
The piece of circuit board used as the backplane was sized to fit in the bottom of a die-case aluminum box that I had handy (about 6" x 3.25" x 2" or approx. 15 x 8.3 x 5 cm - but it could have been a bit smaller) onto which I'd installed two chassis-mount UHF connectors: These connectors were placed rather close to the bottom of the box so that their ground lugs could be soldered to the backplane, providing both the "ground" connection to the copper clad and for mechanical support
Using a VNA, I first adjusted the inductor in section 3 to provide a notch at about 14.24 MHz and then iteratively tweaked the inductors in sections 1 and 5 to provide the lowest insertion loss and lowest VSWR at 15 and 10 meters. When I was done, the insertion loss was just fine - less than 0.5dB - but the VSWR was about 1.45:1 at 15 and 10 meter so I added two more components (the 30pF capacitor in section 7 and the inductor in section 8 of the diagram) to act as a bit of a "tuner" to improve the match: In the figure above you can see an inductor (in section 8) that goes to the right-hand UHF connector (6 turns of the same wire as the other coils on a 13/32" drill bit) and a 30pF disk-ceramic capacitor (section 7) between the PC board "island" to which it connects and ground: With a bit more adjustment of all four inductors this brought the VSWR at 15 and (most of) 10 meters down to about 1.25:1 or better - plenty good enough! The response of this filter is shown below:
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| Figure 4: Insertion loss and VSWR plot of the 15 MHz high-pass filter as plotted by a VNA. As can be seen, attenuation at 40, 30 and 20 meters is well over 45dB with less than 0.5dB at 15, 12 and 10 meters: The VSWR is also acceptably low on these band as well. Click on the image for a larger version. |
Not shown in Figure 3, I later used RTV (silicone) adhesive to stabilize the coils and add support - after tuning, of course: This reduces the probability of the coils being detuned by the filter being jarred or dropped. RTV is fairly low loss (at least at HF) and far superior to "hot melt glue" in this case (it's lighter - and it won't melt!) and unlike hot glue or cyanoacrylate (e.g. "Super") glue, it can withstand mechanical shock without breaking loose - even when cold.
This filter should easily handle 100 watts - and the low loss is largely due to the use of silver-mica capacitors: After all, 500 volt silver mica capacitors - such as those used here - may be found in wide-range antenna tuners made by LDG and the like where they would be exposed to more stress than in the filter. If you are wondering about the use of the small, disc-ceramic capacitors, they are used in "low stress" parts of the circuit - to "trim" the capacitance to the needed value (e.g. a NP0 ceramic in parallel with a 120pF silver mica to get about 130pF) or used to "tune" the filter as in the case of the 30pF capacitor on the output. While it might seem risky to use these tiny ceramic capacitors at 100 watts, a quick look at almost any Japanese-made amateur HF transceiver - particularly those made up until fairly recently - you'll find them sprinkled with these capacitors in the low-pass filters and even for matching in the final amplifiers - both at HF and VHF/UHF - for matching: If it works for them, I'll not worry about using them here in the right places.
Don't forget to change the filter when you change bands!
Final comments
As can be seen from the response plot of Figure 4 this filter will attenuate signals on the bands 20 meters and below by more than 45dB and this should be enough to quash to inaudibility any low-level noise produced by the transceiver at these lower frequencies that might degrade reception on these bands. Similarly, energy from transmissions on 20 meters and lower from other stations will be at a much lower level prior to reaching the front end of the radio using this filter, further reducing the probability that they could overload/cause noise.
One thing that has not been discussed thus far is the fact that harmonically-related frequencies (e.g. a transmitter on 7.05 MHz would have harmonics at 14.10 and 21.15 MHz) are likely to be audible on other receivers, despite heroic attempts to fully-filter them. The reason for this is that these harmonically-related signals will be fairly strong compared to the noise floor of the amateur bands and, unlike the low-level noise discussed earlier, would have their energy concentrated into a small bandwidth.
Such signals are also likely to be radiated not only from the antenna ports, but from other cables connected to the radios themselves - namely the power cables, audio/microphone connections, data and PTT lines which means that a filter on the output won't suppress those other leakage sources. Other than wide-spaced separation (e.g. not placing radios in the same location and moving them as far apart as possible) there's no way to completely prevent harmonically-related QRM other than to coordinate efforts and simply avoid operations that could result in harmonically-related interference.
As it is not yet Field Day, I don't know if this filter will "fix" the problem that my friend was reporting, but should help, and it was quick, cheap and easy to throw together. Footnote 8
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After-Field-Day follow-up
A few days after 2025 Field Day I spoke again with the friend for whom I constructed the filter described above. While 10 meters was mostly dead, 15 meters was reasonably productive at times and operation on that band was successfully carried out - mostly using digital modes.
There was time to do a bit of A/B testing and it was noted that without the high-pass filter, the 15 meter transmitter did produce a very audible "hiss" on lower bands - most notably 40 meters - when it keyed up, but this was totally absent with the filter in place. Additionally, a slight amount of QRM from the lower-band stations (on 80, 40 and 20 meters) transmitting was noted in 15 meter reception without the filter - likely due to strong signals impinging on the radio's internal switching diodes - but this was also absent with the high-pass filter installed.
It's also worth noting that several other tactics were employed to minimize the possibility of QRM (interference) between stations, including:
- Separating antennas as much as practical. Depending on the site - not to mention the "1000 foot" rule - you can do only so much, but it helps to carefully consider the layout of the site to maximize distance between antennas.
- Using band pass filters. These go a long way toward preventing interference to and from radios on other bands.
- Use of mains filters. L/C filtering (e.g. filters using inductors/capacitors) were installed on the long extension cords that fed the individual stations to prevent RF from being conducted on the electrical power leads and for this, "Isobar" plug strips were used. Alternatively, putting a half-dozen or so turns of the extension cord through an FT-240 or "Monster" toroid (using 31 or 43 material for either one) would work as well - but it's recommended that the toroid be protected from damage by putting it in a box (e.g. a plastic electrical box with notches to allow the cord to pass).
- The use of "balanced" 4:1 baluns. As it turns out, most "4:1" baluns are NOT very balanced - and as such they will put significant RF current onto the radio and feedline, not only reducing antenna efficacy but also potentially improving susceptibility of RFI (Radio Frequency Interference) both TO the radio and devices connected to it (e.g. computers, other radios) but also allow RF interference (from generators, switching supplies, lighting, computers, etc.) to find their way onto the feedline. This "un-balanced" nature of most baluns can be proven by noting signal strength of a consistent off-air signal and disconnecting only one side of the feed to the balun: With a truly balanced balun you should see signals reduce by 20dB (3-5 S-units, depending on the radio) or more, but with most baluns you will see only a small (6-10 dB - 1-2 S-units) at most. Of all of the commercial offerings, one of the very few truly "balanced" baluns is the Balun Designs "Hybrid" balun (Model 4116 - link) - but most 4:1 baluns can be made to be balanced by immediately preceding it with a common-mode current choke (e,g, 8-14 turns of coax on an FT240-31 or FT-240-43 toroid).
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This page stolen from ka7oei.com
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Footnotes:
- Unlike most radios, Flex radios do include filtering to prevent low-level noise from being emitted on bands lower than the one on which it's being operated: Specific models of other manufacturers may also include this - although most do not.
- Direct-sampling receivers such as that of the IC-7300 have "different" problems in the presence of very strong signals compared to more conventional superheterodyne receivers: Any signal that hits the analog-to-digital converter can cause overload, no matter the frequency. While a conventional receiver can have a very "strong" mixer and some "roofing" filters in its IF (Intermediate Frequency) stages, this is not possible on a direct-sampling receiver. Instead, it must rely on a rather large number of individual, overlapping band-pass filters to cover its intended frequency range and the ultimate attenuation of these filters may not be "strong" enough to prevent a nearby transmitter on another band from adding to the already-strong melee of signals on the crowded bands during Field Day and causing overload - or, at least, significant de-sensing (e.g. reduction in sensitivity). This property is also what almost certainly makes them very poor candidates for being able to tolerate another local transmitter on the same band (e.g. a 20 phone and a 20 CW/digital station at the same Field Day site). There are strategies that can improve the probability of two stations co-habitating on the same band - mostly having to do with picking the "right" radios (e.g. Elecraft K3S or the K4HD are known to work in this environment as are a few others) - as can the use of parallel-pointed Yagi antennas (see the next section, below) - or very "sharp" band-pass and notch filters can be constructed as described in two articles on this blog, namely: A 100 watt "Helical" resonator bandpass/notch filters to increase isolation of 20 meter stations during Field Day (link) and Revisiting the 20 meter "helical resonator" band-pass/notch filters (link).
- Being able to point beams parallel to each other is at least partly a matter of geography. A station on the east coast is likely pointing their antennas west while the situation would be reversed on the west cost: A station in the middle of the country - with signals coming from potentially all directions - would be less-likely to be able to use this tactic, at least not without a degree of coordination among the individual transmitters/stations.
- A number of different manufacturers make band-specific filters for HF. Depending on the design, these can offer modest (>=30dB) adjacent-band suppression - which is usually enough to solve most interference problems - or much higher degrees of filtering, even more than 50dB. In addition, individual-band "Notch" filters are available from some suppliers that reject a specific band of frequencies which can be used several ways - on a transmitter to suppress any low-level noise that it might be generated on a specific band, or on another station to reduce the levels from a transmitter on that other band to prevent overload - and it can also be used to further-improve performance of a band-pass filter and increase attenuation on that specific band. One of the companies that supplies such filters is Morgan Manufacturing (link): Full disclosure - I know the person that runs this company and am quite familiar with the products. Other manufacturers also make similar, excellent products as well.
- This "hiss" can usually be detected without any sort of special equipment. To do this, one would set up two transceivers in a relatively RF-quiet location (perhaps NOT a suburban home), each on its own antenna spaced within a few hundred feet/meters of each other. On the radio doing the transmitting turn down the microphone gain all of the way after verifying that the RF power output would otherwise yield 100 watts peak when talking. On the receiver, tune in the next band lower than the transmitter and note the noise floor with and without the transmitter keyed up. In many cases, a "hiss" that can mask weak signals can be observed - particularly if using a resonant antenna on the transmitter without an antenna tuner. If you couple carefully into the transmitter (using attenuators or directional couplers) this noise floor can be measured directly with a spectrum analyzer - even the $50-ish "TinySA" is up to the task! It also goes without saying that a transmitter that IS outputting full power will also be prone to producing such hiss as well - not only above and below its actual transmit frequency, but on the "lower" bands as well.
- Testing to determine the efficacy of the built-in tuner as a band-pass filters was done using a Kenwood TS-450SAT, a radio from the 1990s. When the tuner was switched in and "tuned" - even if the load was already matched - it functioned as a low-Q band-pass filter that reduced the broadband noise and adjacent band signals by at least 8dB - and typically 20dB or so. Whether or not this strategy is likely to work on specific radios (e.g. some may switch out the tuner if there is already a good match) would require testing as described above. (These measurements were discussed in a very early entry of this blog - link).
- The most likely components to be damaged when trying to "force feed" RF on the "wrong" band are the capacitors, followed by toroidal inductors being somewhat less-likely - and this will often happen when transmitting is attempted at full power (100 or more watts) rather than at the low power level used for tuning. Usually, the operator realizes the mistake after the tuner fails to find a match, or it does find a match but signals are weak or absent. For this reason, if you are using a filter with a radio and an external tuner it's strongly recommended that you place the filter between the radio and the tuner: This will prevent damage to the filter as the radio will protect itself if it's used on the wrong band, presumably alerting the operator to the problem!
- It took far longer to put together this article than it did to design, gather parts, assemble and tune the filter!
