A "curiously sharp" 40 meter band-pass filter to reduce 41 meter SWBC overload.

At the Northern Utah WebSDR (sdrutah.org) we recently added another server (WebSDR #4) that is connected to an existing east-pointing beam antenna on site. This antenna, it is hoped, will better-allow users to hear stations on the 40-10 meter bands in the eastern U.S. and, to a lesser extent, the DX locations to which it is pointed.

Figure 1:
East-pointing beam antenna at the Northern Utah WebSDR.
This antenna has 10-13 dBi gain and signals in the 39,41
and 31 shortwave broadcast bands can be extremely strong!
Click on the image for a larger version.

As one would expect, this antenna has gain - between 10 and 13 dBi, depending on the frequency - and this has implications when propagation between Utah and the Eastern U.S. is favorable:  Already-strong shortwave broadcast (SWBC) signals become even stronger.

Because the S-meter (signal meter) on the receivers have known calibration it is possible to make an indirect measurement of some of these signals' strength and, at times, individual signals have been observed in the -20 to -15 dBm range at the antenna - these levels being in the "60 over S-9" range.  At times - particularly during evening "gray line" propagation (where both the transmit and receive sites are entering/in twilight/evening) signals can peak significantly.  What's worse is that there may be several such signals, increasing the total RF power impinging on the receiver risking not only receiver overload, but also providing a ready source of multiple, modulated carriers to mix together and reappear within the receiver's passband among the desired signals.

These sorts of signals are far above those that might be expected due to amateur-only transmissions owing to the widely disparate signal levels.  For example, a very well-equipped "DX" stations may be able to run 1500 watts of RF into a (monster!) 15 dBi gain antenna and attain an EIRP (Effective Isotropic Radiated Power) in the area of 50kW, but this does not compare with an SWBC station which may be running 500kW peak (about 125kW carrier) into an antenna with (a conservative) 18dBi gain - an EIRP of about about 32 million watts - a signal level nearly 1000-fold (30dB) stronger than one that would be transmitted by law-abiding amateurs.

What's worse is that some of these SWBC bands are adjacent amateur bands - and the 40 meter amateur - which runs from 7.0-7.3 MHz is no exception as the 41 meter SWBC band is just above it, starting at 7.3 MHz.  With such close spacing, typical filtering in receivers have little hope in effectively rejecting these nearby, strong signals.

Addressing the problem:

There are two time-honored ways of dealing with strong signals impinging on receivers:

• AGC (Automatic Gain Control):  This circuit "monitors" the signal level at the receiver and automatically reduces the gain when they exceed a certain amount.  In the past, this has been applied only to signals within the passband of the receiver's IF to keep the audio level constant regardless of the actual signal strength, but this is also applied to modern SDRs where the level of the entire passband of signals being input to the A/D converter is monitored and adjusted to prevent overload. 

• RF front-end filtering:  With the advent of solid-state radios starting in the 60s and 70s the design of RF filtering used in amateur receivers began to be wideband, typically covering MHz, rather than a narrow peak.  This was done not only because it was easier to do so with these designs, but also because it allowed "general coverage" reception outside the amateur bands and it was significantly less expensive than mechanically-complicated, ganged tuning systems - but it had the down-side that signals some distance away frequency-wise could still cause the receiver to experience overload.  These days - particularly with modern, high-performance direct-sampling Software-Defined Radios (SDRs) - "narrow" filtering is once again being used, along with AGC, owing to the need - more than ever - to strictly control the total amount of RF energy reaching the A/D converter to prevent overload. The receiver used at the Northern Utah WebSDR is a type of SDR where the RF energy is converted directly to audio and then digitized.  This has the advantage of simplicity, but it lacks the "AGC" circuit meaning that it is possible for strong, off-frequency signals to cause overload of not only the RF circuits, but also the audio circuits and the A/D converter.

While it is possible to add an AGC circuit to this receiver system to prevent overload (this has been done with some of the other receivers on site - and is still an option) the first step that we are taking is to build a "sharper" filter.

The "Curiously Sharp" band-pass filter:

Passing signals on the 40 meter amateur band - which ends at 7.3 MHz in the Americas - and filtering out signals on the 41 meter shortwave broadcast band - which starts at 7.3 MHz - is a tricky proposition:  How does one suddenly go from passage of signals to blocking them within just a few 10s of kHz?

Figure 2:
The completed 40 meter band-pass filter
in a Hammond 1590D die-cast box.
Click on the image for the larger version.

The answer is:  You don't - but you do the best that you can!

The limiting factor in constructing a "brick wall" filter - one that has an abrupt transition - is physics and is intrinsic to real-world components:  Real-world inductors have ohmic resistance and capacitors have dielectric losses - to name but two factors - that limit the unloaded "Q" of the circuits.

What does this mean?  A truly "sharp" filter will ultimately be limited in its performance by these factors:  One must trade off insertion loss and/or filter performance in terms of how quickly our band-pass filter cuts off.

Fortunately, the first of these - insertion loss - is pretty easy to mitigate:  Have enough extra signal gain in the receive system to accommodate the insertion loss.  At 40 meters, we have "signal to burn" - partly because our receive antenna has so much gain, but there is also a "strong" (resistant to overload) RF amplifier located near the antenna to mitigate the effects of cable losses at the higher HF bands (10 meters).

Even if we didn't have both antenna and amplifier gain, we could afford to lose a lot of signal at 40 meters:  A system noise figure of about 30 dB (assuming a unity gain antenna) is sufficient to "hear" the noise on even a quiet band, so a significant loss can still be made up by placing an RF amplifier after the filter and still be able to resolve the 40 meter noise floor during quiet band conditions.

Figure 3:
 Schematic of the 40 meter bandpass filter.  This is a 7-element Elliptical (Cauer) filter centered at 7.15 MHz - the middle of the U.S. 40 meter amateur band.  It was originally designed with the aid of the "A.A.D.E. Filter Design" program, version 4.5 being available from the AE5X web site.
The nominal impedance of the filter portion is 800 ohms to permit higher values of inductance and lower values of capacitance in an effort to ease construction and to reduce component losses (e.g. reduce the L/C ratio).
Click on the image for a larger version.

Figure 3 shows the schematic of the filter - and a few explanations are warranted.

• ALL of the capacitors must be either NP0 (a.k.a. C0G) ceramic or silver mica capacitors - preferably the latter.  I did not use any silver mica capacitors, but I used known-good ceramic capacitors from a trusted source (e.g. Mouser-Key) rather than from a random EvilBay seller.
• L1, L4 and L7 were wound using solid 12 AWG copper wire.  The wire that I used happened to be tin-plated, but enameled copper wire will be just fine with only the two ends (and the tap point) being bared for soldering.  If bare copper wire is used it is suggested that it be very clean and sprayed with clear lacquer after construction is completed to prevent oxidation.
• The other inductors were wound using 17 AWG wire, which was on hand, but 18 AWG would be fine.
• All of the inductor/capacitor pairs have their own resonant frequency, noted on the diagram in parentheses.  The 7.15 MHz resonances (C1/L1, C4/L4, C7/L7) will be adjusted very close to the stated frequency but the other resonances (C2/L2, C3/L3, C5/L5, C6/L6) are made adjustable by small ceramic (or air) variable capacitors and must be CAREFULLY adjusted for the proper filter response.
• As can be seen, the filter's in/out ports are terminated with 2dB resistive attenuators to help assure a consistent source/termination impedance to the filter and prevent the likely-imperfect devices to which it is connected from too-badly affecting the response.
• L1 and L7 show taps that are chosen to be at the 50 ohm points.  The "S11" port of a known-calibrated VNA may be used to best-set the 50 ohm points of the taps during filter construction/adjustment.

Constructing the filter:

Figure 5, below, shows the as-built filter:

During construction I used my DG6SAQ Vector Network Analyzer - and a tool such as this is invaluable as it will give "live", dynamic readings to facilitate adjustments.  The more economical (approx. $50 U.S.) "NanoVNA" will work fine (along with the "NanoVNA Saver" program) - and although its update/sweep rate is quite a bit slower than that of the DG6SAQ, it's still usable.  No matter what sort of VNA you might use, be aware that the limited number of data points per scan can "hide" details such as narrow, deep notches - and this is especially true with the NanoVNA.

The "through loss" measurements (in dB) were the most important in this case as the insertion loss versus frequency plots over a range of about 6.5 to 7.8 MHz allowed the "dialing in" of the resonant circuits.  During construction two "bloody ended" coaxial cables were used - one end of each being connected to the VNA and the other end having its ground shield tacked to the ground plane and the center conductor attached to the point under test:  These test cables are visible in Figure 4, below.  This "plywood and foil" test bed allowed an easily reconfigurable circuit design and test bed for ideas - and, most importantly, it helped me determine if a particular design was even practical.

Figure 4:
Early prototype built on a piece of plywood covered with self-adhesive
copper foil.  Originally, L1, L4 and L7 were wound on toroids - but
a switch was made to the larger, air-wound inductors to reduce losses.
This early version used input/output transformers for transformation of
the 50 ohm in/out to the 800 ohm (nominal) impedance of the filter itself -
but this was changed to tapped inductors as that was simpler and
lower loss.  This simple "breadboard" allowed several ideas to be tried
before settling on the final version, giving plenty of room to work.
This picture shows the short pieces of coaxial cable that were
tacked to the foil ground:  These cables connect to the two
ports of the VNA used to analyze the response of the filter.
 Click on the image for a larger version.

The first to be constructed were the large resonators (L1/C1, L4/C4, L7/C7) which needed to be set to 7.15 MHz and for this, two resistors (1k-4.7k - the precise values are unimportant) were connected in series with the center point connected at the "top" end of the parallel L/C network and the "ends" being connected to the VNA's in and out ports.  With this arrangement one can see the "peak" where the L/C circuit resonates - the two resistors minimizing loading - and one compresses/stretches the large inductor using a small screwdriver to increase spacing between turns or a pair of needle-nose to compress them - or, if necessary, removes fractional turns - to "dial it in" at 7.15 MHz.

After these have been adjusted, the other L/C networks are then constructed - and this is where it gets to a bit tricky:  The variable capacitors allow the resulting "notch" to be moved around, but it may be necessary to add/remove turns from the inductor - or add small amounts of capacitance (10pF at a time) to get the circuit's adjustment within range of the variable capacitor.  In some cases, one may temporarily "shunt" (short out) one or more of the series L/C networks to better-visualize the notch that one is trying to adjust.  If you can't find the "notch", don't forget that it may be above/below the sweep range and you may temporarily need to set the start/stop frequencies wired to find it.

As often happens, one's first ideas don't work quite as expected:  You will note that in Figure 4, L1, L4 and L7 are shown as being toroidal inductors, but it became clear that these inductors were just too lossy, a factor that severely affected "Q" and performance.  I ended up using air-core inductors wound from much heavier wire as can be seen in Figure 5 to minimize loss.  Ideally, superconducting inductors would have been used, but for some reason such devices that operate at room temperature aren't available!

Adjusting such a filter requires patience as everything interacts.  An examination of Figure 5 will reveal that each section is connected with jumper wires, allowing isolation of the individual tuned circuits.  Eventually, one can get a "feel" for how the adjustments interact - but it may still be necessary to  disconnect the sections and check/tune them individually back to a starting point if one gets "lost" and the response/tuning gets worse and worse.

Also visible in Figure 5 are shields around the large tuning elements made from pieces of double-sided copper-clad PC board material.  While shielding between the sections isn't really necessary from a performance standpoint, placing the filter - which was constructed on the lid of the Hammond 1590D box - into the box itself causes the filter to detune slightly due to proximity to the enclosure's walls:  The shielding on the sides of the large coils - and the bars across the top - "simulated" the filter being within the die-cast box and almost eliminated the effect, but still allowed access to permit adjustment if the large coils.

Figure 5:
As-built 40 meter band-pass filter.
This filter was constructed on a solid copper ground plane of circuit-board material.  to hold components in place and to isolate junctions from the ground.  "Manhattan" (island) pads were used for junctions that needed to be isolated from the ground:  The "Me Pads" (from "QRPMe") were used.  Blobs of RTV are used to mechanically support some of the larger components.
Click on the image for a larger version.

 

To be clear:  This should NOT be your first band-pass filter as it is VERY tricky to adjust - and you MUST have available a scalar and/or vector network analyzer to properly adjust it!  If both of these do not apply to you it is suggested that you obtain help - or prepare to get this gear and pull your hair out during adjustment!

Did it work?

The answer is Yes.

Figure 6:
A sample passband of the filter during adjustment:  The ultimate adjustment resulted in a somewhat flatter response.
The "upper" notches (L2/C2 and L5/C5) can clearly be seen as can the upper "lower" notch (L6/C6).
The intrinsic insertion loss, including the two 2 dB pads, is around 15dB.  The ultimate rejection is around 65 dB, correlating to a filter rejection of around 50 dB, taking into account the through losses.
Click on the image for a larger version.

 

This filter offers over 20dB of (additional) attenuation below 6.9 MHz and above 7.4 MHz and between careful adjustment of the receive system gain (e.g. just enough signal to comfortably "hear" the noise floor during the quietest part of the day) and the attenuation of the 41 meter signals, overload on the 40 meter receivers on the Northern Utah WebSDR #4 no longer occurs.  If you wish, you can check it yourself, particularly during the evening "gray line" hours when sunset is sweeping across North America at sdrutah.org.

Comments:

The use of a similar filter in ITU Regions 1 and 3:

In Regions 1 and 3 the 40 meter amateur band covers 7.0-7.2 MHz with strong SWBC signals starting at 7.2 MHz.  Narrowing this filter to 200 kHz would require a redesign and would further-push the limits of standard components, but broadly similar results should be possible.  Alternatively, the center frequency of this filter design could be moved down by 100 kHz to 7.05 MHz and offer similar rejection to signals above 7.2 MHz.

Options for even "sharper" filtering:

While the filter described is starting to push the limits in terms of the use of reasonably-obtainable components, there is another option:  A frequency-converting band-pass filter.  For this, a local oscillator and a pair of mixers would be used to convert the 7.0-7.3 MHz 40 meter passband down to a lower frequency where a "sharper" filter would be easier to construct.

For example, using an 8 MHz oscillator would convert the 40 meter band from 7.0-7.3 MHz to 0.7-1.0 MHz, inverting the frequency, meaning that the most critical part of our filtering - that "above" 7.3 MHz - would now be happening below 700 kHz.  Of course, this "converting filter" would have to have decent band-pass filtering of its own to prevent response to undesired signals and the mixer used for the down-conversion would have to be adequately "strong" to withstand the insanely strong 41 meter signals.

Once the filtering is done at this lower frequency, the same 8 MHz oscillator would be used to up-convert back to 40 meters:  With the same oscillator used in both directions, it need not be particularly frequency-stable as any drift would be self-compensated.

This page stolen from ka7oei.blogspot.com

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When Band-Pass/Band-Reject (Bp/Br) duplexers really aren't band-pass

Addendum: 

Since this article was originally published, the availability of low-cost test equipment like the NanoVNA has allowed more-thorough testing of their RF signal paths.

With the advent of inexpensive and "good" test equipment like a NanoVNA, there is little excuse these days for not knowing if one's duplexer has the needed band-pass characteristics described in this article.

* * * * * *

In the repeater world there is a misconception that just because the duplexer may say "Band Pass, Band Reject" on its label - or even in its 'spec sheet - that it really does offer a proper band-pass response over a wide range of frequencies - but this is usually NOT the case.

A close-in look at a typical Band-Pass/Band-Reject duplexer:

Take Figure 1, below, as an example.

Figure 1:
The magenta trace is that of a proper band-pass cavity, the yellow trace is that of a one side (3 cavities) of a 6-cavity Phelps-Dodge "Band-Pass/Band-Reject" duplexer while the cyan trace is the combination of the two.  The top of the yellow peak (with the "1" marker) represents the center frequency of the duplexer with a bit over 1dB loss.

In the analyzer trace above, the YELLOW is the response of one of half of a typical amateur "Band-Pass/Band-Reject" 6-can Phelps-Dodge duplexer tuned in the 2-meter amateur band and from this trace we can see several things happening:

• As there should be, there is a peak at the pass frequency corresponding to the "band-pass" of the duplexer - in this case, a bit over 1dB loss.
• Just above the peak - 600 kHz, to be precise - is a very deep notch - corresponding to the frequency to "band-reject" part of the name.  In reality, the depth of the notch depicted in Figure 1 is about 100dB, but the true depth is not apparent from the trace.
• Once one moves about 1 division (1.5 MHz) either side of the peak/notch frequency, the attenuation isn't that great - only about 20-30dB, and the trace above the center seems to be on a asymptotic trajectory upwards (lower attenuation) as frequency increases.

From the above we can see that while this duplexer offers a "Band-Pass/Band-Reject" response, this occurs only at frequencies very near the input/output frequencies of our hypothetical repeater.  Once you get "farther away", this "band-pass" response diminishes.

On the other hand the MAGENTA trace shows a single band-pass cavity filter.  While its attenuation is not very high at the notch frequency - on the order of 10-15dB - it is apparent that by 2 MHz above the center frequency it is offering greater attenuation than the so-called "Band-Pass/Band-Reject" filter and that below the center, the trend indicates that they might cross over at a point just to the left of the trace.

Comment:  This "Bp/Br" nomenclature is widely applied amongst many manufacturers to duplexers that have the same response as the Phelps-Dodge duplexer above, including Motorola and Wacom - to name but a few.  It is the rare exception to find a "Band-pass/Band-Reject" duplexer that does NOT exhibit the properties described on this page!

Unless you have already installed some band-pass cavities on each leg of your duplexer and/or have done proper sweep responses at frequencies far removed from the designed frequency, you should not assume that your "Bp/Br" duplexer is truly Band-Pass/Band-Reject over a very wide frequency range!

Taking a wider view:

Figure 1 only spans about 7.5 MHz on either side of 2 meters, so let us widen it a bit as shown in Figure 2, below:

Figure 2:
Spanning from 30 MHz to 1 GHz, the same cavities/filters as noted above.  Again, the yellow trace is one half of a 6-cavity "Band-Pass/Band-Reject" duplexer, the magenta trace is the pass cavity alone and the cyan trace is the result of the bandpass cavity and the Bp/Br duplexer cascaded.  It should be noted that at odd-numbered harmonics the pass cavity
will present a narrow bandpass response that can be eliminated with the addition of a simple low-pass filter.

When looking over a much wider frequency range - 30 MHz to 1 GHz - the picture is quite different.  Based on this sweep we can see that our typical "6 can" duplexer - of which 3 "cans" of the transmit or receive side - are represented above in YELLOW and that for the majority of the frequency range there is relatively little attenuation offered overall!  Paying particular attention we see that the attenuation in much of the VHF-low TV band (channels 2-6) and the FM broadcast band is quite poor - on the order of 3-10dB - as is the case over much of the VHF-high (channels 7-13) and large sections of the UHF TV band.

What we can see from this picture is that if we rely on only our so-called "band-pass/band-reject" duplexer on a site with other services such as FM or TV broadcast, or even land-mobile, those frequencies just above the amateur band, such a duplexer offers relatively little protection against those signals getting into the transmitter or receiver.

Why it matters:

One might wonder why it would matter whether or not a duplexer offered good "far-off-frequency" rejection.

In many cases, particularly in mountainous areas, amateur repeaters are co-located at sites with other transmitters and if adequate filtering is not implemented those "other" signals can get into the repeater's receiver and/or transmitter.

The effects of these other signals' ingress into the receiver is easier to envision:  Many of us have observed that, while driving about, our mobile radios have occasionally been overloaded with other signals - the effect being that we are hearing signals on frequencies where they are not.  This phenomenon is the inevitable result of the receiver's mixer - a device that is designed specifically to make new signals out of multiple signals in the first place - synthesizing entirely new ones out of the several that get in via its antenna.

Several decades ago it was common for land-mobile VHF and UHF radios to have receivers that had very tight filtering as there were typically only a few, closely-spaced channels that were used.  By virtue of this extensive filtering it was unlikely that other signals' somewhat-removed frequencies could even get in and cause undesired signals to be generated.  These days most radios have very broad filtering in their receiver inputs - this, to allow a wide range of frequencies to be accommodated.  While convenient, this also has a down side:  Those formerly widely-spaced frequencies from other services now have little impediment and it is more likely that they will get into the receiver and produce undesired, spurious signals.

Many years ago it was also the case that many repeaters used modified land-mobile radios with their extensive filtering, but nowadays many "store bought" repeaters (such as the Icom D-Star and Yaesu Fusion lines) are simply beefed-up mobile radios with "broad as the proverbial barn door" filtering on their receivers.  While this is convenient for the repeater owner to not have to dig up some test equipment and tune up these receivers' narrow filters, this also means is that there are many instances where a club has replaced their old, crystal-controlled analog repeater with a new one - only to find out that it did not work well at all when these off-frequency signals - formerly blocked by the old receiver's narrow front-end filter - clobbered the new receiver.  Worse still, some of these repeaters (namely the Icom D-Star) provided no analog test points where the receiver performance could be directly analyzed to determine if there was a problem, much less its extent!

What's worse is it is often the case that at many sites this sort of interference may be intermittent in nature - occurring only when a certain combination of transmitters happened to be online at once:  With most repeaters using subaudible tones for access, this degradation is often masked since the repeater may stay silent when it is being impacted, the only clue being that some users may suddenly find it difficult to get in to the repeater with a good signal at random times.  In other words, unless one uses the proper test equipment to take and record repeatable measurements at or away from the site, gradual or occasional degradation of the receiver's performance may not be so apparent.

An insidious problem:

While the overloading of a receiver is a familiar problem to many of us, it may not be as obvious that a similar thing can happen in a transmitter.  Like a receiver, a transmitter has the ability to take two signals and produce others via mixing.  For this to happen it usually requires that the "other" signals are very strong - but this is something that can happen at a busy radio site!

As a demonstration of what can happen, it was noted that via a VHF antenna atop Farnsworth Peak near Salt Lake City, Utah - a very busy broadcast site - one could read 100-150 milliwatts of RF on the coaxial cable at the input to the duplexer.  When this energy was analyzed it was found to be a combination of FM broadcast and UHF TV signals - the same transmitters that produce several megawatts of effective radiated power, combined.  If the same 6-cavity duplexer depicted in Figure 1 and Figure 2 was inserted in the line, this power would reduced - but only to the 20-50 milliwatt level!

This power was measured on the feedline of what would be a D-Star repeater, but prior to the installation of that repeater an analog Kenwood TK-740 repeater had been used for several months to assess coverage and performance prior to the installation of the Icom D-Star repeater.

On the day that the D-Star repeater was installed it was discovered that no-one could get into it, despite their running 50 watts.  Upon analysis it was discovered that the 20-50 milliwatts coming back into the coax was causing the Icom repeater's receiver to be deafened (desensed) by about 40dB - a factor of 10,000-fold!  Upon reconnecting the TK-740, no problems were noted and it was realized that the Kenwood repeater had a more traditional, narrow-band helical resonator filter assembly in its front end and compared to the more modern "broad-band" front end of the Icom repeater - which used parts of modified mobile radios - that the power in from the antenna was completely demolishing its receiver!

Figure 3:
A typical Motorola  4-can duplexer for UHF.  Just like its VHF counterparts
it easily passes energy at frequencies above and below its tuned frequency.
Click on the image for a larger version.

The installation of two bandpass cavities on the receive side allowed the Icom repeater to work as well as the old Kenwood analog repeater with its superior filtering - but this brings up the question about what might happen on transmit?

The transmitter can also act as a mixer:  Multiple signals - one of which might be the repeater's output frequency - can combine within the circuitry and instead of only the transmit frequency being emitted, some conglomeration of signals can appear!

In the example of a VHF transmitter we know that while the low-pass filter may remove the frequencies above the 2-meter band - say, UHF land-mobile and UHF TV - it will do nothing to remove energy from FM broadcast stations.  Similarly, if this were a UHF transmitter, its low-pass filter might remove some of the UHF land-mobile and UHF TV energy, but it would have little effect on signals from FM broadcast and VHF high and low band TV.

It might be suggested at this point that the use of an isolator - a device that, while allowing the transmitter's energy to go to the antenna, it directs any power coming back down the coax into a dummy load so that it cannot even get to the transmitter, might be appropriate here - and this would be correct...  Mostly.  While these devices are invaluable - and even required equipment at many radio sites - to both prevent RF from nearby-frequency transmitters from getting into your transmitter - and then re-radiated again and also to insulate your transmitter from a bad VSWR - it is far less-effective when the frequencies that are coming back down the coaxial cable are away from its design frequency.  In other words, while your VHF isolator may work okay from 140 to 160 MHz, it will probably do comparatively little at the FM broadcast band and in the UHF range.

Adding a pass cavity:

It is, therefore, a very good idea to equip any repeater with at least two pass cavities:  One on the receiver, tuned to the input frequency and another on the transmitter, after the isolator, tuned to the output frequency.

If one examines both Figures 1 and 2 you can see the Magenta trace showing the response of a single pass cavity.  When compared to the response of a typical Bp/Br duplexer (the YELLOW trace) the general trend is that the farther away one gets from the pass frequency, the more attenuation it offers.  One quirk of band-pass cavities is that they also have a response at odd multiples of their pass frequency, which means that a 2-meter pass cavity will also pass energy around the low end of 70cm, around 700 MHz, and so-on.  In the case of 2 meters, this spurious response could be eliminated by the addition of a low-pass filter.

Both figures 1 and 2 also show something else:  What happens if you cascade a Bp/Br duplexer with a single pass cavity (the CYAN trace)?  For the most part the overall attenuation of the two sets of filters is complementary - that is, the "best of both worlds."  As can be seen the simple addition of a pass cavity knocks out almost everything that is off-frequency from that which is desired.

Figure 4:
A typical "4 can" (2 on transmit, 2 on receive)
2-meter duplexer.  Even though it is labeled
as a "band-pass/band reject" unit, this refers only
to the two frequencies of interest - the transmit
and receive - and not to the RF spectrum overall!
The plots in Figures 1 and 2 are from a similar,
"6-can" (3 on tx, 3 on rx) duplexer, but the
rejection of frequencies "far removed" from
where they are tuned is comparable.
Click on the image for a larger version.

Bandpass cavities have another important property as well:  Lightning protection.  Because lightning is a broad-band energy spike, it would make sense that if you reduce the passband of the signal path from the antenna, less RF energy, overall, will get in - and the use of a passband cavity also guarantees that there is NO DC path from the center pin of the coax from the antenna to the center pin of the coax going to the radio.  One radio club - the Utah Amateur Radio Club - has several mountain top repeaters and there have been a number of instances where the repeater antenna has taken a direct lightning hit, sometimes destroying the antenna, but never has the attached receiver or transmitter ever been damaged.

Summation:

If you are installing a repeater or other radio at a site with any other transmitters you should not assume that just because the label or specifications of the duplexer say that it is "Band-Pass/Band-Reject" that it will actually do so over a wide range of frequencies.  Again, most brands of duplexers will simply pass, with relatively little attenuation, those frequencies that are far removed from the operating frequencies and the "band-pass/band-reject" nature is limited to the specific frequencies of interest - such as the transmit side of the duplexer passing the transmit signal but rejecting energy at the receive frequency.

Such a duplexer should always be supplemented with at least one bandpass cavity for the transmit frequency and another for the receive frequency to provide additional off-frequency rejection - and adding a simple low-pass filter on each leg won't hurt, either.  While these added elements result in higher signal loss, this need only be 1dB or less in most cases.  Adding this extra cavity will increase the effectiveness of an isolator on the transmitter - which works only well near its design frequency anyway - but it will also prevent excess, off-frequency energy from getting into the repeater's receiver which, these days, is more typically a "mobile" unit with a very broad front end that has been converted.  Finally, the humble band-pass cavity provides good lightning protection, just by its very nature!

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This page stolen from ka7oei.blogspot.com



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