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

[End]

Lower Courthouse Wash is full of beavers!

This past weekend I wandered down to the Moab, Utah area with a friend of mine (Gordon) to meet with my brother and his family - who'd arrived the day before - to do some hiking.

Figure 1:
Gordon and my brother, just entering the wash near
the bridge.

We weren't really considering doing anything too serious so after a bit of thought I suggested Lower Courthouse Wash which lies almost entirely within Arches National Park.  I'd done this hike at least three times in the past  - but not since the mid 1990's - and I knew it to be a nice walk along sandy embankments, occasionally through the cool water flowing over and through large sand bars and embankements - except near the end where the joint grass and trees got thicker, occasionally requiring one to walk through the watercourse to avoid having to push through the thickets.  Starting from an innocuous bridge along the main road in the park, the end of the trail puts the hiker next the main highway, only a few hundred feet from the Colorado River bridge at the north end of town.

That was the plan, anyway.

Leaving home at exactly 4 AM, we drove down and met my brother and his family in Moab for breakfast at the Jailhouse Diner after which we dropped off my Jeep at a convenient parking lot along the Moab cycle trail fairly near the exit of Courthouse wash.  Piling into the other car, we entered the park, drove to the bridge - which now sported a small, paved turnoff for parking - and Gordon, my brother and I wandered off down into the wash.

Right away I could tell that it was quite different - but I couldn't place exactly how:  It had, after all, been nearly 20 years since I'd been there.  It seemed to me that it was greener, somehow, much more vegetation.

Walking downstream I soon realized that this place had changed!

The large-ish, older cottonwood trees were expected, but I was surprised to see that many places where there had once been sand banks were now colonized by a myriad of smaller plants, rabbit brush and the occasional Russian olive tree.

It wasn't until we were about 1/2 mile downstream until we were required to make a stream crossing.  By brother went first into the nearly hip-deep water, managing to cross successfully to the other side, using the conveniently-placed log and the vegetation on the opposite bank as a handhold.  I went next, also making a successful crossing.  Remaining to cross was Gordon who got more than halfway across and suddenly disappeared up to his neck and doing a quick backstroke.  Unfortunately, there was thick vegetation between the bank closest to him and us and we couldn't safely reach him, so all we could do is watch while he managed to stagger backwards to the felled log, pull himself to his feet and then get within safe reach.

Figure 2:
Gordon's first tentative steps into the
water - just before a splash.

Emptying his pockets, and handing their contents to us, I quickly removed the batteries from his digital camera while poured the water out if it, also shaking it out of the memory card and every other available orifice on the camera in an effort to minimize what was already likely to be fatal damage to its mechanism.  Moving down-canyon a short distance to some rocks, we poured several inches of water out of his day pack, separated the now-ruined boxes of raisins and other food - likely contaminated by waterborne bacteria and parasites like giardia - from that which was protected in watertight containers or still dry by virtue of having been near the top and removed the batteries from other electronic items.  The pack was turned inside-out, the wet clothing (jacket, extra shirt) was attached to the pack, and some of the remaining "un-soaked" items were then carried by my brother and I while the pack would dry in the now-increasing breeze blowing up the wash.

"How long do you think it will take for it to dry out?" Gordon asked.  "About 15-20 minutes, I think" I replied.

Continuing down the wash, we made a few other crossings, but none so deep or dramatic, most being no more than calf-deep, but the realization that this place had transformed dramatically since I'd last been there was still dawning on me.

Figure 3:
Once a wide, sandy wash, young trees and
vegetation now rule!

I finally realized how much when we got to a right-hand turn in the stream that I'd recognized as being where I'd ended up waist-deep in quicksand last time I was there, having to hand my gear to one of the fellow hikers while I rolled out if the sand and onto firm ground.  While a fairly barrent sand bar before, it was now heavily grown up with small trees and heavy brush while the watercourse - which had been those years ago, a wet area of sand - was a rather narrow channel of slow-moving water greenish through the vegetation.


Another mile or so down the wash, I looked at Gordon's pack and decreed that it will take another 15-20 minutes for it to dry to which the response was that it had been more than that last time the subject had been brought up.  I pointed out, however, that it all depended on when, exactly, one started timing - and that I wouldn't know that until it was fully dry.

Figure 4:
Cascades over sandstone in lower Courthouse Wash.

Pushing through the ever-thickening brush, we made another knee-deep stream crossing through dark, tannin and algae-stained water and climbed onto a large platform of rock in order to circumvent a nearly impenetrable stand of vegetation - unless, of course, one wanted to wade through slow-moving, slick-bottomed water channel.  Atop the rocks, we sat down for lunch and reassembled Gordon's now (mostly) dry pack.  Briefly putting in the battery, we tried his camera - but it showed no signs of life.

By this time, the partly-cloudy sky of the morning had changed to one that was overcast while the wind blowing up from the bottom of the wash became quite brisk.  The temperature was, perhaps, in the low-mid 70's and while resting, it was getting to be quite cool, but not intolerably so even though I was wearing shorts.  Finishing our lunch, we continued on our way downstream, having to make the occasional knee-deep crossing.

Another mile or two brought us to some familiar territory - a series of cascades over sandstone in a wide portion of the canyon.  Nearby were some very large overhangs that we decided to explore, noting that not only were they suitably large and situated for camping, but a large amount of charcoal evidence indicated that we weren't the first ones with this idea!

A short distance down from this the wash made a sharp turn to the left, requiring us to wade through calf-deep water which immediately became thigh-deep if one paused for more than a second and sunk into the submerged quicksand.  Proceeding downstream from this we saw that there was difficulty ahead:  The channel narrowed considerably, deepened, and footing became precarious.  Getting back on dry ground, I put my boots back on, put down my day pack and set out to see if I could find an alternative route.

Figure 5:
Confirmation of inhabitation
by beavers!

As suspected, the channel got much deeper and probing with a stick showed that it had a rather soft bottom and was likely above the waist - assuming that one didn't sink into quicksand or slip and fall in!  Moving farther downstream, I spotted two logs crossing the stream - a bit too wobbly and precarious for either my brother or I to attempt a crossing, but all around us were knawed-off stumps and felled trees that not only confirmed our suspicions that beavers were about, but provided us with the raw materials to bolster the bridge and allow us to cross with relative confidence that we would make it without having to go for a swim in the still, dark green water.

Continuing on, we made our way down the wash with increasing difficulty, having to cross the stream multiple times and then find a route through occasionally-thick brush.  Up to this point I had been mostly barefoot, but became painfully aware of a lot small thorns in the ground litter that had occasionally required that I stop and scrape the bottom of my feet with my knife to dislodge it.  Although the going was a bit slow, it turned out that the worst was behind us!

Figure 6:
One of, perhaps, a dozen beaver dams of
various sizes that we saw.  This was one of the
last seen before reaching the highway.

As we continued, the clouds thickened and the wind picked up.  It occurred to us that we were, in fact, in a wash that drained a large chunk of Arches National Park and the surrounding area and that being there with increasing clouds probably wasn't the most ideal situation - this notion being occasionally reinforced by the fact that we could often see the remains of past flash floods wrapped around trees at heights that placed the water level as being 6-8 feet above the wash bottom!  Since the clouds looked rather benign and the forecast didn't indicate rain until the late evening, we weren't too terribly worried.

A few more miles brought us to the exit of the wash, a fact reinforced by the fact that ahead, we started catching glimpses of more-distant canyon walls.  Another sure sign of being near the end of the trail is that we also started seeing the occasional beer can and cigarette butt - neither items being frequently seen very far from where one might park a vehicle!

Figure 7:
Informative sign at the bottom of Courthouse Wash.
If you click on the image and inspect the picture
on the sign, you will see what much of lower
Courthouse wash used to look like!

Soon, the tell-tale roar of traffic greeted our ears and the bridge hove into view.  This, too, had changed much since the last time I had hiked lower Courthouse Wash:  The bridge was now much larger and taller and included the Moab bike trail - which had not even existed back then!  Making the short walk back to the Jeep, we scraped the layers of mud off our feet and legs, pulled out a few pesky thorns and then headed back into town to drop off my brother after which Gordon and I decided to do a few short hikes in Arches before sunset.

The upshot:

I would by lying if I said that I wasn't a bit disappointed in what lower Courthouse wash has become in the nearly 20 years since I'd last hiked it.  When I was done, I was nostalgic for the times that I'd made this same hike - mostly in bare feet - while walking along the wide, sandy wash bottom in the sun while the ankle-deep clear water trickled playfully downstream.

I also realize that things change over time and that in nature - as in life - such change is inevitable.  It's difficult to say whether or not this course was "better" 20 or so years ago when it was much more open and, perhaps, somewhat barren:  To be sure, compared to then, life positively abounds in lower Courthouse Wash - but it is distinctly less friendly to hikers!

In doing a bit of quick research on the web - and in my recollection of the book Desert Solitaire by Ed Abbey in which he talks a bit about this same area - it's easy to find documented change in this - and other - areas of red-rock country.  From what I can gather, there was a tremendous flash flood through Courthouse Wash in the mid-late 1960's and, possibly, subsequent burning of much of the debris and it was likely the tail-end of this last major "cleaning" that I had enjoyed in the 1990's.  Now, nature has once again recolonized an area that tantalizes with a fairly reliable source of water - a precious rarity in the desert!

Afterward:

After drying for a few days, Gordon's camera - an old Canon A80 - showed signs of life.  After another week of drying out, it seems to have returned to normal operation with no apparent spots of water in the optics or mechanical problems of any sort!  In the months since, he has take quite a few pictures with no obvious, recurring problems.

[End]

This page stolen from ka7oei.blogspot.com

No Friendship Cruise this year...

It happens every few years:  Winter and spring conditions conspire and there's too little water flow in the Green and/or Colorado rivers at the end of May.  If there's too little water, navigation of the river can become hazardous due to slightly-submerged sandbars and rapids appearing where "flat" water would be during a "normal" year.

What's the Friendship Cruise, you might ask?

Figure 1:
On the Green River near-ish "Turk's Head".
Click on the image for a larger version.

It originally started in the 1950's as a race, the object being to get from the town of Green River, Utah to Moab, Utah via the Green and Colorado rivers.  After a few years of this, in 1963, the "Friendship Cruise" was added as a venue to allow those with power boats (such as those used for water skiing) to participate at a more leisurely race along with their families, providing access to rarely-seen portions of the landscape.  Eventually, the race itself was discontinued leaving the Cruise as the singular event occurring yearly during the Memorial Day weekend.

Unless you've been in rural, southeast Utah it's hard to appreciate the landscape:  The Green and Colorado rivers slice their way through the landscape, spending much of their time at the bottom of 1/4-1/2 mile-deep gorges surrounded by some of the least-inhabited land in the lower 48 states.  Just downstream of the confluence of the Green and Colorado rivers is Cataract Canyon, a fearsome set of rapids that has claimed quite a few lives over the years.

In a remote, desolate place and on the where the only direction that one moves without power is downstream toward dangerous rapids it's vitally important that logistics and safety be considered should someone break down or experience an injury.  Being in a remote area where telephone coverage is spotty at best at "ground level", such coverage is all but hopeless when you are thousands of feet lower than the surrounding landscape!  Even satellite phones don't work too well on many parts of the course owing to the limited view of the sky!

Figure 2:
High-efficiency HF loop antenna on a boat.  These
antennas have been used for a couple of decades
for communications on 75 meters.  They are about
3 feet (1 meter) in diameter.
Click on the image for a larger version.

Very early on Amateur Radio (Ham) operators have been involved in providing communications for this event using their skills and available communications methods to enable coverage over the entire course.  On the river itself there are a number of radio-equipped rescue boats that patrol the river, assisting in repairs, providing emergency fuel, passing messages to/from the "outside" world or, as often happens, towing boats to the nearest place where they may be pulled out of the river and onto their boat trailer.

During the cruise's heyday in the early-mid 1970's there were as many as 700 boats on the river at the same time and careful watch had to be kept on the river to assist boaters in need as well as manage who's boat trailer needed to be delivered to what location to pull it out of the water!  Since that time, the numbers have declined, but a few die-hards and adventurous newcomers still descend on Green River at the end of May - when they hold the cruise, that is!

For decades, the mainstay for communications was on the 75 meter amateur band which, during daylight hours, has an effective coverage radius of a few hundred miles.  Utah, being by itself among the western states, puts this band largely out of reach of most of the country's population centers with Salt Lake being about the only large city within daytime range.  On the boats were mounted mobile rigs with small HF antennas - both loaded verticals and high-Q tuned loops - that provided reliable coverage from anywhere along the course, no matter how deep the canyon.  At night, 75 meters "lengthened" covering large chunks of the U.S. and Canada by nightfall and "local" coverage degraded, but by the time it started getting dark almost everyone on the river had made camp or gotten to the end and pulled their boats out so very little traffic handling was generally required.

At several points along the course - one at each end and two places in the middle - were strategically-placed stations, also equipped with HF communications where river access was possible via vehicle allowing fuel trucks to replenish the boats' gas tanks as well as trailers to allow boats to be taken off the river.

75 meter HF worked quite well for the 30 or so years that it was the sole source of communications on the river, only occasionally succumbing to the odd solar flare that caused all signals on the band to "disappear" for a few hours.  In the 60's and early 70's, VHF such as 6 and 2 meters was occasionally tried, but its signals had difficulty escaping the deep river gorges and the range was limited to only a few miles up and down the river.  By the time the 80's and 90's rolled around there were a few 2 meter repeaters in the general area but their coverage on the river was extremely spotty, again since signals had difficulty escaping the deep, narrow canyons!

In the late 90's, we started to look in earnest at how VHF and UHF might be used in providing coverage of the river, knowing that we faced a significant challenge in finding locations that had any hope of catching the feeble signals emanating from the cracks in the Earth - but that story will have to wait for another day!

Hopefully, the water conditions in 2013 will be favorable for the 50th anniversary of the cruise!

• The official "Friendship Cruise" web site:  http://friendshipcruise.com/

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This page stolen from ka7oei.blogspot.com