My good friend David Cripe (NMOS) has recently informed me about a new product he’s offering to the radio community via his eBay store: Kev-Flex Stealth Kevlar Antenna Wire. Kev-Flex looks like a superb option for field antennas of all stripes especially since it has an incredibly high tensile strength. It’s available in 75′ bundles, but Dave can also cut custom lengths. NM0S is also a trusted retailer in the ham radio world, so you can purchase with confidence.
Kev-Flex is a unique antenna wire manufactured exclusively for NM0S Electronics. The lightweight center core of the wire is made from Kevlar fiber, giving the wire its incredible strength. The Kevlar core is wrapped with six tinned strands of 30 AWG copper. The effective surface of the wire creates an effective skin area capable of handling well over 100W.
The cable is protected from the elements by a coating of UV-resistant black polyethylene. With a total diameter of only 1/16″ (incl. insulation) and a weight of just 16 feet per ounce, the tensile strength 125 lbs allows lengthy unsupported horizontal runs. Kev-Flex is ideal for extremely long LW-antennas and Beverages and is great for balloon or kite-supported antennas. Its low weight and high break-load makes it most suitable for SOTA activations and other field operations.
The outer insulation makes the wire kink-resistant, and its slippery finish makes it ideal for stealth antennas that must be passed through trees or other obstacles without snagging.
This antenna wire is sold in 75 foot long bundles, which is enough for a 40M dipole or EFHW. Two 75 foot bundles would make a great 80M dipole. Custom lengths are available on request.
– Kevlar fiber core wrapped with six 30 AWG copper strands
– Weather-proof black polyethylene (PE) insulation, 1/16″ O.D.
– Weight: 16 feet per ounce
– Breaking-load: 125 lbs
– Velocity factor 0.97
Many thanks to SWLing Post contributor, TomL, who shares the following guest post as a his Backpack Shack 3 continues to evolve:
Backpack Shack 3.0 – Part 3
I have now gone overboard since I think bigger must be better. The temptation was just too great and now there is an MFJ-1979 17-foot telescoping whip antenna in my car (with consequences).
I have a love/hate relationship with MFJ products because of what I think are useful ideas that are made somewhat poorly. But I went ahead and bought the large whip since I figured they could not possibly screw up something so simple, right?
Wrong. As I excitedly tried to screw the supposed 3/8”-24 threaded end into the nice standard Firestik K-11 magnet mount, I realized I was turning and turning it but it was not going in!!! I even had a small steel sliver of metal sticking into my flesh to prove I was not dreaming. The previous day, it had screwed in very tightly, but it did screw in. So, there I was after a long day of work, ready to listen to some SWL-Nirvana and I could not get the blasted antenna into the mount–? That Firestik mount is a VERY standard 3/8”-24 female thread and the other third-party antenna shafts fit perfectly and easily EVERY time I use them. I hate $60 of poor workmanship and MFJ seems to be the poster child of overpromising and underdelivering.
I was determined to make this work, by force if needed. One of the Trucker antenna shafts by necessity had an extra coupling nut on it to allow the extra 18 inch shaft to connect, so I took it off there and tried to thread it onto the MFJ-1979. It barely moved. Not to be thwarted, I dug out an adjustable wrench and 3/8” socket wrench with ½” socket and grunted and twisted and tightened until the coupling nut was threaded all the way “up its shaft”. That is what I feel like telling MFJ! That coupling nut is never coming off and now that I truly have bought it and cannot return it, I might as well use it.
The stainless steel telescoping rod is extremely thin and feels like it can bend and dent with any kind of mishandling. So it resides collapsed in a 27 inch PVC pipe with plumbing pipe foam inside to baby it when it is not being used. It remains to be seen if I can remember to “Handle With Care” when extending/collapsing it. We’ll see.
OK, so using the 18 inch antenna shaft attached to the magnet mount, then the coupling nut on the MFJ antenna, I extended it to a total of about 13 feet. With the DX Engineering Pre-amp turned on, and using the SDR Play RSP2, I was getting many signals booming in. All the usual names we are familiar with – RMI, CRI, Turkey, Cuba, etc. But also the noise level was very high. I know it is summer but I may have been overloading the Pre-amp a little bit. Here is an example, Radio Progresso from Cuba with some very nice acapella music but also a noisy background (plus, a noisy laptop computer pulse!):
So I decided to come back in the morning before my workday started and see if the static crashes would have died down.
The next morning I had everything hooked up again in the same spot at the Forest Preserve (located in a suburb of Northern Illinois). I moved the Cross Country Preselector to be directly connected from the roof, then to the antenna switch on the “Breadboard” (see part 2) to better prevent overloading. I turned on the Verizon battery pack and nothing. No Pre-amp light. Switched it on, off, on, off – nothing. So, I thought I must have burnt it out the previous session?
Later on, I found it was some sort of short in the switch and I will have to move the D-cell batteries to a backup battery pack. In the meantime, I had to do without the Pre-amp and was forced to extend the MFJ antenna all the way. With the 18 inch extension attached to the magnet mount, that was a total of 18.5 feet from antenna tip to the top of my car roof.
This was actually fortuitous since I was already concerned about overloading the Pre-amp or perhaps amplifying background noise. This forced me to test it in a more “barefoot” manner, hearing what it would natively hear without any Pre-amp. It was also lucky there was no wind to blow it over! It seems that if one is in an RFI-quiet area with decent view of horizons, the 20+dB Pre-amp may not be needed, depending on frequency band involved.
I have read that “Norton” style 10 dB Pre-amps and custom handmade transformer baluns are used by Dr. Dallas Lankford in his Low Noise Vertical antennas. I don’t want to get into winding baluns so I am using one Palomar Longwire Balun to simulate the “magnetic” transfer. His design uses two, one 10:1 at the antenna and a 1:1 balun at the feedline into the house. For more reading on LNV antennas, see these references:
I purposely monitored Voice of Korea for their news statement on the De-Nuke talks on the 25 meter band and found it came in great, just as many others have heard it. This was encouraging. Examining carefully the Data file from the SDR, here is what I pulled from it. I am pleasantly surprised and happy with the results; some stations I had never heard before and the language and music are very exotic. All of it was a little more than one half hour of recording time (14 June 2018, 1300 UTC). You may have to crank up the volume on the weaker recordings to hear those properly.
Taiwan International, 11640 kHz, Chinese, Kouhu Taiwan (blasting in strongly plus strong echo of broadcast at top of the hour – is a second transmitter signal going around the earth the other way and getting to me??)
Eighteen feet of whippy rod can sway in the gentlest breeze (consequences of “bigger must be better”). The described setup has fallen over in as little as a 12 mph sustained wind when fully extended because I had the base in a plastic box. I want plastic under the magnet(s) in order to get it off easily and put away out of sight! Now installed is a larger QUAD magnet mount for better stability:
I am using the flat plastic lid from a 20 gallon tote container under the quad mount and a mover’s tie down strap to the main bar of the quad (I have room for multiple straps if needed). Ten foot fits just fine:
Because the backpack and quad mount can fit inside the 20 gallon tote container, this setup can be attached to a picnic table in a state park or campsite if I choose. The Firestik single magnet mount will be recycled as a VHF antenna mount. I can go virtually anywhere now.
Instead of the 20+dB DX Engineering Pre-amp, perhaps one of those “Norton” 10 dB Pre-amps might be optimal (Kiwaelectronics.com broadband-preamp). And I need to figure out why my Verizon battery pack failed as each Tenergy D cell measured fine. Oh yeah, I have to buy an extra coupling nut, too……
Thanks so much for sharing this latest iteration of the BackPack Shack 3.0, Tom! It seems to me, as you imply, your current setup could be installed pretty much anywhere.
I’m sorry to hear about your troubles with MFJ. I’ve only had good experiences with them in the past, but I suspect the specs on the 3/8”-24 thread were simply incorrect or perhaps metric and mislabeled.
Wanting MOAR options for my recent amplified whip antenna experiment, I decided to add a second antenna input to the kitchen cutting board (can I call it a “Breadboard”? – Ha, that’s an electronics joke!). The idea behind it came from realizing that I might not want to spend all my time outside at a picnic table or on the beach, especially if it is drizzly and windy. And I still wanted a better ground for the antenna. So, I thought I could use more Trucker Parts and put an antenna on top of the roof of my vehicle so I could listen in the relative comfort and safety of my small SUV (or even a friend’s car).
Here is the crowded “Breadboard” with some extra items added.
I thought of the vertical antenna as a short longwire and had an old, original RF Systems Magnetic Longwire Balun. That device allows for an improvement in signal/noise ratio (in theory) if used on a longwire. Perhaps it works on this, too?? You can see the gray cylinder connected right beneath the trucker mirror mount on the left (this will not be tested at this time, see External Antenna below). The output goes to a greenish Daiwa switch on the right.
A large amplified antenna has the real possibility of overloading the amplifier. With the Magnetic Balun, I am hoping the VHF band is attenuated enough to preclude any problems because its response naturally tapers off past 40MHz. But Mediumwave is well within its bandpass. I remembered an old Kiwa Electronics Broadcast Band Rejection filter not being used for a long, long time, so I connected that right after the Daiwa switch (the metal box with red plate).
This output then goes to an RF choke just before entering the pre-amp. I figure I will be using my SDRPlay RSP2 and noisy laptop and wanted to try to reduce any interference traveling on the outside coax braid before it gets amplified.
OK, now for the other Daiwa switch selection. The external antenna will be connected and disconnected as often as I use it. I attached two right-angle coax adapters to be the connection point for the antenna. This is so that the physical switch threads do not have to handle that wear-and-tear. You can see it as the fuzzy out of focus thing sticking up out of the left side switch position.
The wire going out the top of the Breadboard goes to a Firestik K-11 magnetic mount placed on top of the roof of the SUV. I also wanted this to be connected to a magnetic balun. I just happened to have a nearly unused Palomar Engineers Magnetic Longwire Balun. It has its own ground lug for use with a counterpoise. Temporarily, I left the 18 feet of wire that came attached to the K-11 Mount and attached an adapter and BNC test lead; on the other end is connected the spade lugs to the Balun (red wire to the lanyard nut, black wire to the ground lug). It all fits neatly inside a Sistema 3 liter container.
The magnet and box self-clamp easily onto the roof of the vehicle. I added a new 18 inch section to the Trucker Antenna Shafts creating a full 72 inch antenna, complete with mag mount, ground plane (car body), and magnetic balun. It is very easy to put up and take down and the box helps keep everything contained.
I am pushing things a bit here. Magnetic Baluns are not really meant to be used on vertical antennas. It probably breaks some sort of Cosmic Electrical Law somewhere that causes electromagnetic waves to get very confused and die a horrible, twisted, circular death. But I figure that it is an unbalanced “line” similar to a longwire antenna; it’s just a little short and goes straight up instead of horizontal! I like the idea, so I am going to run with it.
It goes without saying that the Antenna Shafts, magnet mount, and magnetic balun are weatherproof (but NOT lightning proof!). Take proper lightning precautions and take it down. Even so, I might add a small drainage hole to the box since it did rain a tiny bit during testing.
Secondly, this setup is ONLY FOR STATIONARY VEHICLES!!!! DO NOT TRY TO DRIVE DOWN THE ROAD OR HIGHWAY!!!! The magnetic mount will NOT stay on the car and will damage your vehicle and maybe a vehicle travelling next to you!
As you can see from the picture, my new Tecsun S-8800 is getting a workout while connected to the Cross Country preselector (not shown behind it) and to the backpack next to the back seat window. The Tecsun S-8800 is a nice radio. My copy has a couple of quirks that I might have to send it back (the AM band tunes incorrectly 2 kHz lower than it should and the SW SSB tunes 140 Hz higher than indicated and I have to compensate using the fine-tuning dial for these modes–FM seems correctly tuned).
Other than this, the actual performance is really quite good! DSP does have sharp cutoffs to the IF bandwidth (especially resolving strong station interference when selecting 3 kHz vs. 4 kHz). With all my filters/balun/choke, I did not notice any MW or FM breakthroughs and signals on those bands were nicely contained and “normal”. Interference from my cell phone while looking up internet frequency listings was minimal – seems like the cable shielding, choke, and car roof are doing a good job.
The audio output jacks have very thin clearance between the jacks and the housing of the radio. So for the second time, I will not have recordings since the cable I wanted to use has home theater style construction with very thick plug outer connectors and will not fit!
From an RF-quiet “Forest Preserve” (County park), there were a variety of stations received from the 25 through 19 meter bands (Local time 11am-1:30pm). A few stations I have never heard before until now:
Radio Free Asia in Korean on 11985 KHz (Tinian Island)
Radio China International in Esperanto on 11650 KHz (Xian China)
Radio Farda in Persian on 12005 KHz (Wooferton England) – broadcast opposite my direction
Radio Bible BCI in Somali on 15310 KHz (Nauen Germany) – Strange sounding but interesting Christian Somali music
Radio Free Asia in Chinese on 13675 KHz (Dushanbe Tajikistan)
Voice of Hope Africa in English on 13680 KHz (Lusaka Zambia) – had to use ECSS USB to get away from strong interference from RFA on 13675, fairly good intelligibility (including music)! I wish there was a 6 KHz option for SSB mode since the audio was slightly muffled and could not compensate much with the tone controls. That kind of feature usually comes with radios costing 3X more, however.
Voice of Korea in French (Kujang North Korea) being squashed by Radio China International (Kashi China) in English on 13760 KHz
It was so nice not to be on a beach and have people walk by STARING at me with my weird radio/antenna setup. And I was dry and comfy sipping a cool drink while there was a drizzle of rain pelting the windshield. Downside might be that the car setup cannot always be located optimally if I want to be next to that Very Large Body of Water (Lake Michigan) to help enhance reception but this is not a bad alternative. The next test will have to be during early evening when signals are booming into my location and see if performance holds up under those conditions!
Many thanks to SWLing Post contributor, TomL, who shares the following guest post:
Backpack Shack 3 – Amplified Whip Antenna
So, having enjoyed using the Ferrite Sleeve Loop I created last year, I have wanted something a little more sensitive and less bulky. I will eventually create a much BIGGER FSL antenna on the order of 2 feet long and perhaps 18 or 24 inches in diameter for indoor/attic use. But that is not a priority at the moment.
Since I already have the DX Engineering Pre-Amplifier and the very nice Cross Country Preselector from the loop project, I thought it might be useful to create an active whip antenna for it. And the cool looking Solar Red backpack needed something to do!
Now that the bulky loop was not taking up the main compartment of the backpack, I could think about what else to put in there, like a larger power pack. I scoured FleaBay for ideas and stumbled upon this contraption for backup power to network systems, the CyberPower CyberShield for Verizon.
This has 12 spaces for D-cell batteries and was mounted inside the demarcation terminal to provide backup power for things like cable systems and Copper-to-Ethernet networks. It is not waterproof, so would be inside the premises of the customer getting the internet/cable service. But my Pre-Amp needs 12-18Volts and would love to have nearly unlimited power. So, I bought a used one, cut the end off of the power lead and put on my own 2.1×5.5mm plug (carefully glued down and tie-wrapped). Then I filled it with 1.2Volt Tenergy D-cells.
Everything was just fine until I forgot to double check the polarity of the plug that I had wired onto the end. Plugged it into the DX Engineering Pre-Amp, flipped the power switch and fitzzz…. The Pre-Amp light went on, then off (permanently!).
So, my expensive mistake is that I start using the FREE multimeter I got from Harbor Freight and check the polarity before I connect homemade battery packs to anything!!
DX Engineering charged me $60 to fix my mistake and it is working fine now after I swapped the wires on the plug. Yes, their Pre-Amp is NOT reverse-polarity protected! Disappointing, since the price tag for that device is $148!!! The CyberShield now sits comfortably inside the bottom of the backpack.
Now that the drama was over regarding the Power pack, I could think about the whip. I did not want a wimpy whip! (No one should rightly aspire to this, in my opinion). More FleaBay searches found me looking at Trucker parts. Loaded whips, magnetic mounts, 10 foot tall MFJ telescoping whips, etc was looking a bit expensive.
Besides that, I cannot fit a 10 foot tall telescoping whip into the backpack, I am limited to at most 18 inches (and that is at an angle to fit it in there). But I found an old-fashioned mirror mount that looked promising since it had a nice SO-239 connector at the bottom and standard CB antenna fitting on top of 3/8”-24.
Then I found the 44 inch SuperAntenna with the same threads; then found the replacement Stainless Steel Shafts for a Wilson antenna in different lengths (I ordered the 10 inch version to test). With a couple of rod coupling nuts and I was ready for testing!
I had already scheduled a short vacation to Sleeping Bear Dunes on the thumb of Northwestern Michigan, so I took this test setup with my Sony ICF-2010. This area is a very nice remote National Lakeshore with minimal noise. I tried a beach setting and a couple of hilltop picnic areas (including meeting a local Porcupine) and had very nice reception at all locations. The hilltop locations are approximately 400 – 600 feet above the Lake (yes, the Dunes are THAT big there!).
Meeting a local Porcupine
Later on, I went to Grand Haven, MI on the way home and stopped at their very lovely beach.
Reception was just as good as the hilltop locations at Sleeping Bear! In both areas, I was next to a large body of water (in this case, Lake Michigan) and makes for an advantageous place for DXing! I had also stopped at a Rest Area off the highway and that was a terrible place even though it was electrically quiet but nowhere near the big Lake. I guess the rumors are true about being near a large body of water somehow enhances reception of weak signals–?
I will submit recordings later since I lost the mini-B cable for the Sony digital recorder and had to order a replacement. However, this was a nice project that freed up some space inside the backpack. I will add an 18 inch extension to the whip that will give me a total length of 72 inches. Plus, it is mounted 12 inches up on the poly cutting board and I place the backpack on a small hunters folding chair that is about 24 inches tall. So, the tip will be about 9 feet off the ground.
Not pictured but I was also able to easily fit inside a used CCrane Twin Coil Ferrite antenna for mediumwave use that also performed very well. I noticed that the picnic benches at some locations are made of metal, so that gives me a future idea of trying to leverage that to use as a ground plane somehow. The battery pack is heavy but also gives great ballast to the backpack and will not tip over. Cannot wait for the Tecsun S-8800 to arrive so I can try leaving the radio inside the bag and just use the remote control to tune!
As always, I’m so impressed with your spirit of radio adventure, Tom! I love the fact that your goal is to make a field-deployable DX kit that isn’t cumbersome or time-consuming to set up on site. I imagine you only need a couple of minutes to open the pack and have it on the air.
Those DXing spots are stunning! I had no idea one could find 400-600′ dunes in NW Michigan–! With that said, I’ve heard that part of the state is one of exceptional natural beauty. If you could somehow turn the lake into a body of salt water–thus increasing ground conductivity–you’d really enhance that already impressive reception! I’m guessing that sort of project would be a bit outside your budget! Ha ha! That and the freshwater fish might protest!
To me, there is no better way to enjoy radio than finding a nice RF quiet spot in the great outdoors…no matter where you live in the world. On top of that, Tom, you’re constantly building, experimenting, documenting and sharing your findings–you’re a true radio zealot! Huzzah!
This is just a quick Field Update for my Backpack Shack 2.0 antenna. It is not the most powerful antenna but in the right location it can be useful, especially with using an SDR. It was used during February in two Forest Preserve (County Park) locations outdoors and once from my usual Grocery Store parking lot!
Please excuse some of the computer generated noises (caused by a slow CPU) as well as some audio connector problems on a couple of recordings.
Each Time is in UTC and Frequency in kHz. Where can you hear unique programming like these samples except Shortwave Radio??? Enjoy!
Many thanks to SWLing Post contributor, TomL, who shares the following guest post:
Backpack Shack 2.0
by Tom Lebryk
Like Audiophile speakers, it could be said that “antennas are forever”. They tend to not become obsolete like all of our favorite electronic gear (a good one is worth the trouble). And antennas don’t care if the signals are digital or analog formats. They are “Digital Ready” (LOL)!
Retain the broadband design of the amplified loop on a sturdy form
Shrink the size to fit into a backpack without heavy stand or long pole
Build a modular platform that would allow quick setup
Be something durable that can last me 20+ years of use
Allow the loop to be rotated and tilted by hand
Be easy to hook up to any kind of radio
and later on, Enhance the design as a true Ferrite Sleeve Loop
The Backpack: The existing photo backpack was slightly too bulky. Found on Amazon was an Adidas Excel II XXL backpack on special sale with plenty of tall compartments and minimal padding. It is surprisingly roomy and comfortable to wear with springy shoulder straps and padded mesh backside!
Sturdy Basic Form: The Backpack Shack loop was originally built on 14-inch quilters loops (three of them) in a parallel configuration. I thought to simplify the whole thing and just use one wide loop. But what should I use for a sturdy form? The quilters loops were too flimsy and PVC pipe was too heavy. I stumbled upon a nice company called FlexPVC which allows sales to the public of various kinds of PVC pipe. Their Thinwalled Air Duct PVC looked promising. It is thinner than regular PVC but having standard inside dimensions and comes in custom-cut lengths. I decided 10-inch diameter would fit best inside the Backpack. FlexPVC even sends you a small booklet of the U.S. Constitution and the Bill of Rights with your order! 🙂
Thinwalled PVC form
The “length” as they call it would be my form width for the copper strip. I thought 3-inch would be nice but decided 4-inch was better. Supposedly, the aperture + the width of the “radiant element” is the main design consideration for loop performance. So, I figured that as wide an element as I could get away with was better.
Stable Mounting: Now, how to mount this thing! I eventually went back to my photographic web links and found nice rig equipment for video cameras. The typical construct is made of 15mm tubes of aluminum or carbon fiber (CF) and fit into adapters that allow attachment to other adapters or clamps. Non-metallic CF seemed ideal, so, I ordered a whole bunch of items from eBay to experiment piecing together two 10-inch CF tubes mounted inside the PVC form. Then, I attached two 15-inch CF tubes to the bottom of the PVC with something called a “Cheese Rod” that has multiple holes. Those two tubes are attached to another “Cheese Bar” which is attached to a second Cheese Bar on a cheap two-axis tripod head. This is a simpler photo version with quick release plate that locks pan-tilt separately and only cost $16.
Cheese Rod attached to bottom of PVC
Pan-tilt head assembly with Quick Release plate
For the base, I had an unused Sirui T-2005X 5-Section Aluminum Travel Tripod going to waste, so it was pressed into service. Very good tripod: can hold 26 lbs. (forged aluminum, not cast aluminum), legs can flare out for stability, and folds to 14.5-inches. Now, everything could come apart and fit into the Excel II Backpack!
Critically, the video rig standardization in the DSLR industry allows me to pick and choose parts from any cheap manufacturer but end up with a system that looks and feels coherent, is both sturdy and light, and can come apart if needed. Also, the pan-tilt photo head is really easy to work to get maximum peak or null out of the loop when mounted to a camera tripod.
CF Problem: CF tubes have no internal threads like that of aluminum tubes. So, I attached two, small 3/8-1/4 inch tripod adapters to the ends of a 3/8-inch oak dowel inside each CF tube destined for the inside of the PVC (ridiculously, I used up almost a whole bottle of super glue to get these 4 tiny pieces to attach to the oak dowels). This is definitely a weakness of my design but I could not figure out any other way to get the CF to mount inside the PVC form. Then, added to this is something wonderful I found at Ace Hardware called “speed nuts” to help push ipwards against the incoming stainless steel socket head screws of exact length. With jam nuts, internal lock washers, wing nuts, and strategic use of Thread Locker Blue, I finally had enough confidence that this thing would hold together!!
Speed Nuts pushing upward against incoming screws
Super-glued 3/8-1/4 inch adapter on end of oak dowel inside CF tube
Bottom assembly (Cheese Rod, Cheese Bar, and 15mm Clamp screwed together + wires to a BNC connector)
Ferrite Sleeve Loop: Halfway through this project, I became determined to use the ferrite bars and rods I had purchased from eBay mid-summer 2017 to turn this antenna into a real Ferrite Sleeve Loop but with a broadband design (At that time, I ended up purchasing the very last quantities of 62x12x4mm ferrite bars from the Lithuanian eBay seller, just because they were becoming scarce plus some other 8mm ferrite rods). The Thinwalled PVC is 5mm thick, perfect for this type of application. The video equipment could handle the extra weight. I had just enough ferrites to line the inside of this PVC form with two bars side-by-side all the way around the inside (plus some shorter ferrite rods at the top and bottom). Some quick setting JB WELD Kwik Weld epoxy made quick (and permanent) attachment of these ferrites to the inside of the PVC. Now, the bars stick out from the PVC form by about ½-inch on each side, so I do have to be careful it does not get abused and chip off any of the exposed ferrite.
Soviet ferrite bars and rods, 400 ui (initial permeability).
Note: Using Gary DeBock’s Performance estimate (diameter * length), this calculation predicts that this FSL 10.75-inch loop should perform similarly to Gary’s 10-inch models using 140mm long Russian ferrite bars (mine probably performs less than his since I am not using carefully tuned (to Mediumwave) litz wire on higher permeability 1500 ui ferrites like he does).
Preamplifier: I believe one advantage of building a portable, table-top loop antenna like this is that all the connections are short. This allows me to use a Preamp right at the connection point of the loop. Indeed, this was critical since passive testing (no Preamp, nor ferrites) found that this loop is somewhat deaf at the MW frequencies and uninspiring on the SW bands. This was true even when connected to Antenna A of my SDRPlay RSP-2 and the internal Low Noise Amp cranked all the way up. So, I ordered the DX Engineering RPA-2 Preamp. This adds to the weight somewhat since I also needed a 12V battery supply using a 10-cell holder of NiMH AA batteries and 2.1mm plug.
The question arises that I “should” impedence-match the output of the loop before anything else to increase “maximum gain”. Well, for one thing, a tuner or matching balun would just introduce loss as soon as the wire comes out at the base of the antenna. The slight net increase in gain does not seem worth it; the signal/noise ratio rarely changes when introducing a device that is meant specifically for matching a transmitter to a load. Receiver circuits don’t care as long as there is enough signal to process. That is what the Preamp is for. The Preselector is for rejecting out-of-band (i.e., increasing signal/noise ratio + eliminate overloading the electronics).
Preselector: Now that the signal level was satisfactory, I added on the Cross Country Preselector, which I like very much since it is passive, lightweight, and well made. I had looked at other amplified preselectors but found the schematics showing the preselector came first in the path. I needed the preamp first, so that is how I ended up with separate units. In fact, the reverse configuration performs with worse signal/noise ratio because of the loss inherent in the preselector. In this case, it is definitely needed to amp the loop first with a high quality preamp (high IP3 rating)!
Automatic Bypass: The Cross Country unit has a great feature in the “off” position as an automatic bypass. This feature is very important since I do not need a Preselector in the circuit all the time. The bypass feature also allows the RSP-2 to monitor a large swath of spectrum without having the Preselector cut the bandwidth. The DX Engineering RPA-2 Preamp also has a circuit bypass when the power is off – very nice feature! So, I can keep all the antenna wires connected if I don’t want to use either device on a certain band – necessary for my broadband antenna design and use with an SDR.
Modular Portability: Another advantage of a table top loop is portability. Because of the modular design, I can put this into checked baggage (except for the AA batteries and laptop) and have it available for DXing in unexpected places. It could be useful when traveling and I cannot string wire into a tree but want something better than a whip antenna on a small radio. Everything fits into the bag and can be setup on a balcony, inside a car with a sunroof, or on a park picnic table.
A third advantage is that a short antenna could be clamped to one of the tubes and then connected directly to Antenna B of the RSP-2 for listening to higher frequencies (like a Comet W100RX). This expands the usefulness of this project as a platform for multiple antennas!
Finished Loop and accessories assembled together
Performance: Good on MW and very good on Shortwave. It is not in the league of Wellbrook antennas but it is useful as long as the RSP-2 LNA is kept down around -7 on MW and -4 on SW, else it overloads. The photo gear makes it easier to use than the original loop. I found that one side has a slightly larger receiving lobe than the other which is OK in practice. The null is very sharp and takes a little finesse to null out an offending station by almost 20 dB on MW and 15 dB on SW (the photo head can lock in place). It is handy to have the pan-tilt arm point directly at a station to maximize the null since the arm is mounted perpendicular to the loop. I will look for a clear plastic bag to cover the antenna and electronics to use in wet environments.
A larger loop would work better but this one is to use wherever I can. Also, my work laptop is noisey and shows birdies and spikes here and there on the bands, so I added a large ferrite bead to the USB computer end which helps. But I don’t have to use an SDR, I just have to change a connector and radio. It was expensive and fun to build – I guess I am just LOOPY!
Appendix I, Field Recordings 27-Jan. 2018 between 21.26-22.36 UTC:
Note 1: All Transmitter locations referenced from web site short-wave.info at time of recordings
Note 2: My location in a shelter at Dick Young Forest Preserve (41.84334, -88.38133)
Note 3: Moderate but declining solar wind with no flares, Kp Index = Calm (1)
10 AA Powerex Precharged NiMH batteries for the Preamp + 10x AA snap battery holder + CCTV 2.1mm snap plug
1 Cross Country Preselector
1 SDRPlay RSP-2 with SDR Console software on Lenovo laptop
1 Belkin USB printer cable with large ferrite bead looped through 3 times on computer end
1 Sirui T2005X travel tripod
1 Adidas Excel II XXL backpack (gaudy Solar Orange color!)
Velcro brand 7/8” x 23” One-Wrap velcro strips
Plus shielded cables, BNC and SMA adapters, Thread Lock Blue, tie wraps, rubber bands, super glue, JB WELD Kwik Weld epoxy, speed nuts, jam nuts, acorn nuts, wing nuts, internal lock washers, nylon nuts and screws, and 1/4”-20 socket head screws of various lengths as needed.
What a brilliant project, Tom! What I love is the fact that you consider your unique requirements prior to starting a project and base your design on your specific needs. Additionally, you see each design as an iteration. Fantastic job! No doubt, you’ll log numerous hours with this antenna in the field! Thank you for sharing your detailed design notes, process, list of materials and even audio clips with us.
After comparing the reception of the RSP2 and 1/2 doublet to the reception of the County Comm GP-5 SSB and its little external ferrite-bar, I’ve decided I probably want to make some sort of directional antenna to use on AMBC with the RSP2. A wire loop, perhaps, or some sort of ferrite-bar thing, that connects to the RSP2’s Hi-Z input.
[Perhaps SWLing Post readers can suggest] options and then I can get the needed bits at the 2017 Hamvention.
Post readers: if you can offer Eric suggestions, or point him to antenna plans, please comment! I do believe he would rather build an antenna than simply buy one and he’s looking to permanently mount this antenna outdoors.
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