Many thanks to SWLing Post contributor, TomL, who shares the following guest post:
Backpack Shack 2.0
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)!
The flimsy “Backpack Shack” prototype broadband loop antenna created over a year ago had found a permanent home on the porch for general listening. I started itching to make a new one that was even more portable. To paraphrase an old saying, “The best antenna is the one you have with you!”
The new design criteria were:
- 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! 🙂
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.
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!!
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.
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).
But, “*WHY* do FSL antennas work?!?!” is still the very intriguing question…. 🙂
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!
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)
9.420 MHz – Voice of Greece booming in like it was next door:
9.640 MHz – China Radio International (weakly) in Spanish from Kashi-Salibah western China:
9.620 MHz – HCA, Kununurra Australia beamed at Korea:
9.395 MHz – WRMI booming in:
9.445 MHz – All India Radio clear as a bell, quite nice to hear!
9.490 MHz – KNLS World Christian Radio, Madagascar in Chinese:
9.600 MHz – Vatican Radio, Tinang Philippines in Chinese:
9.610 MHz – Voice of Turkey, Emirler Turkey (endless string & flute music):
15.000 MHz – WWVH, Hawaii (halfway, I checked on WWV at 10 MHz to make sure it was on the air, weird propagation, defintely not due to my antenna!):
Appendix II, Parts List:
- 1 FlexPVC 4-inch Custom cut X 10-inch diameter Thinwall Air Duct PVC
- 1 pair each size of 15mm CF rods 10-inch and 15-inch
- Video Camera Rig parts: 2 Cheese Bars, 3 Rod Clamps, and 1 Cheese Rod
- 1 cheap Neewer pan-tilt photo head with quick release plate
- 10 screw adapters for tripods 3/8-to-1/4 inch
- 1 copper sheet roll cut to size (or Aluminum foil instead)
- 4 Flat Speed Nuts 1/4-20 from Ace Hardware
- 1 Oak Dowel 3/8” diameter from a local hardware store, cut as needed
- 1 DX Engineering RPA-2 Modular Receive Preamplifier
- 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.