Many thanks to SWLing Post contributor, Chris Freitas, who writes:
“I am thinking of the new RSP1A SDR. Would you know of a good indoor antenna that would work well with it?”
Your antenna question is simple, but the answer is complex!
First off, I think the RSP1A is a great choice as it’ll give you proper exposure to the world of SDR (1 kHz to 2 GHz!) at a modest price.
Unlike a portable radio of course, your SDR must be connected to a PC, laptop, tablet or some sort of mini computer like Raspberry Pi. This limits your ability to easily try different antenna locations within your home compared to, say, a battery-powered portable radio. It might take some dedicated experimentation and patience.
Indoor antennas are so vulnerable to the radio noise within your home.
If you live in an off-grid cabin with no radio interference nearby, even a simple $1 random wire antenna hooked up to RSP1A’s SMA connector would yield results. I occasionally spend my summers in an off-grid cabin and it’s simply amazing what you can do with a modest setup when there are no man-made radio noises around.
Listening to the final broadcast of Radio Netherlands in an off-grid cabin on Prince Edward Island in 2012.
But how many radio enthusiasts live in an off-grid cabin? Answer: very, very few! Most of us only get to experience off-grid life during natural disasters when the electrical grid has been damaged in our neighborhoods.
The reality of indoor antennas
You’ve told me previously that you live in an apartment in an urban setting, hence you probably cope with a lot of RFI.
When an antenna is indoors, it is forced to function within this RFI-dense environment. Your telescoping whip or wire antenna doesn’t discern between radio noise and your target broadcast signal. Thus, noise can overwhelm your receiver, essentially deafening it to all but the strongest shortwave broadcasters.
This is why if you had a means to put a small random wire antenna outside–even if it was simply draped outside a window–it would likely perform better than an indoor antenna. I’m guessing this isn’t an option for you, Chris.
A broadband loop antenna (image courtesy of wellbrook.uk.com)
While you can build an amplified mag loop antenna (like our buddy, TomL) it’s not a simple project. Passive single turn loop antennas, on the other hand, are quite easy to build but are narrow in bandwidth (here’s a very cheap, simple passive loop project). You would likely design a single passive loop to serve you on a specific brodcast band and would have to retune it as you make frequency changes. You could build a passive loop antenna for less than ten dollars if you can find a good variable capacitor. Here’s another tutorial.
Commercially produced amplified wideband magnetic loop antennas are not cheap, but they are effective. If you’re a serious SWL, a good mag loop antenna is worth the investment.
Here are a few of my favorites starting with the most portable:
W6LVP sells two versions of the antenna–since you’re not operating a transmitter, this $250 model would be all you need. indeed, if I were in your shoes, this would likely be the loop I purchase–very cost effective.
Wellbrook antennas are the staple magnetic loop antenna for many DXers.
Wellbrook makes a number of loops, but since you have no plans to mount this outside, I believe their indoor model would suffice.
Other loop options
There’s no shortage of magnetic loop antennas on the market, but most are pricer than the models I mention above and I know you have a tight budget. Here’s are some models we’ve mentioned on the SWLing Post in the past:
I hope this helps, Chris! This post is by no means comprehensive, so I hope others will chime in and comment with their experiences. Good luck fighting urban noise and I hope you enjoy your journey into the world of the SDR!
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.
Many thanks to SWLing Post contributor, Kenneth Wigger, who writes:
One Friday evening I contacted Larry W6LVP and asked a question about the kind of coax that he would recommend for his Magnetic Loop antenna. I was surprised to get a response with technical information within an hour or so. With this kind of customer service I decided to go ahead and order one of his antennas later that evening. I actually received the antenna on Monday afternoon within a couple days.
I have extreme electrical noise that was S-8 most of the time on my Carolina Windom and made my radio almost unusable. I temporarily installed the Magnetic Loop antenna on a short 5 FT pole in the backyard. With the XYL as the null monitor at the radio I called her on my cell phone and rotated the antenna by hand and was able to get a sharp noise null of about S-1. Very tight null when rotating just a few degrees one way or the other. Went in the house and couldn’t believe the clear signals that were hidden by the previous high noise level. It reminded me of SWLing 50 years ago as a kid back in the good old days before the electrical noise environment turned so bad!
As I mentioned, the antenna arrived within a couple days and was of high quality construction and packed extremely well for shipment. I had read the previous reviews about Larry’s product quality and customer service and my experience was also very good!!
I am planning to mount the antenna on a Channel Master rotator one of these days to get the full effect of the excellent directionally of this Magnetic Loop antenna. I even read where Broadcast Band Listeners use this antenna to pick up and select between multiple stations on the exact same AM frequencies.
I highly recommend Larry W6LVP and his Magnetic Loop antenna to other Hams and SWL listeners. He responds personally to emails within a business day usually just an hour or two. What more could a customer ask for?
Thank you, Kenneth, for sharing your review! It’s nice to have an alternative to the pricier Wellbrook and Pixel Loop antennas. Someday, I’ll get around to reviewing the W6LPV loop antenna as well.
SWling Post contributor, Ron, recently noted that Jay Allen reviewed one of Paul’s inductively-coupled mediumwave magnetic loop antennas. Jay gave it good marks on performance as it compares favorably with the Grundig AN-200, Select-A-Tenna M, and Terk Advantage AM–even having a performance edge due to it’s larger loop diameter. (Note that Paul makes a number of loop sizes–click here to download PDF of catalog.)
In Canada last summer, I used the PK Loop on a number of field radio listening sessions.
But what really sets the PK Loop apart from its competitors is its durability. PK’s Loops are built to be incredibly rugged. I routinely throw my PK Loop antenna in bags/packs and–unlike most of my other radio components–never worry about how it’s padded or protected. There’s little to damage unless you’re intentionally abusive to this antenna. My Grundig AN-200 antenna, on the other extreme, has exposed coated wires around its loop that I’m constantly concerned about harming in transit.
Ron’s message reminded me that I never followed up after posting a preliminary review of the PK Loop C-LOOP-HDSW6-18 in 2016.
Shortly after publishing the review, I had a fantastic opportunity to evaluate how well the PK Loop would perform in a typical hotel room. My buddies Eric (WD8RIF), Miles (KD8KNC) and I stayed overnight in a hotel on Wright-Patterson Air Force Base during our mini National Parks On The Air DXpedition.
My Elecraft KX2 connected to an external random wire antenna.
The hotel room was indeed dense with RFI.
We hooked my Electraft KX2 to both the PK Loop and to a simple random wire antenna.
Without a doubt, the PK Loop was much better at mitigating radio noise than the wire antenna we hung on the inside of the hotel window.
Unlike most modern hotels, however, this one actually had operable windows, so we tossed the random wire out the window and made another comparison. In this case, the external wire antenna consistently outperformed the PK Loop, no doubt because it had the advantage of being outside the radio noise cloud within the hotel’s walls. It goes to show that outdoor antennas–even if simply hanging from a room window–will almost always outperform comparable indoor antennas.
A late evening listening session on the condo balcony.
Last summer, I also spent two months in a condo near Québec City. The condo was dense with RFI–the PK Loop made the experience much more bearable. The loop couldn’t completely eliminate all of the persistent wideband noise, of course, but it did reduce noise to a level that I could enjoy some of my favorites like RRI, VOG, VOT, REE, WRMI, RNZI and even weaker stations in North America like the BBC and DW.
Even the shortwave version of the PK Loop can null out QRM to some degree by rotating the loop perpendicular to noise. I became quite adept at this by the end of our stay.
Since I purchased the PK Loop, it’s been a constant travel companion and I highly recommend it. I don’t believe you’ll find a more durable or effective portable mag loop antenna on the market.
PK Loops are built by Paul Karlstrand in Australia who has a stellar reputation with his customers. For those of us living outside Australia, there will be additional shipping costs, but they’re negligible and Paul has been exporting these loops for many, many years. I believe I received my loop within a couple of weeks of ordering it.