Man-made RF noise levels have increased dramatically at my place in the past six months. It has become much harder to hear weaker shortwave signals. Even the stronger stations are getting covered in all types of hash from all manner of electrical appliances.
So, I have been looking at ways to reduce the noise problem. I’m currently researching a few possible solutions, including trying a different antenna.
The HF horizontal loop has been around for many years now, but it’s a new antenna for me. I’ve never had a need to try one…..until now! There is some documentation out there praising this antenna’s low noise capabilities. So, it was time to find out for myself and start building an experimental version. So far, the results have been really quite pleasing!
I have prepared a YouTube video (below) in which I discuss the reasons for looking at this antenna, its design, and its installation. I also do some on-air comparisons of my experimental rectangular (!) version of the horizontal loop against my three regular double bazooka (coax) dipoles and the Par SWL End-Fed antenna.
Have you tried this antenna before? Your thoughts and feedback would be most appreciated.
73 and good DX to you all,
Rob Wagner, VK3BVW, is the author of this post and a regular contributor to the SWLing Post. He also blogs at the Mount Evelyn DX Report.
RF filters are used (as the name implies) to filter/remove the frequencies you are not interested in and/or let frequencies you want pass . They come in lots of types. For example a band-pass filter lets the signals in a frequency range to pass through it and rejects/attenuates other frequencies. The opposite of band-pass filter is a band-reject or band-stop filter (also called a notch filter) which rejects/attenuates signals in a specific range and lets other frequencies get through the filter. Lots of different filters are used in SDRs and traditional radios. For example AM low-pass filters (only let frequencies lower than 1.7MHZ or so pass) or band-pass filters for various ham radio bands.
One of the popular use cases for a notch filter is in the FM broadcast range (88-108 MHZ in most parts of the world)
When you live near a powerful transmitter, it can affect the operation of your receiver in other near frequencies (or overload your receiver’s front-end), but I didn’t want the notch filter for this reason. I’ve got a SDRPlay RSP1 (among many other SDRs) which due to its architecture, has some images of FM band in the UHF range (for example in 330-350 MHZ). In fact they’re the images of the product of LO harmonics and FM frequencies.
You can temporarily move/shift the frequency by changing the LO frequency which does not remove them, but moves them around.
Another method to remove these images is using a band-stop filter.
This is the filter I’m using (Thanks to my friend Amirhosein Hasanpur who designed and built it):
Here you can see the effect of using a FM notch filter on my SDRPlay RSP1:
FM, without filter:
FM, with filter:
UHF (images) without filter:
UHF (images) with filter:
Here’s a link to a Zip file containing the PCB (in Protel), schematics (pdf) and S Parameters (pdf):
Note: Like any other SDR test/review, the results depend on lots of different parameters (various gain values, LNA, antenna, software, etc). These pictures are captured with the same conditions just to show the effectiveness of this filter and your milage will definitely vary, but expect a similar outcome. If you live close to a powerful transmitter or use LNAs, you will receive some signals, even when using the filter.
Final note: this issue is solved in the newer version of SDRPlay (RSP2) : it has software-selectable notch filters for FM and MW broadcast frequencies.
I’m very fortunate to live across town from my good radio hobby pal Gary DeBock. He has been responsible for the rapid growth in Ultralight radio DXing and the construction of Ferrite Sleeve Loop (FSL) antennas. Living near each other as we do, I’ve benefited a lot from his expertise and creativity in the hobby. We’ve enjoyed visiting about Ultralight radios and antennas many times over a leisurely lunch. If you’re unfamiliar with Gary’s efforts, just do a YouTube search on his name and see just a few of the many FSL antenna variations he’s built!
Gary uses PVC tubing, “Fun Noodle” foam cylinders, sections of curved foam, and rubber plumbing adapters almost exclusively as the core supporting structures for his FSLs, from small 3-inch models to 17-inch monsters. Gary certainly has perfected his own techniques with these raw materials; he’s an expert in combining them. He even makes PVC table structures to support his FSLs during Oregon coastal DXpeditions, as seen in some of his YouTube videos.
These materials work well, but over time even FSLs as small as 7 to 8 inches in diameter begin to sag and lose their perfectly circular shape. This seems to be caused by the shifting of individual segments of foam which are wrapped around a “Fun Noodle” core and center PVC tubing.
What possibilities are there for other materials in FSL antenna construction? It would be a fun challenge to build a small to medium sized FSL antenna that would have these qualities:
Maintain a circular cross-section without sagging
An appearance less like a threatening explosive and more like a commercial product
Use alternate construction methods for those not skilled with cutting & gluing PVC tubing
I began to keep my eyes open for likely candidates as I visited hardware stores, department stores, and office supply outlets. Eventually some ideas began to gel.
Here is a visual and descriptive list of what I used in this alternate design of Ferrite Sleeve Loop antenna. I won’t go into great detail about dimensions, quantities, and measurements, as other DIYers should be able to easily follow the general idea presented here. This article is mainly to get you thinking about other ways to construct a ferrite sleeve loop antenna.
The Core of the Matter
This 18-inch long, semi-rigid foam roller is six inches in diameter. It’s a workout and exercise aid which I found in the sports department of my local Walmart store. The cost was approximately $13. This one-piece foam is a perfect foundation for holding and protecting the fragile ferrite rods and keeping them in a circular arrangement; since the roller is in one piece there is nothing to shift around, or sag. When placed on this core, the final diameter of the FSL antenna is approximately seven inches.
So, what to put the antenna in? Something needs to suspend and protect the antenna as a substitute for the PVC frame previously used. This Sterilite tote box is the perfect size to hold the antenna. As shown in the photo, the dimensions are approximately 14-1/4″ X 9-5/8″ X 12-1/4″; the model number is 1896.
Early on, I decided that the flimsy “locking tabs” on the cover would not suffice for holding the relatively heavy antenna when carrying the tote by the handle. I drilled holes and attached a dozen small Nylon nuts and bolts to secure the cover. (Nylon avoids distorting the medium wave reception pattern of signals, as metal hardware could.)
An ample quantity of 200mm ferrite rods are needed, plus a air variable capacitor (preferably with a 8:1 reduction drive shaft), and Litz wire. 1162 strands/46 ga. Litz provides the most sensitivity but the coil will cover a greater width on the rods.
Gary likes to use waterproof medical tape, sticky side out, to hold the rods in place, but I like to use Gorilla brand tape, as it is extremely sticky and holds the rods better. My choice for the rod-to-coil spacing material is two turns of 1/8″ thick bubble wrap.
This is the foam core, ferrite rods, bubble spacer, and coil assembly prior to fitting in the Sterilite tote container. Before assembly to this point you’ll need to cut the foam roller to length using a serrated knife or electric carving knife. Two sturdy cable ties help hold all of the rods in place. A better alternative might be strips of 1/2″ wide Velcro straps purchased from a fabric shop or home improvement store.
Another key item to this construction method is the use of plastic drywall anchor screws. These are meant to be pushed and screwed into gypsum wallboard for sturdy attachment of bolts or picture hangers on walls. When screwed through drilled holes in the side of the Sterilite tote, they secure and suspend the foam roller/ferrite sleeve loop assembly. The density of the foam roller is sufficient to give a good grip to the drywall anchors. Eight to ten anchors per side are enough to hold the assembly in place. See the photo at the beginning of this article for a good view of this mounting method.
Every good FSL antenna design needs an official sounding manufacturer! With tongue firmly in cheek I’ve appropriated the name shown on this self-produced label. Clearly, a Ferrite Sleeve Loop antenna from Naughtabaum Ferrite Specialties Ltd. stands a better chance than most of passing through TSA checkpoints, right?
I hope this article has given you some new ideas for FSL antenna designs. There’s certainly room for improvement, including making the antenna’s ferrite rods look less intimidating…less like a bundle of dynamite! Perhaps the entire assembly can be wrapped with something that shields the rods from view, or you could use an opaque tote container rather than a clear model.
Be on the lookout for useful materials to repurpose. Trips to your local home improvement stores, office supply, and other outlets will give you further ideas on how to design your own Ferrite Sleeve Loop antenna.
Guy Atkins is a Sr. Graphic Designer for T-Mobile and lives near Seattle, Washington. He’s a regular contributor to the SWLing Post.
Hi there, I have finally deployed the Medium Wave Circle-design 200 metre Beverage antenna (schematic diagram above) at the woods I use in Oxford, UK. It was terminated at the ‘front end’ with a 650 Ohm resistor into a 1 metre-long, permanent copper earthing rod that I had previously driven into the ground, away from the route taken by the general public. The actual wire was orientated in a generally westerly direction, and thus nulling signals propagating from the east. At the receiver end, I utillised my self-built transformer, wound for a 50 Ohm input impedance (14 turns on the primary), thus making the entire set-up suitable for the Elad FDM DUO. It was quite a pain to set up, taking over an hour to deploy the wire and connect both earthing rods and the transformer! However, intial results are very promising and here I am very pleased to share 3 reception videos with you:
The reception from Colombia and Ecuador was unprecidented and both of these tropical stations sound like local AM signals, aided of course by the Bose SoundLink Mini 2 (the Elad’s stand-alone audio is puny to say the least). The signal from Peru is weak, as would be expected, however, the low-gain/ high SNR performance of this antenna results in audio clarity that is better than I have ever heard previously from this rather exotic station. Further to these recordings, I managed to make two recordings of the entire MW band using the FDM DUO via the FDM-SW2 software and I’m hoping an analysis of the data will uncover some nice MW DX. Thus, all-in-all, a very good result with the 200 metre Beverage – more reception videos using this antenna to follow soon on my youtube channel Oxford Shortwave Log. Thanks for watching/ listening and I wish you all good DX.
Embedded reception videos with text links follow below:
Clint Gouveia is the author of this post and a regular contributor to the SWLing Post. Clint actively publishes videos of his shortwave radio excursions on his YouTube channel: Oxford Shortwave Log. Clint is based in Oxfordshire, England.