Tag Archives: Loop Antennas

Michael pairs the Tecsun PL-990 and the AOR LA400

Many thanks to SWLing Post contributor, Michael Ye (BD4AAQ), for the following guest post:


In the Loop: PL-990 and LA400, a Perfect Match

by Michael Ye (BD4AAQ)

PL-990 and LA400

I have been a happy owner of Tecsun’s PL-880 world band receivers for years. In fact I have two PL-880 radios, one sitting at home and the other staying in my car. So, after Tecsun introduced the new model PL-990 in late 2020, it didn’t take me long to decide to purchase one. In this article I will discuss the Tecsun PL-990 receiver working with loop antennas, while referencing some relevant features of the PL-880.

Overall performance of the PL-990

Merely by its model number, it is easy to regard the PL-990 as an upgraded version of the already highly reputable PL-880. As expected, the PL-990 can very much be regarded as a combination of all the existing fine radio features of the PL-880 AND the music and bluetooth additions, with a number of improvements for instance in shortwave and medium wave performance. The ergonomic design of the PL-990 looks and feels different from that of the PL-880 in a number of ways. Although I may prefer the the more slim and elegant appearance of the PL-880, the PL-990 gives a more rugged and durable feeling, among other improvements over the older PL-880.

Working with loop antennas

The PL-990 and the PL-880 side by side

Living on the twelfth floor of a condominium in the crowded Shanghai, I have often been fascinated with loop antennas. As a licensed amateur operator, I have used the MFJ-1786X and have been impressed with its performance. On reception, I also find loop antennas appealing, as they are able to pull in weak signals while noticeably reducing electro-magnetic interference rampant in the urban environment. I have an unbranded shortwave loop antenna which I believe is based on and performs similarly with the AOR LA320. Despite its excellent performance, it is only good for the 5MHz – 15MHz shortwave range. So a few years ago when AOR launched the new LA400 wideband loop antenna, I bought one, which I often pair up with the PL-880 and other radios for shortwave listening, and get satisfactory results!

Antenna Switch on the PL-990

Now, back to the PL-990. When I first tried the PL-990 with the LA400, the results were generally good but not as good as as compared with using the same LA400 on my PL-880. This puzzled me for a day or so until I realised that the PL-990 actually has an antenna switch which the PL-880 does not have. The switch is used to toggle between an internal antenna (i.e. the built-in ferrite bar/telescopic antenna) and an external one (e.g. the AOR LA400). So a new PL-990 user who has often operated the PL-880 when first using the PL-990 could easily ignore the switch which should be pushed to “Ext” when plugging in an external antenna. This explains why the PL-990 may suddenly appear less sensitive than expected.

“Ext” antenna input for all bands

Contrary to the PL-880 whose external antenna socket is only good for shortwave signal input, the PL-990’s external antenna socket works with all bands, from long wave to FM. I found this to be an important and very useful change, and a pleasant surprise for my LA400, which covers a wide range of frequencies from long wave to medium wave to FM and up to 500MHz.

Once the LA400 is connected, the correct band selected, and last but not least the antenna switch turned to “Ext”, the PL-990 and the LA400 work like a charm in the indoor setting, remarkably better than the built-in telescopic antenna. With the loop connected, while there is not much to expect on the long wave band because of very few long wave stations remaining in the world, reception improves considerably on all other bands including on the medium wave and FM bands, as is also reflected on the upper right hand display of the signal strength and S/N ratio readings. Needless to say, performance on shortwave is as good as on the PL-880, if not better (again, remember to push the antenna switch to “Ext” when using it on the PL-990). Using the AOR loop on the PL-990 for FM reception is somewhat different as there does not seem to be a noticeable tuning point. Simply select the “Others” band, which appears to be broad enough for fair FM reception.

Tecsun AN-200 loop antenna

It is worth mentioning that I have a Tecsun AN-200 tunable medium wave antenna, which I have not used often. As its name suggests, it is for medium wave reception only. I tried it on the PL-990. Works great.

The AN-200 and the PL-990

It is hard to tell which one, the PL-LA400 or the AN-200, fares better, as the signal strength and S/M readings are quite close. They both perform better than the radio’s internal ferrite bar antenna to varying degrees, by improving the signal strength or the S/N ratio or both. The Tecsun loop is a passive antenna, meaning no power is required, making it easy to be used “wirelessly”, by simply placing the loop close to the radio, without having to be connected to the radio via a cable.

Chocolate, our house cat, tries to enhance reception with her tail

It should be noted that in the “wireless” mode of the AN-200 the antenna switch on the PL-990 should remain at “Int” so as for its built-in ferrite bar and the loop to couple with each other.

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Andy seeks advice on directly coupling an AM loop antenna to a crystal radio

Andy would like to couple a similar antenna to a crystal receiver

Many thanks to SWLing Post contributor, Andy, who asks the following question in response to a previous post about building a Milk Crate AM Broadcast Loop Antenna:

All references to tuned loop antennas talk about no real connection to the AM radio, but merely inductive coupling.

However, I made a very elementary crystal radio which has no ferrite core or antenna.

I want this loop to be my primary (only) antenna, so I need to feed it directly to my tuning circuit. So I don’t know if I should take a wire from any particular part of the loop, with another wire to ground… and if these 2 wires should be in parallel or series with the tuning elements of the loop antenna.

Thanks!

Good question, Andy! Hopefully someone in the SWLing Post community can comment with some guidance!

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DIY: How to build a Passive Resonant Transformer-Coupled Loop Antenna for HF reception

We recently posted a tutorial on building a simple Noise-Cancelling Passive Loop (NCPL) antenna. This prompted SWLing Post contributor, Bob Colegrove, to share his excellent article on building a Passive, Resonant, Transformer-Coupled Loop (PRTCL) Antenna:


A Passive, Resonant, Transformer-Coupled Loop Antenna for Shortwave

By Bob Colegrove

Over the years I have resisted the level-of-effort necessary to construct and maintain outdoor antennas.  Rather, I have focused on squeezing out all of the microvolts I could get inside the house. Many years ago I had access to a well-stocked engineering library, and used my advantage to gather information about the theory and development of loop antennas – a daunting undertaking for an English major.  Ultimately, by adhering to a few basic rules, some of them dating back 100 years, I found quite acceptable performance can be had with an indoor passive antenna intersecting just a few square feet of electromagnetic energy.

Theory

There are a couple of advantages of resonant loops as opposed to non-resonant ones.  The first is the fact that the signal dramatically increases when you reach the point of resonance.  The second follows from the first in that resonance provides a natural bandpass which suppresses higher and lower frequencies.  This gives the receiver a head start reducing intermodulation or other spurious responses. The downside of all this is that the resonant loop is, by design, a narrow-band antenna, which must be retuned every time the receiver frequency is changed by a few kHz.  On the other hand, there is nothing quite as rewarding as the sight (S-meter) and sound you get when you peak up one of these antennas – you know when you are tuned in.

There is nothing new about the loop antenna described here.  It’s just the distillation of the information I was able to collect and apply.  There are a number of recurring points throughout the literature, one of which is the equation for “effective height” of a loop antenna.  It basically comes down to the “NA product,” where N is the number of turns in the loop and A is the area they bound. In other words, provide the coil with as much inductance as possible.  Unfortunately, for resonant loops, the maximum coil size diminishes with frequency.

With this limitation on inductance, the challenge becomes minimizing unusable capacitance in the resonant frequency formula in order to get the highest inductance-to-capacitance (L/C) ratio possible.  Some of the unusable capacitance is built into the coil itself in the form of distributed capacitance, or self-capacitance between the coil turns. This cannot be totally eliminated, but can be minimized by winding the coil as a flat spiral rather than a solenoid, and keeping the turns well separated.

The second trick is with the variable capacitor.  Even with the plates fully open, there is residual capacitance on the order of 10 to 20 picofarads which can’t be used for tuning purposes.  A simple solution is to insert a capacitor in series, about ¼ the maximum value of the variable capacitor. This effectively decreases the minimum capacity and extends the upper frequency range.  In order to restore the full operating range of the variable capacitor, the fixed capacitor can be bypassed with a ‘band switch.’ With the series capacitor shorted, the variable capacitor operates at its normal range and extends coverage to the lower frequencies.

Construction

I have constructed similar loops covering long wave, medium wave, and shortwave all the way up to about 23 MHz.  I wanted to optimize this loop for the most active portion of the shortwave spectrum. Consequently, it covers approximately 2.6 to 12.3 MHz.  See Figure 1.

Figure 1.  A Passive, Resonant, Transformer-Coupled Loop Antenna for Shortwave

Figure 2 is a schematic diagram of the antenna.  Cd (in red) is the distributed capacitance of the primary coil, L1.  This is not tunable capacitance, but it still contributes to the resonance; likewise, the 15 pf minimum capacitance of C1.  By adding C2, the minimum total capacitance can be lowered to greatly increase the upper range of the antenna. S1 is the ‘band switch.’  It shorts out the series capacitor, restoring the maximum low frequency.

Figure 2.  Schematic Diagram

Frame – The frame is made from 3/8”-square basswood or poplar dowel (see Specialized Parts).  Two pieces, each 36” long, have been predrilled at ½” intervals to accommodate the primary and secondary coil wire (think of a tennis racket).  It is a good idea to drill holes along the length of each dowel – more than you will need. You may decide to change things later on, and drilling holes in an assembled antenna is not easy.  Also the two dowels are notched in their centers to fit together. See Figure 3 and Figure 4. The clear plastic disk in Figure 4 is a packing disk from a spindle of CDs; it is cemented to the square dowels, and used to hold them at right angles.  Any rigid, light-weight material will do.

Figure 3. Square Dowel Showing 1?2” Hole Spacing and Lacing of Secondary Coil


Figure 4. Cross Members Notched and Square Dowel Reinforcement

Primary Coil – With a coil size 36” in diameter, you likely won’t be able to get more than two turns of wire to resonate at frequencies up to 12 MHz.  This takes into account the precautions described above to minimize unusable capacitance.  AWG 22 stranded, insulated wire was used to lace this coil; ensure the dowels remain at right angles with one another.  Note that one set of holes in the dowel is skipped between the first and second turn.

Tuning Capacitor – Almost any salvaged variable capacitor can be made to work.  For a typical 2-gang unit, the gangs can be connected in series through the common rotor sections and metal frame with the stator terminals of each gang used as the outer terminals.  This will create a lower minimum capacitance as described above.

For the antenna described here, a single-gang, 365-pf capacitor (see Specialized Parts) was used with a fixed mica capacitor in series.  The minimum capacitance of the variable capacitor is nominally 15 pf. Figure 5 shows the capacitor assembly for the primary circuit. Components are mounted on a perforated circuit board, which, in turn, is mounted to the bottom of the vertical square dowel.  A portion of the base can be seen at the rear. A large diameter tuning knob is suggested, as the peak tuning for a properly constructed loop will be very sharp and require a delicate touch. As an option, I have used a planetary reduction mechanism on other antennas to give an 8:1 ratio with the capacitor shaft.

You may notice at high frequencies that the antenna is somewhat unstable with body contact of the knob or around the tuning capacitor.  This is because the resonant circuit is operating at a very high L/C ratio with capacitance at just a few picofarads. Body capacitance will tend to detune the antenna.  It may be useful to extend the knob 2 or 3 inches from the tuning capacitor with an insulated shaft.

Figure 5.  Capacitor Assembly

Secondary Coil – The secondary coil operates at low impedance to feed the lead-in.  There are two extremes governing the size of the secondary coil. A coil which is too small will not pick up much of the magnetic field generated by the primary circuit at resonance.  On the other hand, a secondary coil which is too large will overcouple or load the primary circuit. This will reduce the Q, or sharpness of the tuning.

The secondary coil is 16” diagonal at the largest turn and consists of 7 turns of AWG 20 buss wire.  Buss wire was used so the coil can easily be tapped after the 1st, 2nd, 3rd, 4th, and 6th turn.  The 7th turn is not currently used.  A tapped coil will provide better impedance matches to the lead-in when the antenna is used through a wide frequency range.  The taps are selected with a rotary switch. The taps are connected so that the outer turns are used first, and inner turns connected as needed.  It is important that unused turns remain unconnected (free) rather than shorted. See Figure 6.

Figure 6.  Secondary Coil Switch

Lead-in – A twisted pair of AWG 22 stranded wire is used as the lead-in.  This will be more flexible than coax. The lead-in should be kept as short as possible and twisted tightly so it will not pick up any signal by itself.  This is important at shortwave frequencies. A twisted pair can be fabricated from two lengths of wire with one set of ends anchored in a vise, while the remaining ends are twisted in the chuck of a hand drill.  Most portable radios are equipped with a standard 1/8” phone jack at the external antenna connection point.  So, this antenna is terminated with a 1/8” phone plug.

Base – There is nothing special about the base.  Your only guidance should be to make it as stable as possible.  Since the frame is light, most of the weight will be at the bottom with the capacitor assembly and other parts.  That helps stability. This antenna uses a 5” plastic jar lid for the bottom. Keep the base small, as the antenna will likely be operated on a desk or table.

Operation

The antenna is intended to operate in close proximity to the radio, such as on a desk or table.  There must be sufficient space to rotate the loop laterally. As described, this antenna has a range of 2.6 MHz through 12.3 MHz with a band overlap around 8 MHz.  Depending on your selection of capacitors, your range and overlap may be slightly different.

  1. Tune the receiver to a desired frequency.
  2. Set the band switch on the antenna to the corresponding band.
  3. Tune the antenna capacitor to resonance (peak signal).
  4. Rotate the secondary switch to the position of maximum signal strength.  Begin with the fewest turns (generally one) in the secondary.
  5. It may be necessary to repeak the primary circuit.

Repeat the procedure to test operation of the upper or lower band.

Unlike similar loops for long and medium wave reception, this antenna is not especially responsive to direction for peak or null signal reception.  However, you will find it very useful to reduce or possibly eliminate locally produced noise. Simply rotate the antenna on its base.

Modification

The basic concept for this antenna can easily be extended to higher or lower frequencies.  Removal of the inner turn of the primary will significantly raise the upper frequency; whereas, adding turns will increase the lower range.  Note that the lacing of the primary coil skips one set of holes in the square dowels between the first and second turn. This minimizes distributed capacitance between turns.  This separation should be maintained if additional turns are added to lower the operational frequency.

Specialized Parts

Some sources for square wood dowel and single-gang 365 pf variable capacitors are listed below.  The author does not endorse any of them. Prices for similar capacitors vary widely.

Square wood dowel:

Variable capacitor (365 pf):


Bob, thank you so much for sharing this excellent, detailed tutorial. Although I don’t have the exact same variable capacitor, I have all of the other components to make this antenna. I will have to put this on my Social DX bucket list! Thank you again!


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The Airspy Youloop is a freaking brilliant passive loop antenna

Before I start talking Youloop, I have a little confession to make up front:

At the Winter SWL Fest when I gave a presentation about Portable SDR DXing, not only did I give attendees the wrong name of the Airspy Youloop antenna, but I also configured it incorrectly, hence the poor performance via my Miscrosoft Surface Go tablet PC.

I had assumed the the crossover component of the antenna was the transformer component. I realized the mistake I made when I saw some of the first promotional photos of the Youloop antenna a few weeks ago.

The crossover connects both sides of the loop while the tee junction contains the transformer.

Doh! I’m trying to forgive myself for making such an obvious mistake, but in my defence–and in the spirit of full disclosure–my antenna was a very early sample prototype without instructions, diagrams, etc. so I set it up imagining it being similar to the homebrew loop Vlado and I built. (FYI: When I say “Vlado and I built” it, I really mean, “Vlado built it.”) 

So obviously I made a poor assumption.

Once I assembled the antenna correctly? Wow. Just. Wow!

Youloop: The ideal travel antenna for high dynamic range SDRs

The Youloop, Airspy HF+ Discovery, SDRplay RSPdx, and all cables easily fit in my Red Oxx Lil Roy pack.

The Youloop is truly the travel antenna I’ve always wanted for portable SDR DXing. Here’s why:

  • It’s incredibly portable and can be rolled up to fit in a small travel pouch
  • It has all of the low-noise characteristics of other magnetic loop antennas
  • It’s wideband unlike many passive loop designs
  • It requires no variable capacitor or tuner
  • It’s made of quality components
  • It requires no external amplifier nor power source
  • It takes one minute to assemble
  • It’s affordable (~$35 USD shipped)

The only caveat? To take advantage of the Youloop, you must use a high dynamic range receiver.

Airspy HF+ Discovery SDR

I can verify that this antenna works brilliantly with the Airspy HF+ Discovery.

The AirSpy HF+ SDR

It will also pair well with the Airspy HF+  if you shortcut R3 via the R3 modification.

SDRplay RSPdx SDR

I’ve also used it numerous times with the new SDRplay RSPdx while using SDRuno in High Dynamic Range (HDR) mode. With the RSPdx, I can make spectrum recordings of the entire AM broadcast band. Note that HDR mode is only available on the RSPdx at 2 MHz and below, using the SDRuno app.

I have not tested the Youloop with other SDRs yet. I will soon test it with my WinRadio Excalibur.

So how well does the Youloop perform?

Listen for yourself!

I’m doing a little cargiving family members today. Their home is swimming in RFI (radio interference/noise). In the past, I’ve struggled to make good mediumwave recordings at their home–certainly an ideal situation for a mag loop antenna.

This morning, I wanted to record one of my favorite local AM stations (WAIZ at 630 kHz), so I set up the Youloop in the middle of a bedroom, hanging off a large bookshelf set against an interior wall. In other words: not an ideal situation.

When I plugged in the Airspy HF+ Discovery and loaded the Airspy SDR application, I fully expected to see a spectrum display full of broadband noise.

Instead, I saw signals. Lots of signals:

Sure, there’s some noise in there, but it’s low enough I could even do proper mediumwave DXing on most of the band if I wished.

In fact, if you’d like to experience the HF+ Discovery/Youloop pairing in this compromised, less-than-ideal DXing setup, why not tune through one of the spectrum recordings I made?

Click here to download the spectrum file [885.7MB .wav].

The recording was made on March 30, 2020 starting around 10:50 UTC. You’ll need to open this file in AirSpy’s free application SDR# or a third party SDR app that can read AirSpy .wav files.

I can’t wait to try the Youloop in other locations. Since we’re in lock-down due to Covid-19, I won’t be able to try the Youloop in a hotel any time soon. Almost all of my 2020 travel plans have been canceled.

Highly recommend

If you have one of the SDRs mentioned above, go grab a Youloop. At $35 USD, it’s a fantastic deal.

Click here to check out the YouLoop at the RTL-SDR.com store ($34.95 USD shipped).

Click here to check out the YouLoop at Airspy.US ($29.95 + shipping).

Click here to search for AirSpy distributors in your region/country.

If you don’t have a high dynamic range receiver, note that Airspy is exploring the idea of making a pre-amp for the Youloop. If interested, you might drop them a note of encouragement!

Do you have a Youloop? Please share your comments!


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Coastal DXing with the AirSpy HF+ Discovery and a homebrew passive loop antenna

Last week, we packed the car and headed to coast of South Carolina.

The trip was a bit impromptu but through the creative use of hotel points, we scored a two bedroom ocean front unit with a fantastic little balcony.

The vacation gave me an excuse to test the new passive loop antenna my buddy Vlado (N3CZ) helped me build recently.

The loop design came from AirSpy’s engineer and president, Youssef Touil.

This passive mag loop takes advantage of the new AirSpy HF+ Discovery‘s exceptionally high dynamic range. Youssef had reported impressive results, so I had to build one.

Vlado had a length of Wireman Flexi 4XL that was ideal for this project. The only tricky part was penetrating the shielding and dielectric core at the bottom of the loop, then tapping into both sides of the center conductor for the balun connections.  Being Vlado, he used several lengths of heat shrink tubing to make a nice, clean and snag-free design.

The results were truly exceptional. I spent most of my time on mediumwave from the hotel balcony because I was determined to catch a transatlantic signal.

Check out the spectrum display from my Microsoft Surface Go tablet:

Our ocean front hotel was inundated with noise, but I still managed to null out most of it and maximize reception using the passive loop. I simply suspended the loop on the balcony rocking chair–not ideal, but effective and low-profile.

Want to take a test drive?

If you’d like to experience this portable SDR setup, why not tune through one of the spectrum recordings I made?

Click here to download the spectrum file [1.7GB .wav].

The recording was made on November 17, 2019 starting around 01:55 UTC–I chose it at random and have yet to listen to it myself. You’ll need to open this file in AirSpy’s application SDR# or a third party SDR app that can read AirSpy .wav files.

Stay tuned…

I’m writing an in-depth report of the HF+ Discovery, my experiments with this setup and AirSpy’s soon-to-be-released passive loop antenna for the January 2020 issue The Spectrum Monitor magazine. Spoiler alert: I am truly impressed with the wee little AirSpy HF+ Discovery. It’s a powerhouse!


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Marty needs advice as he builds a passive magnetic loop antenna

Many thanks to SWLing Post contributor, Marty Kraft, who asked that I share the following question with our community:

I’m still working on a receive-only passive hula loop magnetic antenna for my Tecsun PL-660.

After viewing thousands of YouTube videos (LOL), I built the PVC-pipe structure [you can see in the photo below].

But I need some tech help to finish…

The antenna is 90 inches tall; large loop diameter is 40 inches; and small loop diameter is 17 inches. The wiring is 14 gauge braided.

I plan to put the antenna outside on the porch. Then I’ll run coax from the small loop to the receiver inside and use a 365 pF air variable capacitor to tune the large loop.

My first question is, what’s the best coax to use for the 10-ft run from the small loop to the radio inside? Second, will that 365 pF cap tune the entire 3-30 MHz range?

It’s hot here in Louisiana, so I’d really like to tune the capacitor from inside my apartment, also using coax to connect the cap to the large loop. Will that work? Or does the cap have to connect directly to the large loop?

Any other tips or suggestions? Thanks for the help!

Post Readers: If you have any helpful advice for Marty, please comment!

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Ron is impressed with the PK Loop Mini (A-LOOP-MTAM)

Many thanks to SWLing Post contributor, Ron, who shares the following review of the PK Loop Mini (A-LOOP-MTAM):

Standard PK Loose-Coupled Loops (the “A” series) are 14 inches in diameter…they are
for use with a receiver having a built-in ferrite bar antenna.

Don’t think they’re not capable of serious DX…a few years ago a friend in Texas
snagged and recorded France Inter (162 kHz) on a 14 inch standard PK LW
loop using a Sony 7600GR.

But this is about the PK Mini 10 inch loop for Medium Wave (there are none for
other ranges).

The build quality is superb as with all PK loops. The band has two ranges: 525 to
710 and 710 to about 1720 kHz. Paul Karlstrand uses a unique design consisting
of flat computer cable and a custom made circuit board to connect the turns
end-to-end.

The low end simply switches in a fixed capacitor across the variable to lower the
frequency.

The Mini has 25% less sensitivity than the standard 14 inch loop according to the website.
Currently it can be had for $66 USD delivered to your door from Melbourne…this one
was ordered on a Monday and showed up a week later,which is outstanding considering
it had to clear customs in NYC.

Performance is virtually identical with the Tecsun or Terk loops, which are 9 inch loops.
So why buy a PK?

You get what you pay for, or not…the PK is fairly robust compared to the Tecsun or Terk.
It’s made to last and it’s a PK Loop.

Here are some links for comparison purposes:

If you wish to enhance your loop try co-coupling it to your receiver with a Q-Stick :

http://dxtools.com/QStick.htm

Thank you, Ron, for sharing your experience with the PK Loop Mini. It certainly sounds like an excellent option for travelers and, like you, I agree that the construction quality is superb!

Click here to check out the PK Loop website.

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