Tag Archives: Mag Loop Antennas

Bill’s first DX contest using a Panasonic RF-2200 and a hombrew diode/loop radio

Many thanks to SWLing Post contributor, Bill Hemphill (WD9EQD), who shares the following guest post:


My First DX Contest

by Bill Hemphill, WD9EQD

Being a recent new member of NJARC, this is my first time competing in this contest.  I have always been a big fan of BCB DXing and have recently got back into it – especially with the amateur radio bands being in such poor conditions.  The acquisition of a couple of Loop antennas plus two Panasonic RF-2200 radios have just enhanced my enjoyment.

For the contest, I used two completely different radios.  First was the RF-2200 and second was a spur of the moment creation.

The RF-2200 was its usual good performer. While the RF-2200 has a beautiful built-in rotating bar antenna, I enhanced it with the 27” Torus-Tuner Loop Antenna as made by K3FDY, Edmund Wawzinski.  I think I had picked this antenna up at one of NJARC’s swap meets.  So I wish to thank whoever it was that was nice enough to bring it and sell it at the meet.  I have really enjoyed using it.  With this setup, I was hoping that I might be able to pull in Denver, Salt Lake City and maybe even a Mexican station, but it was a complete bust on them.  But I did have a nice surprise in receiving the Cuban station Radio Enciclopedia on 530 in addition to the usual Radio Reloj time signal station.  Following is photo of it in operation:

Originally, I had thought that my second contest entry would be done with a 1962 Sony TR-910T three-band transistor radio.  This radio has a fairly wide dial along with a second fine-tuning knob which would be a big help.  I would have again used the 27” hula-hoop antenna.

But I made the nice mistake of running across Dave Schmarder’s Makearadio website:

http://makearadio.com/

Dave’s site is a wonderful resource for creating your own Crystal, Tube, and Solid State radios as well as Audio Amplifiers and Loop Antennas.  While going down the rabbit hole of his site, I ran across his Loop Crystal Set, #19 Crystal Radio:

http://makearadio.com/crystal/19.php

What grabbed my attention was the wood frame loop antenna which is similar to one I had acquired a couple of years ago at a ham fest:


It was a really nicely constructed, nice swivel base.

I replaced the tuning capacitor with one that has a 6:1 ratio.

At this point I started thinking that I could create something similar with my loop.

I randomly grabbed a diode from my parts box.  Not sure what the exact model is.  (I later found out that it was an IN-34 which is what I was hoping it was.)  Then quickly soldered the diode, a resistor and capacitor to a RCA plug:

I then proceeded to use some jumper cables and just clip it to the tuning capacitor on the antenna base:

The RCA plug was then the audio out (I hope) from the radio.

I quickly realized that I did not have a crystal headset or any headset that would reproduce any audio.  So I used an old Marantz cassette recorder to act as an amplifier.  Fed it into the mic jack and then tried to listen to the monitor out.  Bingo – I could pick up or local station on 1340 really weak.

So I then fed the audio from the Marantz into a Edirol digital recorder.  Now I was getting enough audio for the headphones plus could make  a recording of the audio.

At last I was receiving some signals.  To boost the audio some more I removed the resistor from the circuit.

I found out the I could only tune from about 530 to 1350.  I probably needed to clip the lead on one of the loop turns, but I really wanted to see how it would do at night.  I spent several hours and was just totally amazed at how well it performed and how good the audio was.  The hardest part was when there were very strong signals on the adjacent frequency.  What I found really interesting was that it was not linear in its tuning.  At the low end of the band the stations were more spread out than at the higher end.  This made tuning fairy easy at the low end and very touchy at the high end.  I was able to hear a couple of Chicago stations along with Atlanta and St. Louis.

Here’s photo of it in action:

I have created an audio file of the station ID’s heard with the diode/loop radio.  The audio file is on the Internet Archive at:

https://archive.org/details/bcbstationidsondioderadio

I had a lot of fun in the contest and especially enjoyed trying something really different with the diode/loop radio.  Now I have a whole year to try to think up something really creative for next year’s contest.


Absolutely brilliant, Bill! I’m so happy to see that your ham fest homebrew loop has served you so very well in a contest. I love how you pulled audio from your homebrew, make-shift diode radio as well–using your audio gear in a chain for amplification obviously worked very well.

Thanks so much for sharing your experience, Bill!

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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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Pete seeks rechargeable battery pack options for his Wellbrook loop antenna

Paul Walker’s battery-powered Wellbrook antenna in remote Alaska.

Many thanks to SWLing Post contributor, Pete Jernakoff, who writes:

[…]I’m hopeful that one of your readers might be able to suggest a solution to my problem.

I have an older Wellbrook Communications active loop antenna that is powered by a 12 v AC-to-DC, center pin negative, plug-in power supply (Stancor AC Adapter, was supplied with the antenna).

I’d like to power the antenna from a rechargeable battery in order to make the antenna portable and to eliminate any noise that might be emanating from said power supply (which, as an aside, runs very warm when in operation).

My problem is that I cannot find any rechargeable batteries (lithium ion preferred) with a center pin negative output. All of the ones that I can find online have center pin positive outputs (such as the TalentCell 12V/6000mAh rechargeable battery that I’ve purchased to power my other, more recently produced, Wellbrook Communications active loop antenna whose amp needs a center pin *positive* input).

Thanks in advance for consideration of my request. Btw, love your blog! I’ve been an avid reader of it for quite some time now.

Post readers: If you have any suggestions for Pete, please comment with any relevant links to help him make the purchase. I’m guessing Pete isn’t interested in re-soldering a coaxial plug for negative tip polarity at this point.

By the way, I used a photo of Paul Walker’s Wellbrook at the top of this post because I recall that when he lived in Alaska, he powered his Wellbrook loop with a rechargeable pack (and during the winter, I also recall he struggled to keep it warm enough to provide power for any length of time!). Perhaps Paul can comment.

I’m so glad you enjoy the SWLing Post, Pete!

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John adds a pre-amp and rotatable stand to his homebrew NCPL antenna

A couple weeks ago, we featured John Mills’ homebrew Noise-Cancelling Passive Loop (NCPL) antenna (see photo above) in a post with two other Post community designs.

John recently shared an update to his project:

As promised, some more pictures of my antenna mounted on a rotatable stand. I have used standard (in the UK) 20mm electrical plastic conduit and fittings to make the frame.

Fitted a small plastic box to house the balun and have put a BNC socket on the underside of it for connection the coax cable to my Rx.

I have also been experimenting with cheap low cost amplifiers (LNA) found on eBay (see picture) which do seem to improve the general strength of signals by 10-15db, but the baseline noise also rises.

I did manage to hear a QSO on 160M using one of these which was inaudible without the LNA but I cannot say for sure yet if they are worth the extra noise introduced.

Thank you for sharing your update, John! The plastic conduit support is simple and effective! Indeed, it looks very professional. What I love about your NCPL build (loop, stand, and LNA) is it that it’s all incredibly affordable as well.

Read more about John’s NCPL build in this post.

Post Readers: Has anyone else experimented with implementing a pre-amp in their NCPL antenna design? Please comment!


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More about hacking VGA cables to make binocular ferrite cores

A few days ago, we posted an a short article showing how Oscar hacked a VGA cable to make a binocular ferrite core for his homebrew NCPL/Youloop antenna. Many thanks to SWLing Post contributor, Grayhat, who explored this clever hack a little further:

Hi Thomas, Having some time in my hands Sunday afternoon I decided to try pulling out the ferrite chokes from a VGA cable I had around, and while doing so, I decided to coarsely document the process with some pics.

Figure 1

The first thing to do is use a cutter to carefully cut around the “washer” shaped plastic at the connector end of the choke (fig.1, #1,#2, #3 above), then on the same side, after cutting the plastic also cut the inner conductors (fig.1, #1).

Move to the other side of the choke, gently cut around the “washer” w/o cutting the inner conductors, now pull the cable to extract it from the choke (fig.1, #3), repeat the process for the other choke.

Now look at the “cans” containing the chokes, one side of those will show a “cap” (fig.1, #4), insert a small screwdriver into the center hole and gently ply to one side to raise the cap and extract it (fig.2, #1).

Figure 2

The result will be as in fig.2, where #1 is the closing cap, #2 is the ferrite choke and #3 is the “can” containing the choke. Repeat the process and you’ll have two ferrite chokes as shown in fig.3 (the VGA connector is there to give an idea of the dimensions):

Figure 3

At this point, use some tape (duct tape will be a better idea, I used clear tape just to make an example) to tie the two ferrites together as in fig.4 and you’ll have your “binocular ferrite”:

Figure 4

Willing to use whatever you have there to wind the transformer, you may now extract the tiny insulated wires from the VGA cable (fig.1, #3, see wires) and use them for the windings.

Notice that other cables may use different choke “cans” which may need to cut a larger portion around the flat faces at the ends. But remember that in any case, those are just “snap-in” cans containing the ferrites, so with a bit of attention and patience, it shouldn’t be difficult extracting the ferrites.

Based on a little online research, it sounds like the ferrites used to choke the VGA cables (HDMI ones too) are generally type #31.

Looking at some #31 datasheets it appears that while #73 is works fine at frequencies below 50MHz, the #31 is best suited for the 1-300MHz range.

This means that #31 won’t be the best pick for mediumwave, although if one doesn’t have another choice… well, go for that! Also notice that the ferrite permeability is different:1500 for #31 and 2500 for #73. This means that we’ll need to increase the number of windings to achieve acceptable signal transfer, otherwise the transformer loss will make our antenna deaf.

One might try increasing the number of windings to say 8:8 or 16:16; as long as the winding
ratio will remain the same, there won’t be problems (although the resulting bandwidth will become narrower).

Thanks for documenting and sharing this, Grayhat! Since most of us have more time on our hands at home, I think it would be worth experimenting with the number of windings to see how it affects the antenna performance. That’s a clever thought, too, to use the VGA wires to wind the Balun. As long as the cable is long enough for the amount of turns, it’s certainly the most efficient use of resources!

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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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Hack a broken VGA cable to make a binocular ferrite cores for your NCPL antenna

Many thanks to Oscar (EA3IBC) who shares this simple hack.

I’ve gotten quite a few emails from Post readers telling me that the only thing holding them back from building a NCPL antenna is the BN-73-302 binocular ferrite core needed for the 1:1 Balun. While so much of the world is sheltering at home due to Covid-19, it’s less convenient to purchase one just for this purpose.

But almost everyone has an old VGA or USB cable with ferrite cores they can cannibalize for this  very purpose. Oscar shared this super-simple hack on Twitter:

1. Cut the cable and remove the ferrite cores

2. Tape the two cores together

3. And wind four turns on both sides

Admittedly, this 1:1 balun might not have the same properties as the BN-73-302 from our tutorial, but it seems to be working for Oscar. Check out this screenshot he shared from SDR# while hooked up to his NCPL antenna:

Thanks again!

Readers, you can follow Oscar on Twitter by clicking here.

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