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Mid-2012 I acquired a portable QRP rig: a Yeasu FT-817ND (HF + 2m + 70cm, 5 watt). So I needed a portable loop (40-20 or 40-10). Such loops are available commercially for several hundred euros/dollars. You can make one yourself for a fraction of that price.

For a general introduction to Small Transmitting Loop antennas (STL, a.k.a. "Magnetic Loops"), incl. coupling methods, please see my 80-20 STL page.


Last page update: 5 February 2017


CONSTRUCTION

This STL antenna must be be easily (trans)portable. So, the loop diameter should not exceed about 1 mtr (≈3.2 ft). Loop antenna calculators suggest that a loop of that size can be tune from 40 to 10 mtrs with a variable capacitor of 10-180 pF. See the tables and plot below.


Mag Loop
Mag Loop

Calculator results for a loop with a diameter of 1 m (3.2 ft)

(calculator: ref 2B; 10 mm (3/8") copper tubing, 5 milliohm loss included)

Here are the components that I used for this loop antenna:

  • The loop: soft copper tubing with an outside diameter of 10 mm (3/8 inch). Such tubing is sold in rolls, and used in refrigeration & air conditioning systems.
  • Good coax such as LRM-400 could also be used. It is less expensive than copper tubing, but also less rigid, and not available at my local Do It Yourself store or any other store near my QTH!
  • Two PL-259 plugs for 10 mm coax (note: do not use PL-259 that is meant for another coax diameter - one size does not fit all!). The plugs are installed on the ends of the copper tubing (or coax).
  • To keep loss-resistance low over time, use decent quality connectors, with teflon centers - not cheap plastic that completely melt away when a soldering iron is in the same room. See discussion in ref. 11.
  • Two SO-239 chassis jacks; if using coax, you can short-circuit the braid and the center conductor.
  • Air variable capacitor, ca. 180 pF. The minimum capacitance should be as small as possible, as it limits the highest achievable resonance frequency.
  • I used a 10-208 pF, 1 kV capacitor from MFJ (type 292-2005; I measured 13.5-212 pF). Rotation of this capacitor's shaft is not limited, so it is easy to motorize. My similarly sized  Cardwell / E.F. Johnson type 167-12-1 is equally suitable: 12-202 pF (I measured 10-200 pF).
  • A small ABS-plastic project box (not metal!), for housing the capacitor and the SO-29 jacks. I used a box measuring 11x6x3 cm (≈4¼x2¼x1¼ inch).
  • Coupling - I used two options:
  • toroidal ferrite core (inner diameter large enough to fit over the PL259 plug. I used cores of type FT-140-43 and T-140-61.
  • coupling loop, made of 60 cm (≈2 ft) heavy single-strand installation wire, or thin copper or brass tubing.
  • One square-flange BNC female chassis jack, for connecting the coupling loop to the coax feedline.
  • For the mast (no cross-brace required):
  • Two meter (6.5 ft) of rigid ( = thick wall) PVC tubing. I used heavy outdoor electrical conduit with an outside diameter of 22.5 mm (7/8 inch), and an inner diameter of 12.5 cm (0.5 inch). With thin-wall PVC, the antenna will be swaying around if there is any wind. 
  • Two PVC female-to-female couplers for the above tubing. The tubing is cut in two, for easier storage and transportation. The second PVC coupler is used for installing the coupling loop.
  • Optional: small DC-voltage motor with low rpm, for remote tuning.

Mag Loop

10-208 pF, 1 kV air variable capacitor (MFJ type 292-2005)


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The capacitor and SO-239 jacks installed in the plastic project box

(the FT-140-43 ferrite ring is for transformer coupling)

As stated above, my capacitor has a rating of 1 kV. This is sufficient for operating this loop with up to 5 watt power. If the input power is increased by a factor of N, then the maximum voltage at resonance across the capacitor is increased by a factor √N. E.g., doubling the power increases the capacitor voltage by ≈1.4. The graph below shows the calculated capacitor voltage for 5 and 100 watt input power. It shows that a 1 kV capacitor rating is sufficient for 5 watt in my loop.

Mag Loop

Capacitor voltage as a function of frequency, calculated for my 1 mtr loop

(calculated with ref. 2B)

One important thing that the above graph shows, is the very high voltages across the capacitor at resonance. This is not only important when choosing a suitable capacitor, it is also a SAFETY issue!

Mag Loop

I figured out a new way for mounting a coupling loop to the antenna. The female-female couplers for the PVC tubing are slightly tapered (conical) in the inside. This provides a tight connection when the tubing is fully inserted. I used a half-round file to remove the taper. The coupler can now be slid completely onto the tubing of the mast. It still has a good tight fit, but it can be moved up and down on the mast, and rotated by hand to find the best SWR. This mechanism works very well, even if I say so myself! With a flat file, I made a notch in the outside wall of the coupler, and glued the square flange of the female BNC jack to the coupler. I used 2-component Eccobond® epoxy glue for this. It is strong and waterproof.

Mag Loop

PVC coupler piece, modified for mounting a coupling loop to the antenna mast


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The BNC jack glued to the PVC coupler

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Mag Loop

The modified PVC coupler can be moved up & down on the mast and be rotated for SWR adjustment

(the coupling loop has the standard 1/5 diameter of the main loop)

As shown above, I initially fixed the top of the loop to the PVC mast with a plastic strap. This didn't work very well. So I drilled a hole through the tip of the mast (9 mm drill for the 10 mm OD copper tubing), and then cut a slit to the hole. See photo below. My clip-on method works great!

Mag Loop

The copper tubing is clipped onto the tip of the PVC mast


I have also replaced the coupling loop. My initial experiments were with a loop made of heavy installation wire. I then changed to brass tubing with a diameter of 4 mm (5/32"). This is much more sturdy than the coupling loop made of heavy installation wire. The tubing is easily shaped by hand.

Mag Loop

Coupling loop made of thin brass tubing


Mag Loop

The shaft of the tuning capacitor can be turned without limitation. The capacitor is symmetrical, so capacitance is varied between minimum and maximum by turning the shaft 180 degrees. The tuning range is well over 10 MHz. Due to the high Q of the antenna, the resonance bandwidth is relative narrow. This makes it difficult to tune accurately by hand. So I added a 6:1 reduction gear to the shaft of the capacitor.

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A 6:1 reduction gear mounted onto the shaft of the capacitor


Standard ways for tuning the antenna to resonance at the operating frequency, are to adjust the tuning capacitor for minimum SWR or maximum antenna current while keying the transmitter, or for maximum receiver noise. You can also use a very simple visual tuning indicator. I have added a small neon lamp (standard NE-2 type, 5 mm diameter, 75 Volt gas discharge lamp) to the tuning capacitor. Tune for maximum brightness of the neon lamp ( = resonance) at the transmit frequency. Note that the lamp is only connected to one side of the capacitor! The second wire of the lamp is draped alongside the capacitor, but not connected to it. Some folks connect both wires of the neon lamp to the same side of the capacitor. Even a couple of watts is enough to light up the lamp. Warning: the lamp does get quite hot when glowing very brightly with RF. Maximum brightness can be changed by adjusting the placement of the second wire of the lamp.

Note: at resonance, per definition, the reactive component of the antenna impedance is zero: the impedance is purely resistive. Typically, antenna impedance at resonance is not 50 ohms. Hence, typically, the resonance frequency of an antenna is not the same as the minimum-SWR frequency. For loop antennas, the resonance and minimum-SWR frequencies are often close enough so as not to require further tuning.

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Mag Loop

The neon lamp glows brightly at resonance


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The neon lamp is off when not transmitting at/near the resonance frequency, and when not transmitting at all :-)


As explained on my 80-20 STL page, the voltage distribution around the loop has a maximum across the capacitor, and a near-zero minimum at the point opposite the capacitor. So, half the maximum voltage ( = at resonance) is distributed over half the circumference.

Mag Loop

Voltage and current distribution of an STL antenna


My loop has a circumference of about 3 mtrs (10 ft) and a calculated maximum capacitor voltage of about 1 kV. Assuming a linear voltage distribution, this translates to about 60 volt per 5 cm (2 ") at resonance. So, connecting the wires of a neon lamp to the loop at two points about 5-10 cm (2-4") apart, should also work (possibly with a series resistor).

Mag Loop

Another way to connect a neon "tuning" lamp

(source: ref. 6F, QRP STL wire antenna)

The PVC mast is inserted into an old tripod that I had laying around. I salvaged it from a very cheap telescope - the adjustable tripod was actually the best part of the telescope, hihi. I did have to make a new tripod-hub, to accommodate the mast. I attached the plastic box with the variable capacitor to the mast with velcro® strips.

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My third STL antenna - installed on a tripod

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Tuning the loop by hand is a bit of a hassle, and being near the loop also slightly de-tunes it. So I added a simple DC-motor drive for remote tuning. I bought a tiny 10 rpm / high torque motor on eBay for a couple of euros (search for "12V torque gear motor"). I made a small rigid coupler, to go from the 3 mm motor shaft to the 1/4 inch shaft of the variable capacitor, and of the 6:1 reduction gear that I added to it. Yes, I know: I should have used flexible-yet-torsionally-stiff coupler (I did not have one with the right bore size in my junk box, and did not want to wait another three weeks to get an inexpensive one from eBay). The capacitor will turn at 10 / 6 = 1.67 rpm or 1 rev in ≈38 sec. As the capacitor is symmetrical, it tunes over the entire frequency range in 38 / 2 = 19 sec.

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The tiny 12 VDC motor with its reduction gear down to 10 rpm, and a home-built (non-flexible) shaft coupler (3 mm to 1/4 inch)


The (long) wiring from the12 V supply to the motor is passed through a ferrite ring several times. A small decoupling capacitor (0.01 μF, ceramic) is put across the motor terminals, to suppress receiver noise when tuning without transmitting. These cheap high-speed motors are not brushless, and typically exhibit arcing at the brushes.

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6:1 planetary reduction gear


Controlling the little motor is extremely simple: a 3-position DPDT toggle switch is used for on/off and to change direction:

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Hooking up the motor to the 12 VDC power source


Attaching the motor to the capacitor shaft was straight forward. But the motor has to be immobilized. From a polyethylene cutting board (from the kitchen), I fabricated a mounting bracket. It is a simple rectangular piece of the board (I actually doubled up on the thickness), with appropriately spaced hole - one for the PVC mast, one for the motor. I tapped threads into holes coming in from from the sides, for set screws. Works FB.

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Cutting-board bracket for mounting the motor - holes for the PVC mast and for the motor


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Capacitor + reduction gear + shaft-coupler + 10 rpm motor


I have added a small (2x5cm; 1x2") speed controller circuit card. For fine-tuning. It is a very simple Pulse Width Modulator (PWM) circuit. Quite inexpensive: $4.50 (end-2013 pricing, incl. shipping) - from eBay (search for "PWM DC motor controller"). If using a controller card, it may be necessary to add low-pass filters in the motor leads near the controller, to prevent RF from damaging the controller. E.g., a 100 μH RF choke in series with each wire, and a 10 nF (ceramic) disc capacitor between each wire and ground/common.

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The PWM speed controller card


To be able to dispense with the 10 rpm motor, I also picked up a small 3 rpm 12 VDC motor, though I have not installed it yet...

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Small 12 Vdc / 3 rpm motor


MEASUREMENTS

With the 10-208 pF capacitor, I can tune this loop over a frequency range from 6.2 to 21.2 MHz. According to the loop calculator programs, I should have been able to tune up to 10 mtrs with this capacitor. Somehow I have some stray capacitance, and the effective minimum capacitance is about 20 pF instead of 10 pF...

Below are measurement results for coupling with a standard coupling loop.

Mag Loop
Mag Loop

SWR plots for a standard coupling loop


In the 40 m band, the plot suggest an SWR=2 bandwidth of about 10 kHz. I had to enter an additional loss resistance of 15 milliohm into the AA5TB calculator (ref. 2B), to get the same result...

I have noticed that around 10 MHz, the SWR varies significantly with the direction that the plane of the loop is pointing in. I believe that this is related to the steel in floor of the terrace and in the nearby walls, and possibly the nearby daisy-chain wiring of my terrace lighting system.

I also did some very quick tests with a ferrite transformer coupling. I used two ferrite materials: material type number 31 and 43. I still have to test with material type 61, which should actually be more appropriate above 10 MHz than both type 31 and 43. Ref. 7.

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Toroidal ferrite cores FT-240-31, F-140-43, and T-140-61

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Coupling a coax to the loop with a ferrite core transformer


The graph below shows the SWR sweeps that I measured for various configurations of the ferrite core transformer. This convenient type of coupling appears to be suitable only for a 2:1 frequency range.

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SWR sweep for ferrite core transformer coupling, for 1-3 secondary windings and material types 31 & 43



REFERENCES


External links last checked: October 2015


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©1999-2016 F. Dörenberg, unless stated otherwise. All rights reserved worldwide. No part of this publication may be used without permission from the author.