2015年1月29日木曜日

Building a 7 MHz small transmitting loop antenna, part 5

Part 5 of this series describes some mechanical design changes and first disappointing tests with the loop.

First design change: From copper pipe to copper strap

As described in parts 1 and 2 of this series, the original design for the 3m x 2m loop radiator was to construct it using several 1m-length segments of 6mm-diameter thin-wall copper pipe. However, such a structure, once built, is impossible to move or alter without tedious disassembly.

Since I wasn't completely confident that the loop would work as expected, I decided to go with a more mechanically flexible solution using copper strap instead of copper pipe. The flexible strap cannot support itself and thus requires a supporting frame to be built, on which the strap is draped. I built a supporting PVC frame, 3 meters by 2 meters, as follows (apologies for the blurry photograph):


The bottom half of the support frame is obscured by lower balcony wall, which has been cropped from the bottom of the photograph.

The above photograph shows a top supporting bar spanning the three vertical support pipes.  In the end I ended up removing this top supporting bar because its weight tended to make the whole structure sway excessively in the wind. Removing the top supporting bar means that the uppermost horizontal run of the copper strap is supported only at the tips of the three vertical support pipes. The strap then runs down the two end-most vertical support pipes and is temporarily affixed to the frame with velcro cable ties.

The above setup allows repeated and easy setup and tear down of the antenna as needed for various changes, especially around the capacitor section. As it turns out, this ability to easily tear down the antenna turned out to be vitally important, as the antenna didn't work as expected and required major changes.

Cutting and splicing the copper strap


I used copper flashing with 0.2mm thickness as the loop radiator. The skin depth of copper at 7 MHz is 24.605 micrometers, so the flashing with 200 micrometers thickness is greater than 8 skin depths thick for RF currents at 7 MHz.

The flashing is available in a single sheet of dimensions 365mm x 600mm. I decided to cut this large sheet into 18 individual strips of 20mm width, then to solder all of these strips together to form a ~10m-long radiator.

According to W9CF's strip-to-round-conductor equivalency calculator at http://fermi.la.asu.edu/w9cf/equiv/, a 20mm-wide strip should have RF resistance equivalent to that of a ~6.2mm-diameter conductor, i.e., equivalently low resistance to the 6mm copper pipes I originally planned on using. Note that the calculator assumes that the strip "is much thicker than a skin depth", which is satisfied by our ~8-skin-depths-thick copper flashing.



The following photographs show the time-consuming process to cut and splice the copper flashing by hand. This took a few hours, and my fingers ached for a few days afterwards; cutting this thickness of copper flashing with small shears is tiring on the fingers. 







After joining all the strips into a ~10m roll, the next step was to wrap the copper strap in vinyl electrical tape, to avoid risk of injury on the sharp copper edges when handling the strap.



The last step was to solder the copper strap to the butterfly capacitor and to mount the capacitor in the box.







Failure #1: Not enough capacitance

I draped the loop over the PVC support frame with the remotely-controlled capacitor at the top center of the support frame. I then created a ~2m-circumference auxiliary coupling loop and placed it at the bottom of the large loop radiator.

Unfortunately, my worst fear materialised: the butterfly capacitor did not have enough capacitance to resonate the loop at 7 MHz, the intended frequency of operation. I could resonate it from about 9165 kHz to 15270 kHz, listening for the noise peak in my receiver.

I did a quick tune up test on 10 MHz and 14 MHz and was able to get the SWR down to better than 2:1, but not much better.

To allow operation at 7 MHz, I investigated soldering another smaller butterfly capacitor inside the box, in parallel with the existing butterfly capacitor, in order to reduce the resonant frequency. However, this plan was mechanically infeasible.

As a stopgap measure to allow operation at 7 MHz I tried attaching an additional 150 pF variable capacitor (with a wiping contact, which is not recommended due to ohmic losses) to the butterfly capacitor using alligator clips (again, not recommended due to ohmic losses), and was able to resonate the loop at 7 MHz with the 150 pF variable capacitor almost completely unmeshed. However, I was unable to obtain a low SWR at 7 MHz.

My inability to reduce the SWR to 1:1 on any band prevented me from measuring the SWR bandwidth of the antenna, which might have been an indication of the antenna's efficiency. I did various tests squashing and stretching the coupling loop but was never able to obtain a good SWR. This is one disadvantage of using a coupling loop: it can be difficult to adjust the coupling loop for a good SWR, especially if the loop losses are excessive and unknown due to environmental or other ohmic losses.

Second design change: Capacitive instead of inductive matching 

One alternative to the difficult-to-adjust inductive coupling loop is to use a capacitive network to match the loop impedance to 50 ohms. There are a number of schemes for doing this, and I chose the scheme shown at http://homepage.ntlworld.com/g4kki.william/army_loop_tuner.htm, which resonates the loop with a split-stator capacitor, and feeds the loop through a matching capacitor into one side of the loop conductor (with the coax shield going to the neutral rotor of the split-stator capacitor). Note the above link refers to this as an "Army loop tuner", though it is not the same capacitive network used by the original "Army loop". G8ODE describes the commonly-seen capacitive matching schemes here: https://rsars.files.wordpress.com/2013/01/qrp-loop-tuner-80-20m-g8ode-iss-1-32.pdf.

I built a prototype capacitive matching network on a polypropylene cutting board, and soldered it to my loop conductor. For the loop resonating capacitor, because the 50pF butterfly capacitor had insufficient capacity, I used a dual-gang 2x440pF variable capacitor in a split-stator configuration.

The loop tuning and matching circuit construction looks as follows:


Note that the capacitive matching scheme is not balanced, and so there is a danger of common-mode currents on the outside of the shield of the coaxial cable. To suppress these, I wound a ferrite-cored common-mode choke on a FT250-43 core, using 12 turns of RG58A/U. According to G3TXQ's page at http://www.karinya.net/g3txq/chokes/, this choke should exhibit >4k ohms impedance at 7 MHz.


Although it is preferable to mount the capacitor at the top of the loop (to keep it and the intense electric field away from lossy surroundings), the heaviness of the new capacitor assembly and the choke precluded top-mounting with my somewhat fragile PVC support frame. Therefore, I mounted the capacitor, feedline choke, and feedline at the bottom of the loop, which was still about 20cm above the concrete balcony floor.

Partial success: Perfect 1:1 SWR achieved

After hooking up the loop to my rig, I tuned the split-stator capacitor and quickly found the capacitor setting (approximately 25% meshed) that would peak the receiver noise at 7 MHz. Then, I experimented with different settings of the 150pF matching capacitor. It turned out that setting the matching capacitor to fully-meshed (150pF) gave a perfect 1:1 SWR.

This was my first experience with a capacitively-matched loop, and it was quite easy to tune the loop for minimum SWR. Adjusting the matching capacitor is much easier in practice than adjusting the size of an inductive coupling loop.

There is some interaction between the tuning and matching capacitors, due to the matching scheme requiring that the loop be tuned slightly off-frequency to provide the inductive reactance needed to effect the impedance transformation; see here for details: http://www.eham.net/ehamforum/smf/index.php/topic,100081.msg802902.html#msg802902. In practice, this interaction really isn't a problem. You just tune the main capacitor for maximum noise, adjust the matching capacitor to some approximate appropriate value, and retune the main capacitor to again peak the noise.

Failure #2: Ridiculously high bandwidth

My joy at attaining a perfect 1:1 SWR was short-lived, as I quickly discovered that the SWR was still 1:1 even after retuning several tens of kHz. In fact, the SWR was less than 1.5:1 over the entire 200 kHz of the Japanese 40m band!

This is quite a bad sign indeed. A previous vertical dipole I built for 7 MHz (2m long) had a 1.5:1 SWR bandwidth of about 40 kHz, and that was broader than the 4nec2 prediction of 20 kHz. A small transmitting loop should have even narrower bandwidth. Therefore, a 1.5:1 SWR bandwidth of 200 kHz is ridiculously high and indicates significant losses within the system.

I can think of a number of possibilities to explain the loss:
  1. My split-stator capacitor is too lossy. 
  2. The matching capacitor is too lossy.
  3. The copper strap is inherently too lossy and/or the electrical tape is increasing the losses.
  4. The environment around the loop is too lossy, with the loop running close to the concrete walls.
  5. The environment around the capacitor is too lossy, with the loop capacitor near the concrete floor.
  6. One or more solder joints is faulty. The solder joints include those that connect the short copper strap segments together, those that connect the strap to the capacitor, and those around the matching capacitor.
Possibility 1 I think is unlikely, as I used the split stator capacitor in a previous small transmitting loop that exhibited a narrow bandwidth. Possibility 2 also seems unlikely because the matching capacitor is not part of the resonant circuit and isn't carrying that much current; its losses should not adversely affect the loop efficiency. And, possibility 3 also seems unlikely because I am inclined to believe W9CF's calculations on the RF resistance of strip conductors.

I can think of the following ways to proceed:
  • To attack possibilities 4 and 5, a smaller-diameter loop (reducing radiation resistance) with a wider conductor (reducing loss resistance) could be used. That could allow comparable efficiency to the larger loop while allowing the loop to be positioned farther away from the concrete walls.
  • Alternatively, and still for possibilities 4 and 5, an experiment could be done erecting the loop in an open area away from concrete walls, and observing if the SWR bandwidth decreases. If so, then the environment is a major source of loss. An experiment with top-mounting of the capacitor might also yield insight.
  • To address possibility 6, I could re-do all the solder joints. This will be quite tedious on the shorter copper strap segments, as these have all now been wrapped in and sealed with electrical tape.
  • Alternatively, to address possibilities 3 and 6, an experiment could be done with a continuous conductor instead of my pieced-together copper strap. The continuous conductor could be a wire, a coaxial cable shield, or a long strip of unbroken copper tape. None of these is an ideal conductor for a low-loss small transmitting loop, but using one or more of these continuous conductors with the existing capacitive loop tuner might reveal if the pieced-together and tape-insulated copper strap is excessively lossy in comparison.
  • To address possibility 2, the matching capacitor - currently an air variable capacitor with a wiping contact - might be replaced with the high-Q, no-wiper, butterfly capacitor that proved to be unsuitable as the resonating capacitor.
  • Finally to address possibility 1, a vacuum variable capacitor could be used as the loop resonating capacitance. This would require two matching capacitors rather than just one. And, again, the weight of the vacuum variable capacitor would preclude top-mounting on my current support frame, requiring a bottom-mounted capacitor.
Most likely I will proceed next by re-doing the easily re-doable solder joints. If that does not improve (reduce) the bandwidth, then I will investigate using a continuous conductor.

My current hope is to achieve a narrow-bandwidth with a capacitively-matched loop and bottom-mounting of the capacitor. If I can achieve narrow bandwidth (and reasonable efficiency), then I will motorise the variable capacitors for remote control of tuning and matching.

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