Wednesday, December 2, 2009

DCTL demystified

This is an attempt to explain my understanding of the DCTL (Distributed Capacitance Twisted Loop) antenna. Personally, I am convinced to have understood its functioning. In a second blog entry I will provide detail for a 10MHz DCTL antenna, so you could build one yourself.

Let's start on a simple design of a self-resonant loop antenna. This concept is also known as frame antenna. A sort of magnetic loop without a capacitor but two windings instead. The two windings (parallel conductors) will provide a distributed capacitance which will form a parallel resonance LC-circuit, as we know it from regular magnetic loop antennae.
Let's assume, the feeding is done by a T-match, this would result in the following configuration (red=feedline):

The T-match as got two feed-points to the otherwise closed resonance LR-circuit. Those feed-points are marked in the following sketch:

Assuming that the conductor of the loop between the feed-points roughly matches the feedline's impedance (200-300Ohms), one could squeeze this section into a hairpin structure. The following sketch would reflect this...

OK, the loop is matched now, but what about the resonance? Well, if the loop is cut well, that is the end of the story! Why bother? Yes, really, this could be the end of the story if you intend to use the loop in a relatively narrow stretch of QRG.

However, here is, what was though about in 1994, the end result of the DCTL (as you may find it on the internet).
The sketch show a capacitive (open) stub which terminates the loop in a fashion known from magnetic loops. This stub adds a few pF only, and should be cut to the mid-resonance of the DCTL. The stub lowers the DCTL's frequency, hence, the DCTL has to be cut to a QRG above the aimed operation frequency. Finally, having a DCTL self-resonant, i.e. w/o capacitive stub, above a ham-band, allows for a kit of stubs to cover the whole band.

With this tiny post, I hope to focus your attention of a very portable antenna, which is almost forgotten, probably because it was never understood or decently explained in the first place.


  1. Hi there,

    I'm interested in a portable, collapsible, small, indoor, efficient antenna for QRP (1-5 watt level) work on 40m CW. Tough requirements, but not impossible I think. My last idea was a 1/4 wavelength magnetic loop made of copper foil tape ( The problem with that idea is that the foil tape wouldn't be so easy to set up and take down for portability.

    Another interesting antenna is of course the DCTL; its small size and wire construction make it appealing. I don't yet have my license, so I can't perform on-air tests myself and thus am interested in experience reports like yours.

    One thing I am curious about is the DCTL efficiency. As I understand it, traditional magnetic loop antennas have very low radiation resistance, which requires use of very low-loss conductors like copper tubing (or in my idea, a wide copper strip). It seems that the DCTL, using ordinary wire, would have low efficiency if its radiation resistance is as low as that of a magnetic loop (the radiation resistance being swamped by the loss resistance in the wire).

    So, my question: how efficient is the DCTL? Is it efficient enough for QRP (1 watt) level CW work on 40m? I know I can't work the world on 1 watt with a small antenna, but I'm just wondering if the DCTL is efficient enough for this kind of low-power use.

    Thanks for reading. Your comments would be appreciated! Your website seems to have the most detail about the DCTL antenna being used in practice.

  2. Hi qrp!

    To answer your question, it does work, however, it is not particularly efficient, as you assumed.
    This is presumably why it never got really popular.
    Some 10+ years ago, I built my first one for 40m and had a couple contacts using it. A wire thrown in a tree had better results to that time.

    Your understanding of magnetic loop antennae is correct.
    The DCTL however, is more like an old-style frame antenna. Those are more used for receiving actually.

    There are some pros for the DCTL actually. It is resonant, but not as high-Q than a magnetic loop. Meaning, regular fine-tuning is not required. But still, the resonance helps to keep harmonics low.
    The DCTL can easily be weather-proofed, in particular when it is design for a single range of frequency only.

    For you qrp-plans, I would recommend an antenna known as the Rockloop. Here you can find the version I built:
    I made many contacts using the Rockloop from various locations with good success. The Rockloop is as easily collapsible as the DCTL. If you build one, make sure to use heavy gauge wire, the Rockloop is a pure magnetic loop.
    One downside of the Rockloop would be that it is an indoor aerial only and needs fine-tuning.

    Regarding the space required to install a DCTL, there wont be many more outdoor aerials more efficient.

    That brings me to the last point, I figure that the DCTL will be a very interesting option on the low bands. I intent to build versions for 80m and 160m.
    I will write about those, as soon as there is something to write about ;-)

    72 & 73

  3. Thanks for your comments.

    About the DCTL, I found a guy who said it didn't work well for him when built with wire, so he built a copper tube version that apparently works better:

    About the Rockloop, I'm still a little worried about its efficiency on 40m. Do you have any experience with that?

    Another gentleman has built a Rockloop from copper tape, similar to my idea. He says it works on 160m with 3 watts. Not very portable unfortunately.

    Looking forward to your further DCTL reports.

  4. Yes, the copper DCTL looks cool, not really collapsible though ;-)
    Contacts were made using the Rockloop on 20m 30m and 40m with about 2W in CW, PSK31 and WSPR. Using WSPR on 30m and on 40m, my signal was heard in across the Atlantic when using the Rockloop.
    Back to the DCTL, I think, for bands 30m and shorter, more efficient collapsible aerials are conceivable.