Jason Hsu wrote:

The DCTL Antenna design is at http://www.n5nw.org/antennas.html

The MFJ magnetic loop antennas are quite expensive and don't cover the
low bands. Homebrewing magnetic loop antennas is very tricky due to
the need for WELDED connections and special capacitors that can handle
thousands of volts. These tricky issues probably explain why magnetic
loop antennas are so expensive. (I'm also surprised that MFJ doesn't
make a larger version of the magnetic loop antennas for 80m that's
perhaps 6 feet wide and long.)



The DCTL Antenna is a loop antenna made out of twin lead, and it has
received rave reviews. From what I've heard, it performs well
compared to other antennas of a similar size. (Obviously, it can't be
as good as a full-size loop or dipole.) I believe the DCTL Antenna
uses the shorted stub and open stub as loading capacitors. It's not
as small as the magnetic loop antennas (4-foot wide square for 40m
instead of 3-foot wide loop), but that allows the DCTL to have more
radiation resistance (so that welded connections aren't needed).

Er... The dctl IS a magloop. A piece of paper, some elementary formulae
and
a few minutes of thinking might brighetn things up.

The dimensions of the magloop are more or less given out of the pf/m
beteween the twinlead conductors and the dielectricity constant of the
twinlead
material. A "classic" magloop can be any size.

Radiation resistance of the dctl, is, well, smallish to say the least.



Has anyone here constructed and used a DCTL Antenna? Some questions:
1. How seriously is it affected by nearby metal? I know any antenna
is affected, but some are affected less than others. For example,
monopoles are affected less by nearby metal. This is why apartment
dwellers using a
balcony with metal railings are generally advised to use a monopole
antenna and use the metal railing as the antenna ground. Mobile
antennas are always monopoles instead of dipoles.

The DCTL is not only a magloop, but a magnetical dipole. Magnetic
materials (such as iron) should be kept away! Non-magnetic materials
of course affects, but are not a disaster (given the poor performannce to
start with!).


2. How do you insulate and reinforce the wires at the feedpoint? I
think this could be a mechanical weak spot. Would coax sealer help?
Epoxy glue? Perhaps a non-metallic box with holes drilled in for the
wires?

Self-wulcanizing rubber tape (bicycle repair kit!).


3. Could the length of the open stub LC be adjustable to modify the
resonant frequency? Perhaps one could put in a connector (Molex,
PowerPole, etc.) and then add in a small open stub. Perhaps something
similar could be done with the shorted end as well. Perhaps one could
add ferrite chokes to electrically (but not mechanically) terminate
the stub, as the RF would be mostly on the outer surface of the wires.
With these adjustments, it may even be possible to make the antenna
multiband. Of course, this would require deviating from the provided
formulas.

The stubs can (and SHOULD, IMHO) be trimmed to get the resonance at
favourable frequency. The dctl is very narrow-band. If You have got a
wide-band SVF dip, the antenna Q is bad!


4. How much feedpoint resistance can one expect from the DCTL? The
author suggests using a 6:1 balun. So it sounds like the DCTL Antenna
should be about 300 ohms. 300 ohms is about the characteristic
impedance of twin lead. But I thought that smaller antennas normally
have feedpoint resistances WELL BELOW 50 ohms, which is an even
farther cry from 300 ohms. Or do those stubs work as a matching
network as well?

Feedpoint resistance does not equal radiation resistance. From a matching
point of wiev, what interest us is feedpoint impedance. As we use the
dctl also when there is no resonance, I propose it is feed by a balanced
output transmatch (same principle as a "zepp").

The feed impedance is a function of the twinlead feeder length, and the
stubs
(which are used to match the thingamajig). The conclusion is that feed
impedance
is individual and adjustable!

Radiation resistance is LOW, and remember that 300 ohm twinlead is lossy.

A.N. Onym




Jason Hsu, AG4DG
personal AAAATTTT jasonhsu.com
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