[Antennas] Re Terry N6RY's comments on the compromise of a DCTL antenna like End Fedz

Wed Sep 15 22:31:31 EDT 2010

>>> The DCTL antenna, due to it's small size, is something of a compromise.
> It's a variant of the halo, which is a half-wave dipole with the ends
> bent around until they almost meet. With the DCTL, the ends are wrapped
> even tighter until they get back to the feedpoint, making it look like a
> two-turn open loop (with a small capacitor across the opening formed
> from a short length of open-circuit transmission line). All this
> bending back on itself and the distributed capacity of the twinlead
> lowers the feedpoint Z to well under 10 ohms, so a hairpin (aka beta)
> match with a shorted section of line is used to get the Z up to 300
> ohms. (It is quite possible to adjust the tuning and shorted stubs to
> get a 50 ohm match instead of 300 ohms, with minimal impact on bandwidth.
>> As noted on http://www.io.com/~n5fc/dctl_ant.htm<http://www.io.com/%7En5fc/dctl_ant.htm>, it's down about 1 to
> 1.5 S-units below a full size dipole, and has a much narrower bandwidth,
> as well. I've modeled the DCTL in EZNEC (but with air spaced wires
> forming the transmission line in the loop) and most of the losses are in
> the copper conductors of the loop and are similar to what N5FC/N5ESE
> notes. Increasing the conductor diameter will increase the signal at
> the expense of an even narrower bandwidth.
>> It's interesting that the signal from this loop is a little stronger off
> the edges, with a slightly (~6 dB) weaker signal broadside. The size of
> the DCTL in wavelengths is between that of the very small receiving
> loops often used on the low bands (which have deep side nulls) and a 1
> wavelength "quad" loop (which is strongest broadside).
>> I second the suggestion to use a mobile whip, *** OK, just bear in mind
> that I need to maintain a low profile for many reasons. And such a vertical
> would radiate only 1 or 2 W RF effectively, is that correct? With 100 W in.
> *** which should produce much
> better signals on 20m, even though is it probably half the size of a 1/4
> wave vertical. *** A BTV would be nice but shipping will be a nightmare.
> and then Customs will grab them. DL.. now HS0ZIV has 2 antennas confiscated
> by Customs and it's a living nightmare. (>3 ft coax without a ham radio
> station license? That will be 5 years in jail!!) *** Adding one 1/4 wave
> radial made from insulated wire and
> dropped off one side of your balcony would make a fair "ground" system
> for a mobile whip. An alternative, although not inexpensive, are the
> products from http://www.buddipole.com/ . *** TNX Terry! ***
>> 73, Terry N6RY Chris HS0ZFE, KF6VCI, MI1ESG
>> PS - Putting your keyer paddle or bug on one those older soft rubber
> mouse pads makes it stay put on the desk. TNX, I used such a solution at
> the club station yesterday. Hope K8RA's paddle will give a more solid feel
> than the BY-1.
>>