[Antennas] Introduction and a WTB of a DCTL antenna for 20 m

[Terry Conboy]

  On 2010-09-13 10:10 PM, Chris Wagner wrote:
> Hello Guys,
>
> how you doing? This is Chris KF6VCI now in Thailand as HS0ZFE. In Northern
> Ireland I am MI1ESG. Getting QRV after being QRT for years.
>
> Right now, the Yaesu FT-757 GX on an end fed wire only brings the BBC @
> 6.195 sending from Singapore. What I want is the cW band on 20 m.
> The QRP friends recommend a DCTL antenna for this 3rd floor condo with a
> balcony.
>
> Would someone like to build&  test one, then mail it? Prepaid funds,
> California address. many freak out when someone located overseas wants to
> buy something, anything. (I've had hundreds of scammers respond or post fake
> ads. Motorcycles to laptops. One con man clarified that he is in Edinburgh,
> UK. That alone was a red flag hi hi. BTW, has anyone from Scotland heard
> about the demise of MEGS? Back in 1999 I became member # 700 when I moved to
> Wemys Bay nr Greenock).
>
> Closing, I would like to share that I ordered the P-2 and P-4 paddles from
> www.K8RA.com after watching a first class OP and his Bencher moving a bit.
>
> 73 de Chris
Chris,

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 , 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, which should produce much 
better signals on 20m, even though is it probably half the size of a 1/4 
wave vertical.  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/ .

73, Terry N6RY

PS - Putting your keyer paddle or bug on one those older soft rubber 
mouse pads makes it stay put on the desk.