[Antennas] DCTL Antenna (long)

[Terry Conboy]

At 11:11 PM 2004-02-27, Durwydd MacTara wrote:
 > If the Antenna is resonant and constructed of 450 ohm line, then it 
should have a load
 > impedance of 450 ohms, right? I am feeding it with 450 ohm line and then 
using a 9:1 balun
 > to my 50 ohm transmitter. Or at least that is what I want to do.  The 
original design at
 > http://www.n5nw.org/antennas.html is done with 300 ohm twinlead and a 
6:1 "balun" I want
 > to avoid the rather awkward 6:1 match in the one I build, particularly 
since I dont want
 > to use a tuner.
 >
 > 450 ladder and 300 twinlead have very different Capacitance and velocity 
factors, so I
 > need to compensate in the construction. My question is HOW.

Durwydd,
I've spent a lot of time messing with a model of this antenna, and it's 
quite an interesting problem.  Of course, EZNEC doesn't have the ability to 
model insulation between the conductors, but even without that, some 
interesting points have emerged.

The DCTL is simply a small two-turn loop.  Since each turn is less than 1/3 
wavelength in circumference (actually just under 1/8 wave), it will have 
the classic small loop pattern with nulls perpendicular to the plane of the 
loop.  Instead of the discrete tuning capacitor often used with a small 
loop, the DCTL uses the distributed shunt capacitance of the 300 ohm line, 
in parallel with the capacitance of the stub of open transmission line, to 
bring it to resonance.  The shorted stub across the feedpoint acts as a 
"hairpin" (aka "beta match") matching device to step up the very low 
impedance of the loop to 300 ohms.

I made a 40m model in EZNEC using 0.5 inch copper conductors spaced by 3 
inches, which should be close to 300 ohms.  Being air insulated, the 
velocity factor is nearly 1 and the capacitance per foot is probably 20% 
less than typical twinlead.  As a result, I had to use a 7.03 foot open 
stub (300 ohm, VF = 0.8) to get it to resonate at 7040 kHz.  The 
interesting thing is that WITHOUT the shorted stub, the feed impedance is 
0.23 ohms!  This low impedance isn't a surprise; it is typical of small 
loops.  What is really interesting is that if lossless conductors are 
assumed, the feed impedance goes down to 0.1 ohm!  Remember, this is with 
1/2 inch conductors; using #16 copper wire gives 1.43 ohms (with 1.33 of 
resistive loss).

Even more interesting: the gain drops by 3.5 db when copper is assumed, 
compared to zero loss 1/2 inch conductors.  Using #16 copper wire, the gain 
is down 11 dB.  It can easily be argued that the actual loss may be higher 
than predicted by EZNEC, since NEC2 doesn't account for the "bunching" of 
current on the close spaced conductors.

I don't want to bad-mouth the DCTL, it is just a compromise between size 
and efficiency.  It's good points are that the antenna has a pattern that 
is useful for short haul NVIS contacts and it also has good gain at low 
angles, typical of small loops.  It can be mounted at low height without 
sacrificing performance and it doesn't require a ground system.  However, 
since you're a QRP op, you would likely be much better off with a larger 
antenna, if you have the space.  You probably don't want to burn up so much 
of your limited RF power output in resistive losses.

If the DCTL is still of interest, you can take several approaches.  Among them:

1) You can build the antenna out of 300 ohm twinlead and lengthen of the 
shorted stub "hairpin" to get a better match to your 450 ohm main 
feedline.  You should also be able to shorten the hairpin to get a match to 
50 or 75 ohm coax, too.  (Note that adjusting the length of the shorted 
stub will move the center frequency of the SWR curve, so adjustment of the 
open stub will also be required).

2) Make the antenna itself out of 450 ohm line and be prepared to make the 
open stub a bit longer than the formula and adjust the shorted stub to 
match your line.

3) Make the antenna out of 300 ohm twinlead and use a "series-section 
match" to transform the 300 ohm impedance of the DCTL to 450 ohms.  This 
approach is explained in the ARRL Antenna Book, 18th Edition, p26-4, based 
on the article "Series-Section Transmission Line Impedance Matching" by 
OD5CG in July 1978 QST, p14.  I made an Excel spreadsheet to do the 
calculations.  At 7040 kHz, connect 9.11 feet of 450 ohm (VF=95%) to the 
antenna, followed by 10.81 feet of 300 ohm (VF=80%).  Looking into the 300 
ohm line it will be 450 ohms.  (You also could use a 300 ohm main feedline 
and put the series-section at your transmitter.)  The match bandwidth with 
this 1.5:1 impedance change is MUCH wider than the antenna bandwidth.

BTW, I found an interesting posting from N8UG, "The Wireman", who stated 
that the impedance of "450 ohm" line may be quite different than 450 ohms: 
http://lists.contesting.com/archives/html/Towertalk/1998-07/msg00320.html

And lastly, if you don't like loud music, the DCTL is also described by 
Arnie Coro at:
http://www.radiohc.org/Distributions/Dxers/dctl-antenna.html

Good luck!

72, Terry


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