Rhombic DCTL

Winter and storms arriving, it was time to do something about the wobbly design of the grabber's antenna.

Having had very good results with PE plumbing parts for my weather proof 600m frame antenna, I went to the hardware store today to buy some other material.
Contrary to the 600m stuff, which uses very heavy speaker cable, the DCTL uses lightweight TV twin line and only turn of it. This allows for flimsy 5/8" PE electrical installation tubing. Have a look at the b.o.m.:

My DCTL is 327cm long. An elbow is good for 8.5cm. In order to compensate for the shorter path through the terminal box, the two tubes connected to said box are 2cm longer than the other two.
Consequently, I cut two tubes to the length of 73cm and two to the length of 75cm.

Since today, 1903z, the DCTL is back online, in "tip up" rhombic configuration.

BTW, another reason for selecting this configuration was the wish to lift up the aerial by means of a pole, to improve ground clearance.

Note, the tubes are held together by Duck-Tape. Plumbing glue could be used, I guess, however, tape does not create such a stink ;-)

DCTL vs Full-Size Vertical

I was asked a very justified question, how well the DCTL-antenna would actually work. To test this, I was running the spectrumlab-grabber on a full-size vertical for 30m. The vertical is made from flimsy speaker twin-lead, cut to a length of 7.2m; both leads parallel. The vertical got two radials, made from the same stuff, cut to the same length. A telescopic fishing-rod keeps the vertical vertical.

On the left side of the following spectrum, I can see a spectrum taken with the vertical, the spectrum on the right hand side is taken using the DCTL.

It is somewhat obvious that the vertical delivers more signal. In terms of signal to noise ration, the DCTL is in a 3dB lead. Well, the slightly better S/N-R is not really visible, since I run WSPR parallel to SpectrumLab, it was actually measured. You may however get the impression that the FSK signal on 10140060Hz is somewhat clearer when the DCTL was use.

I have to add that the DCTL is placed next to a wall, the distance I would estimate as 20cm. With the DCTL not being proximate to other objects, and may somewhat higher, signals would probably be much better. Something still to be investigated.

DCTL relocated

The position as shown in the previous blog entry caused the DCTL antenna to pick up a lot of the TV-c###, erghh QRM. Also the position was rather visible and occupied another aerial's "mooring ring". Hence, relocation was required.

The new location is very low profile and still provides very good reception for the grabber receiver. My impression, the new location is much better than the older.

DCTL update

Up to now my DCTL for the 30m grabber was not weather proof. Last week, I weather-proofed it by means of flexible installation tubing. The material I used is supposed to be inside the walls of a building. I supposed, it is not at all UV resistant. However, the materials seem not to have any adverse influence on the performance of the DCTL, as tested indoor. Now, all the tests done, I decided to give the DCTL an outdoor experience. On the northwards face of my house, UV won't be such a big deal, however, I would expect the tubing to discolor and become brittle over time.

indoor DCTL & WSPR on 30m

This is what my tiny subharmonic d.c.-rx heard when wired to the indoors DCTL.

Timestamp	Call	MHz	SNR	Drift	Grid	Pwr	Reporter	RGrid	km	az
2009-12-08 03:36	W3HH	10.140158	-24	0	EL89vb	1	PA1GSJ	JO22da	7276	42
2009-12-08 13:40	W1XP	10.140242	-17	0	FN42fo	5	PA1GSJ	JO22da	5546	51
2009-12-08 14:24	RW6XC	10.140147	-19	-1	LN23as	5	PA1GSJ	JO22da	3059	302
2009-12-08 09:46	I0/N2CQR	10.140152	-24	1	JN61fv	0.02	PA1GSJ	JO22da	1283	334
2009-12-08 08:54	F6EZP	10.140189	-20	0	IN93	2	PA1GSJ	JO22da	1029	21
2009-12-08 07:36	OK2SAM	10.140199	-23	0	JN99du	1	PA1GSJ	JO22da	1008	289
2009-12-08 07:30	OE3EV	10.140193	-6	0	JN88	5	PA1GSJ	JO22da	981	298
2009-12-08 07:42	IQ4DJ	10.140228	-5	1	JN54mq	1	PA1GSJ	JO22da	955	331
2009-12-08 09:44	GM4KGK	10.140281	-6	0	IO68ve	5	PA1GSJ	JO22da	955	131
2009-12-08 10:10	OE3WGW	10.140178	+9	-1	JN88cj	5	PA1GSJ	JO22da	937	300
2009-12-08 09:40	IK1ODO	10.140264	-19	0	JN35sa	20	PA1GSJ	JO22da	814	344
2009-12-08 10:34	DL5MHD	10.140212	-9	0	JN58xc	5	PA1GSJ	JO22da	699	311
2009-12-08 10:34	DG7RJ	10.140240	-23	0	JN58th	5	PA1GSJ	JO22da	665	311
2009-12-08 10:46	HB9TMW	10.140144	-25	1	JN36gq	5	PA1GSJ	JO22da	615	345

So folks, here you have it, you can receive weak signal living in a concrete building in a dens suburban environment.

In another experiment, I will use the E-probe, outside at my balcony, stay tuned for the data.

30m DCTL as presently used for the JO22-grabber

Here come the numbers... (bonk bonk bonk!!! for the insiders):

• radiator total length: 327cm
• matching stub length: 46cm

For the radiator, i.e. loop, this is the actual wire cut length. Meaning, there's about 5mm vanishing into the luster terminal.

As for the stub, same story, however, the stub is shorted, and whatever is needed for that short is not taken into account by that figure. However, I believe, that this would not make a big deal difference in matching anyway...

This is, how stuff is wired up in my (experimental) setup:

The "matching stub" (current center of the loop) is connected to both terminals, either of the loop leads to the respective terminals. The whole thing is matched to RG58 by means of a 4:1 balun using a T50-6 toroid contraption (classic approach).

With this setup my MFJ-analyzer was showing a SWR of 1.2:1 @ 50Ohms. Fine with me. The loop is sensitive to its environment however.

Short note on how I resonated this loop. It was cut to a frequency much lower than wanted, since my aim was a single frequency loop for a grabber. Anyway, the 30m band is not that wide, or is it? With the analyzer engaged, I progressively cut off bits of the "voltage"-end of the loop, until I reached the aimed mid resonance frequency. This should be done symmetrically, in order to prevent noise caused by local E-fields (vacuum cleaners, hair dryers, etc.).

No toroid in the junk box? This could be an alternative to match TV-twin lead to 50Ohmx coax:

It was giving a slightly inferior result, hence, the T50-6 balun won and will stay connected with the grabber for the upcoming future.

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.

bonk bonk bonk... (DCTL)

That would be the echo of me banging my head on the wall... Now that the thing is in resonance, it works brilliantly with my subharmonic direct conversion receiver. Grggrrrrrr, I took it for granted for several weeks, that the dimensions of the loop were ok, and put all efforts to the 6:1 balun... Now that I think of it more clearly (don't you ask me why this didn't happen earlier, you don't wanna know!), it is sort of clear, I am not using 300Ohms window line as I used for the 40m DCTL, but rather EUROPEAN (!!!!) TV-twin, which is 240Ohms (I believe), and thus, also has got a different capacitance per length.
Bonk bonk bonk ...
Bonk bonk bonk!!!!
But now, it seems, I am onto something. Receiving the mystery station from JN44AE now, 2037z.
The DCTL finally usable for QRSS? I hope so, I intended to use this sort of aerial for future journeys, to put signal in the air, or maybe receive occasionally...

hmmmm

Banging my head against the wall... stupid me!!!

The 30m DCTL, even with the new balun I produced this morning (before the O.P. @ work) did not perform. And guess what??? Now that I finally checked it out with my MFJ analyzer, it is resonant 500kHz lower, somewhere around 9.5MHz.... Yep, for a narrow-ish band antenna, this cant do the trick.... Oh man! I wonder how I received Andrew's MEPT with this thing in the first place....

Now, more cutting will be done, and I promise that I will have the aerial as successful as the big brother I once cut for 40....

Still banging my head....

Roll your own!

This is about my newest addition to the spectrum grabber installation. The regular reader will know, that I was mainly using my rockloop (see webpage) for the 30m suburban subharmonic grabber. My rockloop is, however, a multiband (15m to 40m) aerial, which provided me with enjoyable QSOs in its' entire bandwidth. OK, I could build another one, but than again, multiband?

Years ago, I learned about the DCTL (Distributed Capacitance Twisted Loop) antenna. Actually shortly after its' design has been published. To that time, I cut myself a version for 40m from 300Ohm window-line. This loop actually never worked... well, at least not to that point in time.

Just recently I found the bag in which the DCTL was rolled up for storage. That brought the thought that I may possibly made a mistake when making the loop. And yes, in fact, I made a mistake, I forgot the "twist" in it.

Some months ago, I thus fixed it by soldering the leads together which belong together... and... voila, a working 40m DCTL. So far, so good, rolled it up again a was busy doing other stuff.

And now, just recently, I remembered it again (actually I just wanted something to print, in order to test my print-server setup). Only God know why I was hacking "DCTL antenna" into the average search engine and printed just the first thing that popped up:

http://www.radiohc.org/Distributions/Dxers/dctl-antenna.html

Now, that would form a nice and cheap receive loop for the grabber.... And yes, it does!

Here come the dimensions of my 30m DCTL (please refer to the link above to understand what I am writing about)

• Ll = 347cm
• Lz = 46cm
• Lc = none

The 30m version is made from TV twin lead, which is much lighter and easier to roll up for transport as the window-line is.

The lazy dawg I am, I could not be bothered yet to include a balun in the design, I am thus sipping the signals off the loop by means of ordinary RG58 coax. Yes, I do consider using TV twin lead as feedline, however, the stuff is rare nowadays, and I'd therefore probably rather wind a balun.... We'll see....

My 40m version is good for TX, I guess, the 30m version will be as well. I will try to tune (trim/cut) it accordingly and give it a try during my next journey.

Roll it up, roll your own!