KGD antenna – The ‘Sawn-off dipole”

This is where I plan to work on the information available about the KGD Antenna. The centre coil form apparently used to be sold as a kit by QRP projects in Germany.

All this information is sourced from their site. (Here is their English translation of the main page – the pdf manual is not translated AFAIK).

To simplify the process – I hope – I’ve tried generating a rough first pass translation via Yahoo’s Babelfish site. I’ll work on that as time permits.

Just like any text translated into any language and then translated back again, these translations may be more hilarious than informative, but it’s a start…

The part following this English page re-write is sourced from the copy on the main web page. There is some overlap with the pdf file also translated later on this page. It’s a bit repetitive at the moment but the idea is to distill it all down to something practical and unambiguous.

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lightly edited English translated page:

Lots of hams are not able to install a standard antenna. Some German QRPers designed the KGD – that’s a German nickname for a ‘very short dipole’, that can help these friends because it’s really small but very efficient.

The 40 metre version of the KGD is only about 150cm long. The dipole is made from two 20mm diameter aluminium tubes and two 18mm diameter coils.

The middle of the KGD is made from polyamide. The coils are wound with 0.8 mm enamelled wire. To match the 50 ohm coax impedance, we use a capacitor rated at 6 kV.

The centre part of the KGD antenna with coax connected
The centre part of the KGD antenna with coax connected
KGD antenna centre dimensions
KGD antenna centre dimensions

(This diagram shows the dimensions of the turned centre coli former. It’s intended to hold aluminium tubes of 18mm internal diameter – presumably so their outer edge lines up with the end of the coil space.)

There are short tuning rods at both ends of the KGD. Tuning is very reproducible. We tested ten KGD 40m antennnas and they all tuned up within +/- 5 kHz of the same frequency. Bandwith for VSWR 2 is about 45 kHz at 40m.

One of the tuning stubs
One of the tuning stubs

We used RG58 coax to feed the KGD. Our special mast to antenna holder is designed to enable horizontal or vertical mounting on a fishing pole. If the KGD is mounted vertically, the coax must be attached at a 90 degree angle for at least one metre.

The QRPproject KGD is limited for 100 Watt RF power maximum

We ship KGD kits from stock. The coils are pre-wound and the capacitor installed. You only need to add the tubes and the coax.


WARNING:
Take care that no one can touch the KGD during transmit. Even with five watts there are extremely high voltages at the dipole.

The KGD is not longer available.

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KGD – Short ‘sawn-off’ dipole

Drawing on recently published material a small DL-QRp-AG working group developed an antenna for those with restrictions on what they can erect. We call this antenna

the KGD = (K) urz (G) eratener (D) ipol or Short ‘sawn-off’ dipole

The centre part of the 40m version of the KGD antenna
The centre part of the 40m version of the KGD antenna

The 40m band version of the KGD antenna is exactly 150 cm long (4ft 11in)and 2 cm (3/4in) diameter. The special feature of the KGD is that it is incredibly immune to the environment, and it performs well. Essentially it’s a system with two series resonant circuits and a tube (?) capacitor. With a transmit power of just 3 watts fed to the antenna, a voltage tester 10cm away glows like a Christmas tree. In practical testing there was barely an S-point difference with the G5RV on the other side of the house.

The KGD was originally launched in two forms: a simple homebrew version and a kit. This data here outlines the simple homebrew version and there used to be a kit available with the turned centre section made from Polyamide, with ready-wound coils and the pipes prepared and cut to length. So there used to be a choice to build the KGD completely or buy the kit. The kit is no longer available.

KGD antenna centre dimensions
KGD antenna centre dimensions

The coil form of the QRPproject version was made of Polyamide. The spaces on the left and right were supplied pre-wound with 0.8mm diameter (18 awg /21 swg) copper wire. A fixed capacitor was inserted to match the cable impedance to the feedpoint resistance of 20 ohms. The OMs tried to couple the 2:1 balun directly without a fixed capacitor, and likewise coupling with condenser and using a 1:1 balun was quite successful.

The centre section of the KGD can be made weatherproof by covering it with a heat shrink sleeve. If the KGD is to be installed outside permanently, we recommend enclosing the centre section in a plastic tube.

There are tuning rods at both ends of the dipole. They can be adjusted to balance out any possible environmental effects. On the most critical band – 40m – all the KGD antennas tested so far were able to be tuned in each band segment. The range between the points where SWR was 2:1 was about 45 kHz.

RG58 coax is used to feed the antenna. The mast support (KGD-MH) was arranged so that the dipole could be fastened horizontally or vertically to a mast or a fishing rod, or it could even be screwed to to a roof slat. When mounted vertically the coax cable must be led away from the dipole at 90 degrees for a metre or two.

The KGD from QRPproject can handle up to 100 Watts, but as staunch QRPers we consider this a waste of energy 🙂

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If the resonant frequency is below the respective band, then the antenna is tuned by unwinding a turns. NOTE: Always remove only one turn from each side at the same time and measure again.

Pipe length for each side Turns 0,8mm copper wire per side
80m (no kit!) 160cm (20mm ID) 258 turns close wound on 20mm core, adjustment with coupling coil
40m 70cm (18mm ID) 114 turns close wound on 18.5 mm core adjustment also with tap with 1,5 Wdg or condenser 470pF
30m 50cm (18mm ID) 71 turns close wound to 18.5 mm core adjustment with condenser 330pF
20m 35cm (18mm ID) 47 turns close wound on 18.5 mm core adjustment with condenser 220pF
17m 30cm (18mm ID) 35 turns close wound on 18.5 mm core adjustment with condenser 220pF

Please note: production of the KGD has ceased. The only option available now is to build it yourself.

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This is the copy appearing in the pdf at the ‘Baumappe’ link at http://www.qrpproject.de/Media/pdf/kgdmanual.pdf.

40m KGD antenna installed
40m KGD antenna installed

QRPproject

QRP and homebrew international

KGD

(K)urz (G)eratener (D)ipol

Thank you for choosing the KGD.

The KGD was developed by a small working group of the DL-QRP-AG after eager studies of the literature on short antennas.

Our goal was it to build as efficient an antenna as possible for those with few or no antenna possibilities. While the KGD can naturally be used portably, our prime concern however is with a stationary deployment, whether on a balcony or in your allotment garden.

The efficiency of the KGD is enormous. One compares one Standard dipole in SAME height with the KGD, then falls it heavily to recognize large differences. The directivity of the KGD is exactly like standard of a dipole as to regard lying eight. The side absorption must be considered with comparisons naturally.

At ground level it is better to arrange the KGD vertically. That is because of the fact that horizontal waves at ground level very much strongly to be absorbed. In practice we have with list 2m over the ground 2-3 S-stages difference between horizontal and vertical attachment determined.

With vertical attachment the food cable must at least 1 meter in the 90 degree of angles by the dipole to be led away.

During vertical arrangement 2 or 4 dipoles at one can Food cable to be operated, if about 1 m distance between the dipoles is.

An extension of the emitter pipes worsens the total result. In the available form of the KGD that became Relationship from length to diameter to inductance optimizes in such a way that the KGD, differently than a shortened wire dipole only still the electrical field radiates. The emitter pipe places at the same time a top load. The co-ordination staffs to that Ends of the KGD make smaller or increase the overall capacity.

The FuĂźpunktwiderstand of the KGD amounts to about 20 ohms. To Adjustment 50 OHMS of a Koax cable serves a condenser its was computed worth accordingly. The Koax cable without additional Balun directly to the KGD one attaches. Attempts of users resulted in that one under Circumstances with a 1:1 Balun further improvements erziehlen can. It, the KGD without condenser seems interesting to feed directly me a 2:1 Balun. These attempts are however still pending.

WARNING All versions of the KGD form extremely high tensions on the emitter pipes out. It must absolutely provided for it it becomes that during the transmit mode nobody the KGD to affect knows. We point expressly to it that already with achievements between 1 and 5 Watt substantial danger of injury exists!

You should encounter at any place problems or Improvement suggestions have, then use you Peter, DL2FI, it is pleased you to help at any time to be able. You reach QRPeter best by email under the address: support@qrpproject.de or by telephone under ++49 (30) 85961323

The centre part of the KGD antenna with coax connected
The centre part of the KGD antenna with coax connected

Structure of the KGD

The coil for the QRPproject KGD is finished wound, on one Enclosing note is the current is number of turns for everyone Side notes. For the beginning it is advisable, redundant To complete turns up to the target number.

All parts are properly matching bored, turned or milled. It become, as evident on the design, which installs solder taps and the wire ends after careful distance of the Lacquer thereby solders. The condenser provided becomes in such a way in the solder taps with soldered that it bridges the two dipole halves. The final assembly a RG58 becomes cable of the thin Drilling ago by the center section led. Soul and coat are stripped the wire and soldered and bolted with the solder taps as is to be seen in the design. The borehole also the cable end should after soldering with acetic acid-free Silicone rubber to be filled up, around the cable end water resistantly to make. To the other end of the cable a coaxial connector comes after own preference. The length of the food cable is arbitrary.

Next the tubing emitters become on the two sides the center piece pushed. The respective end of reel becomes with the attached Solder tap on the emitter pipe screwed. Absolutely ask those VA provided Screws use. With unsuitable Screw material the danger exists of corrosion through Local cells. Into those Ends of the emitter pipes become aluminum provided Co-ordination staffs pushed in.

One of the tuning stubs
One of the tuning stubs

The KGD is now ready for a first test. If our fixing clip were along-ordered, this is attached at the center piece and the KGD can be put on direct on a Spieth mast or a sturdy fishing rod. With a Antenna analyzer or with the transmitter and one VSWR measuring instrument can be examined now, where the working frequency the KGD lies. After our experience it would have now already in the AFU are appropriate for volume. Through shift the two The resonant frequency knows co-ordination staffs (both always equivalent far) to the desired frequency to be adjusted.

The resonant frequency should lie underneath the volume, a turn on each side of the dipole must completed become. CAUTION, really only one turn complete and then again measure.

For short-time duty in practice it showed up that except the sealing of the food cable no further isolation advisable is. Paint the wire or similar measures shift the resonant frequency frequently very strongly downward and reduce the quality. It is better, with short term one To work enterprise without each further isolation.

If the KGD is to be installed permanently outside, then to recommend we to make it weatherproof by the fact that one at least over the center section a plastic pipe pushes, which approximately the dipole pipes one isolates.

Much success with the KGD and much QSOs

72/73 de DL2FI

© QRPproject Saarstr. 13 12161 Berlin http://www.QRPproject.de Tel: +49(30) 859 61 323 e-mail: support@QRPproject.de