23 April 2007

Piratenfrühling in Deutschland: 48m-Dipol

Von bermuda
Posted by at

5 comments:

Anonymous said...

This is an antenna with coax fed and center (or off-center?) balun. Not the best. Feeding with open-wire line has many advantages. See also "double-zepp" or "Levy" antennas, or large loops, and transmatch units.

Ray

10:14 PM
Anonymous said...

It's not very high is it ?

AW

11:12 AM
Radio Bermudadreieck said...

Hi Ray. Ok, will have a look of others designs of antennas.But after years involved in the F.R.scene it seems to be one of the best and easy to handle antennas.And if you have changing locations, like BERMUDADREIECK RADIO its nice to use this one. But thanks for writings in the comment on the site. DJ Dipol

And second short answer to Mr. AW! Of course its no very high. The cord for the wire on the tree behind is not fixed finally. Normally its between 6 and 8 mtrs high. C.I. would hang the antenna between 2 and 3 mtrs high.So I think I do good work.Hi.
Greetings to you, DJ Dipol

4:00 PM
Anonymous said...

Please, look at the sketch with fixed spacing :

short dipole
____________ ____________
I I
I-I
I I
I-I ladder
I I line
I-I
I I
I-I
I I
(T) trans.
I
I coax

For light portable operation on 48m, I would use two 12m legs, each leg partly in the dipole, partly in the open (ladder) line. Maybe a 12m long dipole and a 6m high ladder line, but the total lenght of each leg is 12m. Then, (T) is a transformer wound on a suitable ferrite core, the best being big toroid. (Try a toroid discarded from inside a junked CRT monitor around a cable, but not that one from the high voltage transformer).

With 12m in each leg, the antenna (including ladder line) is tuned. But it should be matched also, as the impedance is somewhat low and should be raised by the transformer. For operation on almost any wavelength, a longer dipole is preferable and exact multiples of a half wavelength *in each leg* should always be avoided. Then, a transmatch unit is used to tune and match the antenna, but more (costly) components are needed. Look for famous paper "A *balanced* balanced antenna tuner" on the web, look for Johnson Matchbox. MFJ has a new balanced unit designed for power up to 100W approx. Beware, a balun on the antenna side of a classical ATU is not good if the antenna+line system is too much detuned.

For very high efficiency, each leg should be 36m long, maybe with 12m horizontal (in the dipole) and 24m in the ladder line. So that would make a half wave dipole hung a half wave above ground : it's not easy to find trees so high and then throw wires above them! The 'extended double Zepp' is the same antenna with more wire in the horizontal dipole part (and maybe less in the open line). An other way is to change the dipole part in a closed large loop, then the good and bad lenghts for the whole wire (loop and both sides of the ladder line) are exactly the other way from the dipole kind : that makes sometimes more manageable lengths, but setting a portable loop is not so easy. Among many ways to put it, I would try a 50m wire doing a triangular loop in a near vertical plane, with the apex about 16 m above ground and the low side only 2 or 3 m above ground, with a short feed line in the center of the low side. Only one high tree is needed, I would try to throw a guide wire above it with a sling shot, and then raise the antenna wire.

More on the advantages : the open line has almost no losses, even with very high SWR on it (the SWR meter should on the short coax between radio and transmatch). With a transmatch unit, almost any frequency can be used with a single length antenna. The open line doesn't get warm, and has no problem with rain. The open line is lighter than coax, and the center insulator of the dipole is also lightweight. So the center weight on the antenna is much less than with a balun and coax line. Most of the power really goes to the antenna instead of heating the coax. But components in the transmatch should be well sized, especially the variable capacitors should stand high voltage. If all is well balanced, the open line doesn't radiate (not that easy with a coax). Last word: don't mind about the open line impedance, think only about total wire length and how standing waves are settling on it. Any wire spacing is good, provided that the space between (insulated) wires is almost only air (with no water staying there of course).

Ray

11:12 PM
Anonymous said...

Sorry for the sketch, the blank spaces were discarded somewhere on the way. Just think the ladder line is under the middle of the dipole, nothing more complicated.

Ray

11:16 PM

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