The 4x5-El.-12,5-Ohm-Yagis

with a Spider-Beam mast

at

TM7T

 1. Place in the QRP-Contest

(Practical Wireless)

3x5-Element-12,5-Ohm-Yagis stacked by

F/OT3T

in the Practical Wireless Contest 2009

With these Yagis you get the highest possible gain for the given boomlength. 

Vertical stacked arrays have the best effenciency for contest-operation!

Left: The 4x4-El.-Array with the 1,5 m long 12,5-Ohm-Yagis

Type/Boom

For Yagis with a boom of >=1,5 lambda you should prefer 28-Ohm-Yagis, they have only 0,05dB less than the 12,5-Ohm-types with the same length, but have more bandwith and are less critical.

These Yagis were constructed as ultra-light, portable Yagis with extrem high gain. They have small bandwidth and are working from 144,0-144,8MHz with good SWR.

For portable use it is an advantage, to have a small vertical radiation angle and a brought horizontal angle. Therefore it is highly recommended to stack these Yagis as groups. For stacking details of 2 or 4 antennas see the page with the 7-Element-Yagis.

Of cause these yagis will give you a good performance for a base station, too!

This is a solution of

Theo, PA1TK

He is using 25-Ohm-

PTFE-cable KTR-141

Here is the data-sheet of this coax-cable:

LINK KTR-141

The radiation resistance is 12.5 Ohm, to avoid losses you must work  carefully and look for the construction details. The match is made with 2xLambda/4-50-Ohm-cables in parallel. The 12,5-Ohm radiation resistance is transformed to the 50 Ohm feedpoint with the 12,5-Ohm-DK7ZB-match.

The construction was made with "YO" and then fine-tuned with "EZNEC". This method is working exactly, you only have to look for the radiator length, no other correction is to be made.

The parasitic elementes are mounted with the insulated clamps and a 3mm-screw, the radiator consists of 10x1mm-Aluminium tubes. No other fixing is allowed because this will cause severe detuning of the yagis! If you use homemade insulated clamps, the elements must be mounted 3-4mm above the boom. The boom is made of 15x15mm square aluminium. The connection screw/boom (electrical "zero") has no influence to the length of the elements, but any contact by mounting not insulated above or through the boom!

After building the antennas you should measure the SWR, normally you must cut the tips of each side of the radiator about 1-2mm for SWR=1.0 at 144.3MHz.

The given length of the radiator element is from tip to tip, including 10mm spacing for the insulated part.

With Aircell-7 you can run 1KW to a single Yagi. Use a good quality coax (not RG-58!). The length is 42cm (braid!) with H-155 (V=0,82), 44cm with Aircell-7 (V=0,85), with full PE-coax V=0,67 (34,5cm), with PTFE V=0,71 (37cm).

The given length of the radiator element is from tip to tip, including 10mm spacing for the insulated part.

This is the middle part of the radiator in the 2-, 3,- and 4-Element-Yagis.

The matching cable here is PTFE-coax (RG-188) and fits completely in the box (L=37,0cm). The insulator between the two element-halves is made by the rods of a broken tent of my children.....

The radiator box with PTFE-cable by

The mounting clamp for the boom:

Made with an automotive exhaust clamp, cheap and simple.

Another method with homemade clamps (cheap and easy) by Adam, 9A4QV. See further Details on his homepage: