viernes, 10 de julio de 2020

"Blandiblu" 2m portable yagi antenna

Blandiblú ver 2 at Sollube, EA2/BI-033
 
The goal of the project was to make a light and portable antenna for the 144-146 MHz band.  The design of reference was the pretty IZ2UUF 100 gr. yagi, but I chose to make the elements of brass, to be able to solder them later. I don't like aluminium. Although this material can be lighter than brass, it's harder to work with. Finally, the antenna will be a 50 ohm 4 elements vertical yagi on a non-conductive boom.

So, here we go. I found some sort of 25 mm aluminium wire, teoretically covered with a thin copper layer, that later shown not to be able to be soldered at all. After the first try, the elements were so soft that they bent too easily. This was not the most appropiate material at all, and thus, the name of "blandiblú" comes from here (you can search its meaning on the web to find why).

Blandiblú ver. 1 with aluminium "proactive" elements

The boom, from the very beginning, was a 1 meter piece of a broken fishing rod, formerly used as SOTA mast. I have 3 of them, so more designs are to come for sure.
The elements, finally, were 4 x 1m length, 4mm outer diameter, 0.25mm thickness brass rods, which are available at local bricolage stores. They make a good compromise between weight and strength. 

The lengths and positions of the elements were brought from IZ2UUF website and are (all measures in mm):

ElementLengthPosition
reflector10100
driven995460
1st director900660
2nd director840990

The most difficult parts of the antenna were two: the boom_to_mast adaptor and the feeding_point of the driven element. They were made with a 3D printer (this is the bad side of the design, as not everybody has easy access to one). The desing was made using Wings3D free software. Here they are:


boom to mast adapter

driven element feeder (1)

driven element feeder (2)

The 4 mm wide brass rod were drilled to allow 2 x 2mm diam x 10mm length screws, which will later connect the center conductor and the braid of the coaxial cable to each side of the driven element. I used a total of 6 meters of RG-174 from antena to rig. 

Driven element feeder

A short length of  the coaxial (+- 1m) is used to make a choke. This is made of several turns of the coaxial with a length of about 34,5 cm. The number of turns may vary depending of the diameter of the boom, just keep that length. Tape it later to the boom and drive the rest of the coaxial back until it falls behind the reflector. Keep it perpedicular to the driven element. A longer length of the same coaxial is then connected (4-5m) to the talkie or transceiver using SMA type connectors.

The elements "hang" on the boom, so they are not vertically aligned with it. The size of the feeder will give you the amount of deviation. Being of brass, I soldered small pieces of copper wire to set them in their positions. Be sure to mark in some way which side of the rod must go up. 


boom to mast and elements set details


Boom to mast, feeder and elements
 

The holes must be carefully made to mantain the same orientation along the boom. IZ2UUF gives a well explained procedure about how to make this right.

The antenna was first tested on tuesday from Sollube, SOTA EA2/BI-033, and the results were apparently good, making several long distance QSOs. Later this week I will try to use it from other "rf noisy summits" around my home.

Finally, this and other alternative designs were tested using free antenna design software MMANA-GAL. The results obtained are more or less coherent with the actual developement.

I hope all this would be useful to SOTA VHF adicts. See you on the air!

73, Mikel EA2CW

01-08-2020 UPDATE 
Blandiblú version 3, 6 el. 2 m boom

Blandiblú ver 3 at Ganguren EA2/BI-055

After spending some time reading documentation about portable vhf yagis, I finally started the 6 elements version of the Blandiblú (version 3). Based on the invaluable information given by DK7ZB I chose the 2 meters boom 6 elements 50 ohm yagi (see links below).

In this case, the boom is composed by 2 pieces of a fishing pole that fit together , with a passing thru screw to mantain the elements distances. 

The elements themselves were made again of 3mm diam brass tube. Probably in future developements, only the driven element will be brass, and the other 5 of aluminium, to downsize even more the final weight of the antenna

I will also modify the boom to mast adapter to avoid making more holes on the boom, and allowing a fast change between vertical and horizontal polarization as well. 


The sizes of the elements were calculated and adapted from information of DK7ZB's website for this particular diameter of the elements. 

This last version has been already tested successfully on SOTA activations of Jata (EA2/BI-053) and Rubrillu (EA1/CT-071).  From this late one, I was able to make QSO with 2 french operators using only a 5w talkie and vertical polarization, while they were using horizontal polarization and 100 and 50 w respectively. The distance reached was about 485 Km.

At Rubrillu EA1/CT-071 not very well adjusted

Shack at Rubrillu Mt. 5 w for 485 Km QSOs


References:

IZ2UUF 100 g portable yagi
A lot of information on yagi design by DK7ZB (with alternative 4 el. short 50 ohm yagi design)
On-line yagi calculator
VK5DJ yagi design software
Wings 3D easy and free software
MMANA-GAL free antenna design software

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