Arrow Antenna for LEO

After having tasted the communications through radios on VHF and HF HAM bands. I started exploring constructing more antennas and the QUAD to know the difference between their performances and what we could do with lots of left out material, generally seeming useless.

It was an evening before the diwali, around September-October 2009, when i was visiting my friend and my mentor on antennas VU2AF, to check the SWR and performance of a four element QUAD for UHF band (70cm). It was a pleasant surprise having an eye ball with another veteran HAM from Bangalore, VU2RMS; he prompted and motivated me to now move to satellite communications. That set the idea rolling, all what was needed was a dual band radio, which was already in my possession and doing great, and an antenna suitable antenna for LEO (Low Earth Orbiting) satellites and an . After going through couple of sittings on internet, it was arrow antenna, which i decided to home brew. A lot of design and construction details were available, it took me some time to assemble the material.


Features of the cross yagi (arrow)

• A 3/4-inch CPVC plastic pipe as the boom, as opposed to wood,
• 5mm aluminium rods for the elements and are very light weight.
• Feed the driven elements with the "semi-folded dipole" approach, which simplify mechanical construction and impedance matching,
• A home brewed micro duplexer,
• Use of RG-58 coaxial cable,
• Conceal the coaxial cable inside the boom,
• VHF elements are easily disassembled, which makes the antenna much smaller for travelling or storage purposes.

The UHF section is made of 6 elements with gain-optimized spacing elements with equal spacing for the Arrow. And VHF section is made of 3 elements. The antenna is lightweight and approx 300 grams
Construction Details

The suggested boom length is 30 inches. Since the UHF antenna is the longest of the the two, the 4th Director (Dir.4) should start close to the tip of the boom. By measuring 24 inches back from that position, it will give you the position for the Reflector (Refl.) element. This leaves roughly 6 inches for the handle.

As for the VHF antenna, its Reflector element can be positioned forward of the UHF Reflector by 3/4-inch. The table below summarizes the element dimensions and spacing for frequencies of 146.000MHz and 435.000MHz. I suggest you cut the two driven elements a bit longer and then tweak the SWR by trimming the element tip. Use a hard cylindrical object (i used the 1/2 inch grill of my staircase of the right diameter to fold the Driven elements).

Note:

	Element: Refl. Driv. Dir.1 Dir.2 Dir.3 Dir.4
VHF:	Length: 40.50 38.00 36.50 Spacing: 0.00 8.50 20.00 Folded Section Gap: 1.00
UHF:	Length: 13.40 13.00 12.40 12.00 12.00 11.00 Spacing: 0.00 2.50 5.50 11.25 17.50 24.00 Folded Section Gap: 0.50

All dimensions shown in inches. Multiply values by 2.54 to convert to centimeters.

Another important aspect while planning for the material length for procurement (here i had committed an error initially), is the length of Aluminium rods (5 or 6mm dia) while procuring from the shops is given below. Make sure your length of rods for the driven elements are as given

Unit pieces of Aluminium rods-------------- Utilize this length for
(In feet)

5 ----------------------------------------------- Driven VHF
5 ----------------------------------------------- Ref VHF + Dir 1 UHF
4 ----------------------------------------------- Dir VHF
3 ----------------------------------------------- Driven UHF + Ref UHF
3 ----------------------------------------------- Dir2UHF+Dir3UHF+Dir4 UHF


Length of aluminum rod to be cut for Driven element VHF = 4.81 ft (ref. fig. above)
Length of aluminum rod to be cut for Driven element UHF = 1.63 ft (ref. fig. above)


Boom Drilling Technique Care must be used when drilling element holes on a circular boom. I found it almost impossible to drill element holes that would yield elements that are parallel and in the same plane, without using some form of guidance and reference. My trick for getting parallel elements is to first screw down the boom at both ends against a narrow wooden plank or board, and to use a vertical drill press to make the holes. VHF feed point Connection
The feed point connection to the VHF Driven element is done in such a way that it can be disconnected if the antenna is to be disassembled for traveling, which will be the case for most situations. I have used the copper tubes to solder the coax elements to the feed points. Solder two pins to the coaxial cable with the pin ends facing the driven element. This is a temporary but effective connection.Connect the coaxial center conductor to the folded section end and the shield to the long section.See the picture below. Other means of connecting can also be used, including drilling the aluminium elements and using the nut bolts and sleeve to solder the coax elements.

UHF Feedpoint Connection

Since I don't intend to disassemble the UHF section, the feedpoint connection for the UHF Driven element is more straightforward. I simply split the coaxial center conductor and shield. Again, I solder the coaxial center conductor to the folded section end and the shield to the long section.

Parts List

-------1- 3/4" inside diameter CPVC water pile, 4 ft long.
-------5mm aluminium rod total 20 ft long (you can get it cut into lengths as described above for the ease of carrying the stuff from shop to your home)
-------2 Vinyl cap, 7/8" diameter.
-------6' RG-58 type coaxial cable, BNC connector at one end.
-------1 Microduplexer, assembled
-------2 Self-Tapping screws, small size


Some other photographs of the antenna






Microduplexer Photographs






Shall be uploading the construction details of the home brewed micro duplexer in the next post.

My sincere thanks to the following HAMs for their support in the entire process of home brewing the antenna for testing.

VU2AF
VU2RMS
VU2 SGW
VU2 WSM
VU2 IVV
VE2ZAZ
WA5VJB