My Arrow-Style VHF/UHF Portable Satellite Antenna
By: Bertrand Zauhar, VE2ZAZ


One of the pleasure of working the amateur radio satellites is making contacts in a portable application and with QRP power. This requires a VHF/UHF handheld FM radio and a handheld 2-band antenna.

After browsing the various Internet antenna construction sites in search for the best portable satellite antenna for the V/U and U/V modes, I figured I could build something good. So I decided to combine the best ideas of a few existing designs and add my own twist to it. This gave my own "Arrow-Style" VHF/UHF antenna. My sources of inspiration were:

I recommend that you consult the links above before proceeding with the reading of this web page, as I make implicit references to those designs.

FEATURES

So after reading and thinking, I came up with this improved "Arrow-Style" VHF/UHF antenna. The resulting antenna has the following characteristics:
  • I use a 3/4-inch CPVC plastic pipe as the boom, as opposed to wood,
  • I use 1/8-inch brass rods for the elements; they are commonly available in hardware stores and are stiffer than aluminium rods or copper wiring of the same diameter,
  • I feed the driven elements with the "semi-folded dipole" approach, which simplify mechanical construction and impedance matching,
  • I fit a diplexer inside the boom,
  • I use RG-174 coaxial cable,
  • I conceal the coaxial cable inside the boom,
  • I make the VHF elements easily disassembled, which makes the antenna much smaller for travelling or storage purposes.
The antenna performance should match or surpass the Arrow antenna. The UHF section is made of 6 elements with gain-optimized spacing as opposed to 7 elements with equal spacing for the Arrow. This makes my antenna shorter by 6 inches. The antenna is lightweight, only 19 ounces (550g).

Have you looked at the price of the original Arrow antenna? How about working on this evening project and spending only 1/3 the cost?

CONSTRUCTION

Element Lengths and Positioning
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 of the right diameter to fold the Driven elements.




Element:
Refl.
Driv.
Dir.1
Dir.2
Dir.3
Dir.4
VHF:
Longueur/Length:
40.50
38.00
36.50



Espacement/Spacing:
0.00
8.50
20.00



Folded Section Gap:

1.00




UHF:
Longueur/Length:
13.40
13.00
12.40
12.00
12.00
11.00
Espacement/Spacing:
0.00
2.50
5.50
11.25
17.50
24.00
Folded Section Gap:
0.50




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

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 Driven Element Fastening
Since the boom has only a 7/8-inch outside diameter, I added another short piece of pipe to support the folded portion of the VHF Driven element. I used 2 self-tapping screws to fasted the piece of pipe to the boom. (see picture below).

UHF Driven Element Fastening
The UHF Driven element has a gap of
only 1/2-inch, so bending the feedpoint end of the folded section to create a 1/4 inch gap is manageable and allows to support the element without adding another short piece of pipe like for the VHF Driven element.

Element lengthening
If, like me, the only length of brass rod available to you is 36 inches, you will need to lengthen each of the VHF elements by splicing two brass rods together. A good way to do this is by filing off half of the rod thickness for about 1/2 of an inch long at each end with a grinder. Then, position the two ends one against the other and solder the two rods together using a gas torch or a powerful soldering gun. The end result is illustrated below.

VHF Feedpoint Connection
The feedpoint 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 travelling, which will be the case for most situations. I use DIP integrated circuit socket pins for the mating. You need a DIP socket of the machined type, the kind that has barrel contacts. Using a sharp knife, cut the plastic to detatch 4 pins from their socket. Solder two pins with their barrel end facing the coaxial cable. 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 be used.



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.


 
Diplexer
Since most handheld dual-band radios have a single antenna connector (VHF and UHF signals are combined), you will need an antenna diplexer. A commercial diplexer will work fine, but is quite heavy. Remember that you will have to point this antenna by hand for 15-minute satellite passes. Because of this, I elected to build the Microduplexer. I have the capability to make my own PCBs, so I tried to obtain the original layout from the author. It appears that the final layout in a ready-to-make form is no longer available. So I re-imported the Autocad DXF file, re-worked the proposed layout, filled the islands, etc. The final Microduplexer board layout is available below. I provide a Gerber file of the top copper layout. This is pretty much all you need to have this PCB made by a PCB shop. I also provide a PDF file for home-making the PCB. Printing the file with 100% scaling will give accurate results. Note that in order to fit the Microduplexer PCB inside the CPVC boom, I had to file off a bit of both long edges of the PCB. I did this before installing the components.


Coaxial Cable Runs
I run the coaxial cables inside the boom. I drill holes at an angle, close to the feedpoints, where I bring out the coax (see pictures). The Microduplexer is located inside the boom, within the handle portion. this is where the three coaxial cables converge. I bring out the transceiver feedline at the back of the handle.



Other Considerations
  • I installed plastic caps on the element tips to save my eyes and my furniture...
  • I Installed 7/8-inch vinyl endcaps on the boom. Looks neat... I punctured a hole through the rear endcap to bring out the transceiver feedline.
  • I do not fasten the elements to the boom in any way. The elements fit quite tightly when drilling the boom with a 1/8-inch drill bit. Some hot melt glue, choc nut, set screw or clips could be used if you are worried about exact positioning...
RESULTS

During operation, the results have been quite gratifying so far. The VSWR is at less than 1.5:1 on both bands. I have no problem working AO-51 pretty much from horizon to horizon. Of course, some "big guns" will swamp my signal every now and then, but that is not due to the antenna, but rather to the 2 Watts of RF output I use on the uplink.

The cool thing I did this summer was to bring my portable setup to our camping trip...So much fun when wife and kids are gone on a bike ride!

PARTS LIST

  • 1        3/4" inside diameter CPVC water pile, 36" long.
  • 7        1/8" brass rod, 3' long
  • 2        Vinyl cap, 7/8" diameter.
  • 6'        RG-174 type coaxial cable, BNC connector at one end.
  • 1        Microduplexer, assembled
  • 4        DIP integrated circuit socket pin
  • 2        Self-Tapping screw, small size