Intro: Radio Direction Finding Antenna for VHF
I needed an antenna to chase down a noise source.. After much ado I settled on some plans I found on the web, added my own modifications & took some photos along the way. It has a cardoidal pattern with a deep null on the rear. I made a few minor changes to the original design but these were mostly for structural integrity, looks & ease of assembly.
This antenna will DF noise or signals in the area of the 2 meter amateur radio band..(144 - 148 MHz). I have tried it as high as 155MHz with good results. It is flexible so it won't break or bend as will most commercial directional antennas. It also tunes well on the 2 meter band.
You will need the following:
# A tape measure with a 1 inch wide steel tape. (Or a replacement tape)
# 3 PVC crosses for 1/2 inch pipe.
# ~3 ft section of 1/2 inch PVC pipe.
# 2 stainless hose clamps for 1 1/4 dia hose.
# Electrical tape.
# Soldering iron & associated tools.
# 6 feet or more of 50 Ohm coax & connector (BNC, PL-259, SMA...)
# Scissors or small shears to cut steel tape.
# Hack saw or tubing cutter to cut pipe.
# A Dremel tool is handy but sandpaper will do.
# You will need a receiver that has an "S-meter" to locate the direction of the signal.
Step 1: Cutting the Tape Measure
Remove the tape from the tape measure and use the tail end for your elements.. This part sees much less wear & tear than the first 10 ft or so.
Measure your elements to the following dimensions.
# Reflector: 41 3/8 inches
# Driven element: 35 1/2 total.. cut in half for 2 @ 17 3/4 inches
# Director: 35 1/8 inches
These can be cut with regular scissors.. BE CAREFUL.. The ends will be quite sharp. I cut all ends with 45 degree angles. Some folks have taped or dipped the ends in Plasti-Dip but I just sanded mine a bit to take the sharp corners off.
Step 2: Assembling the Reflector & Director..
Mark the center of these elements with a sharpie & slide them through a cross till they're centered. You might need to sand the "stop lip" inside the cross a bit to make this easier.
Cut 4 pieces of PVC pipe about a half inch long. These "keepers" will hold the tape centered in the reflector & director crosses. Sand off any burrs.
I used a "C-clamp" to squeeze the little "keepers" into the crosses but a vice would probably work as well or better.. They can also be tapped in with a hammer if you're careful & use a block of wood or dowel as a driving pin. Make sure your tape stays centered.
Step 3: The Driven Element
The feedline is connected to the driven elements directly through a "hairpin match" This was the original concept and it works well so I followed suit.
Cut a piece of solid wire (14 gauge or smaller) 5 inches long. Form this into a shape resembling a tall "ohms symbol". Strip 1 1/2 inches of cover off the coax & separate the core. Solder the coax to the hairpin as shown in picture.. Trim the excess coax wire close to the solder. The ends of the hairpin need contact with the elements but soldering is extremely difficult on the steel tape. I crimped & soldered some small connector lugs to the ends of the hairpin and these are contacting the steel under the hose clamps. The tape measure sections need to be sanded to remove any coating at the contact area.
Step 4: Setting the Element Spacing
Cut a piece of pipe 11 inches and another at 7 inches. Sand any burrs from the ends. These will set the spacing nicely.. I didn't "glue" anything.. they're held together by a tight fit.
The 7 inch section goes between the reflector and driven elements.
The 11 inch section goes between the driven elements and the director.
Step 5: Tweaking, Tuning & Using
Not only will this antenna function well as an RDF unit, it will also tune well for transmit on 2 meters.
The SWR is adjusted by tweaking the distance between the driven elements. Mine is set at 1 inch for a 1.3:1 match.
The radio I use is an old Realistic HTX-202.. I was getting some RF on the feedline causing the rig show an error message. I wound 7 turns of the coax around the pipe that connects to the reflector & secured it with wire ties and electrical tape.. That cleared the problem.. Depending on your use, you may or may not need this step.
Using the antenna for RDF is easy.. USE the BACKSIDE to find the NULL. (second photo). The front lobe has quite a wide beam width but the null on the cardoidal pattern tells you much better especially when you get closer to the source. Watch your S-Meter as you swing the beam from side to side. The weakest signal strength indicates the direction.
If you bought a 10 foot section of pipe, the remainder can be used as a mast of sorts. Just insert the end of it into the cross that holds the driven elements. With 6-8 feet of coax this allows plenty to reach the rig.
This antenna folds up nicely to fit behind a car seat or for storage.. (third photo)
Step 6: Final Thoughts.
This wouldn't be complete without credit where it's due.
Joe Leggio - WB2HOL started this whole concept and I encourage you to check out his methods and explanations as well as his other RDF projects.
So there you have it. I don't have a cost rundown but 10 bux is a reasonable guess.. Some of the parts were junkbox items. Hope you enjoyed my first go at Instructables