Radio Direction Finding Antenna for VHF

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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.
Joe's Design

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
73..FN

2 People Made This Project!

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36 Discussions

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SHOE0007

1 year ago

Just got this type of Radio which could pick up many stations. How high should the antenna be? 10-15 feet??

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Kevin_F

2 years ago

Great project! Can you supply the measurements in METRIC?

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Musicman41

2 years ago

Question: Why use a tape measure instead of some metal rod? Do they behave differently?

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FN64Musicman41

Reply 2 years ago

Yes.. In fox hunting there are many occasions where you will be going through trees, brush & heavy undergrowth where a rigid antenna element would either be a hindrance or it would break.
The flexibility of the tape will just wrap around you as you go and spring back into shape after you're through a thicket.
Hope this explains it & thanks for the question.
..FN..

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hobbybuilder

3 years ago on Introduction

I have made these before in the past, using a tape measure and a wooden yard stick. The work well for Fox hunts, too.

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FN64

4 years ago on Introduction

@shhammer... Sorry I didn't clarify that in the 'ible.
It will work well for ya but it takes a bit of tinkering to get it together.
Thanx for the comment.

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shhammer5634

4 years ago on Introduction

Appreciate yours and Joe Leggio's work. I've built it but not tested it yet. One point of note. Where talking about "you might need to sand down the stop lip inside the cross", I think you might want to change that to "you WILL need to ..." I spent the better part of an hour trying to force the keepers in without sanding down the lip. Didn't work. Ended up both sanding down the lip and then tapering the keepers some to be able to get them started. Thank goodness for Dremel tools. :) Keep up the great work.
73 ... KD0WSW

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scinutz

5 years ago on Introduction

Question -- If you use EZNEC's Antenna Program to design the spacing for the Yagi Antenna, What would you use for the diameter of the wire.

Basically, I want to design the Measuring Tape Yagi Antenna to be sensitive in the MURS Frequency Range. I want a good Front to Back dB ratio and overall gain for the Antenna. My class has a Rabbit Hunt (*A Fox Hunt but in Clean Room Rabbit Outfits*) and the teacher uses the MURS frequency 151MHz to 154.4MHz. Well, Thanks for any reply.

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FN64

5 years ago on Introduction

@KF7UWU..
I've had great luck with 5-10 watts.. Above that it's hit or miss if I get an error message on my rig. I went to 25 with a Yaesu 1500 & was OK but when I upped it to 50 the rig got all weird on me.. I was within about 10 ft of the antenna at the time so I'd say it was either "near field" RF or common mode currents on the feedline that the small choke coil of coax couldn't stop.
Thanx for checking my 'ible out & 73..

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Jugfet

5 years ago on Introduction

A realy natty bit of junk box tech. I found all the bits under the bench and dug out an old tape i chucked in the scrap bin a couple of days ago.
I squirted 5W of RF into it and opened the local repeater too.
good instructable, best 73's from G7WJJ

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M0HIZ

6 years ago on Introduction

Great! I've been looking for a design for a tape measure Yagi on the Net for a while now, and you have provided the perfect article. I'm going to use mine for QRP (5W) bicycle mobile.

Thanks for the great 'ible!

(I didn't intentionally copy your account pic btw)

73

WE

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FN64M0HIZ

Reply 6 years ago on Introduction

No prob on the avatar... it belongs to the ARRL but is available for hams to use.. Glad you got some ideas.. feel free to make your own improvements as you see fit but do kindly give credit to Joe Leggio who came up with the original concept..
Best 73.. & I'm good on qrz
_. .---- -- .-.. ..-.

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KJ4ZVQ

6 years ago on Introduction

Iv built one of these before when I 1st got started. I'm thinking heavily about adding it to my EmComm box that was created after April 27. Thanks for posting and 73s on future Ibles

Adam
KJ4ZVQ

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davidcw73

8 years ago on Introduction

What are the formulas for the element lengths and the boom distances?  I would like to build one for DFing  ELTs on 121.5 mhz.  Thanks.

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FN64davidcw73

Reply 7 years ago on Introduction

Whereas I started with someone else's design and made my own few improvements on it I have no idea what the original formula might be.
If there's a ham in your are with an antenna analyzer then some experimentation will set you on course.. The short answer is... as the frequency decreases, the element length increases. Make one with ALL elements a couple inches longer then trim to the resonant frequency.
73 & good luck.
Fn

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Cameron657

8 years ago on Introduction

Although I'm not a ham (yet) I like this design, and because it is in the 2-Meter band will it work well for FM? I want to make a portable, strong and cheap FM antenna, and this is just the type I'd need. I use 300-ohm cable for FM and to use this I'd say I would have to remove the 'ohm symbol loop'. So, will it work well for FM?

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FN64Cameron657

Reply 7 years ago on Introduction

It was designed by Joe Leggio for the 2 meter band.. (see link to Joe's page in my article).. Frequency is frequency so the mode should make no matter.. AM, SSB, FM or CW The resonant freq will be the same.
For 2 meter work I'd recommend staying with the 50 ohm coax. Most 2 meter gear is made to a 50 ohm standard.
Strength and rigidity are NOT the strong points... Flexibility & ease of use apply far better.

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mathman47

8 years ago on Introduction

Great idea.  Do you have trouble with the tape rolling or collapsing in the wind?  KC9PYD