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Radio Direction Finding Antenna for VHF

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Picture of 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.
 
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Step 1: Cutting the tape measure

Picture of 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.

nice concept for light weight antenna for transport thanks de VU2PLL

FN64 (author) 5 months ago
@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.
shhammer56345 months ago
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
scinutz1 year ago
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.
FN64 (author) 1 year ago
@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..
Great Instructable! How much power can this handle?

KF7UWU
Jugfet1 year ago
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
M0HIZ2 years ago
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
FN64 (author)  M0HIZ2 years ago
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
_. .---- -- .-.. ..-.
KJ4ZVQ2 years ago
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
davidcw734 years ago
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.
FN64 (author)  davidcw733 years ago
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
Cameron6573 years ago
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?
FN64 (author)  Cameron6573 years ago
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.
mathman474 years ago
Great idea.  Do you have trouble with the tape rolling or collapsing in the wind?  KC9PYD
FN64 (author)  mathman473 years ago
I gotta say Yes and No.. If the convex side faces the wind then yes however the concave side is more wind resistant. Nevertheless, not hurricane proof. LOL
That "flexibility" was one of the things I liked about the idea. If you're fox hunting and need to run thru the brush then hold it in front of you with the convex side forward & it folds around you. Try that with an Arrow Antenna!!!. Similar specs with a 50 dollar price tag!!! One day of fox hunting & you have some scrap aluminum.
73..& sorry bout delay in response..
FN
ignition5 years ago
great instructable, stumbled across it before i was a ham, and was recently looking for a collapsible Yagi, remembered this, and i'll be building myself, and probably another buddy one for emcomm purposes. 73 from KC9OCD
 Same here! (sorry for the 2 year comment latency). I recently passed my foundation exam in the UK and I am just waiting for the RSGB and Ofcom to hurry up processing the paperwork.
welder guy4 years ago
this will work for a 2meter 2watt ht right? cool idea and i think i'll build it.
73's, KC2VDM
FN64 (author)  welder guy4 years ago
It will work fine for tx as well as rx. I wouldn't recommend anything over 10 watts unless you get enough feed line to be away from the near field rf. I don't know how much power it will handle before a meltdown so you're on your own on that one.
Good Luck & 73
N1MLF ..fn64.. Jon
KI6WTX4 years ago
can it be used as a transmitter and well, for 5 watts (what my hand held can put out) it sure would be useful when im just out side of a repeater....
FN64 (author)  KI6WTX4 years ago
Sorry for the late reply OM.. been out of the country.. YES.. it works well. thats the reason for the hairpin match and the choke coil to keep rf off the braid.. I've run 25 watts without problems but when I tried 50 it caused errors in the rig due to near field RF. Should be a great boost for a 5 watt HT.. Thanks for the comment N1MLF...Jon
KI6WTX FN644 years ago
ok, do you think that it would work on 50watts?
Misterfixit4 years ago
KI4PSR, Dave, with the Williamson County ARES group made up a pile of these kits and we assembled and used them for our EmComm Fox Hunts. They work great as a transmitting antenna. I am preparing to build one as a test bench to see just how many watts it will dissipate before melting down. All Hail KI4PSR! 73's Dave N4CVX
ha i used a tape measure to make a di-pole bac in the pirate radio days. cracking idea
In the Image for step 1, shouldn't the reflector be the long element and the director the short one? I believe you have the tag's reversed..
Looks like he must have fixed those tags by now.
hailster6 years ago
I made one of these about a year and a half ago. They do work pretty well for camping or any other temporary set-up. At the end of the boom (reflector side) I added about a 6 inch piece of PVC that then joins into a 90 degree bend. I then added a 1 foot piece of PVC that connects into the 90 degree bend on one side and is not connected to anything on the other. This makes a great handle for the antenna and works very well for securing the coax for storage. Also when using it for RDF keep in mind that cross-polarization drops the signal about 15-20 db. USE THIS TO YOUR ADVANTAGE. KC9FSH
ve2vfd6 years ago
I made one of those 10 years ago and still use it to this day! I made it as a backpacking directionnal antenna and between that and my roll-up j-pole I had all I needed for my 2m portable.
rocketbat6 years ago
oh wow! just what ive wanted to make for a while now! thankyou for posting! a "+" and faved!
HamO6 years ago
Excellent instructable, well done pix. Thanks for putting it on this site. BTW I need FN64 on 2 and 6 meters. I'm in EM20
FN64 (author)  HamO6 years ago
Hey HamO.. Tnx for good rating. I've been lurking here for awhile & figured it was time I gave it a try. We might get a lift on 6 sometime but a peek at the map sez 2 would require the birds. No AZ/EL right now.. that's another project ..but have 3 ele on 6 up @ 30-35.
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