Monday, November 15, 2021

The perfect tape measure beam for RDF?


There are lots of instructions and videos about the 2m tape measure beam available already.  Here are some build ideas based on the different iterations I have built over the years which are originally based on Joe WB2HOL's version before he went SK.

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Mounting the directive and/or reflective elements

Rather than using small hose clamps to secure the elements to the boom, I originally used zip ties. This method worked well, but sometimes warped the elements at an angle.  Using electrical tape works better, is easy to replace, cuts down on weight and improves the radiation pattern of the antenna.

Using electrical tape is inexpensive, secure, neater and
does not influence the antenna pattern like hose clamps do.
(Photo Credit: Steve Bossert K2GOG)

Boom Material 

1/2 inch PVC T and crosses seem the best.  A few antennae I made used 3/4 inch conduit and junctions to improve ruggedness, but 1/2 inch has proven just fine and weighs less.  Only use PVC cement once you are happy with the construction to secure the junctions to the boom pieces.  

Friction fit construction may fail at the worst possible time, so glue your junctions.  The only real major benefit of 3/4 inch construction is that your elements can easily slide into them for storage or transport.  This is still possible with 1/2 inch fittings but is a little too tight and the elements will take up more room during transport.

Here is a 2 element version with the director and driven elements folded up.
(Photo Credit: Steve K2GOG) 

Is a current balun necessary?

After making identical versions of  2 or 3 element antenna with or without the balun, I find the pattern of the antenna is more stable with the balun.   Making sure the balun is close to the feed point is a really good idea.   

I drill two pair of holes a few inches apart to form the balun and the feed point goes into the boom for a clean construction. Drilling the holes as far to the interior of the PVC cross is a good idea in order to leave room for folded element storage.

Where the nut inside the PVC cross is where a crimp ring slides over the screw and is the secured by the nut. Outside the PVC cross is a washer and nut. Outside and on the top of the driven element is a star washer on top of the hairpin match.

Scrap off the tape measure paint on both sides. Use a little De-Oxit to ensure excellent conductivity and to prevent any future rust. The 5 inch hair pin match is made from steel clad copper wire and crimp rings with the insulator removed are used to ensure secure operation. The crimp rings are also soldered to the hair pin match for further reliability.  A flat and star washer keep everything tight together.

How to connect your radio to antenna?

I like making a neat looking antenna and I route the RG-58 through the boom and out the back end.  You can make a long tail with a BNC male on the end to connect to your radio or via an SMA to BNC adapter.  Here is what a 3 element version of the antenna looks like configured that way.

The other option is a little more technically challenging to describe where a female BNC is fitted on to the end of the boom instead.  For the photo below, I also drilled a hole to secure a length of 550 paracord to act as a wrist strap of sorts.  

Using a short jumper cable to connect to the radio is needed, but when traveling, not having a cable dangling is a nice benefit and does not offer that much signal loss or leakage if done nicely.

Make your boom grippy?

Self amalgamating insulation tape works great as a grip and it wont leave a sticky residue behind if you care to remove it later. This same tape is also great for protecting antenna connectors, so pick up a roll or two and I am sure you will find many uses for this stuff. 

Here is a 2 element and 3 element 144 MHz tape measure beam antennae using similar construction practices. Put some electrical tape on the antenna ends to prevent potential cuts or eye pokes.
(Photo Credit: Steve K2GOG)


Many articles like the WB2HOL article cover measurements and you can find what works best for you. The goal of this article was to focus on some construction improvements.

When tuning the antenna it is best to adjust the driven element for lowest SWR at your operating frequency first and then add your reflector or directors after. This should lower the SWR further.

You can squeeze the hair pin match element to get a better match and as long as you are using stiff wire, it will hold its shape. The hardware used in construction of these antenna do not noticeably affect the radiation pattern of the antenna.

If you plan to transmit with these antennae, please ensure you adjust for a good SWR to protect your radio transmit circuitry.

Using these antennae

The original use for this design was for radio direction finding, but both will also do just for for transmitting if you are interested in light weight directional antennae for use with summits on the air (SOTA) or other portable operating scenarios.

Remember, that there is a null in signal off the back of the antenna and to the sides. This will be helpful when finding hidden transmitters.  The null is more pronounced with the 3 element version, but the 2 element design works great for other reasons.

If you are looking for a fun and inexpensive project you can build, please give these a try.

Hudson Valley Digital Network in conjunction with other area amateur radio clubs will continue to hold hidden transmitter hunts, so please check our activity calendar for upcoming events.

Safety first.  Be sure to protect the ends of your antenna elements with
some electrical tape to prevent injury
(Photo Credit: Steve Bossert K2GOG)

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