The 2 Meter Bobtail Attic Beam
An Attic version for limited space applications
by NW9T
SAMUEL J. SALADINO

In a world where the names Quad and Yagi are well known as high performance antennas, a "long" name like Bobtail Beam almost seems like bragging.  It almost seems to insinuate almost at a subconscious level,  that an antenna with a longer name has more to offer than one with only four letters in it's name. 
You quickly scan the article about the Bobtail Beam here  and see a claim of 10 db over a quarter wave vertical or 13 db plus in free space.  Ok.. so you zoom down to the picture expecting to see something similar to a Yagi with 12 plus elements or an 8 element quad... WHAT?  There's only two rows of three element arrays.. basically a 2 element beam?!  You almost get defensive for your old buddies the Yagi and Quad, how dare this upstart claim it has better performance!  You think about it for a few days, wondering if it's even worth your time to attempt to build one of these things.  Sooner or later you figure, what the heck I'll build one...... prove it's a pile of doggie doo, and let everyone know about it and once again sit back and relax as the old Yagis and Quads reign supreme. 

First, a Prototype
First you decide to build a quick prototype, just to see if there's any hint this might be worth the effort to build, even if just so you can defend your old buddies.  You grab some old lumber, and some pieces of wire and cut to length, dangling the elements from the wood like cold spaghetti rather than standing up straight like stalwart soldiers.  You hook up your rig, apply a little power to calibrate your meter, then hit the switch to read your SWR.  You're half hoping the next thing you see is the meter go over to the red area to signify yet another badly designed antenna.  Instead you see the SWR a bit high, but still in the usable range.  Ok, so it's not going to blow up your rig, at least not yet.  You tune around and there, just enough to hear, there's voices.  A slight smile crosses your face as you're sure these are the guys talking on the local repeater only a couple of miles from your house, what a farce of an antenna you think.  But as you look again the frequency you've tuned to is not the local repeater, nor is it one you're even familiar with. A few moments later you hear the repeater ID itself and a quick check online shows it's a repeater sixty miles away!  Now sixty miles may not seem much for someone who is used to running a beam even up a scant thirty feet or so.  But when your antenna farm consists of various wires strung through your attic at less than fifteen feet above the ground, that's quite a shock, especially when your house is located practically in a hole.

The Good Version

Today I finished building a good version of this beam, and I won't go into all the details of the building process as they are pretty well explained on the site here at the Bobtail Beam Project. 

I will give you a few hints that might help you build one as well with how I did things that seemed to make it easier.

The Construction: 
First of all I built mine from 1/2 inch cpvc primarily because I had some laying around here, and I wanted the final version light enough to put up on my 23 foot painter's pole so I can use it portable as well.  It currently rests in my attic with a vertical dipole for 2 meters, a Slim Jim and a three band dipole for 10, 15 and 20 meters. 

I would suggest you don't use the 1/2 inch cpvc unless you plan on leaving it in the attic.  Even at the element length of just over 80 inches it still has a little droop, but not enough to seriously affect it's performance. 
Editor's note: (To help prevent droop, larger element supports are suggested. Also, braces from outer element separators (the short sections) can be added and attached to the support mast. Use your own methods and ideas.)

I drilled the holes for the elements in the fittings for the cpvc, not through the cpvc itself.  I think this made it easier to get the elements to stand straight as well as more accurate to drill and keep the elements in line. 

My frame is made up of four pieces of cpvc, two for the front element supports and two for the rear element supports, four 90 degree fittings, three Tees, two pieces of 1/2 cpvc to make the end spreaders and two pieces to make the center spreader which has another Tee in the center so I can attach it to a mast/pole. 

When I cut the multiple pieces that had to be the same length,  I'd cut one piece to the exact measurements by dry fitting the end pieces and making sure the distance was correct from hole to hole in the fittings. 
Once I had that piece correct, I taped that and the other piece or pieces to it using Duct tape at each end and in the center.  I then used an angle grinder and slowly ground down all the longer pieces until all ends were flush.  This gave me a perfect match in lengths. 
I used 3/32 of an inch copper coated welding rods for the elements.  On each end is a flattened area where they've stamped the product number right in the metal and this is slightly larger than the 3/32 hole you'd drill for the elements.  The rods are 36 inches in length, so simply cut off the flattened area on one end of each rod so it slips through the holes you drill. 
Before you cement the cpvc pieces together, dry fit one more time and make sure the holes are the correct distance.  Using a pen, mark the edge of the fitting you are attaching. This stuff dries fast and you won't have time to check your measurements once the glue is applied.  I'd guess ten seconds tops once the pieces are joined before you can't move them anymore.  Make sure your pieces are oriented correctly with each other. If you're attaching two of the 90 degree fittings on the ends of a piece of cpvc make sure they are pointing in the same direction.  Also I laid mine down on a flat area of the sidewalk to make sure they were both on the same plane.  When measuring the elements, I measured down from the cut end to the point where they were the correct length, then using needle nosed pliers made a 90 degree bend.  Push the elements through the cpvc and you can tape the other end that's bent against the boom to hold it in place.  A 35 watt soldering gun was sufficient to solder the 12 gauge wire I used to the welding rods. 

Where the center feed of your coax feeds the center element of the forward array, solder that at the bend of the element on the bottom of the Tee and trim but leave enough to tape to the boom to hold in place.  Run the wire that feeds both end elements from the braid OVER the Tee and solder it there to make sure it doesn't contact the center feed.  Tape to mast about 1/2 inch behind the center element.   Cut your elements longer than what the measurements are in the article.  It's easier to trim down elements that are too long, then to add length to those too short.  I added six tenths of an inch to both driven and reflector elements so the front elements were cut to 21 inches and reflectors to just short of 22 1/4 inches.  Top end of 2 meters SWR at this length is 1.3:1 and down in the low 145 range is flat at 1:1.  I could probably get it down for the higher section where I operate most, but.. 1.3:1 is more than adequate and I'd rather leave it there than end up cutting of a fraction too much and having to start over. 

Danger! Don't do this if the XYL is looking!

Initial performance? 
Ok I don't have any scientific equipment to give you hard and solid facts, but I can give you comparisons to other antennas I have.  S meters we all know are not that accurate and can't be used to give you real db readings.  However it's easy to tell the difference between a signal that reads an S1 vs one at S7 or higher.  It's even easier to tell the difference between a signal you can hear and one you can't! 

My first test was a bit disappointing.  However, what a surprise was waiting for me!

I have been operating 2 meters from inside my house since last winter, before that I was always mobile when on 2 meters.  Since that time I've become the Net Control Operator the local ARES and Skywarn Nets and it was getting too cold and awkward to do that from my car, so I moved the rig indoors.  I was using a 2 meter vertical dipole I built from just a piece of coax, and it worked.  No gain and my SWR was low and I could work all of the local repeaters. 
A few weeks ago I built the Slim Jim antenna, a design that is also on this site.  Comparing that antenna to the Dipole, I was able to pick up one of the repeaters in the city 60 miles north of here which I couldn't even hear on the dipole. 
So my first test was to try to hit that repeater so I could compare power needed to access it as well as received signal strength.  When I keyed my mic there was no response from that repeater, I even upped the power to 50 watts and still nothing.  I thought maybe that repeater was down so decided to try another repeater there and hit that one with no problem, even down to 25 watts and had an S4 on receive.  In the Bobtail Beam article, it states, the STANDARD Bobtail Curtain has a narrow pattern, and it's even narrower in the Beam version of the antenna... BELIEVE IT! 
After hitting all the other repeaters in that city with only 25 watts and from an S2 to S5 on receive, I knew either that repeater was having a problem or that the pattern of the Bobtail Beam was narrower than I was giving it credit for.  I went back up to the attic and moved the Bobtail Beam no more than one and a half inches at most towards the west, re-keyed the mic and BANG!  Brought up the missing repeater with 25 watts and a received signal of S5! This was why my initial testing was disappointing. The Bobtail Beam appears to be highly directional!

2 Meter Attic Bobtail Beam in antenna farm location

The Comparisons
Although not scientific, these comparisons show the Bobtail Beam is definitely an improvement over the other two antennas I've been running.  Again I just finished the Bobtail Beam antenna so these are only limited comparisons, but all have been extremely positive..

City #1 60 miles N
Vertical Dipole could not hear one repeater, let alone access any of them.
Slim Jim heard one repeater, was able to bring up with 25 watts S3 signal received. Bobtail Beam on above repeater brought it up with 25 watts S5 on signal received. In addition, the Bobtail Beam brought up every other repeater in that city at 25 watts.. for a total of 5 repeaters.

City #2 25 miles NNE
Vertical Dipole could hear faintly but could not access
Slim Jim access at 25 watts S6 receive
Bobtail Beam access at 5 WATTS.... Full Scale on receive!

City #3 80 miles ENE
Vertical Dipole.. HA!
Slim Jim accessed one repeater with S2 received signal
Bobtail Beam.. accessed no less than 6 of the repeaters.  2 of the repeaters
needed 50 watts to access, the rest accessed with 25. Received signals ranged from S1 to S4.

City #4 40 miles SW
Vertical Dipole again.. HA!
Slim Jim access at 50 watts S1 on receive
Bobtail Beam access at 10 watts S4 on receive

Again not scientific but hard to deny something good is going on here.  When I
get it out on the painter's pole to see how it does in a better location I'll be happy to send an update. 
Until then, the Bobtail Beam is easy to build, just take your time and double check all measurements.  It can be built cheaply, and is a great performer, definitely has more than adequate side rejection, but haven't attempted to determine any kind of front to back rejection yet.  The local repeaters are just too strong now so even a 20 db difference in strength front-to-back if coming in at 40 over 9 when pointed at it, I'd still hear 20 over 9 off the back end! 

So is the Bobtail Beam with it's "long" name really an attempt to brag? 
If not, it should be, it's EARNED that right in my household and the attic antenna farm! 

73
Sam Saladino NW9T  Email nw9t AT arrl.net 

Editors note:

The Bobtail Beam out performed ALL of the antennas Sam used in the above comparison as it should have. The Bobtail Beam is patterned from the Bobtail curtain used and highly valued on hf. It has been around since the early days of radio and has won it's place also in many antenna farms.
Many thanks to Sam, NW9T for sharing his fun and expertise with us.
Most photos above make antenna appear much wider than actual size.