Friday, July 6, 2012

2m sleeved dipole

A'ight.  I ran across some articles about off-center fed, sleeved dipoles for 2m and 70cm use.  They're cheap and fairly simple to make, which is right up my alley.  I was going to mess around with using some copper tubing, but this sucker is working well enough I don't think I'm going to bother with the copper (or I'll save it for another project).  Total price for this thing for me was around $12.

The original articles I referenced were KV5R's OCF Sleeve Dipole and W7LPN's 2M/440 Dual Band Sleeved Dipole Mod.  I ended up keeping the idea of the RF choke close to the bottom element, but went back to simply using aluminum tape on the outside of a PVC pipe.  Instructions after the jump, but here's a pic of the antenna in action:


Before I start boring you with the construction notes, I should probably report the results.  This thing does fantastic on 2m/144Mhz and is passable (but not ideal) for 70cm/440Mhz.  While moving the transceiver to 144, I hit the public radio stations and it was a cacophonous blast of music as the antenna was picking up a bunch of stations from out of town crowding out the local stuff.  50 watts on 144 and 440 puts out a commanding signal, but the reception on 440 is about half the strength of what I get on 144.  Will try to do some field tests for distance later on this month and see just what kind of range it's getting.  Also going to back off on the power a bit and see how close to QRP I can get to hit the repeaters.

I was also able to send out a clear signal to 2 EchoLink repeaters in San Antonio (443.575 north side, 444.625 south side), but reception on the northern repeater was a lot better/clearer.  The next step is to figure out the DTMF codes for manipulating the repeaters to setup the link... going to try and schedule a time with W9DAN in Illinois for that.


What I ended up using was:

* about 6 feet of 3/4" sched 40 PVC
* a roll of 1.89" wide aluminum foil duct tape
* 3/4" PVC end cap
* 3/4" PVC T section
* 3/4" PVC threaded connector
* 1" hose clamps
* 1 zip tie
* around 25" of el-cheapo RG-58 crap cable that was made in 1978 from Radio Shack
* PL-259 connector with an RG-58 adapter for the end piece
* T connector that I'm using as a male-to-male bridge for the coax from the antenna to the coax to the transceiver

I should have bought the CPVC in order to provide some extra RF shielding for the feedline through the bottom element, but it doesn't seem to make a huge difference.

Tools I used were:
* PVC pipe cutter
* drill press
* 7/16 drill bit
* 5/32 drill bit
* scissors
* tape measure

I didn't use any PVC cement as friction is holding most of this sucker together.  I did spray some 3M loctite adhesive on the end cap that the antenna hangs from, but that's just because I didn't want it falling down into the neighbor's yard.


I used W7LPN's measurements for the elements: 22 1/8" for the top element, 15 5/8" for the bottom.  Unlike his plans, this one is spacing the elements apart 3" instead 1/4".  Also, instead of using metal tubing for the elements, I'm simply using aluminum foil tape attached to the outside of the pipe.

Since I used PVC, I left about 1" for caps/connectors.  I cut the top element to be 24 1/8" and made the bottom 18 5/8" (extra inch to hold the 4-5 wraps of RG-58 for the RF choke).  I used the 5/32 bit on the drill press to poke out 2 holes in the end cap for the zip tie.  Then I did some quick measurements on the bottom element to figure out where to make the holes for the RF choke and then punched those out with a 7/16 bit.

Here's a pic of the RF choke on the bottom element:

The next step was to dry fit everything and I found it all locked together tight enough that I didn't have to worry about PVC cement.  I ran the RG-58 cable up from the bottom of the antenna into the lower hole and pulled it through until I had about 6 inches left hanging out.  I then wrapped the cable around the bottom and pulled it through the top hole and ran it up to the T connection.

The next step was to put the tape on.  A piece of tape run lengthwise down the pipe will cover up around 55% of the circumference, so 2 pieces will overlap.  Other plans I've seen suggest wedging your feed line in between the overlapping pieces of tape, but I decided to simply use hose clamp since that would make disassembly easier (never know when you're going to strip stuff for parts in the future).

I cut 2 pieces of tape for each element (22 1/8" for top, 15 3/8" for bottom) and slapped them lengthwise onto the pipe.  I then stripped back about 6" of the RG-58, opened a hole in the mesh sheath, and pulled the center wire back through it.  Next I simply clamped the sheath on to the aluminum tape, and did the same with part of the center wire that I had stripped back.

Random Notes

* the top element should probably be cut back to 21 1/8".. this might improve selectivity for 440, but I'm not concerned with it

* I should have probably added another inch at the bottom and run the cable back out the side of the bottom element instead of the bottom of the antenna.  My intent for using the threaded cap on the bottom was to enable me to screw this antenna on to a mast if I ever needed to... running a feed line up inside of a mast cuts down on options for mounting it.

* totally assembly time was around 2 hours, cost was around $12, and could have been done without any power tools.  Not too high a price for the payoff of S9 signals at 20 miles out.

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