A Ten Meter Sleeve Dipole Antenna
This was my first Ten Meter beacon antenna. It made a good showing for itself. The "Sleeve" antenna is probably better known as a VHF antenna, but it has a fairly long history on Ten and Eleven Meters. Especially in the Marine community. Shakespeare made their "Big Stick" fiberglass CB antenna for many years, and as well the Antron-99, both are, I believe, sleeve type dipoles. I know the Big Stick was, I had one. It has a few things going for it.
1. No need for radials. Yes, you can mount three or four radials about a quarter wave below the base of the antenna, but I doubt on Ten Meters, you will see much of a difference.
2. Self Shielding. I still use a coaxial RF choke, but typically a 50 ohm coaxial feed will directly feed this antenna, despite the fact that it is a balanced feed like any dipole. However, the bottom "leg" is a hollow tube and the coax runs inside and connects to the center point from within. That bottom tube actually shields the feed line. Finding discussion about this phenomena on line is rather scarce, but it has been my own observation: I never had RF on the coax surface with a sleeve dipole.... just as I never had that problem with the Shakespeare.
3. All parts come from the local Hardware store. Except for the 102" stainless whip and the screw mount, of course. Those you can get on eBay or Amazon. I used a copper pipe cap which I drilled out to receive the whip's screw mount, and and which screws on to the down tube. Being copper, I was able to solder the coax braid to it directly. The down tube is a 1" diameter aluminium tube cut to 102" or so. I also did an extra maneuver by cutting a 12" piece of aluminum tubing to slide into the bottom of the bottom tube which can slide in or out. It's a "trombone" tuner. You will not set the antenna for minimum swr as a regular centerfed dipole by trimming the ends. Not with a stainless steel whip with a corona ball at the end. But just adjusting the length of one side actually does a pretty good job. I used a length of Schedule 40 PVC to cover the bottom tube. The copper end cap size is selected to fit over the PVC pipe. It is drilled to receive screws on three sides, equidistant, which go through the cap, through the PVC, and through the bottom tube. Make sure those screws are stainless.
4. Very good wind resistance. One of the reasons why this type of antenna is popular with the Marine community. It poses minimal wind resistance. Now, this antenna is not what I call "heavy", but it does have some heft to it. I used U-clamps to bolt the bottom to a mast. I selected a ten foot length of PVC to cover the bottom tube, cutting it about 18" longer than the actual bottom tube inside. Use that 18" to bolt the antenna to a metal mast. This will keep the bottom tube from paralleling the mast itself. On the bottom of the PVC, you could mount a cap with an SO-239 coax female jack. If you want. I just ran the coax out of the open bottom with a barrel connector (double female) and attached my coax feed to it.
That's pretty much all there is to it! I will post a drawing I made some years ago that illustrates how this antenna comes together to see it better, just click on the image itself. Note that in this diagram, I use a copper pipe. Substitute that with aluminium. A lot of freedom here. You can even use flexible metal conduit that comes in rolls. The idea is to make the bottom part of the dipole hollow so the coax can feed through it to the center feed point. :
Here are some photos I took while putting mine together:
This is the copper end-cap that serves as the dipole center. The stainless steel whip mount is mounted as you can see. The coax feeds through the bottom tube and attaches here. The braid of the coax becomes common to the copper end cap itself. The whip is, of course, insulated from it.
Here is a view of the mounted whip, the aluminium tube already inserted into the PVC (which I already had. There's a little rust on it from the last antenna project it was involved with. And, of course, there's the coax which will feed through that bottom tube.
Here it is, coax through the tube, connected to the end cap. The holes are drilled into the cap. When the cap is set on the PVC, I will drill pilot holes through these pre drilled holes, through the PVC and through the down tube. Then I will screw three stainless sheet metal screws through the cap, PVC and aluminium tube.
The end cap is screwed on. I do a continuity check with my VOM to make sure there are no opens or shorts. After this, she's ready to be tuned if using the above mentioned trombone tuner (sliding tube). It may not even be necessary to use one. You'll know when you place an SWR bridge in line. The version I used for the beacon did not need it. An 8 ft tube gives me a 1.3:1. Good enough!! Do the tuning with a short mast and step stool to stand on. When you are satisfied, go ahead and install it on your mast. That stainless steel whip will do just that, whip around for a bit, but it will steady out once the gear is mounted. I had mine on a 20 ft mast. And as I mentioned, I did roll a coaxial RF choke at the feed point: RG58, wound with a 4" diameter, 20 turns, zip tied together.
I replace this antenna with the Ringo AR-10 because the Ringo is lighter. Easier to take down in a tropical weather event. I'm not getting younger. I find the Ringo and the Sleeve Dipole perform equally well. The Ringo may have a slightly lower angle of radiation. But spotters on RBN report about the same level signals.
Check out this antenna, it may surprise you!
73, de wd4nka
No comments:
Post a Comment