Tag Archives: NVIS

Getting ARES Ready…Hurricane Season Approaches

Hurricane season officially begins on June 1st. Our local club is gearing up some activities to support ARES operations in the event of a storm. We will be setting up a 2m digital simplex net on Thursdays at 7pm local time on 146.580 simplex using MT63 2000. Net control will be Cliff, N5CEY.

I will be net control for an HF voice and HF digital net on Saturday afternoons. Times and digital modes are yet to be determined. NVIS operations on HF will be desirable for these nets. I suspect that my current antenna (the DXtreme 53′ end fed) works fairly well as NVIS as it is up horizontal no more than 20 feet up. This net will give me a chance to test the 40m magnetic loop as NVIS. I am also thinking of modifying the homebrew buddipole to work NVIS as well. There is a PDF file that shows a simple way to modify a Buddipole to work NVIS portable. Yet another option is to build an easily deployable NVIS antenna for 40m and 80m per these instructions (thanks DX Engineering).

I am looking into a few choices for HF digital modes. MT63 1000, PSK31, Olivia and NBEMS. I am thinking of eventually asking net participants to limit power to 20W max to simulate off grid operations.

Last but not least, getting more local hams to test their HF rigs on Winlink using WINMOR. I cranked up the IC-7100 this afternoon and easily sent some emails on 40m to a node in Houston on 20W. Here is a good tutorial on setting this up.


40m Magnetic Loop Antenna

Finally got around to finishing the 40m magnetic loop antenna. This antenna is designed to be easily deployed and used as an NVIS antenna with up to 100W. The heart of this antenna is the hi-voltage capacitor I designed and previously discussed. Before setting up the new antenna I made a few changes to the capacitor:

  1. Wrapped exposed copper with electrical tape
  2. Replaced the steel 5/16″threaded rod with a two food nylon rod. This was to minimize coupling effects while tuning
  3. Made a new center post with better hole alignment. This helps keep everything from binding.
  4. Epoxied the capture nut into the center post.

Here is the capacitor in use on the new antenna:


The 40m loop consists of 22 1/2′ of RG-213 coax. The length was selected to match the capacitance of my prototype capacitor. Here is the design points via AA5TB’s spreadsheet:


The rest of the build involved cutting pieces of 1/2″ PVC pipe to length to support the antenna. I hauled the whole thing ip a tree branch with the feed point just about 1″ off the ground. Initially the SWR was a bit high but it settled into about 1.4 when I removed two turns from the toroid. Here is the SWR plot through 50 feet of RG-8X feedline:


Here is a view of the antenna deployed on a tree branch.


I have been running WSPR on 40m now for about an hour and transmitting 1W. Here are the contacts thus far:


This seems to be working well so far. My only complaint is that I have to stand on a ladder to tune it. I’ll need to work on that. I’ll run some additional tests now that its built.

Hi-Voltage Variable Capacitor – Finished!

I wrapped the PE film around the center copper pipe lined everything else up and basically finished it! The feed mechanism works great and is not sensitive to me touching the tuning shaft. I should be able to finish the 40m loop now as the RG-213 that forms the loop will just connect to each end of the capacitor directly. I already have a toroid ready to work the matching so I will just have to size and cut the spreaders.The capacitor will be at the top of the loop this time with the feed point at the bottom. Been a great afternoon of homebrewing and realizing an idea that I had been working on for some time.


Connectors Soldered!

I did manage to solder the connectors to the ends of the copper tubes for the variable capacitor. Don’t even bother trying this with a soldering iron as there is just not enough heat.I clamped the connectors in my vise then stacked the pipe on top. A propane torch did the heating. Warm it up for about 30 seconds then start applying solder. Once it is nice and hot the solder will wick up nicely. Keep applying solder and work your way around the connector till done. Let them cool completely before handling as they are hot! The resultant joints looks mechanically solid. On to the PE film wrapping!


Homebrew Hi-voltage Variable Capacitor is Taking Shape

I mentioned in a post a while back that I successfully built a variable capacitor out of copper pipe and polyethylene sheet as a dielectric. The PE sheet has a dielectric strength of 22 kV/mm and the capacitor I have tested has a gap of almost 2.5mm! The problem that I have been working on is how to adjust this on a 40m magnetic loop antenna that I am testing for EMCOMM NVIS use. My initial tries at this either would not work mechanically or require quite a bit of structure to build. So I sat on this project awhile hoping to find a solution. I was sitting down at church last Sunday and just before the service started…BAM! the answer just popped into my head!

The secret is soldering the SO-239 connectors to the end of the pipe that form the capacitor. In this way, I not only minimize the losses but I can make the capacitor part of the mounting structure. A quick visit to Home Depot during the week yielded various PVC fittings and hardware. Here is how the prototype is looking thus far:


The PVC cylinder in the center forms part of the structure. The screw eye will be used to hoist this up onto a suitable tree branch or other support. The bottom part of this fitting will take a 1/2″ PVC pipe as a spreader which will join two together in the center. The copper pipe is the capacitor. In this test of the mechanism, I have not soldered the SO-239’s in place or have the PE sheet on the smaller tube. The block on the end of the smaller copper pipe has a couple of lock nuts sandwiching a delrin block forming a crude bearing. The other end of the threaded rod attached to a nut force fit into the PVC support. As the threaded rod is turned the smaller pipe will move in and out of the larger pipe giving me variable adjustment. The mechanism works so now I have to figure out how to solder the connectors in place, wrap the small pipe with the 45″ long piece of 4 mil PE sheet and epoxy down the parts that need to stay fixed. I have figured that for a magnetic loop that only works on the 40m band I need an adjustment range of 5pF which is about an inch of slide. The target capacitance range is 37 to 42pF. Each turn of the screw will work out to about 1/4pF so plenty sensitive to adjustment.

Stay tuned as I get out the oven mitt and the butane torch and try and do some heavy duty soldering.

A “Small” Magnetic Loop for 40m NVIS

I have run some design iterations on a magnetic loop antenna for EMCOMM use on 40m with an eye for testing it as an easy to deploy NVIS antenna on this band. I plan to build this using RG-213 as before but with a diameter of just under 8 feet. Rather than use a variable capacitor, I will use a length of RG-8X coax and tune for a fixed center frequency of 7.285 Mhz. This should allow me to operate on any of the regular 40m traffic nets run here in Texas. It will be easy enough to cut several lengths of coax for tuning on different parts of the band fairly easily. See the file below to see my design notes for this antenna:

40m loop notes

My thought is to use a small plastic box to mount the UHF panel connectors and connect the tuning capacitor coax. A small screw eye on the top of the box will allow me to haul this up to height. Rather than use a coax coupling loop I plan to use instead either an FT140-43 or FT240-43 toroid with between 5 and 10 turns of insulated wire evenly spaced to form the feedpoint.As with my current loop, I’ll use PVC pipe and fittings to make a spreader to keep the loop as circular as possible. This antenna could be hauled up by a single line onto a tree branch. I plan to use a painters pole to start with and will experiment with the loop between 1 and 6 feet off the ground. The biggest cost on this is expected to be the RG-213 coax at $32. I expect that this antenna can be made to break down into pieces no longer than 3 feet. I’ll post more info as parts come in and I start building.