Tag Archives: 2m Moxon

2m Tropospheric Ducting!

I finally experienced VHF tropospheric ducting tonight. I worked a station near San Antonio on simplex FM on 2m. I also heard and made contact with several stations in the upper valley that I had never been able to contact before. This was on 75W with my discone antenna up about 15ft. One of these days I need to get my 2m Moxon up to thirty feet and try it with the Yaesu at 75W.


A couple of new projects to think about…

I have been looking into making a custom air variable capacitor for an improved magnetic loop antenna. This looks like it would be entirely feasible and allow me to make a loop that could handle 100W. At 100W the voltage across the capacitor is just under 3.9kV at 14Mhz. The plate separation for this would be about 55-60mil. It would be a fairly large capacitor but it could be made fairly easily from sheet aluminum.

Just ordered a 2.5mm stereo plug to finish my wiring harness to connect my iPhone to my Baofeng HT to use with APRS. Been wanting to try Pocket Packet app for some time now and this will be my chance.

Also finished making an air choke for the 2m Moxon out of a coax patch cord. This is ready to test out with the new mast.

2m Moxon Update

I did some tests of the 2m Moxon with the FT-2900R and an SWR/Watt meter. I wanted to check the SWR to evaluate the overall matching. I set the antenna up indoors on a tripod up about 4 feet and connected it to the radio through the meter. Starting on low power 5W at the simplex frequency of 146.52Mhz I had an SWR of 1:1. Same on 10W. At 30W I was reading an SWR of 1.5:1 and at 75W the SWR was close to 3:1. This was not unexpected behavior as the way I built the Moxon I have the feedline running through the middle of the RF field. This is not an issue with low power but at high power it puts RF on the feedline. I wound about 6 turns of coax on the feedline and the SWR dropped to about 2:1. Adding a snap on ferrite bead with three turns of coax running through it further reduced the SWR at 75W to about 1.6:1. Aside from this issue, which is easily solved with an air choke on the feedline, it looks like the 2m Moxon is ready for testing at some height and will be ready for full power from the FT-2900R.

Treating myself to Early Birthday Presents

My birthday is in a couple of weeks. With approval from the XYL  I am ordering several birthday treats that can be wrapped up before I turn the big five three. First up is a S9V33+ vertical antenna from Alpha. This is a 31 foot vertical wire antenna that can be used from 40m up to 10m and is light enough to be free standing. I will test this out first as an end fed antenna and see if I can get away with not using radials. Otherwise I will install the 4:1 balun and a set of radials and run it as a 1/4 vertical.Will be interesting to see how it does against the loop and the DXtreme. Worst case is I will have a nice light fibreglass mast.

Next on deck is a 30 foot push up mast kit from ebay. It used aluminum poles to form both a tripod and a mast.Plan to use this for additional testing of the 2m Moxon and may install as permanent mast if all goes well. Found a  low cost balun kit on ebay and ordered two. Will wire one up for 1:1 and the other for 4:1 for wire antenna experiments.

The big daddy buy was a Yaesu FT-2900R 2m mobile rig. I plan to use this as a base rig with an AC power supply. Will output 75W….so….a bit more than the 5W Baofeng I am using now. I am very interested to see how this does on the valley-wide simplex net with the 2m Moxon.

Finished some chores around the Shack

Took some time this weekend to fix a few things around the shack. First up was fixing the head of my camera tripod I use with the Buddistick and Magnetic loop antenna. A little epoxy fixed the head. Now just have to sand it a bit so the mounting pipe doesn’t fit down do tight. The magnetic loop antenna needed a reliable way to connect a wooden tuning dowel stick to the tuning capacitor. Electrical tape was what I was using and was quite crude. I took a piece of 1/2″ aluminum rod out to the mini-lathe and about 20 minutes later had a decent little shaft coupler:

Photo Aug 11, 11 07 48 AM

This little gadget really helps with the tuning and makes operating the Magnetic Loop Antenna a lot more comfortable. Essentially a 1″ piece of 1/2″ diameter aluminum rod with a 1/4″ hole bored through it. Then drilled and tapped a couple of holes for 4-40 screws.

Next up was fixing the 2m Moxon. I had not glued the PVC pieces together and even the elements were slipping out of their mounts. A bit of 5 minute epoxy fixed all of that. Thanks to Ross, W3OQ, for providing me with a PL-259 to replace the one I damaged on last use. Antenna is ready for more testing with an eye for nights with good ducting.

Built the 4 element 2m Yagi in Time for “Simplex Sunday”

The 4 element 2m Yagi build when pretty well this afternoon. Here it is up about 18ft on the painters pole:

Photo Aug 02, 5 14 49 PM

This is using an interesting 4:1 balun design using RG-6U coax. I can hit the local repeater quite well with it and am now waiting to try this out on “Simplex Sunday” the local VHF simplex net.

UPDATE: I’d say the Yagi was a fail. I can hit local repeater and local simplex but that was about it. It did seem very sensitive to direction as one would expect from a beam. Biggest evidence that something isn’t working right is that I could not hit the Harlingen or La Feria repeaters. More than likely it needs some tuning and unfortunately I don’t have an antenna analyzer that can deal with VHF. Further evidence that something is not quite right is that 2m Moxon is able to hit both those repeaters without any problems. It will be interesting to see how the Moxon performs on nights with good tropospheric ducting potential.

SImplex Testing of the 2m Moxon

Setup the 2m Moxon on a painters pole late afternoon yesterday and checked into the local daily ragchewers net. Was able to hear the repeater traffic in Brownsville, Harlingen and La Feria at full quieting. South Padre Island was readable but scratchy. Could not hear the Edinburg or Raymonsville repeaters at all. W3OQ in Olimto helped me do some testing. I was able to work the La Feria repeater with no problems with full quieting reported on the local repeater node.

Photo Jul 21, 7 24 24 PMPhoto Jul 21, 7 24 56 PM

On simplex I heard stations from Mcallen/Mission and Weslaco for the first time. The Weslaco station could hear me only once. Stations in the upper valley were fading in and out but could not hear me. W3OE was reporting hearing stations near Corpus Christi and received a text message that he had been hear near Refugio.

So to summarize, the 2m Moxon does show an improvement as predicted by EZNEC. At 5W this is not enough to be heard widely through region on simplex but does give me access to a number of other local repeaters. Note that many of the simplex stations I was trying to work are on 50W and have antennas mounted up around 40ft.

Basis for my 2m Moxon

The basis for the 2m Moxon I built is here (2m-moxon-antenna). This describes the basic approach to building the 2m Moxon as developed in 2008 by the Montgomery County ARES.  I did make some modifications to this in order to deal with the parts I got from Home Depot. The end caps and PVC tees have one length but the PVC crosses are a bit shorter. I used a caliper to measure the length of the Tee sub-assembly then set the crosses to match. I then calculated how much wire needs to be inserted in order to set the right gap between elements which in this design is critical. I used a smaller drill size to make the holes so that the wire would be a fairly tight fit. Then I wrapped electrical tape around the wire to form a “stop” so that the wire would not go in further than necessary.

I cut the connector off of one end of a RG-8X coax cable and fed it up the PVC pipe to the feed point. In retrospect this was a mistake as now I cannot easily add a mounting extension to the cross. I will compensate for this by slitting a short piece of PVC pipe to allow the coax to emerge. I have a slip – thread coupler that will fit on the end of this so I can get the antenna mounted o the painters pole for testing. I am using a bulkhead adapter to connect the two pieces of coax for the feed line and have a small patch cable that connects to the Baofeng HT. Will be ready to test later today at new heights.

Finished the 2m Moxon Today

I finished up the 2m Moxon antenna today. I had bent the copper wire elements a few days ago after having straightened the #8 solid wire. Today just had to make some measurements, cut some PVC pipe, drill a few holes and solder up the feed line. All told a pretty quick build with the main issue taking time to get the measurements right. I called up W3OQ across town for a simplex radio check with the Moxon hooked up to my 5W Baofeng HT. I held it up, oriented vertically as high as I could reach and W3OQ reported full scale (13 bars) at his QTH. My 4W HT with the discone up about 12 feet scored 9 bars. Confirmed that there was directivity. Signal dropped to 5 bars on the backside. Looks like for an initial test it worked fine.

I am very impressed with how compact this antenna is. It is a rectangle only 29 inches by just less than 11 inches. Next steps will be to try simplex again with the antenna mounted up as high as I can get it on the painters pole. Here’s a picture of the completed Moxon

Photo Jul 16, 10 36 58 PM

First Step in Building the 2m Moxon – Straightening the Wires

Here is the first step in building the 2m Moxon, I am using #8 bare copper wire for the conductors. I bought about 20 feet of the stuff at Home Depot for 50 cents a foot but they coil it up like this when they measure it out:

Photo Jul 13, 2 18 24 PM

Cutting a length of about 4 feet ends up with a non-straight piece like this:

Photo Jul 13, 2 19 38 PM

Searching the internet for “straightening heavy gauge wire” shows up several methods. The one I used involves a power drill and a vise and makes short work of straightening the wires. The procedure is simple. First lock down one end of the wire into the vise. Take the other end and chuck it into the drill. Then pull it slightly to get some tension and run the drill in short bursts and just enough for the wire to straighten out. Don’t overdo it as the wire will heat up and too much of this will embrittle the copper. When you are done the wire looks nice and straight like this:

Photo Jul 13, 2 22 20 PM

Nice little procedure that saves quite a bit of effort in straightening the wire. Next up…the PVC frame!