2M7N50 Antenna Project

2m7n50 antenna by G4CQM

Updated 12/05/10 - Read my whole first antenna experience, or jump down to the results here.

I started in ham radio to take part in emergency communications... but I am quickly learning that there is an entire social and hobbiest aspect to it as well. Even though I'm not sure I have time for another hobby right now, the community is very supporting and I am glad for it.

In any case, necessity is the mother of invention, and thus I stumble on my first antenna project. There is a simplex emergency net on FM with HT's that are just out of reach with my packaged RG8X and CP22E 2 x 5/8 stacked GP... some members of the net told me that I'm going to have to do better with my antenna and feed line if I want to participate. After some searching, I decided on the 2M7N50 by G4CQM. The source looks reliable and the product looks perfect for what I want (Great gain 2M FM on a narrow band exactly in my range, not too complex, no matching).

Links below to G4CQM's stuff, with information as it becomes available on what I'm doing with it. This ought to be interesting, considering my gross lack of experience. I hope G4CQM's antenna won't mind being mounted vertically to line up with the HT's in the net... Contact me with any advice you might have.

2M7N50 Project Status Updates

  • Nov 7, 2010 - Butchered G4CQM's metric numbers into standard ones, you can download them here in a formated document. I'll measure in metric as much as I can when building, but this was helpful for trips to the hardware store.
  • Nov 8, 2010 - Tripod from an old roof satellite recovered. I think it will work well. 1" PVC seems to fit right down in there easily and has more than 1' inside the tripod for support, with two holes to bolt through.
  • Nov 9, 2010 - Bought a piece of 1" PVC SCH40 for the boom. Not UV protected and will need to be supported, I know. Got a 5' piece of solid-core for the mast.
  • Nov 10, 2010 - Bought some LMR400 from a local; seems to be low loss, and impedance to match the antenna I want to build. The 2M7N50 pages don't mention anything about velocity in the calculations, so I'm crossing my fingers. Should be here in a few days.
  • Nov 11, 2010 - Bought 21' of 1/4" aluminum tubing, about 1.5mm off from what G4CQM lists for his elements. Best I could do on a budget (did I mention I can't spend too much on this?), only cost about $20. I do realize that element size does require some adjustment in design, but I'm hoping I'm close enough here. I also cut them to size, all the elements are ready except the driven.
  • Have I mentioned that I have no idea what I am doing?
  • Nov 12, 2010 - Reading a bit more, starting to wonder if I should be using a conductive boom. His design seems (he doesn't specify) to use a conductive boom, but with insulated elements. Does that mean I need to do the same, or can I use a non-conductive boom? I found at least one source that said you have to re-calculate even if they are insulated.
  • Nov 15, 2010 - Most of the antenna is assembled. The 2M7N50 measurements are in free space, which I believe means they are calculated with a non-conductive boom (?)... the antenna is also "matched" which I've mentioned above, but I believe this means I can connect the DE without a choke (though I did use a ferrite balun). I think I can have it on the roof today, so we'll see if it blows up my radio.
  • Nov 16, 2010 - Ok, it's on the roof. Some help testing it this afternoon from n0eq, wb2ivu and kf7mim. Seems to be pretty directional, but haven't connected with anyone in Apache Junction or Mesa yet, my target areas. No idea what the SWR is, but my radio was putting 75w in without getting warm enough to trigger the fan and without appearing to cut power. Guess we'll see on the Sunday night net if I can copy and communicate.
  • Nov 17, 2010 - I decided to run my figures through yagimaster, since I did change the diameter of my elements from G4CQM's design. Even with the changes, I thought the numbers looked pretty good still (though, I don't know much about it, so I'm probably easy to please). The graph of results sure highlights how narrow the bandwidth is on this thing, which is intended and therefore not an issue. Images linked below.
  • Nov 29, 2010 - Got a contact from Derek, G4CQM. He gave me some adjusted measurements based on my variance in materials! I will trim the elements down and do some more testing, hopefully with some longer range contacts in the near future.

    2M7N50 with 1/4" parasitic elements, from G4CQM

  • Dec 05, 2010 - SUCCESS! I had my wife with an FRS handheld at my station while I went on the roof to rotate the 2M7N50 during last night's distant simplex net... and I was able to copy net control and some of the check-ins, and succesfully check in myself! This was my goal in building this antenna, but I hope to do much more with it.

  • Results & Summary

    11/29/2010 - I now have updated measurements, will be trimming the elements per those instructions and trying a few more things. Hope to also have a meter to check things with in the next month or so.

    11/28/2010 - After trying several times listening to this distant net that I was hoping to take part in, I think I may simply be out of luck. The antenna does seem to do well (I was able to copy 2 net members pretty well this last attempt), but with them operating very low power on HTs, it just may be beyond the help of any antenna I could build right now. I will say this, the antenna was fun to build, and seems to work well. I'll post once I get my meter in here, to see how the SWR compares to what the computer simulations say it should be.

    If I do manage to get some contacts that are further from me to test with, I'll post the results here.