Helical Quad Antenna for Weather Satellites


Finished and installed antenna!

It didn’t take long after my first successful attempt at receiving weather satellite broadcasts for me to realize that I would need a much better antenna.  I had been using a 1/4 wave whip with a 4-wire ground plane.  There performance out of this antenna was poor.  I read up on QFH (Quadrifiliar Helix antennas) from many of the high quality posts from around the world.  I took what I could from these implementations, and did my best with the supplies I had available.
I ultimately started with whatever PVC pipe I had lying around; it ended up being 1-1/4″ outside diameter garden-variety PVC.  Next, I went onto the excellent QFH antenna calculator.  This site already has the defaults for a good 137 MHz antenna, and I didn’t change them.  Next I measured the diameter of my pipe (in mm) and printed out the drilling templates using their drilling template generator.

Drilling template

Pick a point for the top line of the template and scribe a line perfectly parallel with the axis of the pipe.  This is easy if you use the “estes door jamb trick.”  For those that haven’t built a model rocket recently, you put the pipe in a door jamb crease.  This right-angle will make the pipe perfectly square and you can mark a line.  Also, you can use a piece of angle stock, as this guy did.  Make sure that all your template strips line up with this line.  Next mark the holes’ letter with pen on the pipe.  This is your reference for where the wires need to go.  Finally, drill the holes.  I only drilled the top and bottom holes, and I didn’t make them as big as the template said.  The template (and the calculator) suggest using 3/8″ soft copper tubing, but I didn’t have any of that around, and it costs a lot more than the #10 ground wire that I used.

Spacers installed

I knew I would need spacers half-way through the helical loops, but I didn’t have any of the tubing that most people were using, so I used some acrylic sheet.  I just took the width of the loop and marked those points along a line.  In the center I cut holes for the mast.  I trimmed off the edges with a (mostly) straight line so it tapers to the end.  At the ends, I made little notches to hold the wire, and drilled a hole for some unwaxed dental floss.

Loops held in with floss

Once I made a simple knot with the floss, I hit it with some superglue an called it good.  I don’t have any pictures from forming the loops.  This is an often ignored part of building one of these antennas.  I understand why so many people leave that part out, though.  It’s amazingly stressful, and easy to forget about documentation.  To do it, I started with long pieces of the copper wire.  Use more than you think you need.  Once your wire is cut, measure to the center.  Measure out from the center one loop radius on each side and mark it with sharpie.  Bend one side of the wire as sharply as you can, and slide the longer side of the loop through your lower pair of holes.  With what will be the center of the loop in the pipe, bend the other side of the loop.  Carefully twist the helix, meeting the spacers on the way.  It’s probably not a bad idea to temporarily (make sure the copper can be adjusted) attach the wires to the spacer.  Measure from the mast one loop radius and bend the wire toward the mast square with the upper holes.  Cut the extra wire so that it fits inside the pipe with a little room to spare.  Once you’re sure it’s the right length, bend the wire 45 degrees toward the other loop.  Do this for each side of each loop.

Wiring of the feed point

The picture above is skipping ahead a little, but this way you can see what I mean about bending the wires inside the pipe.  As you can tell, I had a very hard time soldering the feed point.  The copper ground wire is a hard to solder with an iron because it conducts heat readily, and has a lot of thermal mass.  I usually use a torch to solder it.  In this case the solder point is inside the tube almost an inch.  The first time I tried to solder it, I set the end of the tube on fire.  Later, I insulated the coax and tube with aluminum foil.  Though the foil provided sufficient protection, I still charred the sides a bit.  The cable was adequately protected.

Balun

Though the feed for this antenna is technically 50 ohms impedance, it’s a balanced design.  Fundamentally, coax is unbalanced.  By coiling the cable around the mast a few times you’re able to implement a simple balun.  I’m not sure how many times to do it, I’ve seen different numbers different places, so I chose 6; because, why not?  It seems to work fine.

finished antenna

This is the antenna all finished up.  I’ve received several quality satellite images with it so far, but this is certainly not where I’m going to keep it.  First of all, my house is to the north-west of it, and seriously degrades the signal, secondly my wife wouldn’t like that very much, I don’t think.

Spider-proofing

I decided to keep it outside (some people put them in their attic), so I had to take some measures to spider and wasp proof the mast.  I epoxied all the openings for wire and cable at the top of the antenna, including capping the pipe, then I stuffed the bottom with some pipe-wrap foam.

Installing on the barn

I ended up settling on the barn roof as a home for the antenna.  There was a small gap in the plywood below the metal roof cap that I think I can exploit for the feedline.  I unscrewed a section of capping, found an existing hole in the tar-paper backing and went to work affixing the tripod.

Feedline routing

It’s really important to think about water infiltration whenever you do anything to a roof, especially in Oregon.  For a while, I did some contract work installing antennas for a cell phone company, and we installed outdoor antennas on almost any kind of roof you can think of.  For metal roofs like these, the easiest way to make it water-tight is to clean the metal surface off, then cover the place where you’re going to screw the mast to with silicon sealant and screw through it.

Mounting the tripod

This way, water doesn’t have a chance to wick through the roof through the screw threads (or that’s the idea…  I explicitly disclaim any responsibility for roof damage if you follow these instructions).

Feedline inside the barn

Here, you can see the ultimate goal:  Antenna on the roof of the barn and feed line inside!

Receiver ready for the next NOAA satellite pass!

Finally, I screwed the receiver to a joist and routed some wires.  I don’t know what I would do without my insulated staples! 🙂

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  1. #1 by fulopke on February 14, 2012 - 9:48 am

    Hello
    This is a very cool system.
    How to buy this receiver?

  2. #2 by hpux735 on February 14, 2012 - 9:53 am

    The best way is to check https://www.kb9yig.com/ often, and look for the “Ensemble VHF” receiver. They go fast, so you should join the “softrock40” group on yahoo groups. When the kits are available, a message will be posted on the mailing list.

    It’s an involved process, time wise, so be warned. It’s totally worth it, though.

  3. #3 by Richard on May 7, 2014 - 8:40 am

    Hello

    Have you had success with this antenna? Did it turn out to resonate at 137 MHz? Is the design frequency 137 or 137.5 MHZ?

    Does it make a difference performance-wise if you use 1/8 inch tubing versus wire?

    Do you have any advice for easier ways to bend the tubing or wire?

    What type of coax did you use for the balun?

    Do you think this would work OK with an RTL-SDR “dongle” type receiver as sold by Nooelec?

    Thanks.

    Richard
    AE6XO

  4. #4 by hpux735 on May 7, 2014 - 8:53 am

    I haven’t measured the resonance frequency of the antenna. I built it well before I had a spectrum analyzer to perform the measurement, and now that it’s installed on the roof, I’m not really comfortable putting RF power into an antenna aimed at satellites. It does perform very well on 137MHz, however.

    It’s my understanding that the difference between wire and pipe can matter, but the calculator that I used allows you to specify the conductor diameter. ( http://www.jcoppens.com/ant/qfh/calc.en.php )

    To bend it, I just did it by hand carefully and taking my time. I imagine that bending pipe would be significantly more difficult.

    The coax I used for the feed line and balun was LMR-195 from Andrews Microwave. I got it surplus. It’s basically RG-58, I think.

    I’ve been using it with a RTL-SDR dongle with great success: https://twitter.com/hpux735/status/452674180970864641

  5. #5 by Richard on May 8, 2014 - 7:31 am

    Thanks for the reply. Your design looks like one of the easier ones to attempt.

  6. #6 by Richard on May 8, 2014 - 10:47 am

    Should the QFH calculator be set for 137.5 or 137.0?

    Are you using an LNA?

    Nice picture on the Twitter page!

    Could you send me an email so that we can talk that way?

  7. #7 by hpux735 on May 8, 2014 - 2:59 pm

    I don’t think it makes all that much difference between 137.5 and 137.0. I am using a simple pre-amp circuit that I found online ( http://electroschematics.com/wp-content/uploads/2009/04/fm-antenna-booster.jpg ). It works O.k., I had to experiment with the coils a bit to get the tuning just right. I just (as of yesterday) assembled the parts to make some new LNAs with the skyworks SKY67015. I’m expecting to make a blog post about that when I’m done.

    I don’t like to correspond using email, because the point of this whole thing is sharing knowledge with as many people as possible.

  8. #8 by Richard on May 11, 2014 - 4:45 pm

    Thanks, Bill.

    I typed a reply a bit ago, but I don’t see it now. I was at the local hardware store and looked at #4 bare copper wire and 1/4 inch copper pipe. I see now that you used #10 wire.

    I may need help with the QFH calculator web site if I go ahead and build this antenna.

    I think I may use thin acrylic rod or tubing for the central supports. How do you drill a hole in 1 1/2 inch or 2 inch PVC pipe so that the exit hole is exactly opposite the entry hole (using a drill press)?

  9. #9 by Richard on May 12, 2014 - 6:20 am

    I found that one uses a v-block for drilling holes in round stock or pipe. I may need to make one out of wood for drilling large diameter PVC pipe, though.

  10. #10 by hpux735 on May 13, 2014 - 5:46 pm

    I made acrylic supports out of flat-stock and a scroll saw. It didn’t work fantastic. Acrylic is pretty brittle… I would choose almost any other way of doing supports. I think that tubing or rod would be a much better choice.

  11. #11 by Marcel Vienneau on June 30, 2014 - 6:55 am

    Hi

    1st off, I want to say thanks for a simple but yet effective Rx antenna. I just completed mine today but had a hard time to get it together inside the dard PVC pipe. With a little determination a manage to do something similar 🙂

    Now my question is this

    i have a 20 feet of RG8 stiff coax ok

    can i use this to replicate coper tubing

    example

    by soldeing both ends ( shield and center conductor )

    Thanks 4 the help

  12. #12 by hpux735 on June 30, 2014 - 8:01 am

    You’re welcome. I’m glad I could help.

    Yes, you can use coax as the “arms” of the antenna. That is a common method for building QFH antennas. There are examples for how to wire them in such a way. I don’t have any links handy, but they’re out there. Good luck.

  13. #13 by pr2hrf on March 15, 2015 - 1:25 pm

    Hi Hpux375,
    small question : did you need any special tool to bend your copper tubes ? (maybe these tubes were soft enough to bend it by hand)

  14. #14 by hpux735 on March 15, 2015 - 2:20 pm

    I used grounding wire, so it’s solid copper. I used my hands to bend it.

  15. #15 by Tom in Kent WA on July 30, 2015 - 6:37 pm

    No ticket, no website…. just fooling around with RTL-SDR. I can watch every commercial airline in the country (and elsewhere) on FlightRadar24, but it’s not the same as watching 8 or 10 local aircraft on a homemade co-linear. And I can watch up to the minute weather conditions almost anywhere in the world, including over my head, all on Google Earth but, it’s not the same as getting that one feed from a NOAA satellite on a home made QFH.

    I have one question though Mister Dillon. When soldering the two elements together, with your coax, are you connecting one short to one longer element at the top? Or short to short and long to long?

    Thanks very much for the fine construction article and the link to the template builder. But, on that note, no matter what dimensions of pipe I attempted, absolutely none of the templates fit the particular pipe. In every case the circumference indicated for drilling holes was too short for the circumference of the pipe. It was easy enough to work around, I think, we’ll see soon enough.

    TIA
    p.s. I’m going with short to long but I’ll keep it changeable by using some wire clamps viz soldering for the time being.

  16. #16 by hpux735 on July 30, 2015 - 8:39 pm

    @ Tom in Kent WA
    The goal is for there to be one big loop. So, I think it is short to long. It’s interesting that I didn’t think to mention this. This site http://www.g4ilo.com/qfh.html has a diagram that is very hard to follow, but I think it is also short-to-long.

    Good luck! 🙂

  17. #17 by Joseph on November 4, 2015 - 5:07 am

    Your photo of the antenna had the upper holes in the same horizontal lines, where as the drawing on the calc page shows all of the paired holes offset vertically.

    You stated the default conductor dia was 3/8 inch on the calc page.
    with same values as u stated u were using the default displayed to me is 50mm.
    That is not 3/8 copper tubing diameter.

    I am going to use the SD-Play, expecting it to arrive this evening.
    Trying to get a part list and cut pattern for the antenna right now.

    Thanks,
    Joseph

  18. #18 by hpux735 on November 4, 2015 - 2:09 pm

    Yes, that’s right. The top holes are level. That’s a structure that I’ve seen rather often. Usually, the bottom holes are staggered.

    I know that the calculator has changed a bit since I made this page, so it’s possible that the defaults have changed. Also, it seems that some of the translations have gone a way, because I’m getting a lot of German now, and I don’t understand it.

  19. #19 by Subhyal Bin Iqbal on February 1, 2016 - 9:42 am

    Whar RTL-SDR dongle are you using? A link or picture would be helpful.

    Thanks!

  20. #20 by hpux735 on February 1, 2016 - 10:16 am

    I doubt the one I have is still available. They’ve advanced since I bought them. However, this one seems to be well regarded.

    http://www.amazon.com/dp/B00P2UOU72/ref=cm_sw_su_dp

  21. #21 by Subhyal Bin Iqbal on February 1, 2016 - 10:32 am

    And did you use BNC to MCX converter/adapter for connecting the antenna to the SDR?

  22. #22 by hpux735 on February 1, 2016 - 10:40 am

    @Subhyal Bin Iqbal

    I used SMA, but BNC would work fine if that’s what your antenna has.

  23. #23 by Subhyal Bin Iqbal on February 2, 2016 - 7:01 am

    Thanks a lot! And prior to RTL-SDR what receiver were you using?

  24. #24 by hpux735 on February 2, 2016 - 8:23 am

    I was using the receiver in the article. The “softrock”, but it’s no longer available as far as I know.

  25. #25 by Subhyal Bin Iqbal on February 5, 2016 - 3:28 am

    And what was the length of the RG-58 cable for the down feed from the antenna to the receiver/SDR dongle?

  26. #26 by hpux735 on February 5, 2016 - 7:12 am

    Probably about 20 feet.

  27. #27 by paul on April 11, 2016 - 12:35 am

    ive just made my own version of this ant but as i used wood dowels for supports and forgot to make copper links through..but i notice the coax version does not feed through support arms. all a bit confusing but there is different designs going round, but guessthey all work..
    cheers
    paul
    g4sbx

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