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Couple questions about a 2.4 ghz antenna project.

Discussion in 'Antennas, Feedlines, Towers & Rotors' started by KC9LGV, Jan 19, 2009.

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  1. KC9LGV

    KC9LGV Ham Member QRZ Page

    What I am wanting to do is build an antenna for an ATV transmitter, and possibly a future WIFI application out of an 8 foot C-Band dish with the Ku-Band rated mesh on it. I would like to use a high gain 2.4 Ghz patch antenna for the LNB. The patch panel has about 20.5 dBi, and the dish itself gives about 35 dBi of gain. I will be attaching a standard 2.4 Ghz video sender into this antenna with 25 mw of power.

    1. What would the total gain be if the patch panel is placed at the focal point of the dish?

    2. What polarization is a stock video sender's antenna?

    3. What polarization should a transmitter be in this application for other hams to receive my signals properly (linear vs. circular)

    4. Is there anything obvious that I am missing that will make this not work?

  2. N8EKT

    N8EKT Ham Member QRZ Page

    First of all, use RHCP since you will never know what polarity is being used on the other end and RHCP will tend to reduce fade and flutter.

    Second, the gain of a typical 8 foot dish would be about 34Dbi at 2.4ghz.
    But the resulting beam width will be so narrow that it will be useless for anything other than fixed point to point use.
    Dishes are only good for line of site and fixed point to point use.

    Your patch antenna is much better suited for your needs.
  3. N4AEQ

    N4AEQ Ham Member QRZ Page

    I use a dish and small patch for

    I used a small patch ant for many projects, every time Ive looked up the specs
    for H or V they say horizonal is normal with ant jack on the bottom & turn it
    90 deg or to the side for vertical. Video senders Ive seen come in either way
    but seem to be alittle stronger verticaly. Years ago after the war driving craz
    the war spying thing was popular (driving around scanning for 2.4ghz secutirty cameras) I tried it awhile but like all things it was time to move on.
    I also tried on on my 8, solid aluminum C band dish, work great for wifi but not
    much else, I put a ku lnb in its place for FTA stuff & just like the patch antenna
    you could rotate the lnb 90 degrees to H orV.
  4. AI3V

    AI3V Ham Member QRZ Page

    Several things to consider when you build a feed for a parabolic dish.

    1- A good compromise between maximum possible gain, and cleanest pattern (Minimum sidelobes) is to have the feed beamwidth 10 db down in intensity at the edge if the dish.

    {Search for G/T, or Gain/Temperature for more info,This G/T is particularly important when the antenna is pointed at "cold sky", for satellite or EME work}

    2- To calculate this, we need to know the dish's F/D ratio ( the ratio of the focal distance, to the diameter of the dish.) AND we need to know the beamwidth of the feed.

    So, Measure the angle the dish subtends when you are at the feed point, and measure the (-10db) beamwidth of the patch antenna.

    If the patch antenna beamwidth is narrower than the dish, the gain will be down, and the sidelobes will be down. ( I think this is what you have)

    If the patch antenna beamwidth is wider than the dish, the gain will be down, and the sidelobes will be up.

    If the patch antenna beamwidth matches the dish, the gain will be up, and the sidelobes will be down.

    The 35dbi figure for the dish is with a optimal feed.

    A circular polarized feed will change "hand" when it reflects off the dish, RHCP will become LHCP and vice-versa, also, the hand will change with each sidelobe- RHCP will become LHCP on the 1st sidelobe,RHCP on the second sidelobe,LHCP on the third, and so forth.

    Last edited: Jan 22, 2009
  5. AE5JU

    AE5JU Ham Member QRZ Page

    I made a "cantenna" or "waveguide" using two Heinz baked beans cans soldered end to end. The cans were 3.25" dia. Hunts Pasta Sauce is also the right dia. 3.25" dia (83 mm) diameter cans are just about ideal for 2.4 ghz.

    I used an N socket with a piece of brass tubing soldered on the center conductor 1.21" long, which is a 1/4 wave of 2.4 ghz.

    I use pop rivets to anchor the N socket in the can as my hands were too big to get nuts inside.

    That has to be a specific distance from the back wall of the "waveguide" (or tin cans).

    Use this calculator:

    With a 3.25" dia can, put the 1/4 wave element 2.63" from the back of the can.


    Next I had a pigtail made up about 15" long that had an N plug on one end and RP-SMA connection on the other. This will connect to many routers, such as D-Link. Also connects to a USB stick I have.


    So, why so short on the pigtail? Because at 2.4 ghz coax is very lossy. I wanted to minimize that. I have a 10' USB Male-A to Female-A cable that goes from my laptop to the USB stick. Then the short pigtail connects from the USB stick's RP-SMA to the can's N connection.

    I found this USB wireless adapter on ebay that has a RP-SMA socket on it and normally uses a little "rubber ducky" antenna as usually found on wireless routers.


    A coat of gray paint on the cans make it look a little more presentable.

    I have the can ty-wrapped to a camera mount, and that can sit on an old video camera tripod to hold it steady.

    I almost forgot... about 12 db gain and 30 degree wide radiation pattern.
    Last edited: Jan 24, 2009
  6. KC9LGV

    KC9LGV Ham Member QRZ Page

    I did some informal tests by just taping the sender antenna on the scalar ring of the dish and driving around and was impressed with the results. I could achieve about a country block of distance with the receiver sitting on the top of my truck and still have a static free display. The beam width is fairly narrow, but not horrible. It is about half the distance between two telephone poles of clear signal at 1/2 mile away.

    I had looked into the specs of the dish and possible patch antennas and found that the patch antenna beamwidth is going to be quite small compared to the area i will need to light up the entire dish. (15 degrees vs. 90 for the dish) The waveguide antenna is a good idea, and the can could be shallowed up to give the appropriate beamwidth to light the whole dish. Could I make the polarity changable between linear and circular by moving the N plug (SO-239 in my case) from the side of the can to the middle of the back of the can? Also could the brass rod be extended to 1.83 inches for a 5/8 wave and gain an additional 3db or would that throw off the front to back reflection calculation?
  7. AI3V

    AI3V Ham Member QRZ Page

    You are on the right track.
    2 ways to make CP with this style feedhorn,

    1- Install a second identical probe, the same distance from the back wall, but at right angles to the first probe, feed both probes with equal amplitude, but 90 degrees phase difference (quadrature hybrid) (note probes may NOT touch each other.)

    2- Install a second probe, 1/4 wavelength ahead of the first probe, and at right angles to it, feed them equal amplitude, in phase. (ordinary 2x divider)

    In either case, you would switch between LHCP and RHCP By swapping the coax from the dividers to the probes.
    No, the gain is set by the opening of the horn, less gain- just a can, more gain- the can with a attached funnel in front.

    Also, the length of the probe is not very critical at all, because the ratio of the length to diameter is very small (compared to a 80M dipole!!!), The VSWR bandwidth of the probe is VERY broad.
  8. AE5JU

    AE5JU Ham Member QRZ Page

    I was thinking just the can, no dish. That would give you 12 dbi and 30 degrees.

    I don't think so. I know the 1/4 wave element has to be a specific distance from the back of the can by some formula, but I don't know the math offhand. I also know it relates to the diameter of the can, and that there is an optimum size for the can vs freq. For 2.4 ghz, the 3 1/4" dia can is just about idea.

    I don't know. I just went by the calculator at the site I provided a link for above.
  9. N4AEQ

    N4AEQ Ham Member QRZ Page

    build alot of cantennas but

    Ive noticed they work better if you can find a can that doesnt have ribs on
    the sides or on the bottom, I hever could find a flat bottom tin can so I used
    a 3.5 dia flat aluminum disk I had in the junk box. I matched a can that had the same dia & glued it to the bottom. After all this is a waveguide type antenna and all surfaces need to be clean and straight, any can will work ok
    but the little things help get a few dB more.
  10. AE5JU

    AE5JU Ham Member QRZ Page

    Yeah, but the can manufacturers just won't cooperate and make cans with no ribs.

    I used what I could get. It works. :D
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