Satellite Antenna Design Questions using 4NEC2

Discussion in 'Satellite and Space Communications' started by KG4GUF, Jan 1, 2010.

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  1. KG4GUF

    KG4GUF Ham Member QRZ Page

    Hello, I've recently decided to try my hand at satellite work and found the great program 4NEC2.
    I started with a 2m circularly polarized crossed-yagi antenna with 5 elements and a boom length of slightly less than 1.5 meters (I'd rather not go beyond this if I can). When I got it figured out a bit better, I got an antenna with a freespace gain of 9.52dBi and I think 20 ohms input impedance and nearly perfect circularity. The first question, is how exactly should I do matching? The design I have on my screen right now has a T-match which gives input impedance of 187 ohms (close enough to 200). Should I just worry about the antenna and match it myself when I build it or should I try to work the T-match into the 4NEC2 design?

    Second question: my 3db beamwidth is 65 degrees right now freespace horizontally and vertically. Is there a way of decreasing this without substantially lengthening the antenna? I had thought a plane reflector, but haven't been able to try it yet.

    And finally, I'm also going to design a 70cm circularly polarized crossed-yagi antenna so I can use both it and the 2m at the same time (one uplink, one downlink). Is there a particular advantage to increasing the gain of the 70cm antenna vs. the 2m? Either one could be used to transmit or receive so it's not really question of if I want better receive quality for instance. I guess I'm really wondering if there's something about 70cm vs. 2m that would make increasing the gain of the smaller antenna more advantageous rather than trying to keep the two antennas with the same gain.

    I know I could operate from my rubber duck antenna on my handheld, or buy/build something simpler such as the arrow, but I like the challenge and I may yet try simpler stuff first before jumping in head first with the two bigger antennas. This is one of the advantages of comptuer-based design, I can experiment, see what works and what doesn't without wasting materials and money (time on the other hand is a different story :p)
    Thanks for any help,
  2. KA9VQF

    KA9VQF Ham Member QRZ Page

    I think I would use a quagie antenna instead of the yagi. Sure the boom would be a bit longer but I think the gain improvement would make up for it.
  3. WA9SVD

    WA9SVD Ham Member QRZ Page

    A Quagi isn't a circular polarized antenna, and likely has little if any gain over a Yagi of the same number of elements.
    While it's true a crossed-element (i.e. circular polarized) Yagi will have 3 dB less gain than a single polarized antenna of the same element number, the ADVANTAGE (particularly for satellites) is that you won't have to worry about polarization alignment in normal circumstances.
    As for boom length, that should be quite similar between a Quagi and Yagi of the same element number, if properly designed..
  4. AI3V

    AI3V Ham Member QRZ Page

    This is where most attempts to make a circular pattern with 2 orthogonal polarized Yagi's fail.

    The CP is VERY sensitive to mismatches between the 2 antennas, their matching networks, and the phasing harness used.

    Very slight errors will give you rather poor elliptical polarization.

    My advice, make each antenna as mechanically prefect as you can. Then adjust the matching sections for EXACTLY the same impedance. Then adjust the phasing harness (don't just cut and hope) for what you think is CP.

    Then rotate the antenna around it's polarization axis and MEASURE the circularity (or lack thereof).


    There is less thermal noise to hamper reception at 70 cm, and a antenna of the same physical size will have more gain at 70cm than at 2m. You also want to pay attention to the sidelobes of the antenna, they do nothing but add noise to your receiver. The concept you want to read about is G/T, or antenna gain/antenna temperature.

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