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Background - UHF - 434-438MHz Two identical crossed Yagi's mounted on rotator.

Currently, I have a low noise preamplifier after combining cables from the two antennas (T connector + matching stub).

I'm considering adding two separate identical low noise preamplifiers right after each antenna(on the antenna boom itself) and combining their outputs later with a T connector.

I am currently calculating link margin for one antenna + preamplifier till the T connector and directly adding 3dB gain after the T connector (in the post-preamplifier feed line) to add the gain from the second identical antenna system. However, this 3dB gain does not seem to affect my SNR much.

Am I adding the gain from the second antenna at the right place? Is the second system helping at all or should I use just one antenna with a preamplifier immediately after it?

user80551
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2 Answers2

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I'm guessing by crossed Yagi, you mean two Yagis with a 90 degree phase shift, creating a circularly polarized Yagi.

A circularly polarized antenna does not get you any gain. If the other end is linearly polarized, then there's a -3dB loss. The benefit is the loss is same no matter the orientation -- horizontal, vertical, or something in between.

If you use circularly polarized antennas on each end, and they are the same sense (right-hand or left-hand), then you avoid the 3dB loss. That's not the same as adding 3dB to your link budget. The link budget would be the same if you used two vertically polarized antennas on each end.

If I were adding a preamplifier to a circularly polarized antenna, I would add it after they have been combined. A couple reasons:

  • the two preamps may have different phase shifts or gain, which would mess up the phasing of your antenna.
  • the impedance the preamp sees (in input or output) might not be 50 ohms, and that may not work so well.
  • the preamp will probably alter the impedance seen by the T, and depending on just what kind of T you have, that may mess it up.
  • again depending on what you are using for the T, the isolation between the antennas may not be great, and this will result in one preamp trying to drive against the output of the other, which could result in oscillations or failure.
  • preamps generate noise, and two preamps generates more noise.
  • preamps cost money, and two preamps cost more money.
Phil Frost - W8II
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Use of 2 preamplifiers should be done symetrically. The usual reason to place the amplifier closer to the antenna is to avoid amplifying noise induced into the feed line and thus increase the S/N ratio BEFORE the extra noise is added improving the overall signal quality. If the end result is the same signal level it makes no difference except for the possible S/N ratio improvement.

Keith Martineau
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