Quote:
Originally Posted by 69harley
Gents,
My favorite antenna guru had this to say about the dual loop antenna posted in this thread.
We might have an opportunity to play with this in the coming months, thanks for bringing it to our attention. It's an interesting structure.
Begin forwarded message:
We have two loops separated by 7 inches or so and fed 180 degrees out of phase.
Whatever the impedance of the loops at the operating freq. (240-320 MHz), the coaxial cable "sees" 1/2 that. Thus if the input impedance to each loop is 100 ohms, the coax connector will see 50 ohm load.
The spacing of the loops is 7 inches or so, and the wavelength is (lets say 280 MHz -- mid-band) approx 42 inches. Thus the spacing is 7/42 wavelengths = 0.17 wavelengths. This is too small to form a "beam" antenna. But; note that the loops have horizontal polarization along the horizon (if held with the plane of the loop horizontal) (it behaves as a magnetic dipole oriented vertically). The loops also have a polarization null directly overhead. And, since they are fed 180 deg. out of phase, they have a null at the horizon. The final pattern is a broad lobe at 30 to 60 degrees or so from vertical and nulls overhead and at the horizon. The polarization is "horizontal" from the point of view of a satellite near 45 degrees from user overhead.
So, we have a donut pattern.
Caution, there is also a downward mirror image pattern. Its reflection from the ground is messy.
Knowing the pattern can we estimate the gain? Not off the top of my head. I will go out on a limb and guess the gain is near 4 dBi minus losses due to the impedance mismatch.
-End of quote.
We built an antenna like the one in the pictures and will run it through testing in the test chamber in the next couple of months.
I will post the test results when they are completed. Not to discount any of the claims here, but claiming a gain above 10db for this antenna is ridiculous and irresponsible. Especially in this forum.
This in no way implies that RF engineers and their antenna modeling software programs are always right. According to most of them my 113 foot long piece of wire laying on the ground should never work, but it does.
I will post the formal results when they are completed.
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We have this antenna scheduled to go into the test chamber next week. I think the 44 db gain claim is way off and is borderline silly to claim in this forum.
We built one of these, and were able to close the loop on a good SC 25K bird. We did some preliminary testing using a PSC-5D and a 117F in manpack mode, with and external 25 watt amplifier (RAMP-25) and an external 100 watt amplifier (TSE-AM-SAT-100). In all three modes we did side by side testing with the experimental antenna, an AV-2055 without extenders and an AV-2011.
The 117F and PSC-5D were able to max all loopback tests with the 2011, regardless of amplifiers used or not used. With the 2055, loopback test were consistently in the 80 percent range. The experimental antenna averaged 65 percent.
The 20011 has 15 db of gain, the 2055 without extenders is 5 db, we guess the experimental to be around 4 db of gain.
This antenna works, but nowhere near the 44 db claimed.
Like I said, next week it will go into the RF test chamber, the results will be posted here.