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Re: Quadrifilar home brew



Dave,
Plots I have on some commercial QFH antennas show gains up to 6 dBd as you
loose 3 dB for not being circular. Two main types exist with reasonable F/B
ratios. The best F/B ratio is a 1/2 turn 1/2 wavelength per side (1 wave
length loops) gives 140 degree beamwidth @-3dB, being circular that is still
3 dB more than a dipole at +/- 70 degrees. F/B can be over 25dB The Axial
length of this antenna is 0.3 wavelength.
If you go to a 1 wavelength per side or two wavelength loops with a full
turn you have an antenna with a peak gain @ 25 Degrees elevation with a -7
dB at 90 degrees, but when the satellite overhead it is more than 7 dB
closer (+6dB =1/2 range) The Gain point is changed by the diameter to length
ratio, but best F/B is .6 axial wavelengths and a F/B of 17 dB for this
design.

The first design is best if you do some pointing , but not critical pointing
+/-20 Degree accuracy gives optimum gain during the pass.
The second design is the best choice for a fixed straight up antenna. It
works well down to less than 10 degrees, good signals above that elevation
and no overhead or rotational fading.
Both antennas have front to back ratios that offer good reduction of ground
noise which is important for frequencies above 2M. Two or more Antennas on
different bands can be constructed on a common mast with the smaller wave
length antenna in side the larger one. All QFH antennas exhibit a very good
volume to gain ratio making them compact for the gain that they offer. They
also are circular polarized on the sides giving good reduction on Multipath
in metropolitan areas when used with satellites that are like polarized. .

Art KC6UQH

----- Original Message -----
From: "Dave Guimont" <dguimon1@san.rr.com>
To: <amsat-bb@amsat.org>
Sent: Monday, March 14, 2005 3:57 PM
Subject: [amsat-bb] Quadrifilar home brew


> Apparently some people are having success after constructing the
> quadrifilar antenna described at:
>
> http://home.san.rr.com/doguimont/uploads
>
> The antenna construction file is Quadfil1.zip and describes measurements
> for 2M, 435 and wx antennas.  Very inexpensive (the N connector costs more
> than the rest of the antenna) and easy to build, but measure the lengths
> carefully, particularly at 435, and obviously a short and low loss feed
line...
>
> The users keep asking me for gain figures, and I have absolutely none...
> Coverage is nearly hemispherical, and estimating 2-3 db at boresite...with
> an estimated beamwidth of 150f...
>
> I have a narrative for some comparisons at ANTEVAL.TXT.
>
> Be very happy to answer questions.  I've not tried it on AO-51, but it
> should work very well reading reports on HT users...I'm very interested in
> your successes or failures...
>
> It worked, but producing a very weak signal, when AO40 was at minimum
> squint angles...Another reason for me being anxious to do the testing on
> P3E..!!!
>
>
>
>
>
>             73, Dave wb6llo@amsat.org
>                     Disagree: I learn....
>
>                Pulling for P3E...
>
>
>
> --
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> ----
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