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Re: S-Band Antenna Comparisons, etc.



At 02:21 PM 1/7/01 -0600, you wrote:
>Cliff, you are incorrect.  The linear polarisation is determined by the 
>"Dipole" Element in the center and not by the direction of the grid.
>
>The reflector has nothing to do with it.
>
>Dave

Well, it is certainly true that the polarization is determined
by the feed but most of the BBQ "dishes" I have seen are only 
parabolic sections and not a full (round) dish so the gain 
would be a lot lower if you didn't use the direction of the bbq rods.
Comments?
73,
Tony

>At 14:17 1/7/01 -0500, you wrote:
>>Cliff,
>>
>>The bar-b-que style of dish that Jerry is using can, I believe, only
>>support linear polarization, in the direction of the bar-b-que rods.  It
>>could support any polarization, however, if it was lined with metal
>>screen.  That shouldn't be hard to do, by the way.
>>
>>
>>Clifford Buttschardt wrote:
>> >
>> > Jerry, it is not clear to me if the dish antenna is linear or were you
>> > able to compare both with RHC?  Truthfully, I have had a devil of a time
>> > with the small dish and about ready to go to the helix regardless of gain.
>> > Cliff K7RR
>> >
>> > On Sun, 7 Jan 2001, Bob Snyder wrote:
>> >
>> > > I did some quick helix antenna runs using NEC.  These runs use an
>> > > infinite backplane, so they are only approximations, but may be useful
>> > > for comparisons.  The 7-turn helix you describe (with 44 mm dia. i.e.
>> > > 1.106 wavelength circumference; and 37 mm spacing, i.e. 15 degree pitch)
>> > > has 11.9 dBi gain.  The 16-turn helix (I assume it is built to nominal 1
>> > > wavelength circumference and 12 deg pitch) has 14.8 dBi gain.  The
>> > > parabolic reflector on the 7-turner may be enhancing it's gain somewhat.
>> > >
>> > > 73,
>> > > Bob KD1VV
>> > >
>> > >
>> > > K5OE@aol.com wrote:
>> > > >
>> > > > I did some antenna experiments this AM when AO-40 came over my 
>> horizon (1200
>> > > > utc, MA=4.5).  Those window-shopping the various options may be 
>> interested in
>> > > > these observations.  What can be determined from current 
>> observations with
>> > > > respect to the final satellite configuration is subject to considerable
>> > > > speculation:  these observations were taken with a squint of 
>> greater than 140
>> > > > degrees on AO-40's low gain antenna at low power.  What one can 
>> extrapolate
>> > > > to a favorable orbit, orientation, and squint, plus the change to the
>> > > > high-gain antenna and the high power amplifier, is clearly just 
>> speculative
>> > > > at this time.
>> > > >
>> > > > In a quasi-designed-experiment, I used several antenna and 
>> downconverter
>> > > > combinations using a Drake 2880 and my Yaesu FT-100 as a receiver, and
>> > > > recorded the results enumerated below.  One unknown in my 
>> measurement system
>> > > > is the accuracy of the meter on the Yaesu.  I have previously 
>> "calibrated" it
>> > > > using a variable power 2 m FM signal, but only in the crudest sense 
>> (after
>> > > > all, I am an "amateur").
>> > > >
>> > > > 1.  The 24 dBi dish antenna (bar-b-que style, borrowed from KK5DO), 
>> does
>> > > > indeed offer a 12 dBi gain (2 full S-units) over my 16-turn helix 
>> (estimated
>> > > > at 12 dBi).  This was consistent with-and-without a preamp.  Note 
>> to those
>> > > > without automatic antenna tracking:  this antenna's beamwidth was 
>> so narrow,
>> > > > that 12 dB of signal was lost in +/- 5 degrees of movement.
>> > > >
>> > > > 2.  The Conifer preamp does NOT provide the rated 10 dBi gain, but 
>> appears
>> > > > much closer to 6 dBi (when close-coupled to the Drake).  It was 
>> difficult to
>> > > > judge the impact on S/N ratio with this setup, but I believe it was
>> > > > substantially improved, even though the noise floor is up 6 dB as well.
>> > > >
>> > > > 3.  At MA=18, I could detect the beacon with a coffee can and 
>> preamp/Drake
>> > > > combination, but it did not move the S-meter.  I could not hear the 
>> beacon
>> > > > without the preamp.
>> > > >
>> > > > 4.  Using an unmodified Drake 2880, I could not detect the beacon 
>> at 121.3xx
>> > > > MHz with both the dish and the preamp in line.  This surprised 
>> me.  If I have
>> > > > the frequency conversion for the stock crystal wrong, please let me 
>> know.
>> > > >
>> > > > 5.  In comparing the 16-turn helix I have been using for a month to 
>> a much
>> > > > smaller one built for me by my friend Oleg, RV3TH, the two were fairly
>> > > > comparable.  The interesting thing is that helix is only 
>> 7-turns!   Oleg's
>> > > > helix employs some non-conventional metrics:  the diameter is about 
>> 44 mm and
>> > > > the turns spacing is about 37 mm, plus it uses a 100 mm semi-parabolic
>> > > > reflector.  I will report more on this after I completely 
>> reverse-engineer it
>> > > > :-)
>> > > >
>> > > > For those interested in seeing the hardware, I have put a few 
>> pictures of
>> > > > this equipment at:
>> > > > http://home.swbell.net/k5oe
>> > > >
>> > > > 73,
>> > > > Jerry, K5OE
>> > > >
>> > > > ----
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>
>
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>

Tony AA2TX@amsat.org
North Andover, MA

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