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Re: Antenna Questions



>Jon, NA9D wrote: {edited}
>Now, the next step would be to add Yagis.  
{snip}

>M-Squared makes a CP antenna now as well.  These are good, but lack
>polarization switching (nice, but not totally needed).  However, if you want
>to save money, a standard yagi will work fine as well, but with diminished
>performance.

EB: I believe M2 is now offering a polarity switch option for their CP
antennas...yes linear yagis will work but you sacrifice 3 dB gain due to
polarity mismatch with a circular polarized signal.  However, the satellite
signal shifts polarity with orientation and this is not as critical as it
sounds if you have a little extra gain to play with.  A Cushcraft 13B2 or
equivalent should suffice for AO-40 and usable with AO-10 part of the time.

>A CP antenna is certainly best as that's what the satellite uses.  But
>again, it's not necessary.  You'll just lose some gain in the overall link
>equation, but a standard yagi is far better than an egg beater, I think.

EB: Definitely if you want to work AO-10, 40, or the FO birds.

>There are also multiple lengths and sizes of the yagis for OSCAR work as
>well.  Bigger is better for AO-10, but for AO-40 you'll be able to get along
>with much smaller antennas.
{snip}

>Don't worry about preamps yet.  I have worked most of my satellite QSOs w/o
>preamps.  They do help, but start somewhere and work up.  You can always add
>them later.

EB: Here I disagree.  If you were to chose between an expensive CP
satellite antenna/no-preamp, and an ordinary 2m yagi/with-preamp, you will
find the second much superior.  One of the most frustrating things I have
encountered is the station with a loud signal on AO-10, who cannot hear
hardly anyone.  If you have over 30 foot coax without a preamp, you are
handicapped in hearing.  Also consider how good the receiver is that you
are using...the FT-847 with internal preamp/on is only 8 dBNF {measured
with HP NF meter}. Got to hear them to work them!  An ARR P144VDG costs $90?

>In terms of helix antennas, I don't think anyone builds them commercially at
>UHF.  But I may be wrong.  I've never used one so I can't comment on how
>they work.

EB: I built my own 437 MHz 8-foot Helix {don't remember how many
turns...theoretical gain 12 dBi}.  Worked OK for uplink to AO-10, but a
Hygain CP yagi was much better.  Old Antenna Labs and Directive Arrays
{link thru Downeast Microwave} make helical beams.  The main disadvantage
is you cannot switch polarity.  Also they are a bit bulkier than a yagi or
even a CP yagi.

EB: I now have the M2-436CP42U/G monster for 432, and four M2-2mxpol-20
x-yagis for 2m {definitely overkill for satellite work}.  A trick I use to
avoid needing low-loss coax to the transmitting antenna [assuming-modeUV]
is to place my 50w 432 PA right at the antenna in a wx-tight box.  It makes
me equivalent to the guy using a 100w PA in his shack but losing half his
power to coax loss going to the antenna.  I have a 8 amp/12v power supply
in the wx-tight box to power the PA.

EB: The most difficult thing right now will be deciding whether to build
for AO-40 or AO-10?  You'll probably know the answer in a few days! 

Ed
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