[amsat-bb] antennas and radios for that which is presently on orbit (or soon will be)

Paul Stoetzer n8hm at arrl.net
Tue Mar 24 10:36:22 UTC 2015


100W EIRP is, in most cases, more than enough - and is generally the
most power recommended to be used on any of the linear transponders.
Of course, if your path to the satellite is blocked by trees, this
calculation may change.

Let's look at the link budget for what is probably the worst case
downlink - for SO-50 at the horizon.

SO-50 TX +24 dBm
TX Antenna Gain  0 dB
Path Loss -155 dB
RX Antenna Gain  +10 dB

                -121 dBm

This is probably just below usability at the horizon for most
receivers, even assuming you have zero feedline loss, so you'd need to
add more antenna gain or a preamp. If you're beaming through a tree,
subtract at least 10 dB from that signal strength. Polarization
mismatch can also affect the signal you receive with periodic fading
if you do not have switchable V/H or RHCP/LHCP polarization.

For a real world example, my portable station - an Arrow antenna, 6 ft
of LMR-240, High Sierra Microwave preamps for both 2m and 70cm, and 2
FT-817s - can hear and work all the satellites to the horizon (when
I'm sufficiently clear of obstructions and in a low noise area).
Sometimes I could use more transmit power - maybe 10 or 20 watts, but
the five watts does it in most cases. On Saturday morning, I worked
SP3QDM on FO-29, at a range of 6,913 km from here in Washington, DC,
using this portable setup
(https://twitter.com/PRStoetzer/status/551437882205171712), so it
definitely does not take large antennas or high transmit power.


Paul Stoetzer, N8HM
Washington, DC (FM18lv)

On Tue, Mar 24, 2015 at 4:57 AM, Eric Fort <eric.fort at gmail.com> wrote:
> This raises the question of where, when, and why are rotors and directional
> gain antennas required and/or necessary.  sure,it's preferable not to be
> radiating power in a direction not directed to the transponder but I'm more
> so inquiring about the limiting factors here.  If I make the assumption
> that I have 100W or even 1000W at the antenna feedpoint,I likely can
> generate enough EIRP to reach the sat with full quieting (or is getting
> enough EIRP a problem here?)  likely more of the issue is one of effective
> rx sensitivity and system noise figure, along with sufficient rx antenna
> gain to hear the down link.  It would be helpful for someone to demonstrate
> the required up link and down link link budgets and how they apply here.
>  where if at all is a setup with rotators and directional antennas required
> to work current or expected sats and when can a modest station with
> sufficient uplink power and excelent rx sensitivity/selectivity be expected
> to get the job done?
> Thanks,
> Eric,
> On Mon, Mar 23, 2015 at 10:54 PM, Clint Bradford <clintbradford at mac.com>
> wrote:
>> Modest yagis are more effective than eggbeaters ... Computer-controlled
>> yagis with an elevation rotator is an "ideal" setup for many.
>> You will receive some excellent suggestions from those with more elaborate
>> systems than mine
>> Clint K6LCS
>> Sent from my iPod touch.
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