One of the interesting
aspects of a mode B ( U/V ) transponder is :
because of the
2m output a lot of starting radio amateurs ( novices ) can receive
easily the signals, coming from the satellite transmitter. The path
loss values ( dB ) and the doppler differences ( f ) are small.
On 70 these
are larger. A receiving is more difficult. And perhaps the used
TRX produces now also more rxnoise.
Besides, the behaviour of 70cm waves is more critical, also concerning
antennae : adjustments, tunings etcetera, and unwanted objects,
causing reflections.
Moreover, the
2m transponder output power is sometimes higher ( can be more easy
higher on these frequencies ). Comparing for instance, SO-50 *
and HamSat, maximal ca 7 times ( ca 8.5 dB ), 140 mW or 21.46 dBmW
versus 1000 mW or 30 dBmW, in theory of course, if only one station
would work via the HamSat transponder.
The Unit dBmW
( or shortened : dBm ). This site contains a few small javascripts.
Which have sufficient exactitude and carry out desired calculations
( for example concerning 50 Ohm impedances ). Click here
or here.
Or see : Java Applications - OnLine Calculations - Relative & Real
Power or Voltage & Impedance REF .....
No QRM from
VHF telephones because of the 70cm uplink !
Assume, another
radioamateur, in the direct neighbourhood,
is working over the satellite. Because of the use of mode B no desense
filters are required because third harmonics do not disturb now
the reception. The harmonics are not
present in the rxband.
Now, it is not
difficult to receive the stations, which work via the satellite.
A very nice and easy way of 'making acquaintance with' a satellite
!
*
comparing
SO-50 and OSCAR-E ( a splendid project
! ) even delivers ( in theory ) a much bigger factor, maximal ca
57 times ( ca 18 dB ), 140 mW *
or 21.46 dBmW versus 8W or 39.03 dBmW. However, both satellites
have downlinks on 70, so here are no differences between ( downlink
) path loss.
*
If
you want to hear a short audio part, received from the SO-50 transmitter
( 140 mW, maximal 250 mW ), click here ..... The file concerns a receiving with help of a small
antenna under roof and without preamp, of course the signal is weak
now. The audio is only available if you visit the PA3GJE or PC1L
website. For the moment it does not work via the I-MODE website.
Another important
point is :
because of the
higher required conditions on 70 it is necessary to make dopplershift
corrections, to use rotator interfaces and more. A 70cm uplink is
more difficult. UHF must be prepared carefully. Because of the above
mentioned reasons the novice starts not at once certain transmissions
over the satellite. First he will only listen. And more learn before
he uses actively any satellite.
All this means
more operating practice. The satellite traffic will become more
correct.
And when the
time is ripe to start a transmission via the satellite ( so, 70cm
uplink mode B ), the absence of third harmonics in the rxband
is an advantage. A desense filter is not needed.
Finally, if
you want to see a few formulas concerning lineair mode B transponders,
click one of the lower submenu buttons.