L-Band IF Application Guide
Page L-Band - 9
to receive their minimum voltage plus enough margin for variation with time and
temperature.
2.
At L-Band frequencies the loss variation with temperature can be extreme. For example a
200 foot length of RG214 cable (double shielded, ½ inch class) has approximately 20 dB
of loss and a variation vs. Temperature of 0.2% (of dB) per degree Centigrade. If
operating in an exposed environment (like a desert) where the temperature may vary
approximately 20 deg. C from day to night that could represent a variation of almost 1 dB
over a 12 hour period. In a 20 dB loss cable the attenuation change is then approximately
.04 dB per deg. C, or 0.8 dB over the full 20 deg. C change. First, this probably says that
the cables must be either buried in conduit or shielded from the sun if run on a cable rack
to minimize variations. A better quality cable such as Times LMR-400 cable would also
provide a significant improvement.
Note: Temperature variation on SCPC links can easily be remedied by using the
AUPC option available for the PSM-4900 modems.
3. The transmit and receive cables must be separated and definitely not tied directly
together with “tie-wraps”, especially on longer runs. This is because of the tremendous
difference between the transmit and receive levels possible. This is made worse on long
cable runs because the modem end will have higher transmit levels and the receive will
have lower levels than on a short run. The better cables in this regard have double
shielding (two braids or a braid/foil combination) and a shielding efficiency of 100 dB or
better. A good note here is that with the typical LO frequencies as shown in the example
above, the transmit and receive L-Band frequencies are widely separated. If the signals
were within the LNB stability/drift frequency limits there might be a tendency for the
receive to attempt locking to its own transmit signal.
4. Considering the L-Band IF range is 800 MHz or more spanning close to an octave, the
variation in loss between the high and low ends of the IF range may be significant.
A nominal design point may be to allow for 10 to 15 dB of total cable losses and select cable that
will reliably achieve this. A more accurate “rule of thumb” would be to design for a total gain from
the antenna to modem receive input of 40 dB. For example if the receive LNB has a gain of 60 dB
and there are no other losses then the cable can have a maximum loss of 20 dB (60 – 40 dB). In
formula form this is:
Loss(cable max) = Gain(LNB) – 40 – Loss(misc)
in dB
Or for the transmit side the cable loss should not keep the input to the BUC from reaching its
Power input for 1 dB compression point Arbitrarily allowing a 3 dB margin in the modem output,
this formula would be:
Loss(cable max) = +2 – BUC Pin(1dB) – Loss(misc) in dB
Notice that we are assuming no miscellaneous losses in the transmit or receive side, but there
may be other losses such as a splitter or output sample port used.
Trying to buy too cheap a cable will only result in problems that are more expensive to fix than
using the proper cable to begin with. Remember that the L-Band design allows for moderately
inexpensive cable in exchange, especially as compared to the typical requirement for either
expensive outdoor converters or very expensive heliax / waveguide with indoor converters.
The PSM-4900L provides a Type “N” 50 Ohm impedance on its transmit cable and a Type “F”, 75
Ohm receive cable connection. Most BUCs are also 50 Ohm, while most LNBs provide a 75 Ohm
impedance and use Type “F” connectors.
Several cable types are shown below with typical maximum recommended frequency, size,
losses per 100 feet at 1.2 GHz, shielding efficiency, and relative approximate costs per foot.
Recommended cables are shown with asterisks. Since maximum loss is preferred to be 20 dB or
