SatSig topic:

replacing 1.8m antenna with 2.4m

(Read 12950 times)

Mar 23^rd, 2014 at 6:42am

Dear all
please advice ma as soon as possible i work with hub system with antenna 7.2 m and 2.2 kw buc for the hub and 1.8 m for the vsat terminal with 5w buc
can I replace 1.8 m antenna with 2.4 m ?
if yes can I use the same buc ?

Thank you .

Mar 23^rd, 2014 at 10:25am

If you use a larger remote VSAT dish, to get the same receive result at the hub, you will need less power from the BUC, so the modem will put out lower power and the BUC will put out less than it did before.

In the other direction, from the hub to the remote, the hub will also be able to reduce its transmitter power, while getting the same C/N at the remote site as before.

Alternatively, if the outlink carrier from the hub to the remote remains unchanged, the C/N at the remote will improve, making less bit errorsm and less outage time in rain.

In the direction towards the hub, if the remote power is kept the same, then the carrier level on the satellite will increase. You need to ask the satellite operator for permission to do this. Check first.

The difference from 1.8 to 2.4m diameter is +2.5 dB.

Note that the gain of your dish depends on it being assembled correctly, with correct feed, in focus, and with no dish distortion. Such problems can easily reduce the gain by 3 dB.

If you feel you have poor performance with your 1.8m dish, check its rim is flat using the cross strings test and check its pointing.

See:
https://www.satsig.net/pointing/rear-side-struts.htm
https://www.satsig.net/vsatflat.htm

Best regards, Eric.

Mar 23^rd, 2014 at 11:02am

Thank you Eric for fast response

I want to replace only one VSAT terminal from 1.8 to 2.4 m and the rest of the terminals remain with the old 1.8 m so how to determine buc power for the new 2.4 m terminal ?

Thank you .

Mar 23^rd, 2014 at 5:29pm

Just use the same 5 watt BUC but with lower drive power from the modem, reduced by 2.5 dB. i.e. about 2.8 watt out, instead of maximum of 5 watt.

A 3 watt BUC would do, but consider the spares situation if you have 5 watt BUCs at all your other sites.

Best regards, Eric.

Mar 25^th, 2014 at 8:51am

Dear Eric
Thank you very much for your support, your feedback is really helpful.

Apr 3^rd, 2014 at 2:06pm

Dear Eric
if I have 5W BUC needs 48 w to work what happen if I give the BUC less power say 35 does this mean it can transmit with less power or it will not work at all ?

See the link below datasheet of the buc.

https://www.vikingsatcom.com/pdf/NJT5669%5E70%5E75%5E77.pdf

Thanks .

Apr 9^th, 2014 at 11:15am

I think the correct answer is to contact the manufacturer NJT for advice.

My comments:

Beware of damage to the power supply. The power supply should be rated above its potential load. If the power supply is operated at its maximum it is critical that it is properly cooled, as overheating will greatly reduce its reliability. If the power supply is significantly overloaded and it has built in over current protection then it may trip off under overload conditions. Read up on the power supply specification.

To get maximum power to the BUC, use thick copper cable with low DC resistance. If the power supply volts is adjustable set it to the highest voltage compatible with the BUC specification, so as to keep the current down. The BUC has a wide range of input voltage as it (probably) uses a switch mode power supply inside itself to generate the final DC voltages and currents for the BUC electronics.

My experience with BUCs is that the current taken varies with the transmit power. If you gradually increase the power power in 1 dB steps and measure the current, you can plot the curve.

If you provide the BUC with limited power then its maximum output will be reduced. I would measure its new gain compression point P-1dB and make sure I never operated above this.

There is risk of non-linearity causing transmit interference on the immediate adjacent frequencies due to carrier spectral regrowth, plus intermodulation if you transmit multiple carriers.

There will be capacitors at the output of the power supply and also inside the BUC. The transient response to an overload may be to output the intended power for only a very short time, followed by decay to the overloaded condition. This may affect the transmitted burst such that its amplitude envelope is abnormal or its frequency chirped, causing rx burst errors at the hub.

If the power supply module also serves other purposes, e.g. +5V, +12V then these supplies may be affected by voltage jumps when the BUC transmits a burst. This can cause receive errors at the site when the site transmits a burst.

Summary: Using an inadequate power supply is not recommended !

Always switch OFF at the AC mains power wall switch before doing anything with the coax cables, as transient voltages may damage either the BUC or the modem.