improving TX power

Transmit Pointing
Accurate pointing is really important as the higher frequency transmit beam is narrower than the receive beam.

Using the receive signal, the top of the beam is quite rounded and it is not possible to find the beam centre simply by adjusting for the maximum. The signal measurments you make are subject to small fluctuations all of the time and you cannot centre the beam sufficiently accurately.

To peak up the elevation.  Mark one flat on the elevation adjustment nut with a felt tip pen. Now adjust the nut till you get a significantly degraded receive signal, approx 3 to 5 dB down.  Record this value carefully, averaging over some time. Note the exact postion of the nut and the marked flat.  Now carefully wind the nut till you are on the opposite side of the beam and adjust to exactly the same degraded level as before, while counting the turns and flats.  By calculation, half the count of turns and flats and then adjust the nut back to the exact centre. This works fine for elevation, where there is no backlash in the adjuster.

For azimuth, the same method will work if you always move the antenna in the same direction, which is difficult, to remove backlash.  Alternatively, if your azimuth adjuster has two nuts along a screw, then space the nuts wider apart and repeatedly swing the dish sideways to gently rest against each nut alternately. Adjust the nuts till you get exactly the same significantly degraded receive signal at each nut.  Then wind both nuts in by the same amount of turns and flats.

Undistorted dish
For the best transmit gain the dish shape must not be distorted.  Most VSAT dishes have a rim that is intended to be flat.  Stretch tight threads or fishing line up/down and across the front.  The lines should just touch (<1.5mm for Ku band). If they don't, you will get low gain.  Some dish designs have short rear side struts which need to be tight to prevent the sides of the dish being pulled forwards by the weight of the feed assembly. Read more: Correcting dish distortion.

Modem output power
The BUC is an amplifer with a rated maximum output, e.g. 2 watts (+33 dBm).  To make it give out its rated power it needs an input level of say -22 dBm.  You must not exceed this value as it will cause interference to other people on the satellite and may also permanenty damage your BUC.  
Normally, under clear sky conditions, the BUC will be operated several dB below its maximum. e.g. -6 dB down or 0.25 watts. During heavy rain the hub observes the signal degradation and tells your modem to increase its output by up to 6 dB.  This is called closed loop uplink power control.  The hub should know in its database what is the maximum value it is allowed to tell your modem to use. This maximum value is derived during commissioning when the hub adjusts your CW levels in steps and finds the BUC -1dB gain compression point (rated maximum power).
The cable between your modem and the BUC has a loss, e.g. -3 to -9 dB, depending on its length.  Since sites have different cable lengths and since BUC gain varies from BUC to BUC (e.g. BUC specification: 55 dB +/- 5 dB) modem output levels will vary significantly from site to site.
A cable with internal corrosion due to moisture will have an extra high loss and must be replaced or have several metres of the bad end cut off.  

Best regards, Eric.

The procedure for installing iDirect is here:
https://www.bnlsat.com/idirect_installation_guide.pdf

The transmit power output of iDirect modem is adjustable -35 dBm to +7 dBm according to this:
iDirect Evolution modem specification

So I can't see why the hub considers your -15.5 dBm as a problem.  The hub need to measure your BUC -1dB gain compression point and then compare that figure with the actual -15.5 dBm.  Hopefully your transmit clear sky operating point, which will give the nominal 9 dB S/N at the hub, will be well below the BUC -1dB gain compression point , which will allow you perhaps as much as 6 dB increase during heavy rain.  The modem can go up to +7 dBm, so the modem output capability is not a factor in the situation.

Sites with long, high loss, cables will need higher modem output levels.

Best regards, Eric.

The hub wants all bursts from remote sites to arrive at the hub at the same level into the hub receiver line card. It therefore adaptively tells remote sites to decrease or increase their modem output levels so that the bursts all arrive at the hub with say 9 dB C/N.

During a rain fade the hub might tell a site to increase its level by up to 6 dB; when the sky clears it will tell the modem to reduce its transmit level.

Prior to going operational,  the hub needs to know and record, for each site what minimum and maximum modem output levels to apply. The hub must never tell a site to transmit at such a high level that the BUC -1dB gain compression point it exceeded. The minimum value should be such that the bursts will successfully be received, albeit with poor quality and high error rate. Once operational, the hub will adjust the remotes to increase or decrease their levels to keep all the bursts at 9 dB C/N at the hub line card.

Actual levels will vary from site to site, particularly due to different cable lengths and losses in those cables. Position of the site in the uplink beam coverage map, the BUC power rating and dish size are also factors.  If a site's modem output power gradually creeps up, over several months, then the cable may be corroding due to moisture or the antenna may be becoming mispointed. Corrosion can have strange effects as it also affects the DC power supply to the BUC and once this voltage/current goes down the power output capability of the BUC decreases and the -1dB compression point comes down and the transmitted signal becomes distorted.

It is bad if the BUC is ever put into saturation (above the -1dB compression point) as this causes interference and damage to the BUC. The hub may do this as it interprets a distorted signal is weak/low level and then tells the modem to transmit at even higher level. But, this will not happen if the site has been tested and the -1 dB compression point set as the maximum value the hub will ever tell that site to use.

Read more: https://www.bnlsat.com/idirect_installation_guide.pdf

This specification for the iDirect Evolution X3 says that the output power may be adjusted from -35 dBm  to +7 dBm.

Best regards, Eric.