The 13.6v limitation for the ALT-512 is primarilyfor performance reasons, not for safety. It is also partly for summer heating reasons in locations prone to very hot summers.
The developer’s primary goal was to build a top performing QRP transceiver in terms of its ability to receive weak signals in the presence of strong near-by stations.
The ALT-512 is an SDR transceiver using Direct Conversion with a “Virtual Intermediate Frequency” (V-IF) just a few kHz removed from the operating frequency. This is very similar to a technology known as Homodyne.
The developer’s goal has been achieved by using high-precisions components in critical stages (i.e., SMD Capacitors with 0.01% tolerance) and very careful adjustment and balancing (in software) of dozens of parameters. In many cases it is working with voltages of 0.0012v (1,2 millivolts).
The accuracy of tuning and balancing these parameters affects most of the critical specs of the radio. Maintaining this balance depends highly on the stability of the supply voltage.
Although voltage regulators are commonly thought of as being very stable, in reality their stability is a percentage of the applied voltage. When working with small voltages such as 1,2mV, the difference in regulated voltage when applying 13.6 volts to the regulator compared to applying 10.5 volts to the regulator, is significant, but tolerable. A higher supply voltage results in performance degredation.
By limiting the range of the voltage source, you limit the maximum variance of the regulated voltage applied to the critical stages of the radio.
The ALT-512 has been carefully tuned ("balanced") with a supply voltage of 12.6 volts. The radio achieves its ultimate performance level at this voltage.
Thus the recommendation for 12.6v.
Its specified usable supply voltage range is 10.5v to 13.6v.
What happens if you disregard the supply voltage limitations and run the radio at 13.8 volts? Does the radio go up in smoke and self-destruct?
Of course not, but the outstanding specs of the radio in terms of Dynamic Range, Carrier Suppression, Filter Shape Factor, etc., begin to deteriorate. For instance, the ALT-512 has excellent carrier suppression (about 65dB). When run at 14 volts, this drops by 3 or 4dB. It still has a very good suppression, but the radio is not operating at its fullest potential.
Of course. if you run higher voltage, and the ambient temperature outside is high, there is a chance of over-heating the radio when running it at full power.
The ALT-512 is a QRP transceiver and easily achieves QRP power levels. Increasing the supply voltage to increase output power is counter-productive and may result in reduced performance.
If you increase the power this way, it is no longer a QRP radio!
When a higher voltage is the only power source available, the logical solution is to lower the supply voltage with one or two diodes. (See figure 1 below).
These Power Adapters are simple to D-i-Y, and if there is enough demand, Aerial-51 will supply the adapters through its Reseller-Partners.
The ALT-512 was designed by successful contesters for the purpose of winning contests. Its primary focus is on performance. In order for it to run at its maximum capability, maintaining a tight supply voltage range is necessary.
Simple, Easy Solution:
Install a diode inside of the power supply, between the regulated 13.8v output and the front-panel + connection.
This diode reduced the voltage from 13.8 to 13.17 volts.