Electronic – How are RS 485 transceivers implemented

If sending data, a higher transmission frequency means a higher data rate. Consider an FM transmission - it "deviates" a carrier between two different frequencies in the spectrum (\$f_0\$ for data 0 and \$f_1\$ for data 1) and, for a given frequency deviation (measured in hertz), the receiver is able make a more robust distinction between the two frequencies when there are more cycles of the carrier wave to detect.

On the other hand, at higher frequencies, the attenuation between transmitter and receiver is greater. This is known, due to work originally done by Friis. Link loss for a radio system is:

Link Loss (dB) = 32.4 + 20log\$_{10}\$(F) + 20log\$_{10}\$(d)

Where F is in MHz and d is distance between the two antennas (kilometres). This is the standard free-space link loss equation - there are anomalies at some frequencies that make it more effective to transmit at a higher frequency but this is a massive subject.

These are two reasons that I consider significant but, there are plenty more that are important. For instance, a higher frequency means smaller antenna size generally but, lower frequencies can be received, due to ionospheric effects at points on the earth well below the horizon (higher frequencies tend to be more-of-sight).

Given that we tend to be "running out of" clean and utilizable air waves (quicker than we are depleting our fossil fuels!), any disadvantage to using higher frequency is a moot point.

Shortening an antenna from its ideal length is not a problem providing you accept and possibly counter the limitations that shortening brings. Here are some words from this site that tell you the story: -

A shortened dipole is simply a dipole antenna that has been shortened. Since it is shorter than its resonant length, it will not be resonant and will exhibit both resistance and reactance at the feed point. Shortened antennas tend to have a capacitive reactance and therefore need an inductance to cancel the capacitance and bring the antenna back to resonance. Normally the resistive impedance also drops as the antenna is shortened, so additional impedance transformation will be needed to effectively match the antenna.

A shortened dipole will act similar to a full sized resonant dipole in many ways. The effect of ground will still be important, current will be maximum in the center and very close to zero at the ends with maximum voltage at the ends and minimum in the center. If the antenna is center fed, it will still be balanced, with equal voltage and current distributions on both legs.

From the principle of conservation of energy, we know that if we can feed energy into an antenna, it will radiate. We also know that with a suitable matching network, we can feed power into nearly anything, including a shortened dipole.

The article goes on to demonstrate how the impedance of the antenna becomes reactive and how the idealized "50 ohm" input resistance becomes significantly less when shortening is done excessively.

Here is another excellent site that explains you can make an antenna from any length.

In short, any length works providing you can get the power into it that you require for transmission. Reciprocity means that a badly shortened antenna works just as "inefficiently" as a transmitter and receiver. There is no silver bullet of length - optimum is quarter, half and full wave antennas but don't let this stop you running the antenna at significantly shorter lengths.

One caution - if you are wanting to transmit several hundred milli-watts or above, not having an ideal antenna (proper length or compensated by matching networks) you may damage your transmitter output stage.