When it comes to frequencies above 30 MHz, is there really a right band to use?   After all, propagation is usually line of sight (except for the occasional band opening on 6 meters or 2 meters).  Daytime/nighttime differences are really not much of a factor, or are they?  And does the population density of an area affect Amateur Radio operation?

Actually, there are a number of factors that impact reliable communications above 30 MHz.  Here in Florida, and in many other coastal areas, the impact of temperature inversions and proximity to large bodies of water will introduce tropospheric ducts that will carry VHF signals well beyond normal line of sight distances.  If you don’t believe me, just look at the list of APRS stations you are able to hear directly on 144.390 MHz, the national APRS network frequency.  It is not uncommon for stations located along the Gulf Coast to hear each other directly as signals are “ducted” over the waters of the Gulf of Mexico.  You will get that same kind of “ducting” along Atlantic Coastal Counties as well.  And such ducting may not be limited to VHF, either.  On a recent business trip from Fort Lauderdale to Melbourne, I carried a Broward County UHF repeater up I-95 until I reached my destination. 

VHF Sideband operators here in Florida relish the many “ducts” that carry their weak (and sometimes not so weak) signals the entire length of the peninsula and then some, allowing them to make contacts in Georgia, the Carolinas and Alabama.  Time of day can be important as nighttime brings a reduction in solar attenuation, even up into the 70 cm band.

But it may not be a long distance you are looking.  Some Ham operators, particularly those working a disaster response situation, may be more interested in reliable, shorter distance communications.  In this case, a 70 cm UHF frequency may be just the ticket.  Tropo-ducting occurs less frequently in the UHF band.  Depending on the location, there may be more repeaters available to use.  And lest we forget that the 70 cm band offers a number of simplex frequencies/channels between 438-440 MHz that are there for the using.  Some newer model portable radios also will allow you access to the 420-430 MHz portion of the band.  Just be careful you are not stepping on an ATV signal there.

UHF also offers better building penetration, both into and out of.  This is important because most commercial and residential structures in Florida are made of steel and concrete.  A UHF signal’s shorter wavelength allows the signal to penetrate these structures more effectively.

Of course, there are exceptions to this.  If you are operating in a densely forested area, you may find that 2 meters (VHF) or 222 MHz will work better for you.  The canopy of the woods (you know, the branches and leaves), may block higher frequency signals.  If you have set up a temporary base station to operate from, you may want to experiment with the height of your antenna.  Sometimes, keeping it below the canopy works better for shorter range comms.

The orientation of your antenna is a critical communications factor.  Nearly ALL repeaters operate with antennas that are VERTICALLY polarized.  Holding your antenna at an angle will attenuate the signal you transmit and receive.  I have often wondered why more Amateur and Land Mobile (Part 90) repeaters don’t employ circular or elliptical polarization.

Antenna gain is also important.  You will usually get more gain and effective radiated power (ERP) with a like sized antenna on UHF than on VHF.

If you want to calculate the likely transmit and receive footprint of your station, use the formula SQRT(1.414*your antenna height [in feet]).  SQRT=Square Root.  The product of this formula will report your distance in miles.