[HCARC] DEE-BEE's, Amps and Antennas

[Kerry Sandstrom]

No Gary, there isn't a conspiracy.  Instead of fooling around on the 
internet asking questions, everyone else is operating and gaining 
experience!

By the way, you really know how to irritate me.  It isn't DeeBees its dB, 
never plural, and it stands for deci Bel.  The basic unit is the Bel, named 
after Alexander Graham Bell.  It is a logarithmic unit for ratios.  Again it 
is never plural.  It was originally used in acoustics but it can be used 
anywhere you are dealing with ratios.  There are numerous similar units you 
will come accross such as dBm and dBV.  dBm is an absolute power referenced 
to 1 milliWatt.  For instance 0 dBm = 1 milliWatt.  o tells you the ratio is 
1 and m tells you the reference is milliWatts.  20 dBm is 100 milliWatts. 
20 dB tells you the ratio is 100 and m again tells you it is referenced to 1 
milliWatt.  For dBV, the reference is 1 Volt and 20 dBV is 10 Volts (Yes, 
that is 10 V and is not a typo).  dB is a power ratio.  For voltage, 
remember that power goes as the square of the voltage so if your impedance 
is constant, 10 times the voltage is 100 times the power.

The power output of amplifiers and/or the gain of antennas must be used with 
extreme caution.  This may be difficult to believe, but the manufacturers 
stretch the truth whenever they give numbers like power output and antenna 
gain.  I don't believe any manufacturer of ham antennas has a real antenna 
range.  A real antenna range for HF requires a couple of hill top locations 
several miles apart looking accross a deep valley.  Anything less will 
produce gains corrupted by ground reflections.  Even if the gains were 
measured accurately, something I really doubt, unless you put ypur antenna 
up in the same relationship to other conducting structures like the ground, 
you will have different results.   Until a few years ago QST would not allow 
manufacturers to publish antenna gain figures in their QST ads because they 
were so misleading.  The other ham magazines had no qualms about gain 
figures in their ads, but that doesn't mean they were any less misleading. 
More recently QST has started publishing gains.  That doesn't make it right, 
it only shows how far the league has strayed from their original intent. 
They make there money from advertising not from being honest!

Amplifier power outputs are equally misleading.  There are really three 
numbers that count: peak power output, intermodulation distortion at that 
output, and average plate dissipation.  Typical grounded grid amplifiers 
have about 10 dB gain.  If you want 500 Watts out, you probably need 50 
Watts to drive the amplifier.  Grounded cathode amplifiers can be designed 
to have more gain, but instead of getting more output, you actually require 
less driving power.  The standard 3rd order IMD level for ham amplifiers has 
been -30 dB or so.  If not carefully operated, the IMD level can rise 
quickly to -25 dB or higher.  This is not good.  Some modes like PSK 31 
require better than -30 dB IMD levels so you don't see people running 
amplifiers with PSK 31.  The average plate dissipation is a critical factor 
in tube life.  Once you exceed the manufacturers average plate dissipation 
rating for the type of service, you are cutting into tube life.

Some years ago there were several articles on station design.  What was true 
then and is still true now, is the cheapest dB are those from going to the 
maximum legal power.  The initial 5-6 db for an antenna are also relatively 
cheap, but once you've got them, adding 3 dB at a time by increasing the 
antenna size (proportional to gain, you need to double the antenna size for 
3 db of gain) or antenna height gets very pricey very quickly.  The 
advantage of the antenna is it helps you on both transmit and receive so you 
can hear the station your trying to work better. There is no similar 
advantage on receive from adding a more powerful amplifier.

No you haven't made any math mistakes.  I think you will find that the cost 
of an antenna is more than you think when you consider the tower, rotator 
and the antenna.  Also you must be very wary of published gain numbers.  I 
don't believe any tribander has 8.9 db of gain when installed in a typical 
ham ststion.  I'm sure others will have other opinions, but that's mine. 
You can have a lot of fun with a standard 100W XCVR and a good dipole or 
vertical and you can get on now!  You won't be a big contest station or a 
big DX'er but I'm not sure either one of those is a worthy goal.

Kerry