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Decibels / Station Upgrades

posted May 11, 2018, 11:23 AM by Charles Boling
Signals strength/loss is very often expressed in decibels.

Riiiight! What's a decibel?


Mathematically, a bel is just the common logarithm of a number, and a decibel is 1/10th of a bel, 2 bels = 20 decibels.  A decibel represents a ratio;  it always compares two things -- like if I exclaim "This radio has twice the power!" and you wonder "Twice the power of WHAT?"

A short table of commonly-used values:
3dB = 2x (roughly)
10dB = 10x
20dB = 100x
30dB = 1000x

Gains/losses add (or subtract) together w/ decibels; for example, if you cut your radio's output power in half, you lose 3dB.  If you run it through a long coax cable that also loses half your power, you lose another 3dB; thus your total loss is 6dB (i.e. 4x weaker) -- your signal is 75% weaker (or 25% of its original power) at the receiving end. Incidentally, 6dB is the standard amount for one "S-unit" on a radio's received signal strength meter, or "S-meter".  So, if you manage to increase your power output by, say, 16x -- 16 is 4 x 4, or 3dB + 3dB -- you'll be 1 s-unit stronger at the other end (assuming that the receiving radio had a properly calibrated S-meter. Often they're very sloppy).

So how much stronger is an S-9 signal than an S-0 signal? If each of those 9 S-units represents 6dB or 4x increase, then the total gain is 6 x 9 = 54dB, or 4^9 = 262,144x.

Sometimes you'll see power expressed in terms of dBmW (decibels compared to 1 milliWatt; 1 Watt=30dBmW) or dBmV (power of received signal compared to one that produces 1 millivolt in a 50-ohm circuit; 1W = 77dBmV), but that's all I'm going to say about that, since I'm not addressing electrical engineers.

Practical examples

Everyone wants to get more effective power out of their radio so that they can talk further, but what can we do that's cost-effective?  Keep in mind that it doesn't help to yell louder if you can't hear the reply! You can shout across a large field using a megaphone, but if you want an answer back, then either the other person has to use a megaphone too, or you need a listening device (ear horn, anyone?) to hear them.  How many decibels do we get from some common things?

 FROM
 TO/after/with dB
 Low Power (0.5W) High Power (5W) HT
 +10
 5W HT
 50W Mobile
 +10
 50W mobile rig 200W VHF Amp
 +6
 100W HF rig
 1500W legal limit
 +12
 70cm rig
 100ft LMR-400 coax
 -3
 70cm rig
 100ft RG-58/U coax
 -11
 "Isotropic Radiator"
 1/2-wave Dipole antenna
 +2
 1/2-wave dipole   
 1/4-wave ground plane
 0
 1/2-wave dipole     Nice omni base antenna
 +6
 1/2-wave dipole
 14-element Yagi
 +13
 1/2-wave dipole
 Typical Rubber duckie
 -6
 Rubber duckie
 worn on belt at waist
or inside vehicle
 -15
 Good weather, best conditions
 Angry propagation gods, trees/antennas moving, etc.
 -30

Not listed in the above table is cost. a 150-300W VHF amp might run you $500 for a one-way 6dB; $150 invested in a better antenna/mount might get you the same 6dB gain both directions. Considering cheap coax? Consider again if you're working VHF/UHF -- losses are a lot worse than at HF, so extra money spent may be well worth it.  This is especially true when you consider that the higher the frequency and the higher the power you're at, the more expensive an amplifier is needed for any particular gain.  Legal-limit VHF amps are out of sight (in more ways than one). Even a 1kW amp can set you back $4k for "only" a 13dB gain for your 50W radio -- the same gain you could get by spending, say, $500 on a 14-element beam and a rotator.  (You may also be more able to make your own antenna than you are your own amplifier.)  Oh, with that amplifier, you might want a good low-noise receive pre-amp, and maybe a switching system to protect it from that 1000 Watts -- better figure another $1k just to be sure.

That's not to say amplifiers don't have their place. I live in an RF-quiet rural area, and talk to many people that are in cities with noise levels easily 6 s-units (36dB) above mine. They can't hear anything!   A linear amplifier that I can switch into the circuit when I want is an easy way for me to send them more power (while the amp adds noise and reduces my own receive sensitivity by at least 6 dB) and evens things out just enough to hold a conversation when conditions otherwise didn't quite permit it.  Many other times, it doesn't do me a lick of good.

Something else to point out is that conditions are extremely variable. Signals to my base station often fluctuate 30dB (5 s-units) and more, as trees our blowing near my antenna, the tower that a repeater's on is moving slightly, causing alternating constructive/destructive multi-path interference, rain is absorbing signals, etc.  So depending on where you're starting from, even an expensive equipment upgrade to the very best you can get, probably isn't going to change a signal from "can't hear them a bit!" to "consistent armchair copy" -- at best, it'll make them usable, and it'll take those marginal signals and make them great.

Let's try a couple of example station upgrades:

5W HT w/ rubber duck (near body) in house
+10dB upgrade to 50W mobile rig
+15dB mag-mount antenna on a cookie sheet/fridge
=25dB gain, or roughly 300x the effective output power. Wow!

50W 2m radio in house w/ mag-mount antenna on cookie sheet
+6dB Better antenna
+10dB Moved antenna outside and higher
-3dB 50ft RG-58 coax @ 148MHz
=13dB, or 20x gain. This is equivalent to a 1kW amplifier -- except that you've also improved your receive gain by at least 20x!

Note: In the typical house with lots of RFI-generating devices, moving your antenna up and outside can make a HUGE difference on receiving.

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