Tech Talk continues after the Wednesday evening net.  The subject this past Wednesday was "defining dB" or at least it was supposed to be.  Some technical difficulties caused parts of the discussion to be lost.  If you wonder what you missed, here it is in print.  This was from an article which originally appeared in the Sept. 06 issue of the SARS newsletter.

At Monday's SARS meeting, Arnie N6APA demonstrated a new miniaturized "fox" that he had recently bought. This device was about the size of a Bic Lighter and was programmed to send a CW message and a series of tones on a particular frequency.
When asked what power output the device produced he looked it up and said "12 dBm". (I think that was the number) but how does one relate that to the power produced by a typical ham radio?

The conversion is really quite simple once you understand a few terms. Firstly, dBm stands for Decibels in miliwatts, so one dBm is one miliwatt.

Let's look at some dB tables. In the handbook they look pretty complicated but you only need to remember 4 values and you can do power conversions like an expert, even in your head.

The values you need to remember are, on the power side (we'll talk about the voltage and current side later) are one, three, six and 10. That's all; 1, 3, 6 and 10

Here's what those numbers represent in decibels:

One is a gain in power of about 20%, or more exactly 1.259. If you multiply one miliwatt (one dBm) times 1.259 you get 1.259 miliwatts. So far so good, right? If you feel like rounding your numbers for simplicity, no one will care because the difference is so small unless you're in a lab it won't matter. One dB is "about" 20% so a gain of 1 dBm produces "about" 1.2 miliwatts.

The next number we remember is 3 dB. 3 dB represents a power gain of 1.995 or nearly 200%. If we have a gain of 3 dBm, we are looking at nearly 2 milliwatts.

Now we take the next memory position: 6 dB. 6 dB represents a power gain of 3.981 or nearly 400%. If we have a gain of 6 dBm, we see nearly 4 miliwatts.

And finally we need to remember 10 dB. 10 dBm represents a power gain of 10 times or 1000%. A gain of 10 dBm delivers a blistering 10 miliwatts.

Can you see something to compare to this in the Ham world? My hand held radio on low power puts out 300 miliwatts. How many dBm would that be?

Ok, we see that 10 dBm is 10 miliwatts, so if we have another 10 dB gain, that's 10 times 10 or 100 miliwatts.
in other words, 20 dBm is 100 miliwatts. If we add 6 dB to that, making 26 dBm we would have 400 miliwatts. That's MORE power than my hand held produces, so how many dB do I need to subtract from 400 miliwatts to get to 300? Let's do it the easy way and go back to 20 dBm. We said that 20 dBm was equivalent to 100 miliwatts. Let's instead of adding 6 dB to that, add only 3 dB. Remember what 3 dB is? Yep double. So 23 dBm is about 200 miliwatts. (199.5 to be exact).
Ok, we're getting there. Let's add one dB to 199.5 miliwatts. Do you remember that one dB is "about 20%" or exactly 1.259? Multiplying 199.5 mW times 1.259 gives us 251 miliwatts (21 dBm). Let's add one more dB. 251 mW times 1.259 gives us 316 miliwatts and that's about as close as we can get without using a scientific calculator but isn't that good enough? The next time you're in a computer store, grab a wireless device and look at the power output. It will usually be in dBm and if it's running in the max it should say something like 400 dBm. Can you do that in your head? One times 10 is 10 times 10 is 100 times 4 (6 dB) is 400. Wow! that device is running 400 miliwatts.
So, what is Arnie's fox running at 12 dBm? One times 10 is 10 times 1.259 (one dB) is 12.59 miliwatts. Close enough?

in Summary: one dB is 1.259 or about 20%...3 dB is 1.995 or roughly double.....6 dB is 3.981 or just about 4 times and 10 dB is an even ten times the value.

If you want to see the entire table go to the September 2006 newsletter and print out the page on defining dB. You will notice that the values on the far right are V2/V1 or I2/I1 and they are different. Why? Because power is derived from BOTH voltage and current (P=IxE) but if you want to do voltage gains in your head you still need only remember 4 key points. Check it out.

How about decibel loss? It's the same 4 numbers but in a different format. In my General course I introduce the reciprocal which is one of the most useful arithmetic functions you can use. Any time you divide a number into 1, you get it's reciprocal. Take 3 dB for example. Divide 1.995 into 1 and you get .5012. When you look on the gain side of the dB chart you will see that .5012, which is the reciprocal of 1.995 just happens to be -3, or a loss of 3 dB.

Loss calculations are pretty important when you're dealing with antennas. Antennas can produce gain, which offsets the loss caused by the feedline. Anytime you come out on the positive side of these dB calculations you will be radiating more power than your transmitter is producing. Pretty cool.

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By now you are probably familiar with "Google Earth" and if so you are also aware of a number of Amateur Radio related features offered by this amazing program.  I recently heard a ham in Missouri talking to a mobile station in Lake County and she was actually following along using the APRS overlay.  Even though the satellite photos are a few years old, a station running an APRS setup will appear on them just as if they were new.  Fascinating!

But here's a fairly new application:  Viewing the ionosphere in near-real-time and seeing how the total electron count, from solar radiation is affecting propagation at any given time.  Pretty to look at too even if the band is dead.

Here's the website of the Space Environment Corporation where you can see the Google Earth feature demonstrated and get a download if you should need it.

If you already have Google Earth installed, go to the link above and click on Total Electron Count to see the overlay in action.

Thanks Howard, K6IA

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DStar update;  Gateway-2 and DPlus.  Gateway-2 or G2 is out and most DStar repeaters are now running it.  When the W6CO system comes on line, it too will be running G2.  DPlus is a software application installed on the Gateway computer which allows a number of new features, the most prominent being the ability to link repeaters and the creation of International Reflectors.  You are already familiar with repeater linking;  The W6CO system routinely links Mt Veeder and Mt St Helena for nets as well as the W6LI repeater in Vallejo for the Thursday night net.  DPlus allows existing DStar repeaters to link without regard for location because the linking is done via the internet.  It's quite common to find the W6DHS DStar repeater in Volcano, California linked to the W0OMD repeater in Ozark, Missouri.  Several of the users on both repeaters chat regularly as well as other visitors linking in via their Dongles.

A very recent addition to DStar, made possible by G2 and DPlus is the world wide reflector system.  Here are some excerpts from the UK-IT (UK Interconnect Team) DPlus Reflector User Guide.

"Okay, so what is a dplus Reflector? A Reflector acts as a central conference hub where multiple DStar
Nodes/DV Dongle users can gather and all talk to each other. There was a recent World Wide
Net; the objective of the net was to see how many D-Star Nodes and Dongle Users could connect to a
Reflector to gauge scalability and performance. Twenty eight D-Star Nodes, and eighteen Dongle
users connected to the Net. The system performed perfectly, and no performance issues were noted."

The User Guide goes on to describe the feature in more detail.  One of the analogies they use for comparison is the "conference room" feature of Echolink.  It works a little like that although in practice it's quite a bit more sophisticated.  Each reflector is divided into 3 modules, referred to as Module-A through Module-C, but unlike the repeater stack these designations do not refer to a frequency band.  They refer to actual subdivisions within the reflector that can be set aside for specific purposes.  For example, Reflector-6 (REF006) in London has Module-B set aside for German language users.  REF005, also in London has Module-A setup for users within the UK and Module-B is setup for French language users.  Any DStar user can connect to any Reflector and module but be prepared to hear conversations in other languages!

Connecting to a reflector is quite simple.  Simply set your radio with the reflector number in the "Your" or "UR" slot and make sure your Rpt2 slot is set up for the gateway you're using.  Here's an example:

To Link:

YOUR:	REF005AL
RPT1:	K6MDD  B     (The B goes in the eighth position.
RPT2:	K6MDD  G     (The G goes in the eighth position.

When you key the mike, an announcement will come back telling you that the reflector is connected.  At this time you move to a different memory location where YOUR is set to CQCQCQ.  It will look like this:

YOUR:	CQCQCQ
RPT1:	K6MDD  B     (The B goes in the eighth position.
RPT2:	K6MDD  G     (The G goes in the eighth position.

Notice that the only thing that has changed is the YOUR call.  RPT1 and RPT2 remain the same.  To disconnect the reflector set YOUR to 7 spaces followed by the letter U, like this:

YOUR:	*******U       (Use spaces, not asterisks)
RPT1:	K6MDD  B     (The B goes in the eighth position.
RPT2:	K6MDD  G     (The G goes in the eighth position.

That's all there is to it.  In many cases the repeater you are listening to is already connected to an interesting reflector.  If that's the case, just set your radio as in frame 2 above and you're all set.  Anyone you hear will hear you.

A lot of this is going to change because there are still a lot of loose ends.  For example, if you connect to REF004, is there a discussion in progress?  You won't interrupt anything if you connect up but you might want to know who's in QSO before you do.  Many more changes are coming.

Read the entire guide at:  http://www.ukit.org.uk/documents/UKIT_dplus_Reflector_User_Guide_V1_1.pdf

Links from previous newsletters:

Photocredits: KE6O "Yountville Mustard"