bill wrote:
I am new to Shortwave and I am trying to understand the technical
stuff. I have been lurking for several months and have some basic
questions that I hope to get answered or at least directed to an on
line reference. I have googled and tried searches but I just don't have
the background to understand what I am searching for. I am a graphic
artist looking at retirement in 6 months and I don't have a real strong
technical background.
Sherwood engineering has a chart that rates receivers.
http://www.sherweng.com/table.html
I think that I understand some of the tests but am at a loss for
others.
Noise Floor (dBm) is very clear. This is the weakest signal the radio
can receive for some minimum signal to produce some unspecified signal
to noise ratio. Is there an industry standard SNR?
The noise floor should be the "signal" that is present with just a 50
ohm resistor attached at the input of the radio.
AGC Threshold (dB) The signal level at which the AGC starts to reduce
the gain?
100KHz blocking dB, the level in dB of a signal 100KHz from where the
receiver is tuned to produce a 1dB reduction in gain. More dB is
better.
Sensitivity (uV) ????? does this go with "100KHz blocking dB"?
Generally sensitivity is the signal level required to read a specific
signal to noise ratio.
LO Noise Spacing (dBc) ???????
KHz ???? I guess these two numbers are paired and the higher the dBc
and the smaller the KHz the better.
I found this at wiki
http://en.wikipedia.org/wiki/DBc
but I don't have a clue what they are saying.
Front End Selectivity: The type of front end a a general; rating as to
it's effectiveness.
Filter Ultimate (dB): The maximum attenuation produced by a filter. A
ham friend thinks this shows leakage across the filter. A higher number
means less leakage and greater out of band attenuation.
Some filter designs roll off for ever (until the noise of the radio
dominates). Others reach a plateau of maximum rejections. Generally
sharper filters have this plataeu. You more or less have this
interpretted correctly.
Dynamic Range Wide Spaced at some KHz: The range, in dB, between the
weakest
signal and the strongest signal that are separated by some, most
commonly 20, KHz.
Higher dB means better. 20KHZ is better then 100KHz.
Dynamic Range Narrow Spaced at some KHz: The range, in dB, between the
weakest
signal and the strongest signal that are separated by some, most
commonly 2, KHz.
Higher dB means better. 2KHZ is better then 2KHz is better then 3, 4 or
5KHz.
I am currently using a DX398 that I bought at a flea market. I
understand that this is a
basic radio and I am thinking about moving up to a better receiver.
Before I commit any
real money for a modern radio I am trying to understand what the
numbers mean and
not be taken. Since I am getting ready to retire, money is an issue and
I may have to consider a used radio. At this point I am not trying to
get suggestions for a specific radio, I am just trying to gain a modest
understanding of what numbers are important. I enjoy the "official"
broadcasts from other countries and I am beginning to understand
utility reception. Is is interesting to listen to airplanes over the
oceans, and some of the maritime conversations are salty to say the
least. I am using a 30' wire that goes out the window to a tree, and I
have bought the coax to errect a better antenna. The computer tech at
work is also a ham so he is teaching me some simple things like how to
solder. He wound a 9:1
transformer and helped me mount it in a weather proof box and made me
an adaptor cable
from very thin flexible cable, rg174 I think, to go from thicker coax
that comes from the grounding block to my DX398. He warned me that
thicker cable might drag the light weight radio off the desk. Another
friend helped me drive a 8' ground rod and we mounted the ground block
and used #8 solid wire to hook it to the rod. I bought a couple of SW
antenna kits from a Lexington Radio Shack store that is going out of
busines and this week end I will put up the antenna. The ham at work
advised me to also buy a 75 ohm adjustable atenauator in case my DX398
is overloaded with the real antenna. At 70% off this stuff was almost
a steal.
The ham at work gave me an older scanner so I am trying to learn about
Vhf as well.
So while I am pretty green, I am learning. I have learned you can have
a lot of fun even if
your don't really know what you are doing.
Bill
Lancaster Kentucky
Ditto on Dave's comments regarding the AGC. OFF is good for beacons,
fast is for SSB communications and digital modes, and slow is for
shortwave broadcast.
The Sherwood list is useful, but you will be listening to shortwave
broadcast with a wide filter. So you might want to look at the list
with that kind of stacking.
If you get a 7030+NB, you have bought your last shortwave radio (unless
you want to listen to more than one station at a time). Unfortunately,
the Bush weak dollar is going to kill you. The Drake R8B, though not in
the same league, is a better buy. It's a real bugger if you plan on
programming it with a PC and homebrew code. The 7030 is more
straightforward in that respect.
Many radios are designed with "holes" for extra filters. Thus you can
take an OK radio and plug in a sharp crystal filter if you really want
to chase DX in the presence of strong signals. These filters are quite
reasonable on the used market. People buy them but have problems doing
the installation, and just dump them. Or they want a different shape.
Buying used gear takes time. You need to search ebay, Craisg's,
whatever periodically. I do this from time to time, but you need to
consider what your free time is worth.