RadioBanter - View Single Post - RRS - The Topic is Shortwave and the Goal is to Talk About (Write About) Shortwave related things . . .

December 5th 05, 05:15 PM posted to rec.radio.shortwave

In article . com,
wrote:

Telamon wrote:

What is counter intuitive to you about either kind of
splitter/combiner?

On the news group we should take care to reference typical natural
HF noise floor and receiver sensitivity in dBm so the addition of
components like splitters can make sense in an antenna system way.

-- Telamon Ventura, California
------------------------------------------------ I had inteneded to
be able to post on line reffences to every dB figure I use, but I had
to go open my big mouth first.
-----------------------------------------------

The common wisdom is that 4dB, I am calling it 4 becuase it is
slighltly over 3dB, of additonal loss will impact reception of weak
signals. In most listening locations the limit to recption is the
local noise floor. Even for a modest receiver like the R2000, the
receivers noise floor is significantly below the atmospheric noise
floor. I have a 1970 copy of ""Reference Data for Radio Engineers",
but I am trying to find a on link refference that everyone could see.
My "Refference Data" says the winter noise floor in my location
should be around -111dB with a 3KHZ BW at 10MHz. Sherwood Engineering
rates the R2000 as having a -130dBm noise floor in SSB, nominal 3KHZ
BW. I have checked 5 R2000s and they all at least meet this
minimium spcification. Most are slighlty better.

So disregarding any local QRM, we have: A system noise floor of
-111dBm. A receiver with a noise floor -130dBm. We have a margin
19dBm.

It is my contention, based on several years of tests, in a
laboratory, and "real world" tests overthe last 6 months, that
adding an additional 4dBm of attenuation will have "no" effect on
reception.

Repeated tests with a remote RF source, a micropower ISM/HiFER
"beacon", with the beacon RF output reduced to where the audio is
barely just intelligible show that an additonal 4dB of loss has not
effect.

A friend's daughter is a speech pathologist/therapist. She recorded a
20 pairs of words to test for "Percentage Articulation Loss of
Consonants" (AKA %AlCons). Words like copy and coffee.

I edited these into sets with each word being first in one set, and
second in another. Each group consisted of 4 words with a modulated
CW group ID at the end. The procedure consisted of reducing the RF
level of the beacon to the minimim level that I could get 90%
accuracy. Then adding the 4dB pad to see if it effected the
accuracy.

"Common sense" argues that any additional loss whould have an effect
on reception. My test results proved to me that that is just not
true.

If one has a locaton where the noise floor approaches, or is lower
then, the receiver noise floor, then and only then will addtional
loss effect reception.

I worked out a formula but ASCII is very poor for sub/super-script.

A wordy version could be: As long as the additional loss in dB is
less then the margin of RecNF-AntNF, then reception will be
uneffected.

This set of experiments also showed that the local noise floor is the
single most imoportant factor in SW/HF recption.

For instance, if the expected noise floor is -111dB, and the receiver
NF is 130dBm, but the real noise floor is -100dBm, then nothing below
-100dBm will be detected. Fairly obvious but not often realised.

My HiFer "beacon"/"remote RF source" consists of a Rio PMP300, a SSB
exciter and buffer feeding an electronic attenuator that is
controlled by a DTMF from a 2Mtr HT. The voice samples are 256K MP3
files normalised at 60% with the MCW at 95%. The unit is located
about 1.5 miles from me feeding a modified CB ground plane. I manualy
adjusted the RF output level for the lowest signal that my wife could
understand. This way I could add known amounts of attenuation via my
HT DTMF. It is true that such a close RF signal had no multipath to
complicate the issue, but I think my effort demonstrates why a few dB
of additional loss have minimal effect on HF reception.

It is also worth noting that adding more antenna wire did not effect
the detection of the weakest signal because more wire gave more
signal, but also gave an matching increase in noise. A test loop
antenna did allow the output level to be reduced another ~10 dB while
keepng the same level of intelligibility. A small active dipole
achieved the same ~10dB improvement. I would love to test a Welbrook
loop. At best my simple loop was a marginal antenna. Such a remote RF
surce also allowed me see that a PCR1000 really benefited from a
MW/BCB reject filter.

Let the flames roar.

Very good Terry I don't see any reason for a flames. You post good
information and real world experiences. What you posted above is a good
example of what people reading the news group need to see in order to
make educated decisions about antenna systems.

I just add that the natural noise floor is not flat and Terry's examples
above are at some particular SW frequency. If I see a good natural noise
figure chart link on the web I'll post it here.

Well, I'll just add more comment. What the information above alludes too
and that is as you decrease the bandwidth of your receiver the noise
floor should go down. Narrowing the bandwidth decreases the amount of
spectrum that goes through the receiver and that lowers the total
background noise at the receiver output. You will notice that Terry
references the filter bandwidth in his examples.

--
Telamon
Ventura, California