Reciever sensitivity questions ...help me understand

[ghutch]

Help me understand these specs pls. Reciever specs posted on Sherwood Eng.'s website on reciever rankings list shows radios rated from best down. As I look at these numbers I have a couple questions?
use the Drake RC4/CF as the example so we are talking about the same units

1. Noise floor....my understanding is these numbers are the internal noise of the radio itself?? internal generated noise. Is this correct? Using the RC4 floor of -138 dbm, the higher the dbm the better or quieter? correct?
2. Sensitivity, using the RC 4 value of .15 uV it will take a current or signal of 0.15 Uv rec'd by the antenna to be converted into audio? in perfect conditions? is this correct?
3. Selectivity using the ultimate filter listed as a base(RC4 listed at 130DB) so this radio would regect any signal up to 130 db 20 khz from the dersired or center freqency? correct?

IF my ideas are correct....this 20+ year old radio has better sensitivity than most more modern radios? based on .15uv it has no pre amp so its sensitivity is better than all but 4 radios listed as better.....what i'm I missing or not following in these test papers? :headbang
I understand the radio is a modified RC4, but if the tech in these radios is that good why is it a struggle for much newer radio designs to get this type sensitivity without using a preamp inserted?? Is it the tube vs soild state ?
the R390 collins boat anchor was built in the 60's and rate very high on this list stock ?

 

[Captain Kilowatt]

You are correct in your thinking except the preamp thing. What we now call a preamp is really just the receiver's first RF amp with the ability to switch it off to avoid overload. The older receivers did not have that ability and would often overload unless the rf gain was rolled back.

 

[BJ radionut]

Interesting is it not!...All the Techno goodies on these "Hi-Tech Rcvrs" and they don't "Hear" any better than a 40+ yr old Rcvr...
However the "Dynamic Range"(lack of better term) that the audio can be changed and enhanced...added to the "Gain+Bandwith adjustments" to the different stages of the Rcvr, is what makes the "Newer" Rcvrs more desirable to some....

I think my old R4B...Was the "sweetest" sounding Rcvr I ever owned....
All the Best
BJ

 

[ghutch]

If these radio ranking are based on science....Sherwood has a great reputation .....the older units are in the ball game!!
next question is
1. sensitivity at .15 is better than a unit with .30 or .35 ...it ability to take a very low voltage off the antenna and boost it to a readable signal ?? Correct
2. The selectivity is the "window we listen thru" or the freqency measured in kc's we tune in on a set freqency?? Correct
this selectivity is based on the radio filters ability to attenuate the unwanted signal close to in freqency or next to our desired signal?? correct?

3.Can a radio be made more selective or more sensitive by using an antenna that is tuned to resonate on the desired freqency we want to operate on or listen to? thus the antenna is attenuating the non desired freqency or its voltage on the antenna is smaller due to the antenna being non resonate at a desired freqency?? ....would this hold true on Dx signals? or weak signals.
4. can filters be built for a small section of freqencies such as 80 meters only to improve the selectivity of a radio on a desired freqency? or would a pre selector or antenna tuner get the results and block to some degree the unwanted signals.
i see pre amps built for freqency ranges (Mirage) that boost the desried signal without boosting the noise....these are designed to work only on a band or set of freqencies....does anyone have first hand knowledge if these are worth the buck?
my long wire SW antenna is working but the radio selectivity is lacking.(Icom R71A) seems the noise is covering the desired signals....hear them at times but the signal is just barely above the floor noise thus un usable.

 

[NeoHam]

I too have a question. How can my Radio Shack 2067 with a sensitivity of .5 uv sound WAY better than my Yaesu FT-1802 with a sensitivity of "better than .2 uv" off the same antenna? Do I have this backwards? Shouldn't the Yaesu work better?

 

[Moleculo]

3.Can a radio be made more selective or more sensitive by using an antenna that is tuned to resonate on the desired freqency we want to operate on or listen to? thus the antenna is attenuating the non desired freqency or its voltage on the antenna is smaller due to the antenna being non resonate at a desired freqency?? ....would this hold true on Dx signals? or weak signals.
4. can filters be built for a small section of freqencies such as 80 meters only to improve the selectivity of a radio on a desired freqency? or would a pre selector or antenna tuner get the results and block to some degree the unwanted signals.
i see pre amps built for freqency ranges (Mirage) that boost the desried signal without boosting the noise....these are designed to work only on a band or set of freqencies....does anyone have first hand knowledge if these are worth the buck?
my long wire SW antenna is working but the radio selectivity is lacking.(Icom R71A) seems the noise is covering the desired signals....hear them at times but the signal is just barely above the floor noise thus un usable.

3. No. The selectivity capability of the receiver is independent of the antenna used. Selectivity is about the rejection of adjacent signals. If you use an HF antenna that is resonant at a specific frequency, it will still hear just as good 5khz away from it's resonant frequency. A receiver with excellent selectivity will be able to discern or filter the adjacent frequency being received.

4. Yes, to a point, but that is not you would usually want to implement RX filtering in a transceiver because it would make the rig fairly useless outside of the filters passband. Instead, look at how roofing filters are implemented in transceivers like the Elecraft K3 and you'll see how it can accomplished in a useful fashion.

 

[Moleculo]

I too have a question. How can my Radio Shack 2067 with a sensitivity of .5 uv sound WAY better than my Yaesu FT-1802 with a sensitivity of "better than .2 uv" off the same antenna? Do I have this backwards? Shouldn't the Yaesu work better?

Sensitivity is about what it can hear, not how good the audio quality is produced out of the speaker.

 

[Captain Kilowatt]

Sensitivity is about what it can hear, not how good the audio quality is produced out of the speaker.

Exactly. The receiver bandwidth of the scanner is likely wider than the Yaesu which makes for better audio but lousier ability to filter out interference from adjacent signals.

 

[ghutch]

First Thanks for the replies.....
sensitivity is the ability to take a weak signal voltage and make it "hearable" or "useable" in a QSO. got that
selectivity is the radios ability to select a single station out of the spectrum....or it ability to hear a signal and reject all or most of others. got that
what hangs me up:bdh: is looking at a single band radio spec's (10 mtr mobile for example) I see radios with .15 specs and then compare the radio with a high end broad band reciever and see lower specs in the range of .2 to .3 before using a preamp. My icom R71A does not hear as good on 10 meters as a rci 2970 for example using same antenna. even with the narrow filter and preamp in play on the icom it does not pickup a weak signal as good as the 2950. local strong signals....it hear fine....as well as the 2950. weak Dx forget it the 2950 smokes it.
do roofing filters make a K3 more sensitive than a good design single band radio....the specs do not show it to have as good a sensitivity? and at time not as good even with a preamp in play. I guess what I want is a broad band reciever with the sensitivity and selectivity of a single band radio....may be cost prohibitive ?? :w00t:
it does seem with as many folks involved with Ham, CB, SWL radio companies could build radios with much better specs....the drake rc4 the old collins 390A's are 40-50 year old and seem to be able to do as good or better

 

[NeoHam]

Yea, I am beginning to think those numbers are BS. My scanner can pick up many weather stations and repeaters the Yaesu can't even detect. The ones it can pick up that are scratchy are clear on the scanner. It has nothing to do with selectivity.

 

[ghutch]

After looking at some single band radio specs it appears the single band radio design is more sensitive AND selective.... IF design it is for a narrower set of freqencies.
I guess the broad band radios have to compromise the design to cover a larger range. It still is a shame the radios (r390-drakes etc) are 40-50 years old and have better specs than a new and improved models.:thumbdown:
With that.... a question for the guys that understand radio design.

1. where is the radios selectivity narrowed?? in the first IF filters or the second, third or fourth depending on the conversion design? At what point is the (SSB filter as an example) bandwidth narrowed?
2. What is a "Roofing Filter" and what does it do for us? in the rdaio designs
3. What is the reason for UP conversion vs Down conversion ? i see rdaios with both types?

just working to have a better understanding of the processes!!