http://elecraft.85.s1.nabble.com/A-comment-about-Receiving-tp6216105p6219989.html
receiver design. The Elecraft receivers have all active receiver stages
first stage. There was a time when pre-selectors were available for
>
> Hi Don,
> That is why I mentioned the case of:
> >I still don't understand the mechanism of how a preselector improves
> >S/N unless its BW is better than the BW of the receiver- and I can't
> imagine
> >that situation either.
> Now, if you are saying the wideband noise may be desensing or causing
> IM- I understand. That is one of the reasons we have passive filters
> up front in a rig.
> But I still don't understand how a preselector with say 50KHz -3dB BW
> can improve S/N in a modern communications RX-
> Dale W4OP
>
>> Dale,
>>
>> The bandwidth can make a difference - the noise is broadband while the
>> signal is narrowband.
>> If the bandwidth of the receiver input is wide, then a preselector will
>> make a difference because it decreases the total noise that the receiver
>> is handling.
>>
>> Note that many(most) tuners will not produce that pre-selector effect.
>> Only those with a resonant circuit will do that. Most common T-network
>> of L-network tuners create either a high pass or a low pass filter.
>>
>> 73,
>> Don W3FPR
>>
>> On 3/29/2011 9:55 AM, Dale Parfitt wrote:
>>> I believe I understood all of your comment Ron.
>>> But as our noise floor at HF is limited by external noise (atmospheric,
>>> manmade and galactic) I don't understand how efficiency, within reason,
>>> comes into play for a receive antenna. Certainly, for signals above the
>>> noise floor, adding in 3 or 10 of attenuation makes no difference in
>>> the S/N
>>> ratio, and since the preselector cannot discriminate between signal and
>>> noise, I still don't understand the mechanism of how a preselector
>>> improves
>>> S/N unless its BW is better than the BW of the receiver- and I can't
>>> imagine
>>> that situation either.
>>> One of my best low band antennas is a rotatable half sized Flag
>>> whose gain
>>> runs around 30dB below my sloper.
>>>
>>> Dale W4OP
>>>
>>>> The antenna tuner acts as a pre-selector so to speak, by making the
>>>> sytem
>>>> of
>>>> the antenna, the transmission line, and the radio, resonant, and
>>>> matched
>>>> impedence.
>>>>
>>>> All of this adds up to minimize losses in a couple of ways. The
>>>> impedence
>>>> matching means that the xmission line and antenna are as close to the
>>>> input
>>>> impedence of the radio (nominally 50 Ohms these days). If it is not
>>>> at 50
>>>> Ohms then you do not get maximum power in to the rig based on simple
>>>> simple
>>>> Ohms law power.
>>>>
>>>> A highly simplified way of explaining resonance part of "a resonant
>>>> antenna"
>>>> is the act of making sure all of the current is in phase with the
>>>> voltage
>>>> so
>>>> that when the power does get to the reciever, that it is in a
>>>> useable form
>>>> to drive things. It is somewhat the reverse or inverse of a good
>>>> spring
>>>> and
>>>> shock absorber system on a car....if you have things wrong, and drive
>>>> along
>>>> a rough road with bumps spaced jsut right relative to how fast,
>>>> then all
>>>> heck breaks loose; maximim power transferred, and that is a resonant
>>>> system.
>>>> If things are not resonant, well, you get a smooth ride in your
>>>> car, or in
>>>> the case of your radio, poor reception (and transmission).
>>>>
>>>> -ron WT5RZ
>>>>
>>>> --
>>>> View this message in context:
>>>>
http://elecraft.365791.n2.nabble.com/A-comment-about-Receiving-tp6216105p6217679.html
>>>>
>>>> Sent from the Elecraft mailing list archive at Nabble.com.
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