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Author Topic: Receive vs. Transmit  (Read 894 times)
WZ6F
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Posts: 98




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« on: August 08, 2005, 05:56:57 PM »

Hello,

Why does the fall-off of receive capability "seem" to be far less steep as compared to SWR tables that indicate a fairly large attentuation with respect to worsening SWR? In other words, my receive capability seems to be much better than what my SWR attenuation tables indicate. Why is this? I hope this makes sense.

Answers Appreciated,

Alan
WZ6F
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W2AEW
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« Reply #1 on: August 08, 2005, 06:10:00 PM »

I would suspect that it has to do with the fact receive performance is not strongly dependent upon effecient POWER transfer, but rather getting sufficient voltage to the receiver.  Therefore, impedance matching is not generally very important in receive strength.  There are exceptions however, but generally this is true.

Transmit performance, on the other hand, is much more highly dependent upon POWER transfer rather than voltage transfer.  Thus, power loss directly affects transmit efficiency.
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KA0GKT
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Posts: 555




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« Reply #2 on: August 08, 2005, 06:20:57 PM »

On most modern amateur transceivers, there is an FET front end to the receiver.  An FET has a high input impedence, so its gain is effected less by the mis match than other devices otherwise would be.

On older receivers, there was a "Preselector" control where you tuned the front end of the receiver, matching the antenna to the lower impedence input.  On transceivers, this usually was ganged with the driver tuning, so as you tuned the transmit sriver circuitry, you were also peaking the receiver input.

This is not dissimilar to tuning the output tank circuit of a tube-tyhpe amplifier, whether it be an external linear or the output of an old FT-101 (or my even older FTdx-100)  The preselector is a receive-match as opposed to a trans-match, both tune the antenna (There's a whole 'nother concept).

73 DE KAØGKT/7
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N0TONE
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Posts: 173




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« Reply #3 on: August 08, 2005, 10:29:35 PM »

You didn't mention whether this was HF or VHF.  If HF, though, the answer would be thus...

Your antenna and radio receive an electromagnetic stew containing a mix of signals (desired), broadband noise/hiss (undesired) and static/thunderstorm crashes (undesired).  For any of today's radios, or for that matter any radio made in the past 30 years, this muck arrives at the receiver with great strength compared to the very tiny signals which the receiver is capable of receiving.

What you hear psychoacoustically is a signal, with some noise in the background.  If you attenuate the electrical energy from your antenna to the receiver by, say, 20dB, due to poor SWR, then both the desired signal and the undesired noise are attenuated by the same amount.  Your ear still hears a signal with noise in the background.  If you attenuate even further, and you finally reach a point where the intended signal strength starts competing in weakness with the radio's internal noise, then you will begin to notice the difference.

This is one reason why we cannot evaluate antennas on the basis of reception - receivers are so sensitive that they can hear quite a bit with no more than the thought of an antenna connected.

AM
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HB9PJT
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« Reply #4 on: August 09, 2005, 01:33:18 AM »

Alan, when transmitting you see a much greater attenuation then on receive because of the SWR protection of the transmitter. On receive my expirience agree with the theory. Compare the following numbers with your S-meter:

SWR of 1:10 attenuates about 3 dB
SWR of 1:100 attenuates about 14 dB

An S-unit should be 6 dB but a lot of S-meters have about 3 dB per S-unit.

73, Peter - HB9PJT
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WB2WIK
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Posts: 20542




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« Reply #5 on: August 09, 2005, 07:47:45 AM »

And there are still more reasons.

One that nobody mentioned is that "SWR" is different for transmitting and receiving, because the source and the load are reversed.  When transmitting, your SWR is the mismatch between your line and your antenna.  When receiving, your SWR is the mismatch between your line and your receiver, because for receiving, your receiver is the load.

Amateur receivers are all optimized to provide about a 50 Ohm impedance to their signal source, so if you're using coax, the SWR between your line and your RX is very low.  The "FET" explanation is irrelevant, since it doesn't matter what your front end device impedance is, it's tuned to match and look like 50 Ohms using either narrowband or broadband circuits -- but in all cases, even if the FET impedance is a million Ohms, it wouldn't matter, because it's adjusted to look like 50 Ohms anyway.  Same as was done with vacuum tube receivers for 75 years.

WB2WIK/6
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WB6BYU
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« Reply #6 on: August 09, 2005, 02:00:43 PM »

Two more different answers...

First, many of the charts you see floating around are wrong.
They assume that all reflected power is lost: this is not
the case.  With a proper conjugate match, most of the
power reflected from the antenna will be reflected back
from the rig again.  (All the power is either radiated
from the antenna or absorbed by the coax cable during
multiple reflections along the cable.)

Second:  generally (especially for HF) the limiting factor
in reception is the background noise level.  When the
level of the transmitted signal is above the background
noise you can hear it.  But BOTH the desired signal and
the background noise are picked up by the antenna, so will
be attenuated the same amount by the coax feedline.  If
you start with the desired signal 10dB above the background
noise, then it will still be 10dB above it even after
lossing 10dB in the cable (because the noise was attenuated
by the same amount.)

That may not quite be the situation on VHF, because the
receiver internal noise level becomes a major contributor
and that noise is NOT attenuated by the feedline.  But
much VHF work uses FM, which is nearly immune to differences
in received signal strength until the signal gets very
low.  That is one reason why it is so popular: strong
and weak signals come out at the same strength.  So,
until you get down to 20dB or so of the receiver
threshold, you won't notice any difference in received
signal loudness with a stronger or weaker signal.
Eventually the background noise comes up on the signal,
but many communications are carried on many dB above
the minimum limiting level.
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KA0GKT
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Posts: 555




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« Reply #7 on: August 09, 2005, 06:35:28 PM »

WB2WIK writes:
"...The "FET" explanation is irrelevant, since it doesn't matter what your front end device impedance is, it's tuned to match and look like 50 Ohms using either narrowband or broadband circuits -- but in all cases, even if the FET impedance is a million Ohms..."



Not entirely true.  If you look at the schematics of some receivers, the input matching for the receiver is often a resistor.  Now if a 50-ohm resistor shunts the 1-megohm gate Z of a FET, the impedence the antenna would see would be along the line of 49.9975 Ohms resistive.  So long as a voltage is generated across the build out resistor(s), the FET will amplify nicely.

This is not to say that a tremendous mismatch won't affect reception, it is just with todays broadband front-ends, it doesn't make all that much difference.


73 DE KAØGKT/7
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WZ6F
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Posts: 98




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« Reply #8 on: August 09, 2005, 08:09:25 PM »

Thank you all for the great replies! I enjoyed all: delving into FET characteristics, voltage aspects, loads, hard number attentuation which I intent to check out with the others, signal vs. background discussion, etc.

Oh yes, I am using a Kenwood th-f6a with a tuned hf whip. It works better than expected!

Thanks Again for All,

Alan
wz6f
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WB2WIK
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Posts: 20542




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« Reply #9 on: August 10, 2005, 08:12:14 AM »

>RE: Receive vs. Transmit  Reply  
by KA0GKT on August 9, 2005  Mail this to a friend!  
WB2WIK writes:
"...The "FET" explanation is irrelevant, since it doesn't matter what your front end device impedance is, it's tuned to match and look like 50 Ohms using either narrowband or broadband circuits -- but in all cases, even if the FET impedance is a million Ohms..."
Not entirely true. If you look at the schematics of some receivers, the input matching for the receiver is often a resistor.<
 
::Can you provide an example of this?  I've never seen the input of any receiver shunted by a 50 Ohm resistor.  That would add 3 dB of noise to the receiver NF, and zero selectivity.  Every receiver I own (8 of them) has a input matching network at the front end, and the better ones are tunable and bandswitched.


 
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WB2WIK
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Posts: 20542




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« Reply #10 on: August 10, 2005, 08:14:35 AM »

>RE: Receive vs. Transmit  Reply  
by WZ6F on August 9, 2005  Mail this to a friend!  
Thank you all for the great replies! I enjoyed all: delving into FET characteristics, voltage aspects, loads, hard number attentuation which I intent to check out with the others, signal vs. background discussion, etc.
Oh yes, I am using a Kenwood th-f6a with a tuned hf whip. It works better than expected!<

::I'm amazed.  My TH-F6A, connected to any sort of decent HF antenna, goes into immediate terminal overload and doesn't receive anything at all due to complete saturation.  It does work okay with a whip.  Of course, you can't compare anything about the F6A's ability to receive versus transmit on HF, since it doesn't transmit on HF....

WB2WIK/6
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X-WB1AUW
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Posts: 559




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« Reply #11 on: August 10, 2005, 05:24:45 PM »

I think he was referring to most, if not all of the material about SWR and line loss, centers on loss of xmitted power.

I think all of the graphs I�ve seen on this, refer back to an article in QST (HandBooks and ON4UN book).  I�ve never seen the original article.

I�m pretty sure ON4UN explains the addition loss is because the net difference between the voltage (and amperage) peaks and troughs increases; never seen anything about the effect of signal being reflected back from the amplifier and either a reduction or increase in SWR loss.

If I understand correctly, the amount of wattage not transferred from the antenna into the coax, because of mismatch is rather low; the SWR between the rig and coax is too low to create loss; a 20 over S9 and 10 over S9 have the same signal to noise ratio.

Bob
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WB6BYU
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Posts: 13017




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« Reply #12 on: August 12, 2005, 12:23:44 PM »

As I mentioned before, some of the "SWR vs. efficiency
tables" published are total... um... well, let's be polite
and just say they are inconsistant with common principles
of electrical engineering.

A feedline can have an SWR of 20 : 1 and still be more
efficient than one with an SWR of 1 : 1.  If the table you
are looking at is NOT the type that shows the additional
attenuation of a line as a function of both the matched
loss of the cable and the SWR, throw it out.
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WZ6F
Member

Posts: 98




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« Reply #13 on: August 14, 2005, 04:26:17 PM »

WB6BYU,

Thanks for the information. It sunk in the second time! This sure is fun with the antenna projects/experimentation.

73's,

Alan
wz6f
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