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Author Topic: Feed Line Loss - reciprocal?  (Read 1207 times)
K8QLW
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Posts: 37




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« on: September 05, 2009, 11:39:34 AM »

Feed Line Loss - Reciprocal?


I was checking coax feed line loss for 30M using http://ocarc.ca/coax.htm.

Just to keep it simple I plugged in the power as 100 watts and the SWR as 1:1 and the cable length as 100 feet.

Tandy RG-58 showed a loss of 24 watts (24%) while Beldon RG-8 showed a loss of 12.009 watts or about 12% for the same length.

Question:  Are those losses reciprocal?  In other words would a received signal be attenuated by the same percentage?  Always wondered about that.

Thanks,
Kent (K8QLW)

(Posted in CW and QRP Forums also)
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AA4PB
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Posts: 12768




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« Reply #1 on: September 05, 2009, 12:06:01 PM »

Yes, the loss affects the received signal by the same amount (%wise or dB) as it does the transmit signal.

On the receive side however, what determines the ability to copy a signal is the signal to noise ratio rather than the absolute strength of the signal. If the external noise is the limiting factor, as it is on all the lower HF bands, then you may not notice the difference on receive because the cable loss attenuates the noise by the same amount that it attenuates the signal.
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WB6BYU
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Posts: 13112




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« Reply #2 on: September 05, 2009, 02:38:57 PM »

Generally speaking, yes, the loss applies both to transmit
and receive.

But on receive the SWR on the cable depends the input
impedance of the receiver, not the antenna.  So if you
are using an old receiver with a 600 ohm input impedance,
the SWR on the coax may be 12 : 1 on receive even though
it is 1 : 1 on transmit.  In that case the losses will be
higher on receive.

But we are less likely to notice the receive coax loss (at
least on HF) because, as was already stated, the signal to
noise ratio is set by the background noise that is picked
up along with the signal.  10dB of attenuation makes all
the signals weaker, but won't change the signal to noise
ratio until the background noise is weaker than the
noise generated by the receiver.  (On VHF/UHF, where the
background noise is very low, the feedline loss can be
an important limiting factor in your ability to hear
weak signals.)
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WX7G
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Posts: 5947




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« Reply #3 on: September 07, 2009, 10:28:34 AM »

Not only is the feedline reciprocal but the entire path. From your TX to his RX and from his TX to your RX. If you both run the same TX power and his is S-7 at your QTH, you will be S-7 at his QTH.
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K8QLW
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Posts: 37




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« Reply #4 on: September 07, 2009, 01:24:48 PM »

Thanks much gentlemen for the input. (No Pun)

kent
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AA4PB
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Posts: 12768




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« Reply #5 on: September 07, 2009, 06:29:05 PM »

If you both run the same TX power and his is S-7 at your QTH, you will be S-7 at his QTH
-----------------------------------------------------
True (assuming that the antennas, receivers, and s-meters are the same) but if his noise level is S1 and yours is S6 then he will copy a lot better than you will. Thats not related to the coax loss but it explains why some people think that antennas work differently on transmit than they do on receive.
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WX7G
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« Reply #6 on: September 08, 2009, 06:35:03 AM »

AA4PB. I did not say S-meter, I said S-unit. An S-unit is commonly defined like so:

S-9 = 50 uV at the receiver input. Each S-unit represents a 6 dB change.

And yes The S-unit was originally defined 'by ear' but the numerically defined S-unit has been with us for decades.
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WX7G
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« Reply #7 on: September 08, 2009, 06:44:12 AM »

AA4PB. It does not depend on the antennas. This is the point I was trying to make.

Say we have a 5 element Yagi up 100' at one end, a dipole up 10' at the other. Both stations run 100 watts. The signal strength at the RX is the same at both stations!

Now trade antennas and the signal strength remains the same.
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K0OD
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Posts: 2539




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« Reply #8 on: September 08, 2009, 07:02:26 AM »

"An S-unit is commonly defined like so: S-9 = 50 uV at the receiver input. Each S-unit represents a 6 dB change."

Defined by whom? Certainly not any major maker in the past 50 years, except perhaps now by Flex.
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N3OX
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« Reply #9 on: September 08, 2009, 07:44:06 AM »

Until all popular radios have 6dB S-units, we should stop using them to compare things.
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73,
Dan
http://www.n3ox.net

Monkey/silicon cyborg, beeping at rocks since 1995.
WX7G
Member

Posts: 5947




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« Reply #10 on: September 08, 2009, 10:16:11 AM »

defined by whom?

Some links with the 6 dB definition:

http://www.astrosurf.com/luxorion/qsl-db-power-units.htm

http://en.wikipedia.org/wiki/S_meter

And the ARRL Handbook, if I'm not mistaken.

The definition of an S-unit and how manufacturers calibrate their receivers are two different things.

Elecraft sets each S-unit to be 5 dB in the K3 transceiver.

So perhaps as N3OX says we should stop using the S-unit. We could discuss the TX/RX reciprocity question in terms of signal power in units we can agree on, such as dBm.
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G3TXQ
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Posts: 1506




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« Reply #11 on: September 08, 2009, 10:58:31 AM »

>> Defined by whom? <<

IARU !

"IARU Region 1 Technical Recommendation R.1 defines S9 for the HF bands to be a receiver input power of -73 dBm. This is a level of 50 microvolts at the receiver's antenna input assuming the input impedance of the receiver is 50 ohms.

For VHF bands the recommendation defines S9 to be a receiver input power of -93 dBm. This is the equivalent of 5 microvolts in 50 ohms.

The recommendation defines a difference of one S-unit corresponds to a difference of 6 decibels (dB), equivalent to a voltage ratio of two, or power ratio of four.

Signals stronger than S9 are given with an additional dB rating, thus "S9 + 20dB", or, verbally, "20 decibels over S9".

Steve G3TXQ
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K0OD
Member

Posts: 2539




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« Reply #12 on: September 08, 2009, 12:49:09 PM »

Read the FULL definition from http://www.astrosurf.com/luxorion/qsl-db-power-units.htm:


"What represents 1 S-point in dB ? Each S-point corresponds to a current or voltage ratio of 2 and a power ratio of 4. 1 S-point or S-unit represents thus a power ratio of  6 dB (and rather 4 dB in average in some Japanese RTX in the lower part of the meter)."


There it's defined as 6 dB except when its 4 dB. LOL! I believe Collins made some futile efforts to define it years ago too. I know some early receivers (HRO5 Huh) had meters without S-units)

The S-unit really belongs in the ham radio's dust-bin. Maybe contesters have been onto something for the past 30 years.
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WB6BYU
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Posts: 13112




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« Reply #13 on: September 08, 2009, 03:33:40 PM »

The last time I measured the calibration on a S-meter on
a commercial ham rig there was a 12dB difference between
"S1" and "S9+40".  How do you expect manufacturers to get
S units right when they can't even measure dB?

But perhaps that explains the gain claims of some antennas...
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AA4PB
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Posts: 12768




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« Reply #14 on: September 08, 2009, 05:51:32 PM »

Until all popular radios have 6dB S-units, we should stop using them to compare things.
------------------------------------------------------
I don't know, S-meters are good for relative comparisons and they can give you a "general" idea of the amount of difference. The problem comes in when someone tries to use them for absolute measurements.

If suddenly every S-meter was accurate to 0.1% we'd still have a problem because of differences in antennas, feed line loss, propogation, etc.
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