TX vs. RX Coax Losses

[ab5r]

We are all used to looking at various sizes of coax when constructing our stations. And, we have all read comments suggesting this type vs. another type. It seems that the rule of thumb is to use larger coax and shortest run possible.

These assumptions almost consider the losses of that type of coax for a given frequency at a particular power output. There are many tables and charts giving these figures.

Please notice they specify POWER and FREQUENCY for you particular usage.

QUESTION: What about RECEIVE ONLY usage of coax where there is no power applied? Would the only loss be the resistance of the center conductor? I have always considered the power application since I am a ham, but I do not really know the answer and tend to believe loss would NOT be significant with a receive only application; except for an extreme length of coax.

Anyone KNOW?

 

[n5ims]

Loss is loss is loss. Transmit or receive, it's the same. While on transmit you'll have a much greater loss of power, that's only because you start out with much more. Say your coax will have 3 dB of loss at a specific frequency.

On transmit, you may start with 50 watts of power from your transmitter and by the time the signal reaches your antenna there will only be 25 watts reaching your antenna (you lost half your signal strength). Quite the loss!

On receive, you may start with 50 microvolts of signal at the antenna. By the time that signal reaches your receiver, you'll end up with only 25 microvolts of signal (you lost half your signal strength). It was only 25 microvolts of loss, but still it was half what you started with.

The loss is due to the resistance of the center conductor, but this is only a very small part of the issue. The "holes" in the shield will allow signal to leak out (or in) causing loss. The loss is also due to the interaction of the center conductor and the coax shield. The type and construction of the dielectric (that plastic insulator that's between the center conductor and the shield) is also critical. What you have is the signals bouncing between the two conductors causing distortion and loss. Better coax will have a better design that's designed to reduce these interactions giving lower loss.

If it was only as simple as the resistance of the center conductor.

 

[Thunderknight]

It is recriprical. Meaning the loss is the same either way at a given frequency. In fact, generally speaking, any passive component in an RF system has recriprical losses.

The power handling has to do with not melting the dielectric

 

[nd5y]

On transmit, you may start with 50 watts of power from your transmitter and by the time the signal reaches your antenna there will only be 25 watts reaching your antenna (you lost half your signal strength). Quite the loss!

On receive, you may start with 50 microvolts of signal at the antenna. By the time that signal reaches your receiver, you'll end up with only 25 microvolts of signal (you lost half your signal strength). It was only 25 microvolts of loss, but still it was half what you started with.

You can't mix power and voltage. The decibel ratio calculation is different.
If you measure 50 W at one end of the coax and 25 W at the other end, that's -3 dB loss (half the power).
If you measure 50 uV at one end of the coax and 25 uV at the other end, that's -6 dB loss (half the voltage).
If you measure 50 uV (-73 dBm) at one end of the coax and 35.5 uV (-76 dBm) at the other end, that's -3 dB loss (half the power).
And if I remember right the above is only true if the source, load, and coax impedances are all the same.

 

[ab5r]

Then, I guess, that the old garden hose analogy sticks; the larger the diameter of the hose, the more water flows through. The larger the coax, the less the loss for a given frequency.

I still doubt that the "bottom line" loss with a RX coax makes that much difference; depending upon overall length, of course

 

[wyShack]

for the most part, receive losses only make a difference for 'weak signal' work (i.e. aircraft at a distance). another area where it makes a BIG difference is satellite reception- as the signal form the satellite is barely above the noise, the loss in your coax can make the difference between hearing the 'bird' or not. when you realize that the squelch in a typical scanner/receiver often 'needs' several db to 'open', worrying about 1 or 2 db is a little overboard if most of your monitoring is all 'full quieting' or S9+.

 

[chetp]

For weak signal applications in the VHF and up range where signal to noise is very important, loss in front of the first gain stage adds to the noise figure of the system. So it can make a very big difference. That is why placing the preamp right at the antenna is a good idea.Then you can use lossy coax from antenna to radio without sacrificing system performance.