Re: Some calculations about GAIN, LNA, ATTENUATORS, COAX...


doug
 

On 11/17/2017 02:14 PM, Eric Oyen wrote:
YW. :)

having been a ham for a number of years (25 currently) and an avid CB radio enthusiast and SWL before that, I have had to learn a lot of stuff about coax cables, connectors and proper matching to the line. receive matching isn't nearly as critical unless you want maximum transfer from the antenna to the line. Connectors do add some loss (depending on type). your typical UHF type connector (pl-259/so-259) add about .5 Db per connector. throw a barrel connector into that and you will have 1.5 Db loss at that point. Barring any other issues (such as line age, water infiltration, etc), a good system will get most of the signal from the antenna to the receiver with little noticeable degradation in signal.

SOme exceptions do need to be noted to the above. An antenna that is substantially out of resonance will appear to give larger losses at the feed point. In the case of rg-58/u cable, that loss on HF can be rather large (to the tune of 12 Db or more). this is usually only for transmitted power from the line to the antenna and may have minimal effect on receive. I would recommend a little friendly reading at the ARRL website (more specifically the publications "ARRL antenna handbook" and "Antenna compendium". both have extensive information on coax types, VF figures, losses, etc.

DE n7zzt Eric

On Nov 17, 2017, at 9:08 AM, Ruben Navarro Huedo (EA5BZ) wrote:

Tnx for your opinion Eric
Eric is far too pessimistic, and in a certain case, wrong. To wit:

At HF (i.e., below 30 MHz) the attenuation of UHF connectors is probably < 0.2dB. It might go to 0.5 dB at 144MHz. I personally HATE UHF connectors,
primarily because they are hard to mate properly (and some don't mate at all--some Chinese ones are made wrong) and it also bugs me that they are
NOT 50-Ohm matched. The point is that at HF and low VHF the mismatch does not cost much. K2RIW has a big paper on that--look him up!

Now as to cable loss: Cable loss does not depend on whether you transmit or receive, unless you transmit so much power that the cable dielectric
gets hot and melts, in which case, the loss will become infinite shortly. Think of the cable as an attenuator. It doesn't care which end is which, or
which direction the signal is moving. However, it will contribute a higher loss if connected to a mismatched load, so be sure your antenna has the
lowest SWR possible. The reason is that the resistive loss of the cable conductors will increase with a mismatch because the conductors will see
peaks in current at the maximum current points of the standing wave. However, a 12dB loss could only result in a very long cable with a high loss
at the frequency of interest combined with a very high VSWR at the antenna. This would be because of bad design right from the start! No-one
would ever design such a system! Note that the SWR mismatch will affect the total loss in EITHER direction. The cable is passive!
As to receive and transmit, assume the SWR is 1:1. Again, the cable is just an attenuator. As such, its loss at the frequency of interest will increase the
system noise figure by the effective attenuation of the cable at the frequency of interest. Suppose you have a 100 foot length of LMR-240 cable, and you want
to communicate on 145 MHz. Assume Type N connectors, which are virtually lossless at 145 MHz. The matched loss of this cable at 145 MHz is 3.5dB/100 feet.
If the noise figure of your 2-meter receiver is 2dB, then your system noise figure is 5.5dB. If you are transmitting 100 Watts, then your antenna will only
have about 45 W into it. Obviously if the antenna is mismatched, the system loss will be greater, in BOTH directions, transmit and receive!

--Doug, WA2SAY, retired RF engineer.

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