Transmission
Line losses, revisited
Jeff , VE1ZAC
It has been my
experience with a lot of hams, that they are needlessly focused on SWR and
transmission line losses.
For example, figuring
out that a RG-58 loses 10% of the power in line losses seems to freak everybody
out. Off they rush for cable that is 4 times as expensive, harder to install
and much harder to hang from a wire antenna.
For what ?
Well for .1 of an S unit…
that’s what ! I
am not kidding…. .1 of an S unit at the receiver at the other end is probably
not detectable by an average ham ! Oh, you say the
crappy RG-58 example I gave above is fine for 80M…. but it’s useless at VHF.. right ? Is it? That same 100
foot piece of RG-58 will induce a .65 S unit loss at 2M. That’s slightly more than ½ and S unit. Think
that’s significant? Only if it’s a really marginal contact.
Most of the time, it isn’t even going to be noticed.
Look at this
chart:
|
|
Losses
expressed as S units at listeners receiver* |
||||
100
feet of: |
80M |
40M |
20M |
10M |
6M |
2M |
RG-58 |
0.1 |
0.14 |
0.2 |
0.29 |
0.38 |
0.65 |
RG-213 |
0.06 |
0.09 |
0.13 |
0.19 |
0.26 |
0.46 |
300
Twin |
0.04 |
0.07 |
0.1 |
0.19 |
0.31 |
0.5 |
450
Ladder |
0.02 |
0.04 |
0.07 |
0.1 |
0.13 |
0.22 |
|
|
|
|
|
|
|
*
Based on 1 S unit being 6 dB, at S9 and below. |
|
|
|
|||
Based
on 50 ohm source and load. Losses can increase with higher SWR |
(These numbers generated with TLW software)
I have figured the
losses as S unit drop at the receiving end. Even arguing about further fudge
factors and other installation factors, it is obvious that we all worry way to
much about losses that aren’t of any apparent interest to the ham at the other end
of our QSO !
Note: There are
lots of potential errors in this chart if the receiver’s S-meter reads
differently. But not that much difference. The point is… these losses aren’t as
apparent to the receiver. In fact, they have way more affect on the transmitter
ham who is panicking about losing 10, 20 or 30 watts
of his 100 watt output in the transmission line. I believe we all overdo the transmission line
needs in many, many cases.
Another reason to
look at smaller transmission lines, besides cost, is the weight and wind load
of a transmission line when it is attached to wire antenna. Smaller diameter
cables induce much smaller loads on the wire elements, and will last longer.
Is the use of an
amp a big factor? Not as much as one would think. RG-58 sized high quality cables, such Super RG-8/U
is rated for 1500 watts. Very few amps put that kind of power in the coax. Yes,
the fatter cables like 213 will have higher power capabilities, but often it
isn’t required. Especially if one is using an amp with less than 1000 watt output.
Don’t disregard the simpler transmission lines. Even 300 ohm twin lead has wonderful characteristics, and is really underutilized by the amateur community. If you can find a source of it… have a look for the next low cost antenna project you tackle.
Addendum:
Since putting this note on the site, I have received several very thoughtful emails either expressing astonishment at this view of transmission lines, or agreeing with me that hams should be thinking about the "End-for-end" channel, rather than one end of the communications link. That's good to hear. I really appreciate the feedback. I admit, it colours what I might write or publish next.
S-Meters
Ahhh yes, the maligned S-Meter. And I am guilty of maligning the things, but they have their uses ! What's really needed is to understand what they are telling us, and find out how they are really calibrated. Coincidentally, Greg Ordy ( this guy rocks !) has an excellent article entitled 'S Meter Blues" which covers the whole subject very well, and provides some calibration for a few popular rigs, including the Pro 3 ( I have one of these things). I put a direct link up under the "Links" section. I also suggest you look at the article "Practical Gain" on my list.