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 unitÖ thatís what !I am not kiddingÖ. .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 80MÖ. but itís useless at VHF.. right ? Is it? That same 100 foot piece of RG-58 will induce a .65 S unit loss at 2M.Thatís slightly more than Ĺ and S unit. Think thatís significant? Only if itís a really marginal contact. Most of the time, it isnít 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 arenít 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 receiverís S-meter reads differently. But not that much difference. The point isÖ these losses arenít 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 sizedhigh 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 isnít required. Especially if one is using an amp with less than1000 watt output.

 

Donít 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 itÖ 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.