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[TowerTalk] transmission line loss and SWR

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Subject: [TowerTalk] transmission line loss and SWR
From: (Tom Rauch)
Date: Tue, 25 Sep 2001 19:56:20 -0400
>     Third, per W8JI's notes, the coax won't handle the power.
>    *********He has no knowledge of the size of coax I used--as usual. 

I picked the largest, lowest loss, 92 ohm coax in any of the dozens 
of catalogs I have. The reason loss is so high and power rating very 
low in 90-ohm coax is because the center conductor has to be 
"hair-thin". 90 ohms is about the practical limit of building a coaxial 
cable, and a very thin conductor (compared to cable diameter) is 

~75-ohm cable impedances result in lowest LOSS and ~50-ohm 
cables result in highest POWER rating for a given cable size and 
dielectric type!!!!

CATV systems...where power is low and loss is very 
important...use 75-ohm cables. Transmitting systems with high 
power where loss is less important than power rating use 50-ohm 

> don't you ask before going off in another tangent?  It's true that
> coax used as a 1/4 wave matching stub has SWR on it and that 
> the losses some but not for the full length. 

This is another good point.

SWR on a matching section is the same over the entire length of 
the matching stub. SWR does not change along a length of uniform 
impedance line, except for the decrease in SWR caused by loss in 
that line!

The reason for that is line SWR must always be referenced to the 
surge impedance of the system AT THE POINT OF 

We have a 90 ohm line. We have a 160 ohm load. The SWR in the 
line is 160/90, or 1.777 to one.

1/4wl away from the load the line reaches minimum impedance, 
and the SWR is still 1.777 to one.

So a 90 ohm line with a 1.777 to one SWR will have an impedance 
of....90/1.777 = 50-.64 ohms!!!!!

Now when we connect a 50 ohm line to the 90 ohm line, the 50 
ohm line's SWR is 1:1!!! Yet the 90 ohm line has a 1.77:1 SWR on 
it over its length..

See how neat this all is?

By the way, MAXIMUM loss in the 90 ohm line occurs at the 50 
ohm end of the line! That's because current is highest at that end of 
the line, and current related losses dominate transmission lines at 
HF by a large margin over other losses!

In a short transmission line, loss actually decreases with a 
mismatch in a direction that increases SWR.  




>    .
>     You have to go to RG133 (unavailable anywhere I could find on
>     using the GOOGLE search engine. It is 0.4" diameter stuff with a
>     big solid copper center conductor.  Any guess on the $$/foot if
>     you could find it). 
>    ********You fail to realize we in Seattle have access to Boeing
>    Surplus 
> and coax, wire and aluminum that makes your mouth water.
>    Making your favorite (open wire line) is impossible for 90
>     ohms.  The closest one can come is 110 ohms with 1/4" copper
>     tubing close spaced.  Thus another design iteration.....
>    ********I never said I made 90 ohm open wire line for that
>    application.  
> But if I wanted something for that range or higher there is a trick
> you don't known of just paralleling 2-300 ohm 1/4 waves for a 150 ohm
> stub.  That will invert 450 ohms down to 50 ohms balanced.  Or
> paralleling 3-300 ohm stubs for a"100 ohm 1/4 wave stub".  That will
> invert ____ohms down to 50 ohms. I'll let you do the math. Or parallel
> a 300 with a 150 ohm line there was at one time--for another "100 ohm
> 1/4 stub."  How do you like them design apples?  There is no
> dielectric like around 300 ohm amphenol around 2 copper tubing's
> attempting to make low Zo 1/4 wave stubs.  Therefore if you know your
> coax cable formula for Zo and related math you talk about all the
> time, I'll ask you how that dielectric changes the Zo of the line? 
> This is a "Pop Quiz!"  You Show me YOUR math.  
> I mentioned one lead of an open wire line plugging into a coax switch
> and the other lead through a series Xc to the case ground.  That
> particular feedline came from a 80m dipole and also a 40M horizontally
> oriented quad loop.  A single L wire can also connect to the coax
> switch center through the Xc also--I did it here.  The other coaxes to
> the coax switch acted like radials. 
>  It's a useful concept. Perhaps you could suggest a few?
>    Anybody in the US can work JA with great signals when the band is
>    open
>     on 10M using a wet noodle antenna.  I have worked a JA here who
>     was running 500 microwatts.  It wasn't his antenna, my antenna. 
>     It was propogation.  It is more a matter of knowing when the
>     propogation is right.
>    **********Get off it Brain.  I'll be happy to compare with your
>    "wet 
> noodle" any time.  Your example is a "wet noodle."  I gave an
> important point you totally missed again.  I said "I was still 20/9
> when the strongest JA's had dropped to S6 or less (band going dead)
> and I was in contact with the last signal heard around S1-2 before
> total collapse which was often the HL in Korea."  No one else ever
> called them when I finished.  Seattle has plenty of witnesses for
> this.  Some burnt rubber over here to check it out or called me on the
> phone.  I could hardly explain it myself but it happened and will use
> this beam again this winter for further comparisons.  It's Kicks Butt.
>  If you are going to claim great knowledge of band conditions, learn
> how to spell Propagation. 
>    Tests all done in the 30's?? Hmmmmm.  I guess you didn't use any
>    coax
>     then. Seems to contradict all you have said.  My recollection of
>     history is that coax as we know it was not put into service until
>     a few years pre-WWII.   If you did, I can understand why you hate
>     the stuff. No comparison to today's products.
>    ********You can't get anything right can you?  I didn't say ALL the
>    tests 
> were run in the 30's--my 1st ones were and 4 or 5 since.  Many used
> the L longer than a 1/4 wave in the 30's with the series Xc as it is a
> clever way to raise the Rr to any higher value which is generally
> lower than 50 ohms.  I didn't say I used coax then either.  I have
> never said I hated coax either.  I've just pointed out how well open
> wire line works, lasts and how cheap it is and suggested tuners that
> work great.  I use all kinds of coax.  About 40 years ago I purchased
> 4000' of Times Wire&Cable Teflon coax RG-8 size that cost Boeing
> $1.75/ft--double shield silver plated and all that--11 cents a foot. 
> It has the same loss as Pollyfoam.  Eat your heart out.  It's awesome
> stuff.  I just got 250' of a recent Teflon coax for 50 cents foot that
> I think had a $1.60/ft price.  I use coax and open wire line where it
> works best.    
>    Brian it's very clear to me everyone who E-mailed me, you have
> "intentionally butchered and misstated" the information I took a lot
> of the time to send you.  Your statements indicated you lacked the
> basic insights to grasp how it worked and I have forgotten your call. 
> With all your "40 years of extensive experience of ham radio reading
> you bragged about" you said "last year you had never seen or heard of
> any of my articles--even though they had been discussed and referenced
> repeatedly on TT."  Your current post shows your reading hasn't
> improved.  I asked you to contact me privately if you had any
> questions a week ago and you didn't.  Your and Tom's nonsense has been
> in full display again for reasons known only to you.  It's very
> obvious Tom put you up to this. I will state here that both your
> personal conducts are highly unethical and libelous. This conduct has
> no limits on TT.  I am telling you and Tom "never to comment on my
> Posts or Articles anywhere in Ham Radio or use your practices on
> others again".  Tom was told this in E-mail last year when he kept
> rewording my posts and would make up theory (from some book he
> couldn't produce?) to support a different version of a subject at the
> time on TT.  Tom was also told to "stop his making up his total bold
> faced lies about what I do and say."  Yet he has continued to do so. 
>    The information given Freely by many on TT is prepared in usually a
>    short 
> period of time and is not a Doctor Thesis.  Errors in numbers can
> occur.  Some data is well documented and some used to give their
> opinion but would "get Flamed" as the term is frequently used.  It
> discourages many contributions.  Even documented data that is easily
> proven by those with proven RF skills and insights is literally
> attacked by you and Tom in clever ways even after repeated requests
> I've made to try it.  Unfortunately Reflector Administration has
> alowed your practices to continue when told repeatedly of this and not
> allowed proper defense of legitimate material without any
> justification. If you and Tom can't understand the most elementary of
> antenna concepts, unload your technical deficiencies elsewhere. 
>  Getting new concepts to the ham public is difficult enough with your
>  total 
> nonsense.  Actually try sometime before you jump on it.  Don't you
> ever get tired of being wrong?  If you violate my request again you
> will have a very "Uncomfortable Legal QSO."  Do I make myself
> perfectly clear?  K7GCO   
>          73 de Brian/K3KO >>
> List Sponsored by AN Wireless:  AN Wireless handles Rohn tower
> systems, Trylon Titan towers, coax, hardline and more. Also check out
> our self supporting towers up to 100 feet for under $1500!! 
> -----
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73, Tom W8JI 

List Sponsored by AN Wireless:  AN Wireless handles Rohn tower systems,
Trylon Titan towers, coax, hardline and more. Also check out our self
supporting towers up to 100 feet for under $1500!!

FAQ on WWW:     
Administrative requests:

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