[Elecraft] OT: pwr rating of RG6, etc

[Mike Fatchett W0MU]

Samuel,

Because a radio is poorly made and starts shutting back at an swr of  1.5:1
does not mean that the coax cable is not efficiently transferring the power
to the antenna.

Many superstations use 75 ohm hardline obtained from cable companies and
those stations get out just fine, ok they get out better than fine.  Radio
shack used to sell some RG-8 cable that was cheap.  The braid, if you were
lucky, covered 50 percent of the dielectric.  There is cheap coax out there.
I have purchased 1000's of feet of RG-6U at Home depot and it is high
quality belden cable.

SWR is not loss.  SWR is a standing wave ratio.

Just because you have 50 ohm coax does not mean that you have 50 ohms at the
end of that coax.

An SWR of 1:1 does not guarantee that you have maximized the power transfer
from the rig to the antenna.  If just means that the rig is seeing a 50 ohm
load.  You can cut a piece of coax at a particular length that will make a
poorly matched antenna fool the radio into thinking there is a good match.
The SWR will be low but the antenna will not work very well.


On 11/13/07 8:16 AM, "Samuel Strongin" <kf4yox at embarqmail.com> wrote:

> I have a great idea don't use the proper cable at all just use the cheapest
> piece of junk and try to justify it. The fact of the matter 1.5 does
> matter.For example Icom radios fold back when seeing a swr of 1.5 to one.
> CHEAP is not a reason to use something yes it will work so what. Is it the
> proper cable to use no.  Rg6 gives you a starting loss at 1.5. Rg 59 will
> work to on 75 meters it will even handle some power would I use it No.
>        Good luck using the wrong cable. We don't have pi networks any more.
> Solid state devices want to see the proper load. They arn't rated 50-75
> ohms. Yes it will work so will a coat hanger with a tuner as antenna.
> ----- Original Message -----
> From: "Don Wilhelm" <w3fpr at embarqmail.com>
> To: "Samuel Strongin" <kf4yox at embarqmail.com>
> Cc: "DOUGLAS ZWIEBEL" <dougzzz at gmail.com>; <elecraft at mailman.qth.net>
> Sent: Tuesday, November 13, 2007 9:10 AM
> Subject: Re: [Elecraft] OT: pwr rating of RG6, etc
> 
> 
>> You use it because of 2 characteristics.  1) It has lower loss than an
>> equal physical size 50 ohm cable - and that is true even considering the
>> added loss due to mismatch.  2) It is conviently available and
>> inexpensive.
>> 
>> Considering the added loss due to a 1.5 SWR, look at the charts.  If the
>> matched line loss is 2 dB, the loss when the SWR is 1.5 is only 2.1 dB.
>> Even if the matched line loss is 10 dB, having an SWR of 1.5 adds only 0.2
>> dB to the total loss.  A 1.5 SWR is insignificant when considering line
>> loss, especially at HF.
>> 
>> The facts do not support the popular view that an SWR any higher than 1.0
>> is a bad thing.  In most cases, a transmission line SWR of 2.0 or below
>> will work fine.
>> 
>> Now, there is another part to consider - the feedpoint impedance at the
>> output of a transceiver may have to be transformed to something closer to
>> 50 ohms for proper, full power operation of the PA - but that is the job
>> of a tuner, it has nothing to do with feedline loss beyond the tuner.
>> Unfortunately, these two parameters get mixed together and the result is
>> 'numerical soup'.
>> 
>> 
>> 73,
>> Don W3FPR
>> 
>> Samuel Strongin wrote:
>>> RG6 is 75 ohm cable. Why would you buy a great radio and worry about
>>> performance,then go use the wrong cable. You start out at a 1.5:1 swr.
>>> Idon't get it.
>>>                  kf4yox
>>> 
> 
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