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Re: [TenTec] OT SWR VS Power Loss

To: tentec@contesting.com
Subject: Re: [TenTec] OT SWR VS Power Loss
From: "Dr. Gerald N. Johnson" <geraldj@storm.weather.net>
Reply-to: geraldj@storm.weather.net, Discussion of Ten-Tec Equipment <tentec@contesting.com>
Date: Fri, 04 Jul 2008 12:37:09 -0600
List-post: <tentec@contesting.com">mailto:tentec@contesting.com>
On Fri, 2008-07-04 at 10:59 -0700, Jim WA9YSD wrote:
> This is interesting, 1:1 is 10w 2:1 is 5w, 3:1 is 2.5w and 4:1 is 1.25w 
> mismatch that radiates from the antenna if you just use ratios.  So some 
> where in between is what is actually happening.
And its all noise. The posted data is wrong except for the 1:1 line. All
the rest is fiction.

> Where does all this lost RF go?

It is sometimes absorbed by the PA, sometimes reflected again towards
the antenna.
> Some of it radiates from the coax, ground or where ever it finds a near 
> resonant
> route, or just heat or any other combination.  Mainly trouble. The rule
> of thumb like Ken and the Doc say, is anything that makes up 1/8 wave length, 
> like on cable between
> equipment becomes a point of RFI hots spots like a tingle of RF on the
> base of the key or what ever. 
> This is just interesting.  Every one has some type of a mis match on their 
> system of one type or another.  It has haunted me time and time again over 
> the years.
Not necessarily. Some antennas a perfectly matched at some frequency,
more often very closely matched by an antenna tuner which is nearly all
reactive matching so it doesn't dissipate power. Some times a "mismatch"
is intentional like using 75 ohm coax with a 50 ohm radio and antenna.
When that line is any multiple of a half wave long, the radio sees the
antenna match, not the coax. I've run that situation on VHF and UHF for
decades with no detected loss in receiver or transmitter performance,
and no detectable improvement by including a matching network. The
slightly lower loss of the 75 ohm coax can make up for the mismatch loss
and the PA has enough mismatch tolerance to put out full power.

Finding that match is a greater problem than it appears, because of
imperfect instrumentation. It takes a 30 dB reduction in reflected power
to achieve a 1.002:1 SWR, yet few ham grade wattmeters or SWR bridges
have that much directivity. Lack of directivity means that forward power
is read on the reflected power range. What that means is that when we
adjust the antenna match for ZERO reflected power we actually have some
reflected power to cancel the erroneous coupling to the reflected power
circuit from that lack of directivity, so that the meter says zero but
the actual reflected power is NOT ZERO. Most of the time it IS GOOD
ENOUGH, but its not the perfection displayed.

>  Keep The Faith, Jim K9TF/WA9YSD
73, Jerry, K0CQ

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