[Antennas] Hardline Questions

2003年10月28日 16:03:04 -0500

Dear Don,
Gee, I really hate to burst that bubble, which is one of the myths about
transmission lines.
The truth of the matter is that the transmission line's characteristic
impedance and the load terminating that line (the antenna) are what
determine VSWR. Don't argue with me about it - just look up the formula for
SWR in any reference, such as the ARRL Antenna Book.
The point that I will make is that REGARDLESS of the transmission line's
characteristic impedance and REGARDLESS of the terminating impedance (so
long as they don't go to impossible extremes) can be transformed to almost
any impedance that you need for the transmitter. So the fact that the line
is 75 ohms does NOT prevent you from providing a perfect match to the
transmitter NOR does it change the fact that the tuner will have NO effect
on the VSWR on the line. Again, don't quibble with me about it - just look
up the facts and also read the comments of the other contributors.
----- Original Message -----
From: "Donald E. "Buck" Stiles" <[email protected]>
To: <[email protected]>; "Robert Lay" <[email protected]>
Sent: Tuesday, October 28, 2003 2:14 PM
Subject: Re: [Antennas] Hardline Questions
> Bob,
>> Regardless of all others factors, the transmitter is looking for a 50 ohm
> load but the transmission line is 75 ohm. It is impossible to achieve
below
> a 1.5:1 swr. It may be higher, but never lower...
>> The may be insignificant at HF, but quite the opposite at VHF/UHF if you
are
> counting watts.
>> ----- Original Message -----
> From: "Robert Lay" <[email protected]>
> To: "Donald E. "Buck" Stiles" <[email protected]>
> Sent: Tuesday, October 28, 2003 12:16 PM
> Subject: Re: [Antennas] Hardline Questions
>>> > Don't forget, Tim and Don, that the 1.5:1 mismatch may or may not occur.
> > First of all, do you know what the antenna impedance is - maybe it's
> closer
> > to 75 ohms than it is to 50 ohms, in which case the VSWR would be
closer
> to
> > 1:1.
> >
> > Furthermore, in a typical installation, there is at least 1.5:1 VSWR on
> the
> > transmission line - generally much higher. The fact that we put a tuner
at
> > the transmitter end to transform the impedance seen by the rig to 50
ohms
> > (thus giving us the "perfect match" to the transmitter), does NOT change
> the
> > fact that the VSWR on the line has NOT changed at all. It is STILL
> > determined by the line impedance and the antenna impedance.
> >
> > Bob Lay (W9DMK) in Dahlgren, VA
> > http://www.qsl.net/w9dmk
> >
> >
> >
>>
Bob Lay (W9DMK) in Dahlgren, VA
http://www.qsl.net/w9dmk