[Antennas] Hardline Questions

Tue, 28 Oct 2003 17:16:43 -0500

Buck ..etc.
In fact, if your antenna is adjusted for a feedpoint impedance of 75 
ohms [a distinct possibility], and the feedline is ALL 75 ohms, your 
tuner can match the 50 ohm output of your transmitter/amplifier to 75 
ohms .. and you would/could obtain a 1:1 SWR, even if the SWR meter you 
use is designed for 50 ohms, IF that meter is between your 
transmitter/amplifier and the tuner!
Don
N8DE

Robert Lay wrote:
> 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
> 
> 
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