[Elecraft] Feedline Loss With Mismatched Loads

[Jim Brown]

Loss in a feedline due to mismatch can easily be computed by several 
means if the antenna feedpoint impedance and the feedline loss vs. 
frequency and length are known. Those means include:

TLW, a simple Windoze program by N6BV, retired editor of the ARRL 
Antenna Book, and included on the CDROM that comes with the Antenna Book.

ZPlots, an Excel spreadsheet by Dan, AC6LA.

TLA, a simple Windoze program by AC6LA. Both of Dan's programs are 
freeware, google his call to find his website.

SimSmith, an excellent freeware Smith Chart program by AE6TY. It runs in 
Java, so is cross platform.

Data to plug into these programs can be exported from EZNEC and other 
antenna modeling programs. Data for real antennas can be measured using 
good quality vector impedance analyzers (they must provide R + jX, or Z 
and the phase angle of Z).

If used correctly, all of these programs will provide correct answers. 
That can be a big IF -- it's the old GIGO (garbage in, garbage out) rule.

It IS, for example, good engineering practice to use a low loss line 
that is mismatched to an antenna with a tuner in the shack IF the line 
is short enough, and the line loss is low enough.  Phil Salas, AD5X, 
published a method for doing exactly this, using 50 ft or so of 1/2-in 
hard line to feed a 43 ft vertical.

The technique of using high impedance open wire line to feed a dipole of 
random length is another example, but I consider it no longer good 
engineering practice because it is not practical to choke it to reject 
receive noise. Off-center fed antennas have the same problem, no matter 
how they are fed. A major advantage of coax-fed, resonant antennas is 
that they CAN be choked.

73, Jim K9YC

On Wed,4/27/2016 8:36 AM, Ken K6MR wrote:
> “P.S. I don't subscribe to the notion that quality coax runs of < 150 feet
> make it "ok" to have the ATU in the shack while operating an antenna on
> multiple bands....what technical evidence of that posit do you have to
> share????”
>
> Jim:
>
> Google “transmission line bounce diagram”. This is a simple concept easily proven by mathematics.
>
> On a practical level, this is the concept behind the use of open wire feed lines. It’s been done for decades. The type of feed line does not change the theory.
>
> Ken K6MR
>
>
>
>
>
>
> From: Jim Rodenkirch<mailto:rodenkirch_llc at msn.com>
> Sent: Wednesday, April 27, 2016 06:21
> To: elecraft at mailman.qth.net<mailto:elecraft at mailman.qth.net>
> Subject: Re: [Elecraft] Sherwood's receiver performance table updated
>
> "It all depends" is/was the premise for my reply, Don.
>
>   If new and old hams don't understand the potential problems with long runs
> of coax to/from an antenna they want to operate on multiple bands and an ATU
> in the shack they will be surprised at how inefficient their system is.
>
> P.S. I don't subscribe to the notion that quality coax runs of < 150 feet
> make it "ok" to have the ATU in the shack while operating an antenna on
> multiple bands....what technical evidence of that posit do you have to
> share????
>
> Note 1: I had a 43' vertical with top loading wires in a NORD-style config
> with my external ATU at the base of the vertical and 120' of coax back to
> the shack ---- never saw a VSWR delta of more than .2 between what was
> "seen" at the input to the tuner and what was "seen" at the xmtr while
> operating on 160 through 20 meters.
>
>
>
> --
> View this message in context: http://elecraft.365791.n2.nabble.com/Sherwood-s-receiver-performance-table-updated-tp7616652p7616802.html
> Sent from the Elecraft mailing list archive at Nabble.com.
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