[Boatanchors] OT: antenna couplers?

[eugene at hertzmail.com]

Thanks for the note, Sandy. And a great big thanks to everyone else who
answered my question! 

Some good things to think about. I happen to be running an OCF dipole. I
swept it to see where its resonant by using a return loss bridge. Here
is a photo of the plot.  

http://www.hertzmail.com/w2hx/antenna/SA-Plot.jpg

You will see the low points are the points with the least reflected
power, hence the most forward power, hence best resonance, hence best
impedence match to 50 ohms (that's the sweeper's source impedence.)

You can see that even out of the box, the OCR has some resemblance to
the ham bands. So the tuner doesn't need to do too much work.  As I
mentioned, my question was mostly hypothetical.

The next step is for me to convert the db readings on the SA into SWR.
There was a great article in the Jan issue of QST that explains how to
do this. That's my project this weekend!

Thanks again, everyone!
Eugene W2HX


-----Original Message-----
From: Sandy [mailto:ebjr37 at charter.net] 
Sent: Friday, January 09, 2009 11:59 AM
To: eugene at hertzmail.com; BOATANCHORS
Subject: Re: [Boatanchors] OT: antenna couplers?

Firstly, you didn't say what the antenna configuration was.  That aside,
a VSWR meter is usually engineered for a particular impedance.  The
antenna, in this instance being an unknown entity, could be very
"reactive" 
inductively or capacitively, since your SWR bridge is giving you a high
SWR. 
The antenna tuner/matcher is compensating for the reactance, and should
be used between the transmitter and the antenna tuner.  If you intalled
the SWR meter on the output side of the tuner the SWR would still be
high there. 
High VSWR on the feedline will affect losses.  Usually coaxial feeders
exhibit higher looses with increasing VSWR than open wire or "ladder
line" 
type feeders, as the cable/feedline losses are usually less with
parallel wire feeders.

A lot of other factors come into play.  What type antenna is it?
(Dipole, end fed wire, off center fed, vertical? some sort of "array" or
"beam"? 
Etc.)  What type and length feeder is being used?  With no tuner at what
frequency or frequencies is the VSWR high? (Over say 3:1)  What is the
length of the antenna and height above ground?  Is the antenna being
used on more than one band?  Etc.

73,

Sandy W5TVW



  ----- Original Message ----- 
From: <eugene at hertzmail.com>
To: "BOATANCHORS" <boatanchors at mailman.qth.net>
Sent: Thursday, January 08, 2009 8:50 PM
Subject: [Boatanchors] OT: antenna couplers?


> Sorry for the OT.
>
> This is a basic question about tuners/couplers.
>
> Hypothetical Premise: My antenna gives a "bad"match to my transmitter
at
> a given frequency (say 4:1 SWR). So I put an atenna coupler/tuner
> between the transmitter and antenna.
>
> I know that if the tuner is working well and is able to match the
> impedences, I will see a very good match between my transmitter and
> antenna coupler, maybe as good as 1:1 or 1.1:1. I would see that by
> using an SWR meter between my transmitter and the antenna coupler.
>
> The question is, what would I see if I put the SWR meter between the
> antenna coupler and the antenna? What would I see Would I still 4:1
SWR?
> Or would it still look like 1:1?
>
> Thanks for indulging (and yes, I will pull out the arrl handbook, but
> its nice to chat with some folks now and then)
> 73 Eugene w2hx
> _______________________________________________


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