[Boatanchors] OT: antenna couplers?

[Glen Zook]

The purpose of the antenna coupler (tuner, whatever) is to match the transmitter to the feedline.  It has no effect on what you would "see" if you put your SWR bridge on the output of the tuner.  That impedance does not change.  What does change is the SWR that the transmitter "sees" and therefore should make the transmitter much "happier".  The transfer of energy into the antenna should be more efficient than if the transmitter is not able to "match" the impedance of the antenna.  However, there will be losses in the coupler so there will not be a 100% transfer of energy.

Remember that the SWR bridge "assumes" (usually) that the transmitter wants to see a 50 ohm load and then the SWR reading is a ratio of what the SWR brige is expecting to see and what the actual impedance is.

Many of the older transmitters could handle a "mere" 4:1 mismatch without any problems (probably 200 ohms in this case) and therefore an SWR bridge was not in general use until transmitters started having a fixed, or close to fixed, 50 ohm output.  My WRL Globe Champion 350A can match up to at least 300 ohms, my Heath DX-100 can match up to at least 600 ohms, and my Hallicrafters HT-20 can match up to at least 2000 ohms.  That means at least a 6:1 for the Globe Champ, 12:1 for the DX-100, and 40:1 for the HT-20!

In the "goode olde dayes" amateur radio operators didn't worry about SWR.  If the transmitter could "load" the antenna and they made contacts they didn't worry.  If the transmitter didn't load the antenna very well then they made adjustments to the antenna.  From May 1959 (when I got my Novice Class license) until after I graduated from college in 1967 I didn't even own an SWR bridge.  After I graduated from college I eventually did build a "monimatch" type of SWR bridge but didn't really use it for quite some time.

Glen, K9STH

Website:  http://k9sth.com


--- On Thu, 1/8/09, eugene at hertzmail.com <eugene at hertzmail.com> wrote:
 
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)