[BC] "VSWR" Metering

[Richard Fry]

Chuck Dube:
>May I boil it down to "a vswr meter is valuable for viewing the
>antenna system match to the transmitter in general (for those
>of us without access to a TDR)...

A TDR with the bandwidth/resolution needed to show discrete discontinuities 
along the line (bad bullets, split bullets, dents in the outer conductor, 
sagging inner conductors, sooty or otherwise contaminated insulators, water 
in the line, etc) always will have a very high mismatch from the antenna, 
itself.  An antenna is a relatively narrowband device, and looks like a bad 
match to a wideband TDR pulse - even if the antenna has a perfect match 
across the FM channel.

>Which brings us to the next question- if indeed you see a "good"
>reflected power indication at the tx output, but still suspect something
>is awry, what, in your opinions, are the best places to look (and let's 
>say
>that you don't have the ability to climb a tower and check the vswr at
>different points nor have access to a TDR). Efficiency?

Check that the SWR metering system is working right.  Did the slug(s) get 
zapped by lightning?   Are the right slugs used?  Is the SWR meter 
calibrated properly?  Does the indicator you're using that makes you 
suspect high antenna system SWR have a problem itself? Tx efficiency rarely 
enters into this.  And if that was the cause of your problems you should 
already know it by PA voltages, currents, tuning and loading that are 
different than normal.

Transmitter and "Bird" type SWR metering is OK for SWRs greater than about 
1.3:1.  Below that they tend to have progressively high errors.  They are 
not precision instruments.   If you really want to evaluate an FM antenna 
system, that needs.some special test gear and an experienced "antenna 
person."  Fixing the problems you find, and optimizing that antenna system 
takes all that plus a tower man or two, and maybe some parts.

RF