[BC]

[DANA PUOPOLO]

The absolute way to tune an antenna system is with a Network Analyzer.
Several times now, I've found VSWR problems with antennas who showed zero
reflected power on Birdwattchers and the VSWR meters on transmitters. In every
case, a noticeable reduction of multipath occurred. In the case of KIIS in Los
Angeles, they had been running RCA 10 kw tube rigs at 7100 watts out since the
late 1970's which both showed zero reflected power on their VSWR meters, yet
when we put their new SS Nautel on the antenna it folded back to 600 watts!!
We had a Jampro engineer fly in with their Nework Analyzer and it showed the
return loss of the system to be equalivent to a 1.7:1 VSWR. They had been
running for 25 years this way! We refurbished and retuned the 2 bay Penetrator
antenna in the field and when we were done we had 7100 forward watts and under
12 watts reflected. The arms were all four inches too long, and the gamma
match was off by over three inches. As found, the antenna was resonant on
101.9; they are 102.7. That morning when I drove home listening to the
station, areas where multipath was bad before were completely clean! AND..I
wasn't the only one who noticed it either...Rick Dees' board op mentioned on
the air how good the station sounded on his way in (He lives in Malibu and
there used to be places on his drive in where  multipath had made KIIS
unlistenable - but not any more!).

It's the best 3500 bucks any FM station can spend....

-D

 

------ Original Message ------
Received: Sun, 16 Oct 2005 09:44:59 PM PDT
From: Alan Alsobrook <radiotech at bellsouth.net>
To: Broadcast Radio Mailing List <broadcast at radiolists.net>
Subject: Re: [BC] "VSWR" Metering

Unfortunately efficiency of the transmitter will remain stable when 
looking into one of these situations. I'm not sure how good the results 
will be if you look at it with a TDR as well. your best results will be 
to sweep the line with an FDR (Frequency Domain Reflectometer) to do 
this properly you would need to sweep the line on commission and on 
regular intervals there after. If you don't have access to one of these 
devices you might want to call someone who does. Even if you don't to 
periodical sweeps (highly recommended) at least if you have a base sweep 
you'll be able to see a problem if it occurs. If you look at Radio Guide 
August issue on page 26 Gary Minker wrote an article about this. (it 
didn't get as technical as I would have liked). I've meet Gary a couple 
of times he's got some really really nice equipment to do extensive and 
detailed sweeping. I have good basic equipment myself. Also to be a good 
sweeper takes some time and practice, so it might be better to have 
someone else come and do it for you. I'm getting pretty good at it but I 
know there is still much I have to learn.

Now to actually answer your question, about the only place you'll for 
sure be able to see a problem like this, in AM you have your line meter 
and hopefully a base current meter if your base current meter indicated 
a drop in power going to the tower while the transmitter is still 
showing full output then you will know of a problem. For FM it gets much 
trickier an indicator may be a loss of coverage, or a fire on the tower.

cld at admin.umass.edu wrote:
> 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? 

-- 
Alan Alsobrook CSRE AMD CBNT
St. Augustine Fl. 32086 904-829-8885
aalso at Bellsouth.net


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