VSWR Overload?

[Robert Bass]

Can someone explain what VSWR Overload is? What causes it and what affect does it have on the transmitter? Is this the same thing as reflective power? I tried a google search for “what is VSWR overload”, but that wasn’t very helpful.

Thanks

R

 

[radiosaur]

> Can someone explain what VSWR Overload is? What causes it
> and what affect does it have on the transmitter? Is this
> the same thing as reflective power? I tried a google search
> for “what is VSWR overload”, but that wasn’t very helpful.
>
> Thanks
>
> R
>

I'm assuming you're having them...so:

You're very close. VSWR is the ratio of forward power to reflected power. If you're having VSWR overloads, there is too much reflected power coming back from the antenna system. Maybe...

Possible causes:

(Assuming no component failures within the TX)

1. You really do have too much reflected power.
2. The meter calibration for your reflected power is incorrect
3. The overload is set too tight.

Numbers 2 and 3 can be corrected easily and cheaply. #1 is a bit more serious.

You need to verify with a wattmeter if you actually have high reflected power. It can cause tube damage or other nastiness if not corrected.

High VSWR can be caused by a failed or failing antenna or transmission line, problems with the tuning system, or (rerely) metallic objects near the antenna (broken brackets, etc)

Sounds like you might want to get an engineer to check the system out. It may save you a lot of headaches. However, It bears repeating...Don't adjust the meter or overload limits until you are sure you don't really have a problem.



<P ID="signature">______________
Never hold a cat and a dustbuster at the same time.</P>

 

[k9ez]

> > Can someone explain what VSWR Overload is? What causes it
>
> > and what affect does it have on the transmitter? Is this
> > the same thing as reflective power? I tried a google
> search
> > for “what is VSWR overload”, but that wasn’t very helpful.
>
> >
> > Thanks
> >
> > R
> >
>
> I'm assuming you're having them...so:
>
> You're very close. VSWR is the ratio of forward power to
> reflected power. If you're having VSWR overloads, there is
> too much reflected power coming back from the antenna
> system. Maybe...
>
> Possible causes:
>
> (Assuming no component failures within the TX)
>
> 1. You really do have too much reflected power.
> 2. The meter calibration for your reflected power is
> incorrect
> 3. The overload is set too tight.
>
> Numbers 2 and 3 can be corrected easily and cheaply. #1 is a
> bit more serious.
>
> You need to verify with a wattmeter if you actually have
> high reflected power. It can cause tube damage or other
> nastiness if not corrected.
>
> High VSWR can be caused by a failed or failing antenna or
> transmission line, problems with the tuning system, or
> (rerely) metallic objects near the antenna (broken brackets,
> etc)
>
> Sounds like you might want to get an engineer to check the
> system out. It may save you a lot of headaches. However, It
> bears repeating...Don't adjust the meter or overload limits
> until you are sure you don't really have a problem.
>


And to pile on.....

VSWR is USUALLY a mismatch in the impedance of the antenna system. That means that the feedline, or the antenna could haveproblems.

Question...

Isthis for AM or FM?

There are a nunber of reasons why an antenna system could become "mis-tuned".

 

[radiosaur]

> Question...
>
> Isthis for AM or FM?
>
> There are a nunber of reasons why an antenna system could
> become "mis-tuned".
>

I guess I should have asked that as well. All of what I said still applies but the way to go about correcting it is quite different




<P ID="signature">______________
Never hold a cat and a dustbuster at the same time.</P>

 

[Robert Bass]

Thanks for the informative reply! In this case it’s an FM station, but honestly I was primarily asking from a just-curious standpoint. I’ve seen the sticker next to a LED indicator on the Burk ARC-16 remote control unit where I work. Actually in this case, there are three separate VSWR Overload monitors. There’s one that just says VSWR Overload. There’s also one that says Main VSWR Overload and another that says Aux VSWR Overload.

Main and Aux simply refers to the Main and Auxiliary transmitters, which are on the same antenna. Main is the primary transmitter, and Aux is the backup. As I understand it now, both xmtrs cannot be on-air simultaneously. However I understand there is a desire to install a backup antenna as well, which I presume would be run on a separate line, theoretically allowing both xmtrs to run at the same time.

R

 

[tpt]

The third overload may be a device like a Bird WattWatcher.

It's desilgned to trip quickly in case of a high vswr condition, such as ice.

Don't know where you are, but in some parts of the country it's easy to have a rainy day in the low thirties begin to build up quite a layer of ice high up on the tower where the antenna is. Best to shut down, rather than burn out a feedline or damage the transmitter.

 

[k9ez]

> Thanks for the informative reply! In this case it’s an FM
> station, but honestly I was primarily asking from a
> just-curious standpoint. I’ve seen the sticker next to a
> LED indicator on the Burk ARC-16 remote control unit where I
> work. Actually in this case, there are three separate VSWR
> Overload monitors. There’s one that just says VSWR
> Overload. There’s also one that says Main VSWR Overload and
> another that says Aux VSWR Overload.
>
> Main and Aux simply refers to the Main and Auxiliary
> transmitters, which are on the same antenna. Main is the
> primary transmitter, and Aux is the backup. As I understand
> it now, both xmtrs cannot be on-air simultaneously. However
> I understand there is a desire to install a backup antenna
> as well, which I presume would be run on a separate line,
> theoretically allowing both xmtrs to run at the same time.
>
> R
>

Usually the AUX antenna is for when tower work needs to be done (or the main antenna goes down), not for running both transmitters.

 

[Robert Bass]

North Central Texas…

And actually the station I work for has a brand new antenna (actually it’s about 10 months old now). The existing one was damaged beyond repair in a lightning strike last summer. The new one is more efficient and has heating elements to eliminate ice buildup.

R

 

[Grrradio]

Some quick math on the matter - VSWR is usually expressed in a ratio of the form X:1, where X can be any real number above one (integer or not). The percentage of actual reflected power can be obtained by dividing (X-1) by (X+1), then squaring the result and multiplying by 100.

For instance - if the SWR is 1.6:1, (0.6 / 2.6) = .23077; .23077 squared = .05325, meaning 5.325% of the transmitter power output is being reflected. For an SWR of 3:1, (2/4) squared = .25, meaning 25% reflection. It's one of those misleading logarithmic scales - the arithmetic equivalents go up pretty quickly! 3:1 and even 4:1 are not unheard of in mobile ham or CB applications, but hopefully, your broadcast installation will rarely have to deal with figures greater than 2:1. (There are, however, a few stories of antenna feedline actually "flashing over" at SWR's above 10:1, due to the peak amplitudes of the standing waves generated in the coax by reflected power! For a full-powered class C with, say, a TPO of 12.5 kW into a bay system providing 9 dB of gain... 10:1 would mean about 9500 watts of RF coming back down that cable!)

As far as the transmitter itself... the only transmitter I've seen "up close and personal" was at my college FM station in Iowa, so I'm not familiar with any FM equipment stronger than 500 watts. I believe ours was rated to operate with a SWR as high as 5:1. (However, I'm not sure when the actual overload protection kicked in and began to ratchet down the power.) Many British community stations use equipment from an outfit called NRG, which claims overload protection as high as 10:1 for its 40-watt transmitters.