SWR/Return loss question???

FM does it with frequency!
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waveguide
Posts: 26
Joined: Thu Feb 17, 2011 3:28 pm

SWR/Return loss question???

Post by waveguide » Fri Mar 31, 2017 1:55 pm

Hi everyone,
Im seeing a small amount of reflected power on an antenna system - 1500watts FWD - and 20w REF - its at the top of the band and 106.9Mhz so I'm thinking maybe 106.9 is approaching the upper limit of the antenna (Sira dipole).
I managed to reduce the reflection from 20watts to 4watts by shortening the feeder between the filter and the trasmitter. (This took a while to do!!!)

Am I right in thinking the reflected 20watts is still there, its just been nulled out by adjusted the phase/length of the feeder - the transmitter cant see it. Is this an acceptable solution?
(The filter has tested ok)
Also, is it worth looking at investing in a matching unit to reduce the reflection?
Some thing like this? (PDF Link....) http://www.sira.mi.it/download.php?doc= ... 3/1332.pdf

Thanks - hope this makes sense!!!! :)

TPT
Posts: 649
Joined: Mon Dec 03, 2007 3:18 pm
Location: St. Marys, WV

Re: SWR/Return loss question???

Post by TPT » Fri Mar 31, 2017 2:47 pm

Sometimes a short jumper like you described may actually be the cause of the reflected power. Changing the length--e.g., changing the wavelength of that short piece of transmission line may improve the overall match of the system.

Lee_Wheeler
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Location: Kansas City

Re: SWR/Return loss question???

Post by Lee_Wheeler » Fri Mar 31, 2017 10:36 pm

Trimming coax to deal with a bad match really doesn't change the load whatsoever, what it can do is alter the relationship to the directional coupler with the standing wave reflection. What it essentially does is trick the measurement rather than changing the load.

If you do add a fine matching section it should go at the load end (antenna). Putting it at the generator end (transmitter end) only matches the transmitter to the fine matcher with all of the bad load left beyond it.

You mentioned a filter, is this an external filter bandpass or band reject filter that would be installed for a multiuser site or are you talking about the actual low pass filter for the transmitter? If it is an additional external filter then its tuning could be off and cause the 1.26:1 VSWR you are seeing. It would be a valuable test to look at the reflected power of just the filter into a dummy load but tuning a filter usually takes a network analyzer since you need to make sure it doesn't change the filtering characteristics along with the load characteristics.

...Lee

waveguide
Posts: 26
Joined: Thu Feb 17, 2011 3:28 pm

Re: SWR/Return loss question???

Post by waveguide » Sat Apr 01, 2017 1:11 pm

Thanks,
Its actually a combined site, two hybrid combiners - there was three FM service, now there's only two. The end of chain wideband input is unused.
It's probably an idea to look at doing a full sweep of the antenna system - and the combiners - one by one.

rich wood
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Location: Madison, WI
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Re: SWR/Return loss question???

Post by rich wood » Sun Apr 02, 2017 8:45 am

Howdy,
Changing the length of the cable to the combiner changes the phase angle to the reflectometer. The VSWR is still there, it just "magically" disappears. The readings you saw before the jumper change are a VSWR of 1.2:1. The Sira Dipole spec sheet shows the upper portion of the band for that antenna having a typical VSWR of that level. So, the VSWR is still present but, if the coverage is OK, stay with the setup for now.
A frequency specific antenna is usually a better choice.

waveguide
Posts: 26
Joined: Thu Feb 17, 2011 3:28 pm

Re: SWR/Return loss question???

Post by waveguide » Wed Apr 05, 2017 1:40 pm

Thanks everyone. That helps a lot.
That level of reflection doesnt concern me too much, but I'd like it to be 0!
The transmitter were using (Rhode and Schwarz) has a very low threshold for VSWR protection! It'll go into protect mode for the smallest reflection!

TPT
Posts: 649
Joined: Mon Dec 03, 2007 3:18 pm
Location: St. Marys, WV

Re: SWR/Return loss question???

Post by TPT » Wed Apr 05, 2017 2:07 pm

What happens if you terminate the unused port to the combiner?

waveguide
Posts: 26
Joined: Thu Feb 17, 2011 3:28 pm

Re: SWR/Return loss question???

Post by waveguide » Wed Apr 05, 2017 2:21 pm

No change with the unused port open - or terminated with a dummy load.
My understanding is its recommend to terminate it with a test load. Is this correct - and why?

Thanks

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