Nicom 900 link - STL Antennas Can you Mix and Match

Gotta watch those Fresnel zones!
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abwdvm
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Nicom 900 link - STL Antennas Can you Mix and Match

Post by abwdvm » Fri Jul 27, 2018 8:26 am

What could it mean if the audio quality if better when the STL receiver freq is set .100 lower than the transmitter?
Better but not broadcast-able.
technician at an 1000w AM station in Texas

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abwdvm
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STL Antennas, can you mix and match?

Post by abwdvm » Fri Jul 27, 2018 8:30 am

We had a pair of OMB STL antennas. The tower climber said one of them was damaged slightly where the cable attaches, so he happened to have another antenna he said would work.

The link is not working, there is audio, but not broadcast-able. The audio improves when there is a .100 difference in the TX and RX freq.

I think we should go back to the old 'damaged' paired antenna.
technician at an 1000w AM station in Texas

grich
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Re: STL Antennas, can you mix and match?

Post by grich » Fri Jul 27, 2018 10:40 am

Antennas are antennas, to a point. If it's designed for the frequency, has the same gain as the original, and is installed so its polarity matches the other end, it should be good.

Is this the transmit or receive antenna?

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Re: Nicom 900 link

Post by grich » Fri Jul 27, 2018 10:50 am

Need more information...especially being aware of the other thread about antennas.

One thing I've ran into over the years with most of the transmitter shacks I work in not being air conditioned, is receiver drift if the shack temperature gets up around 100. I've had to add supplemental fans around the receivers to keep them happy (Marti STL10 stuff).

Has this path ever been good? If so, do you have a record of the received signal strength when it was good? What is the signal strength now?

Wouldn't hurt to put a frequency counter on the transmitter and make sure it's within tolerance.

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abwdvm
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Re: Nicom 900 link

Post by abwdvm » Fri Jul 27, 2018 10:55 am

A/c working fine.
This link was workable a few weeks ago, when RX moved to new location , and both RX an TX adjusted by tower crew, this problem began.
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Re: STL Antennas, can you mix and match?

Post by TPT » Fri Jul 27, 2018 11:06 am

All I see on the OMB site are Yagis for UHF --not US 950 MHZ band. Picture of the existing /replacement antenna?

You may have had enough change in the path to need a higher gain antenna system. Scala Pr-950 or Marti SC-48 would be recommend for medium lengths paths.

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Re: Nicom 900 link

Post by TPT » Fri Jul 27, 2018 11:08 am

Not enough antenna gain...the receiver afc can't really lock on, perhaps.

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Re: STL Antennas, can you mix and match?

Post by abwdvm » Fri Jul 27, 2018 11:23 am

omb.jpg
Only photo I have
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Re: STL Antennas, can you mix and match?

Post by grich » Fri Jul 27, 2018 2:16 pm

OMB RR antenna, 13dB gain, rated for the 950 band. Don't see yagis much in this band, the typical STL antenna around here is some sort of parabolic design.

The picture shows horizontal polarity...the other antenna should be mounted the same way, with the flat sides facing up and down.

So, the path has never worked from the new TX location? What is the path length, and do you have line-of-sight between antennas?

ncradioeng
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Re: Nicom 900 link

Post by ncradioeng » Fri Jul 27, 2018 2:18 pm

Did you do a path study to see if you have line of sight including Fresnel zone clearance to the new site?

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RGORJANCE
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Re: Nicom 900 link

Post by RGORJANCE » Fri Jul 27, 2018 2:38 pm

When they remounted the receive antenna, did they change the polarization from it's original position?

If they went opposite, that definitely could explain the signal change.

Fossil

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Re: Nicom 900 link

Post by ncradioeng » Fri Jul 27, 2018 3:04 pm

You can go to Google Earth, find the TX and RX points, use the ruler to draw a line between them and then look at the elevation profile viewer to see if you have an obstacle. This is at ground level so you have to factor in the antenna heights.

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Re: Nicom 900 link

Post by kkiddkkidd » Fri Jul 27, 2018 8:32 pm

abwdvm wrote:
Fri Jul 27, 2018 8:26 am
What could it mean if the audio quality if better when the STL receiver freq is set .100 lower than the transmitter?
Better but not broadcast-able.
I have seen more than one STL RX or TX drift off freq. As a matter of fact, I just found a Moseley 60x0 PLL TX that was operating on the exact correct freq but was programmed for the next step up.

If your RF level is bad, all bets are off.
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kkiddkkidd
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Re: STL Antennas, can you mix and match?

Post by kkiddkkidd » Fri Jul 27, 2018 8:36 pm

abwdvm wrote:
Fri Jul 27, 2018 8:30 am
We had a pair of OMB STL antennas. The tower climber said one of them was damaged slightly where the cable attaches, so he happened to have another antenna he said would work.

The link is not working, there is audio, but not broadcast-able. The audio improves when there is a .100 difference in the TX and RX freq.

I think we should go back to the old 'damaged' paired antenna.
Post the coordinates of the RX and TX so that someone can run a path study. PM if you wish.

Or use UBNT's path study site

https://link.ubnt.com/

to run your own...
--
Kevin C. Kidd CSRE/AMD
WD4RAT
AM Ground Systems Company
http://www.amgroundsystems.com
KK Broadcast Engineering
http://www.kkbc.com

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Re: Nicom 900 link

Post by DaveSt » Sat Jul 28, 2018 12:34 am

The performance of a receiver can be different with a very weak signal strength. So appearing to work better with a 0.1MHz offset may not be an indicator of any problem. If you are old enough, recall the days of analogue radios in cars. You often had to tweak the tuning as you moved to areas with weak signals to get more usable reception. For this case, you need to get a good enough receive signal. That means identifying the problems in the current setup, whether they be antenna or path related. None of this is hard to do. Even Google Earth as stated earlier can help.

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