ERI COG-1084 question

FM does it with frequency!
Post Reply
User avatar
NECRAT
Site Admin
Posts: 3069
Joined: Sat Nov 17, 2007 9:13 pm
Location: Taunton, MA
Contact:

ERI COG-1084 question

Post by NECRAT » Sun Nov 18, 2018 12:42 pm

So here in Bahstahn (Boston), we have a 3 station master FM antenna around 1140 ft up a tower. The antenna has two "bays" and is an omni ERI Cog-Wheel. (1084CP-2)

All three FMs on it are full Bs. All are around 9kW give or take 200 watts. Two of the frequencies, 94.5 and 98.5 both radiate very nice clean signals. All the way to my house, 30 miles from the tower, I have no issues with multi-path, get a solid HD lock, etc.
The third, however, 103.3, isn't the same story. It suffers bad multi-path in places the other two don't, and I can't get an HD lock on it 15 miles away.
I have confirmed with the engineer at 103.3 that they are at full power and on the master antenna.

So my question is, as an omni directional antenna, shouldn't the radiation pattern match for all three stations? Or do broadband FM panel antennas tend to have a little frequency specific directionality? (I suppose it could be something else but I haven't heard any other radio signals).

Now side notes. The antenna was designed for more signals, going as high as 106.7, and is built as an aux for those as well. The antenna has also had a lot of issues in the past, including jumpers burning up and the power divider failing as well. Also 103.3 is a "new" add to the antenna, moving to it around 2009, where the other FMs have been on it much longer, so the combiner was added on to for 103.3. 103.3 used to be on a side mount Jampro 4 bay antenna.
http://www.necrat.us

"Arguing with an engineer is like mud wrestling with a pig. After a couple of hours, you realize the pig likes it"

TPT
Posts: 820
Joined: Mon Dec 03, 2007 3:18 pm
Location: St. Marys, WV

Re: ERI COG-1084 question

Post by TPT » Sun Nov 18, 2018 2:54 pm

Brand of transmitters in use for the three stations?

Even at a relatively modest ERP (3200 watts) I noticed a dramatic decrease in multipath when I finally retired an old Haris 2.5H in favor of a solid state transmitter.

DaveSt
Posts: 23
Joined: Mon Jul 23, 2018 2:21 pm
Location: UK

Re: ERI COG-1084 question

Post by DaveSt » Mon Nov 19, 2018 1:25 am

There are lots of things that can cause problems like this. Answering your question though, panel antenna systems will have a small frequency dependent variation, but it should not be significant. Unless that is you have an intended null in the pattern to protect another station. Variations in that could be very noticeable depending on the frequency. ERI should have radiation patterns at different frequencies or at least be able to give some guidance.

It is worth a quick check to see that there is no co-channel interference or if there is another strong station on 92.3 to 92.9MHz (10.7 relationship). After that, you will need to get the test equipment out. Particularly a spectrum analyser. Hopefully there is a directional coupler on the combiner output so you can compare levels of the stations there. Also have a look at the modulation with the spectrum analyser so you can see the analogue and HD modulation are correct. It is also worth checking that there is no significant or varying reflected power on the transmitter.

Good luck with this, finding these problems is often not easy.

sallen
Posts: 90
Joined: Sat May 09, 2015 8:39 am

Re: ERI COG-1084 question

Post by sallen » Mon Nov 19, 2018 9:44 am

I think that is the same style of antenna that just burned up in a spectacular morning antenna fire here in Seattle. This was a 32 element ERI antenna. You can see some pictures at the local SBE website, sbe16.com.

Antenna is a total loss. Stations affected are broadcasting from Aux sites. One of the Cog elements fingers fell off the antenna and hit the ground. Most of the fiberglass radomes burned and dripped hot goo onto the transmitter building, what a mess. Lucky your not putting 150KW into your master antenna when the power divider and jumpers burned up.

User avatar
NECRAT
Site Admin
Posts: 3069
Joined: Sat Nov 17, 2007 9:13 pm
Location: Taunton, MA
Contact:

Re: ERI COG-1084 question

Post by NECRAT » Mon Nov 19, 2018 2:07 pm

DaveSt wrote:
Mon Nov 19, 2018 1:25 am
There are lots of things that can cause problems like this. Answering your question though, panel antenna systems will have a small frequency dependent variation, but it should not be significant. Unless that is you have an intended null in the pattern to protect another station. Variations in that could be very noticeable depending on the frequency. ERI should have radiation patterns at different frequencies or at least be able to give some guidance.

It is worth a quick check to see that there is no co-channel interference or if there is another strong station on 92.3 to 92.9MHz (10.7 relationship). After that, you will need to get the test equipment out. Particularly a spectrum analyser. Hopefully there is a directional coupler on the combiner output so you can compare levels of the stations there. Also have a look at the modulation with the spectrum analyser so you can see the analogue and HD modulation are correct. It is also worth checking that there is no significant or varying reflected power on the transmitter.

Good luck with this, finding these problems is often not easy.
For clarification, this isn't my station or antenna. Mine is much much higher in frequency (and about to go even higher, thanks Repack!)
http://www.necrat.us

"Arguing with an engineer is like mud wrestling with a pig. After a couple of hours, you realize the pig likes it"

User avatar
NECRAT
Site Admin
Posts: 3069
Joined: Sat Nov 17, 2007 9:13 pm
Location: Taunton, MA
Contact:

Re: ERI COG-1084 question

Post by NECRAT » Mon Nov 19, 2018 2:07 pm

sallen wrote:
Mon Nov 19, 2018 9:44 am
I think that is the same style of antenna that just burned up in a spectacular morning antenna fire here in Seattle. This was a 32 element ERI antenna. You can see some pictures at the local SBE website, sbe16.com.

Antenna is a total loss. Stations affected are broadcasting from Aux sites. One of the Cog elements fingers fell off the antenna and hit the ground. Most of the fiberglass radomes burned and dripped hot goo onto the transmitter building, what a mess. Lucky your not putting 150KW into your master antenna when the power divider and jumpers burned up.
You mean this one?
https://www.broadcastengineering.info/v ... 37&p=43029
http://www.necrat.us

"Arguing with an engineer is like mud wrestling with a pig. After a couple of hours, you realize the pig likes it"

Kelly
Posts: 509
Joined: Thu Aug 21, 2008 5:32 pm
Location: Washington D.C. Area

Re: ERI COG-1084 question

Post by Kelly » Sun Nov 25, 2018 7:33 pm

TPT wrote:
Sun Nov 18, 2018 2:54 pm
Brand of transmitters in use for the three stations?

Even at a relatively modest ERP (3200 watts) I noticed a dramatic decrease in multipath when I finally retired an old Haris 2.5H in favor of a solid state transmitter.
Chances are what you noticed by going to a solid state TX is a reduction in amplitude phase distortion or probably AM noise, not multipath.
Skype:kellyalford Twitter: @KellyAlford

Ray
Posts: 61
Joined: Sat Mar 22, 2014 10:04 am

Re: ERI COG-1084 question

Post by Ray » Mon Nov 26, 2018 7:22 am

I've designed many similar high power UHF/VHF antennas, just not for FM broadcast. Over a design bandwidth, there is a spacing compromise between the dipole-reflector and also the overall size of the entire array. Often the panel has higher directivity at the higher frequencies resulting in less uniform pattern coverage for the array. A good MOM program will model the performance without much effort. I'm sure ERI has done an analysis already.
Ray

TPT
Posts: 820
Joined: Mon Dec 03, 2007 3:18 pm
Location: St. Marys, WV

Re: ERI COG-1084 question

Post by TPT » Mon Nov 26, 2018 9:01 am

Well familiar with the areas of multipath in our area--narrow river valley at 600 ft. AMSL surrounded by hills hitting 1200' amsl. Replacing the old transmitter with a solid state transmitter reduced those effects. Now--incidental AM very likely a factor, causing multi-path to become much more apparent, especially in a stereo signal.

Post Reply