OMB GP-6

FM does it with frequency!
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kkiddkkidd
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OMB GP-6

Post by kkiddkkidd » Thu Nov 02, 2017 11:13 am

Hi all,

Does anybody have a manual for the above antenna? Specific tuning info? Any experience with it? Any advice other than cutting it up for scrap?

I contacted OMB and have been waiting on a return call from tech support for almost 2 weeks.

Any suggestions on sweeping the power divider with a PowerAim120? 6ea 50ohm loads on the output ports? I have never swept this type of power divider before... The PowerAim is a single port device.

I have a customer with one of these showing high VSWR. Not astronomically high but about 1.6:1 (if you can believe the old Collins TX metering). Which is too high for a new Nautel...

This site was built as an absolute low bid about 20yrs ago and has been horribly mis-maintained since. The previous owner operated it as a hobby and never spent a cent that wasn't absolutely necessary and sometimes didn't spend that. New owners are trying to make it look like a real radio station again but a new antenna is not in the budget right this second.

It has what appears to be a 1-5/8in input to 6-way 7/8in output power divider with 6ea 1/2in equal length jumpers to the bays. 2 or more of the jumpers have been replaced over the years (2 damaged jumpers are laying in the TX building) and the local tower guy thought that he had replaced the power divider at some point. No idea how carefully they cut the replacement jumpers.

We have little to no info or knowledge of the actual installation other than the local tower guy who is very non-technical.

Regards,
--
Kevin C. Kidd CSRE/AMD
WD4RAT
AM Ground Systems Company
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KK Broadcast Engineering
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Deep Thought
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Re: OMB GP-6

Post by Deep Thought » Thu Nov 02, 2017 11:50 am

Without knowing the power split between bays or the normal bay impedance this is going to be a blind walk down a tightrope. However, you should be able to get some kind of an idea of the health of the thing by measuring one bay at a time. It probably won't be 50 ohms because the other five aren't participating, but comparing it with the others might give you some insight as to whether one of them has wandered off.

Same with the power divider. Terminate all but one of the outputs with 50 ohms and connect an antenna bay to the sixth. Move around the outputs with the antenna and see if the input impedance changes when the antenna changes ports.

I suspect that the jumper replacement may have had more of an effect than anything.

Their US office is in a Miami industrial park out by the airport. If you haven't heard back in two weeks I'd try again along with sending an email.
Mark Mueller • Mueller Broadcast Design • La Grange, IL • http://www.muellerbroadcastdesign.com

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kkiddkkidd
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Re: OMB GP-6

Post by kkiddkkidd » Thu Nov 02, 2017 2:02 pm

I had wondered if each bay would individually measure 50 ohms as mounted or if there would be some amount of mutual Z interaction between them.

As far as I know, it is an even power split between the power divider outputs but that is purely an assumption.

I am also concerned about the jumper lengths. After seeing some of the other patching and BS that has been done around this property, I wouldn't be surprised if the length was whatever they had on hand. Coverage seems to be pretty good considering the TX is only running about 75% power and the VSWR is high. We are talking about a site where nothing was ever really repaired for 20 years and there are patches covering patches.

I have troubleshot, swept and tuned quite a number of rigid interbay antenna's but this is the first full power, cheap, flex interbay that I have troubleshot in 30+ years. I have installed a few low power xlators w/ power dividers and exactly one full power with the Shiveley VersaTune with flex interbays.

Any other info on this antenna is very welcome...

Regards,
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Kevin C. Kidd CSRE/AMD
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davek
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Re: OMB GP-6

Post by davek » Thu Nov 02, 2017 4:10 pm

I'd be suspicious of the replacement jumpers too. The damaged ones would give you a good idea what their length is supposed to be.

I'd also want to know how well the connectors were installed to the new jumpers. I'm not trying to pass comment on the ability of the previous tower workers, but I have come across my fair share of tower workers that aren't particularly well versed when it comes to proper installation of connectors, resulting in either poor return loss or ingress of water over time.

It will be interesting to know what you find. Great view from the tower, too!

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kkiddkkidd
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Re: OMB GP-6

Post by kkiddkkidd » Thu Nov 02, 2017 6:37 pm

Dave,

I agree and if you look at the connections in the previous image, there are a couple of the interbay feeders that are pretty rough looking. No idea if those are the original feeders, rebuilt feeders, randomly cut feeders or what.
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Kevin C. Kidd CSRE/AMD
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Re: OMB GP-6

Post by ChuckG » Fri Nov 03, 2017 2:52 am

Is that the ring stub antenna with the beercan attached to the rear of the bay with the RF connector? If so those are 50 ohm bays. I pulled one off of a translator with no innerbay anything at all, feedline connected directly to the single bay.

That far connector looks really ragged... i bet there's all sorts of headache hiding under all that tape
<><><><><><><><><>
Chuck Gennaro
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kkiddkkidd
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Re: OMB GP-6

Post by kkiddkkidd » Fri Nov 03, 2017 6:24 am

ChuckG wrote:
Fri Nov 03, 2017 2:52 am
Is that the ring stub antenna with the beercan attached to the rear of the bay with the RF connector? If so those are 50 ohm bays. I pulled one off of a translator with no innerbay anything at all, feedline connected directly to the single bay.

That far connector looks really ragged... i bet there's all sorts of headache hiding under all that tape
It is in fact a ring stub w/ PVC beer can covering the connector. I'm glad to hear that they should resemble 50ohms... There is no telling what kind of nightmares lurk in this system. Whoever installed the 1-5/8in feed line connector on the bottom screwed that up and has what appears to be ROLLS of black electrical tape and hose clamps wrapped around the connector in an attempt to stop a whistling leak. And failed... Of course reinstall kits for the 20 year old Andrew 87c connector are on backorder for weeks.

Does anybody have an extra reinstall kit for an Andrew 87c connnector that they would want to sell and ship today? I am going over for a couple of days next week to install a new VS2.5 for another associated station and finish rebuilding the tower light controller.

Thanks,
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Kevin C. Kidd CSRE/AMD
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Ray
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Re: OMB GP-6

Post by Ray » Fri Nov 03, 2017 8:35 am

Here are some random ideas. I know you have lots of experience with this type of problem.

It looks like the Heliax is bent very close to the connectors in a couple of cases. I've seen the shield fracture many times when this happens causing intermittent VSWR problems. I've had to travel half-way around the world more than once for this simple issue.

I'd probably try to measure the length of each phase cable in-place to check for gross differences. 1 degree phase delay is only about 1/4 inch in Heliax for the FM band, so the tolerance is not to bad. The best way to measure the phase length is a vector NWA. Since I don't own a portable version, I have a double balanced mixer, power divider, and RF cables in a kit. The mixer outputs a DC voltage proportional to the phase shift of the test cable. Very simple, but seems to work well. The kit also has some various cable lengths with electrical delay previously measured on the lab NWA for reference.

There was an good article in QEX magazine a couple of months ago about weatherizing RF cables. The author made about 10 LMR400 cable junctions and immersed them in water buckets for a month with various degrees of protection ranging from nothing to heat shrink (with glue). The heat shrink provided the best protection. Connector seal also worked well. Electrical tape was in the middle as I recall.

Ray

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kkiddkkidd
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Re: OMB GP-6

Post by kkiddkkidd » Fri Nov 03, 2017 12:54 pm

Ray wrote:
Fri Nov 03, 2017 8:35 am
Here are some random ideas. I know you have lots of experience with this type of problem.

It looks like the Heliax is bent very close to the connectors in a couple of cases. I've seen the shield fracture many times when this happens causing intermittent VSWR problems. I've had to travel half-way around the world more than once for this simple issue.

I'd probably try to measure the length of each phase cable in-place to check for gross differences. 1 degree phase delay is only about 1/4 inch in Heliax for the FM band, so the tolerance is not to bad. The best way to measure the phase length is a vector NWA. Since I don't own a portable version, I have a double balanced mixer, power divider, and RF cables in a kit. The mixer outputs a DC voltage proportional to the phase shift of the test cable. Very simple, but seems to work well. The kit also has some various cable lengths with electrical delay previously measured on the lab NWA for reference.

There was an good article in QEX magazine a couple of months ago about weatherizing RF cables. The author made about 10 LMR400 cable junctions and immersed them in water buckets for a month with various degrees of protection ranging from nothing to heat shrink (with glue). The heat shrink provided the best protection. Connector seal also worked well. Electrical tape was in the middle as I recall.

Ray
Thanks Ray. Is your portable kit something that you built?

Not only does a couple of the bay feeders have kinks but it appears that there is a small sideways kink in the 1-5/8in feed line right at the connector to the power divider. It looks like someone may have tried to align the feed line flange with the power divider by cranking down on the flange bolts AFTER the line and power divider was secured.

Of course, the tower boy didn't get any decent pictures of that...
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Kevin C. Kidd CSRE/AMD
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Dale H. Cook
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Re: OMB GP-6

Post by Dale H. Cook » Sat Nov 04, 2017 7:25 am

Ray wrote:
Fri Nov 03, 2017 8:35 am
... various degrees of protection ranging from nothing to heat shrink (with glue). The heat shrink provided the best protection.
That does not surprise me. For many years I was CE of a class C FM on Poor Mt. near Roanoke, VA. It is the highest mountain in the Roanoke area, at 3,928 feet, and catches all the bad weather (which is why the 200 foot broadcast towers there look like the top 200 feet of a 2000 foot tower). At that site I had two 950 MHz STL antennas, four RPU antennas, and a TSL antenna. I used adhesive heat shrink on all outdoor connections and never had a problem with water penetration. I don't know, however, how well it held up in the really long term as I replaced all of the RG-214 jumpers about every five years, so that was how long the heat shrink had to last.
Dale H. Cook, Contract Engineer, Roanoke/Lynchburg, VA
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Ray
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Re: OMB GP-6

Post by Ray » Tue Nov 07, 2017 4:38 pm

The phase kit is home-made. Several good articles about how to use a double balanced mixer with a 2-way power divider to provide a output voltage proportional to the relative phase between the two signals at the LO and RF ports of the mixer. I have several lengths of RF cable cut and calibrated on my lab NWA in the kit for phase references.

Here is one link, there are many others:

http://www.rfcafe.com/references/articl ... ectors.pdf

I was thinking about your problem, and I would be tempted to make some new Heliax jumpers up and do a wholesale replacement. If you have the Heliax stripping tool (used with a drill motor), you should be able to make four jumper cables in a couple of hours. The tool saves lots of time. By carefully cutting the cables to the same length, I can usually get the phase well within 1-2 degrees.

Ray

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kkiddkkidd
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Re: OMB GP-6

Post by kkiddkkidd » Wed Nov 08, 2017 8:28 am

Thanks Ray,

I want to verify that the feed line, power divider and each individual bay is OK before going to that extent but I think replacing all of the jumpers with known good, correct length jumpers will correct most of the remaining ills.

I have contacted OMB twice now and still have not received a call back from anyone that understood the situation (or English for that matter).

I am going to look into your balanced mixer rig.

Regards,
--
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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