950 MHz STL Dish on AM Tower

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Ryan Williams
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950 MHz STL Dish on AM Tower

Post by Ryan Williams » Wed Dec 02, 2015 4:57 pm

The wooden pole that has held our STL dish for more than 3 decades came crashing down.

I've looked into replacing the wooden pole, but with freight, it is very very expensive.

What do I need to know about mounting it on one of the hot AM towers? Is that more trouble than it is worth?

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Chris Arnesen
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Re: 950 MHz STL Dish on AM Tower

Post by Chris Arnesen » Wed Dec 02, 2015 5:01 pm

Sincerely,
Chris Arnesen, CBNT, CRO

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Shane
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Re: 950 MHz STL Dish on AM Tower

Post by Shane » Wed Dec 02, 2015 6:03 pm

The isocoupler may change the base characteristics of the tower. I would call that a major project but others may not. Certainly been done before.

Check with your local power utility. Not only will they likely have some poles but you might be able to have them install it for you.

How tall was the pole? You can get 40 feet with a couple of 2X4s. Select a couple 22 feet long with very few knots and have the yard rip each into 2 2X2s. You can find the rest of the instructions in an old ARRL handbook but the gist of it is to bolt three of the 2X2s together overlapping about 2' (thus resulting in a height of 40'), then spread out the bottom 2 and use cut pieces of the 4th 2X2 as spreaders and lag bolt on the spreaders.

Tie ropes to the top and tie them off as guys perpendicular to the spread after raising it.

You will need to mount your STL antenna on it before raising it. I'm hoping it's something small and not an 8 foot parabolic dish!

This would get you by until a pole can be delivered.

Personal experience: Resist the temptation to skip a step and just use 4 2X4s! It'll be too heavy to support itself and possibly kill or injure someone. I have made one out of 2X2s with no deleterious effects.
Mike Shane, CBRE
---Omaha---

sallen
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Re: 950 MHz STL Dish on AM Tower

Post by sallen » Wed Dec 02, 2015 6:28 pm

If it's a directional AM or even non directional day and directional night you will have to do at the minimum a partial proof and of course the isocoupler cost. If this was just a simple one tower non directional day/night then all you need is the isocoupler and very little paper work. If you use a consultant them most likely your station is directional and they would know what paper work and measurements need to be made to make sure you have not disrupted your pattern so it will pass through the FCC.

The part that drives up the cost for putting it on a directional array is the consultant preparing the paper work and labor hours doing the proof. That's on top of the cost of the isocoupler, transmission line and labor to install on the tower. If your an owner operator you might be able to do the actual labor for the proof yourself and have the consultant look it over and submit it.

TPT
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Re: 950 MHz STL Dish on AM Tower

Post by TPT » Wed Dec 02, 2015 6:47 pm

Directionals will add complexity, but assuming a single tower AM, a short triangular tower for the STL is the best long term solution.

I wouldn't use wood unless you are out in the desert. You can get 10 foot Rohn 25G tower sections for around $120 each. Dig a hole down about 2~3 feet (depends on your dirt--we often run into rock more than 2 feet deep around here). Put the first section in the hole and mix up a batch of Quickrete. Level the tower section & orient a flat side in the direction your STL antenna will aim. Pour in the concrete. Brace the section if necessary so it stays level as the concrete sets. Wait a day or so then stack two or three sections on top of that first section. That should get you a tower as high as your wooden pole. Generally Rohn 25 should be stable under 50 feet without the need for guys, as long as you have enough concrete around the bottom section. (You could also buy a mounting plate for around 200 dollars--you install mounting bolts in your concrete base to hold the plate in place). Look at the DXengineering.com site.

Next step: buy some copper strap from Georgia Copper; then some #10 copper wire, ground clamps and ground rods from your local electric supply house. Drive the ground rods around the base of this tower, bury some strap around the base & tie the strap and #10 to the ground rods and to the legs of the tower. If you use a base plate you can run strap under the plate before you stack tower. You can use roofing tar to paint the visible copper & discourage copper thieves. While your new tower is relatively short at AM frequencies, it will still suck up a fair amount of RF, so the grounding is to drain off that energy & prevent this short tower from re-radiating the AM signal. Mount your STL antenna and coaxial cable & you are back in business.

This may cost a bit more than a new wooden pole--certainly more than the 2X4's--but the tower won't get blown over in a storm or rot out like the wooden poles. It will certainly be cheaper than the cost of a 950 isocoupler, nor will you need to remeasure the base impedance of the tower ( which would also be expensive if you need to hire someone to do it).

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kkiddkkidd
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Re: 950 MHz STL Dish on AM Tower

Post by kkiddkkidd » Wed Dec 02, 2015 6:54 pm

Ryan Williams wrote:The wooden pole that has held our STL dish for more than 3 decades came crashing down.

I've looked into replacing the wooden pole, but with freight, it is very very expensive.

What do I need to know about mounting it on one of the hot AM towers? Is that more trouble than it is worth?
Ryan,

I notice that you said "one" of the hot AM towers which suggests that this is a DA. You will almost certainly be better off replacing the wooden pole than trying to use an iso-coupler or iso-coil to isolate the STL line. Adding anything across the base insulator or a DA will not only affect (possibly dramatically) the array tuning. Once you get it retuned THEN you have to do a partial proof and file the pertinent paperwork.

Figure $1000-1500 for an isocoupler or isocoil PLUS array retuning PLUS partial proof PLUS filing fees.

Check with your local power utility for pole costs. Not sure where you are located but around here, the power system has provided poles at what I thought was very reasonable costs ($500ish for a 50fter installed). Also, I bought 6x6in poles 30ft long last year for about $250ea if that would be tall enough.

What is your location and path length? If you had suffucent signal strength prior to the collapse, you might be get by with a shorter pole.

Good luck,
--
Kevin C. Kidd CSRE/AMD
WD4RAT
AM Ground Systems Company
http://www.amgroundsystems.com
KK Broadcast Engineering
http://www.kkbc.com

Ryan Williams
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Re: 950 MHz STL Dish on AM Tower

Post by Ryan Williams » Thu Dec 03, 2015 10:54 am

I'm very appreciative of all the ideas and leads. Still waiting to hear back on another quote for a wood pole replacement.

I thought about TPT's idea of getting a Rohn tower bracketed to the transmitter building. But is putting up a 50 ft metal structure going to be a problem for the tuning? I figured that is why wood was originally used.

Kevin, I'm in Texas and the path is a little over 11 miles long and we haven't ever had any problem with the signal, so it is possible the overall height could be reduced.

This collapse is at our night site (we have two separate AM locations) and I have an FM translator that covers most of the populated areas at night. That's why I am not quite in panic mode, but I do want to get AM nighttime service restored as quickly as possible.

awsherrill
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Re: 950 MHz STL Dish on AM Tower

Post by awsherrill » Thu Dec 03, 2015 1:04 pm

If your translator can be received reasonably well at your AM nighttime site, you could use an FM tuner as a temporary "STL". That would get your night AM signal back on while you get everything else fixed.

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davedybas
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Re: 950 MHz STL Dish on AM Tower

Post by davedybas » Thu Dec 03, 2015 4:40 pm

I notice that you said "one" of the hot AM towers which suggests that this is a DA. You will almost certainly be better off replacing the wooden pole than trying to use an iso-coupler or iso-coil to isolate the STL line. Adding anything across the base insulator or a DA will not only affect (possibly dramatically) the array tuning. Once you get it retuned THEN you have to do a partial proof and file the pertinent paperwork.
Putting an Isocoupler on a DA tower is not any big deal...if it's done correctly. There are quite a few AM stations operating as DA's with STL dishes, Cell Equipment and FM Arrays on them. I've had the opportunity to visit them and there have been no problems when the STL Feedlines are bonded to the tower and the Iso-Couplers are grounded on the transmitter side...The change in tower impedance has always been small.

But putting in a Wooden pole could be Cheaper !

ChuckG
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Re: 950 MHz STL Dish on AM Tower

Post by ChuckG » Fri Dec 04, 2015 12:03 am

The only problem I've run into using wooden poles is getting up there later on if repairs are needed. Bucket truck time. Other than that, the local power company sold one out of stock, dug the hole and planted it for me. $750. YMMV
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W2XJ
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Re: 950 MHz STL Dish on AM Tower

Post by W2XJ » Fri Aug 26, 2016 3:59 am

Thats what spurs are for or climbing rungs.

rich wood
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Re: 950 MHz STL Dish on AM Tower

Post by rich wood » Fri Aug 26, 2016 10:05 am

Howdy,
First question: Did the pattern change when the STL pole came down? Yes the pole is "transparent" but was the feed line? Even properly grounded the feed line would want to become an AM radiator.
Second, putting the dish on hot tower is a multi point process. The feed line would need to be bonded at the antenna, the tower base and every 1/16th of a wave length for the operating frequency. Feed lines on hot towers used to be isolated with a 1/4 wave short or bazooka feed.Unfortunately the stand off insulators for this method have stop being produced. The isocoupler is the next best choice. Getting the feed line from the tower to the building will require some digging and hopefully you will avoid digging up the existing feed lines/control lines etc. Damage to the ground radials is easily fixed with a torch and some 15% silver solder rods.
Be sure to do before and after partial proof field readings and file a change with the FCC.
But, before starting any of this, is the array in tolerance and stable? If not fix the issues first before adding or changing anything.

W2XJ
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Re: 950 MHz STL Dish on AM Tower

Post by W2XJ » Fri Aug 26, 2016 3:31 pm

Rich

You're over thinking this. A wooden pole might be 40 feet. I consider anything under 45 degrees insignificant. Practice from building numerous large high power critical arrays supports this. A 40 foot pole is 45 deg around 3 MHz. Taller structured can easily be detuned.

awsherrill
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Re: 950 MHz STL Dish on AM Tower

Post by awsherrill » Fri Aug 26, 2016 3:38 pm

This all happened over eight months ago according to the date stamps. Wonder if the OP has resolved the problem by now.

ChuckG
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Re: 950 MHz STL Dish on AM Tower

Post by ChuckG » Fri Aug 26, 2016 3:40 pm

W2XJ wrote:Thats what spurs are for or climbing rungs.
I maintain that is still what bucket trucks are for. :lol: I'd get halfway up and my back would lock up. The paramedics would then bring a bucket truck anyway.
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