Distance between transmitter and tower

[jorgeSWL]

I would like to know if it is a high importance what the distance is between the transmitter and tower. We have a Nautel 10kw AM transmitter and a Tunipole system installed in our tower. We have three wires going to the middle of our 180ft tower. We broadcast on 1380 in Honduras, C. A. What should the distance be between the transmitter and the tower? Thanks a lot!

[KA0HCP]

Hello Jorge, Welcome!

I will let some of the experience broadcasters answer your question. 

Here is a link to the antenna for those interested:
http://www.lbagroup.com/products/tunipole-and-combipole-antenna-systems

[flintstone mop]

I would guess as far as you can afford feed line. Too close and 10kw of RF might cause RFI to the control circuits or audio processing equipment that would be feeding the transmitter. Audio processing equipment is usually the rack right next to the transmitter.
Fred

[jorgeSWL]

One detail I omited is that we are only licensed for 3kw. However, we are presently using only 1.5, for it seems like something gets damaged every time we want to go up to 3kw.

Our transmitter was originally installed at about 150ft from the antenna. A technician came and said it was located too far away. He said we had to build something at about 50ft from the tower, so we did it. But for some reason we are not able to go up to 3kw again. Everytime we do, some elements get damaged. Is the transmitter too close?? Would we need to move it again?

[flintstone mop]

You will need to explain what is getting damaged when you try to go to higher power.
Seems like an "engineer" that doesn't know very much.
Your first location 150 feet away may have really needed to be 250 feet distance.
When you start running very high power like that the match between the antenna and the transmitter needs to be very low. Almost 1:1. Not for the losses in the transmission line, but high SWR at high power (more than 1.5KW) can cause serious heating in the P.A. tank circuits and any tuner circuits to match the transmitter to the antenna system.
Who installed the antenna system?
Do you have some way to test the antenna system that it is resonant for your operating frequency?
You would need some broadcast test equipment to check out the antenna. Unfortunately the familiar Ham gear MFJ 259 will not go that low.
It would be a shame to see a nice transmitter damaged if the antenna system is not resonant for 1380. Are there ground radials installed around the base of the antenna?
Fred

[WQ9E]

Do you have some way to test the antenna system that it is resonant for your operating frequency?
You would need some broadcast test equipment to check out the antenna. Unfortunately the familiar Ham gear MFJ 259 will not go that low.
It would be a shame to see a nice transmitter damaged if the antenna system is not resonant for 1380. Are there ground radials installed around the base of the antenna?
Fred

Fred,  An antenna noise bridge like the MFJ-202B would be an inexpensive way to check antenna match at that frequency.  http://www.mfjenterprises.com/Product.php?productid=MFJ-202B


I agree with the other posters that moving the transmitter/studio closer to the tower is not a good idea.

[W3GMS]

One detail I omited is that we are only licensed for 3kw. However, we are presently using only 1.5, for it seems like something gets damaged every time we want to go up to 3kw.

Our transmitter was originally installed at about 150ft from the antenna. A technician came and said it was located too far away. He said we had to build something at about 50ft from the tower, so we did it. But for some reason we are not able to go up to 3kw again. Everytime we do, some elements get damaged. Is the transmitter too close?? Would we need to move it again?

Hi Jorge,

You really need to find another engineer.  This is not a complex subject and the fact that you can't run your licensed TPO needs to be addressed.  Seems like your tuning unit at the base of your stick may need some work.  With the proper instruments its not difficult to find where the problem is.  Check with other stations in your area and maybe you can borrow their engineer / contractor.

150' is not an unusual amount of distance for a transmitter to be from the antenna.  The more loss in your transmission line means the transmitter has to deliver more power to make up for the loss.  Your CP should list the amount of antenna current at the base of your antenna.  The less loss in your transmission line simply means your transmitter does not need to run as much power to get the rated antenna current out at the antenna.   

Joe, W3GMS

[DMOD]

Hi jorge,

I would recommend you register over at:

http://www.broadcastengineering.info/index.php

because it is here many BC engineers tackle these kinds of problems every day.

But you will need to provide more information.

FWIW, I have had many 10kW transmitters within 20 feet of a tower and never had any problems with a properly built transmitter building and properly designed ATU.

Phil - AC0OB

[jorgeSWL]

Damaged parts: Modulator Driver, RF Driver, Power amplifiers, and power supplies.

The reason the technician gave for moving it closer was that we were losing too much because of the big length of the line going to the antenna and the transmitter was overworked because it had to put out much power to get it to the antenna.

The same technician decided how to install the unipole system, and had a tower person do it.

So everybody agrees that our AM transmitter is too close to the antenna?

[jorgeSWL]

Thanks, DMOD!

[jorgeSWL]

Thanks, Fred, Joe, and everybody with difficult names!

[steve_qix]

There is no good reason I can think of to move anything.

I've been chief engineer of a lot of AM broadcast stations.  One of them had the antenna tower 10 feet from the transmitter and at least one of them had at least one of the towers HUNDREDS of feet away.  At others, the tower was a couple of hundred feet away.  It varied.

At these frequencies, good feedline will exhibit very low loss.  Heliax was used at most installations I was involved with.

Have you actually measured the power loss in the feedline?  It shouldn't be much at all if all is working correctly.  I don't know what kind of cable you're running, but with something like LMR600, 150 feet of it at these frequencies is going to show a very low loss - maybe .5 dB - probably less.  You might be using even better cable than that.

[DMOD]

Damaged parts: Modulator Driver, RF Driver, Power amplifiers, and power supplies.

The reason the technician gave for moving it closer was that we were losing too much because of the big length of the line going to the antenna and the transmitter was overworked because it had to put out much power to get it to the antenna.

The same technician decided how to install the unipole system, and had a tower person do it.

So everybody agrees that our AM transmitter is too close to the antenna?

The distance shouldn't matter with a properly sized transmission line.

Nautels are usually hardy units and you have to severely abuse them to get them to fail.

I think you need a BC engineer.

Phil - AC0OB

[KA2DZT]

With the amount of different areas of the xmtr system being damaged, I would take of close look at the grounding system (if there is one) being used at the xmtr site.  Especially, with respect to whatever grounding is at the tower.

Just my 3 cents.

Fred

[jorgeSWL]

There is grounding for both the Antenna and the Transmitter... also the tuning unit has it's own grounding system....

[M1ECY]

From your description, it sounds like the three "ground" systems may not be interconnected,

Over this side of the pond it is common practice to tie all of the grounded systems to the station ground mat, this is different to the power company ground connection.

As others have said, you need to consult a professional BC engineer for the correct information.

As for ground systems, a short description.

I used the term ground mat - this is a large network of conductive materials buried below the ground, and will have several large copper tape type connection points, where principal RF type structures are "bonded" (connected) to, this may also form part of the radial network, as referred to in other posts. It is certainly usual to connect the antenna tuner ground point to this, and if the tower itself is not the radiating structure, then the metalwork of the tower is usually also bonded to this, as part of the lightning conductor system.

If you are using unbalanced feeders (Coaxial cables) to feed the Antenna tuner, then you will also be connecting the ground point of the transmitter to this same ground system, using the outer braid of the cable. if this is the only way that the transmitter is seeing the ground mat, you may find that the length of feeder is actually trying to radiate - this is a bad thing.

The power company ground is a different thing, and is there to provide protection in the event of an electrical fault - our system in the UK is probably different to yours, but over here the neutral connection is bonded to this point at the local substation (I guess power pole out your way), assuming that your power network has this safety feature built in, there will be no grounding at RF of your transmitter from this ground connection - this will then allow the transmitter to send any stray RF the wrong way, and back towards the power company!

Given the amount of damage you appear to be seeing when you wind the wick up, I would say that there are some very fundamental problems with the system as installed, some of which might be quite dangerous.

I would think the length of feeder used is of little importance if you have a remote tuning network, so long as the antenna system provides a good match to the transmitter, the feeder could be feet long, as opposed to hundereds of feet - I can understand wanting to have the audio chain well seperated from the strong RF field, to minimise interaction problems.

Some proper idea regarding the installation - pictures and drawings would allow a better understanding of your situation, however, getting someone to site that really knows and understands broadcasting requirements would be the best bet.

Cheers
Sean

[jorgeSWL]

We are using a Heliax between the transmitter and the antenna. We believe the three grounds are interconnected. We also have the net of copper wires underground... radials? Do you mean all grounds should be connected to the radials? Not sure about this, but we will check.

[M1ECY]

Ok, so now it is easier to visualise the system.

Heliax, even the small stuff will have next to no loss at the frequency you are running, so even a thousand yards or so wouldnt make much difference to you.

Your antenna and radial system will need to be checked out properly, radials can be tied into the ground mat, but they may be tuned radials, to act as part of the matching of your antenna system, if this is the case, then they will need to be separated.

Would be very interesting to know how the match to your antenna system really is, I would think that it is a mile out.

Cheers
Sean

[W8IXY]

FWIW:   I was twice the CE of a 50kw station, non-directional 24 hours.  The half wave tower was less than 10 feet from the transmitter building.  There was no transmission line.  The tower was fed at the base by a copper pipe from the bowl insulator on the transmitter building, with a one turn "lightning loop", fastened directly to the base of the tower.

Proper installation of the transmitter, antenna system and ancillary equipment is a must to keep RF where it belongs - into the tower and not into audio wiring or power mains.

73
Ted W8IXY

[jorgeSWL]

So much appreciate this advise... So probably we don't have to move the transmitter back after all! Just hope to get an honest technician that will do the right work for the right price. Thanks to all for your input!

[KA2DZT]

There is grounding for both the Antenna and the Transmitter... also the tuning unit has it's own grounding system....

This may be the problem, too many separate ground systems.  All the grounds should be tied together with heavy copper wire and all the grounds should be bounded to the building AC service panel main ground.

Maybe some of this already exist at the site.

Fred

[flintstone mop]

Jorge. I hope you find your problem. Please reply with results.
The responses were interesting for the distance of transmitter and tower. I think signing into another broadcast forum would be the same responses you got here. A lot of Ham ops are in broadcasting, either in production or the engineering side. Some maintain high power low power radio stations AM-FM and TV.

One reply was for an antenna bridge from MFJ...go back to that reply and ask your boss to buy that unit. You must find out if that antenna system will accept power at 1380. There is no such thing as install an antenna system and the engineer leaves the site. There must be an ATU at the base of the tower.
Grounding was another interesting reply. High power RF needs to see good bonding of everything involved in the transmitter shack and antenna system. Hopefully the damages done to the Nautel were not extensive and expensive to repair.
 
Short story: During my smokey dayz at a college radio station there was a female FCC licensed "engineer" who was managing the install of a new 20kw FM transmitter. A new FM outlet for Pacifica Radio network. One of those Voice of the People stations in the late 1970's.
She made a miscalculation in the electrical connections for 3 phase power. When the trasnmitter was energized..... kaaaplow!!!!!!
Power supply wiped out completely and extensive damage to the power company feed.
She was quickly retired.....I was kinda in that hippy culture during those times, but this girl definitely was out of her league.
Fred..........keep us posted please.

[jorgeSWL]

Hello to all and thanks to all who have been helping us with your advise.

Just an update. We have visited some AM stations around our frequency and about same power and more, and they have their transmitters close to their antennae, too. We are trying to find out about the engineers working for them and trying to get one of them to come and help us.

We'll keep you posted. Thanks so much!

[jorgeSWL]

Here is a link to an album I hope you are able to see. It contains pictures of our transmitter, tower and tuning box, Please, let me know if you were able to see them. Thanks to all!

https://www.facebook.com/teresa.jimenezdemunoz/media_set?set=a.10202445620117886.1073741828.1115560458&type=3

[flintstone mop]

ya won't see much unless you have a face book account.
What I see is that the antenna/tuner needs a lot of work and clean up. Your engineer guy did that?
I hope you are prepared to get a real engineer there and reorganize the antenna and tuner.
Fred