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WTB: 10kW FM TX & Antenna Array = 50 kW ERP
- Thread starter TomZ
- Start date
I say we need more info ;D (Type and length of cable, arrayspecs...)
A lot more info is needed. ERP is the product of TPO (-) feedline losses (x) Antenna gain. We need to know what type of feedline is used and how long it is. Is the licensed ERP really 50kW? Is the antenna non-directional? Are there any other losses in the system such as surge arresters or xfer switches or cavities? If it's a typical installation, with circular or H + V polarization, with a 10kW transmitter achieving 50kW, you'll need at least 10 bays, preferrably 12. How does the licensee feel about a bigger electric bill using more power and fewer bays? How's the coverage been with what you were using?
Also, you haven't mentioned what happened to the system. A big antenna array is fairly expensive to replace. The licensee may want to consider repair vs replacement unless there is a known defect in the antenna system. In my experience, improper assembly results in a lot more failures than does poor design. Often, an improperly assembled antenna will work for a while, then start to have problems, eventually failing completely. A nearby class A had this very thing happen over the summer. They'd been on the air for about 5 years when the antenna suddenly showed high VSWR while simultaneously losing pressure. Lightning was suspected, but an inspection found that a fitting had been installed backwards during installation and had finally cracked, letting water into the antenna, resulting in a destructive arc. Instead of buying a new antenna, the old one was brought down and rebuilt for about $1,000 vs $4,000 for a new one. The manufacturer also updated the design to the current version with some improved fittings that cannot be installed backwards. Another common failure mechanism is when antennas are assembled on the ground, then hoisted up intact instead of assembling them in place. A tower crew in Texas was killed about 30 years ago hoisting up a fully assembled 12 bay antenna when the hoist failed, dropping the antenna on a guy wire and collapsing the tower. That should never have been tried but it's a common practice, despite the dangers to the men and the antennas.
Most antennas can be repaired for a fraction of the cost of a new one. It's usually also possible to change the number of bays, if desired. We once converted a 10 bay ERI into a 6 bay and used one of the spare bays as an emergency antenna. We also rebuilt a 10 bay ERI that had burned up badly after being shot with a 30 cal. jacketed slug. Methyl ethyl ketone (MEK) is your friend on such jobs.
If the antenna was damaged and arcing occurred, check the coax carefully for signs of soot because soot from these arcs is highly conductive and is hydroscopic as well. If soot is present, the feedline may need to be replaced, at least partly, unless it is rigid line that can be taken apart and cleaned.
Most antennas can be repaired for a fraction of the cost of a new one. It's usually also possible to change the number of bays, if desired. We once converted a 10 bay ERI into a 6 bay and used one of the spare bays as an emergency antenna. We also rebuilt a 10 bay ERI that had burned up badly after being shot with a 30 cal. jacketed slug. Methyl ethyl ketone (MEK) is your friend on such jobs.
If the antenna was damaged and arcing occurred, check the coax carefully for signs of soot because soot from these arcs is highly conductive and is hydroscopic as well. If soot is present, the feedline may need to be replaced, at least partly, unless it is rigid line that can be taken apart and cleaned.
I love these posts... it's the same as saying "I need a muffler for my car, what do you recommend?"
Of course, we don't know the: Year, make, model, engine etc.
We are happy to help on the boards here, but please be specific with questions like this. In your case Tom, we need the size of the line. Is it air or foam (likely air at your TPO)? How long a run from transmitter to antenna? How many bays are in the current system? Are they full wave or half wave spaced? C-Pol (you never know, could be a noncomm with a channel 6 problem)?
In short, "us engineers" are happy to help, but help us help you with details of situations like this!
Of course, we don't know the: Year, make, model, engine etc.
We are happy to help on the boards here, but please be specific with questions like this. In your case Tom, we need the size of the line. Is it air or foam (likely air at your TPO)? How long a run from transmitter to antenna? How many bays are in the current system? Are they full wave or half wave spaced? C-Pol (you never know, could be a noncomm with a channel 6 problem)?
In short, "us engineers" are happy to help, but help us help you with details of situations like this!
Thanks everyone for your interest and I agree (lots of miles driven yesterday...)
The station was (mostly) Armstrong engineering and the antennae were damaged by ice falls and they have been removed from the tower. Additionally, the top 100 feet of (nitrogen filled) coax has been replaced.
From the FCC:
It's been on the air for awhile.
I'll be on the road most of today; but I will appreciate every response.
The station was (mostly) Armstrong engineering and the antennae were damaged by ice falls and they have been removed from the tower. Additionally, the top 100 feet of (nitrogen filled) coax has been replaced.
From the FCC:
It's been on the air for awhile.
I'll be on the road most of today; but I will appreciate every response.
We're really trying to help here. I asked you some very specific questions that pertain to information I need to be able to answer your question.
The antenna is Armstrong. That means nothing. How many bays? Input flange size? How long is the transmission line and what is the diameter of it?
If you can answer the questions in my previous post, I can help (as can others here too). But without that, it's as specific as "do you have tires for my car?" and leaving it at that.
The antenna is Armstrong. That means nothing. How many bays? Input flange size? How long is the transmission line and what is the diameter of it?
If you can answer the questions in my previous post, I can help (as can others here too). But without that, it's as specific as "do you have tires for my car?" and leaving it at that.
I like these guys for used gear, as far a used antenna's that could be a bit more difficult.
http://fmamtv.com/index.html
http://fmamtv.com/index.html
Okay... 10 bay CP antenna is what you need, pressurized with a 1 7/8" input. Sounds like with the TPO you have that it is full wave spaced.
The thing to do is have tower riggers go up and inspect it. It may only be a couple of bays that are bad and not the whole antenna. A used 10 bay is hard to come by, and when they come off towers it's usually because they're shot to s***.
The thing to do is have tower riggers go up and inspect it. It may only be a couple of bays that are bad and not the whole antenna. A used 10 bay is hard to come by, and when they come off towers it's usually because they're shot to s***.
A 10 bay full wave spaced antenna has a gain of about 5. With feedline losses, your 10kW transmitter is probably running 11kW just to make licensed power. You're also getting close to the safe power handling capability of the coax, so you can't cut the number of bays down and go with more power.
I'd think about fixing the existing 10-bay and using the saved money to buy a good used 20kW transmitter (that can run well at about 1/2 power) and an automatic transfer switch so you can keep your Armstrong rig as a spare.
I'd think about fixing the existing 10-bay and using the saved money to buy a good used 20kW transmitter (that can run well at about 1/2 power) and an automatic transfer switch so you can keep your Armstrong rig as a spare.
And one that's on the right frequency is even harder to find. I've installed numerous used transmitters but never a used antenna. What happens when you pay a rigger a king's ransom to take the old one down and put up the used one only to find that it's no better than the one that's up there now? Add my name to the list of "fix the current antenna" voters....or at least get a definitive diagnosis of exactly what's wrong with it before making this major decision.WNTIRadio said:
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