Archive for the ‘Transmission Line’ category

« Older Entries

Day Four at NAB (for me anyway)

April 17th, 2012

Today started off with a breakfast with Dielectric where they told us about modern design tools for making antennas. Now they use computer modeling which saves a lot of time and trail and error. Then they went on to talk about the FCC spectrum grab, I mean repackaging and how sharp tuned mask filters will be needed to squeeze everyone into the few channels left over. The difference  between a 6 pole and 8 pole filter is quite amazing when talking about adjacent channels of different power levels. Dielectric also talked about their low power TV filters and antennas and how they put the same effort into them as their big brothers.

Oh yes, I got a cool medallion commemorating Dielectric’s 70th year. Check it out on Pinterest

Then it was off to the show. I learned more about fiber optics and keeping them clean and how an Ethernet to F/O box costs almost the same as a network switch with F/O SFP. But I found that Yellow Brick sells one for about $550 instead of $850.

I saw Sisvel show has a Titled approach to 3D transmission. I talked about this last year when I first heard about it but this was the first time I could see it in action. Looked good.

Talked with the Dolly guys, they have the Dolly Drive which combines Cloud storage and local backup as well as incremental and shared storage. They make a good case for an integrated approach. Still don’t know the cost.

And I talked with Derek at Fis Blue about F/O and tactical fiber, they also make cleaning kits and tools for F/O. I will recommend we get some for the tower.

Then the station called cause they could not get the playlist to work on the Matco, they powered it off and on but it would not turn on, until I told them about the power switch on the front behind the little doors,then it came up.

My boss’ station in Kansas is having problems so I went over to the booth of the MFG and talked to them. I saw the transmitter for the first time even though I have been asked many questions about it. The problem still exists.

Be the first to like.
Unlike
Share

Comments Off »

Posted in Broadcasting, Educational, Engineering, Fiber Optic, Transmission Line

Tags:

Wired Network for iPad

March 26th, 2012

We are looking at using an iPad to display data while on the tower, to do this we would need a wired network connection since a wireless network won’t  (may not ) work. But I cannot find any adapters to allow a wired network connection for the iPad. It seems that the common solution is to use a pocket wireless router that plugs into the RJ45 and provides a wireless network for you. The iPad to USB adapter will not work with a USB to Ethernet adapter as I have read, so thats out.

The reason for this is FileMaker Go, it’s an App that lets us access a FileMaker Pro file on your iPad or iPhone. I created a data base in File Maker to keep track of all the antenna and cables on the tower, it uses seven data bases to contain all the data. If the iPad is on the same network as a computer with the data base on it can be accessed on the iPad. If not, you can load the the files on to the iPad or iPhone and then you can load the changes back to the computer based data base.

The reason for all this is so I can go put the tower and compare the data base with whats really up there.

Be the first to like.
Unlike
Share

1 comment »

Posted in Antenna, Broadcasting, Computers, IP, Networks, Tower, Transmission Line

Tags:

The Basics of Impedance Matching

March 17th, 2012

Fig 1. Maximum power is transferred from a source to a load when the load resistance equals the internal resistance of the source.Electronic Design has a nice two part (so far) tutorial on Impedance Matching. The tutorials give you a very good grasp of impedance, useful for all types of circuits and situations. The comments have about as much information as the tutorial it’s self, as you can see from this one:

As pointed out by Martin, amplifiers driving loudspeakers are NOT impedance matched, for all the reasons he cited. I’d like to add that another misconception held by many non-audio folks (and audiophiles, too, thanks to makers of “floobydust” audio cables) is that an audio cable is a transmission line. The purpose of an audio signal cable is to deliver maximum VOLTAGE, not power, to the destination. Toward this end, IEC standards for professional line-level interfaces call for a source (output) impedance of 50 ohms or less and a load (input) impedance of 10 k-ohms or more. Further, it takes an audio cable some 4,000 feet long to just have noticeable transmission line effects at 20 kHz. All shorter cables are accurately represented by a lump of C, R, and coupled L. Therefore, unless you’re the telephone company with mile-long cables, source and load impedances do not need to be matched … to 600 ohms or any other impedance. 

Bill Whitlock, president & chief engineer of Jensen Transformers, Inc. and AES Life Fellow.

Click below for the Impedance Matching tutorials -

Impedance MatchingPart 1 

Impedance MatchingPart 2

Impedance MatchingPart 3

Be the first to like.
Unlike
Share

2 comments »

Posted in AES / SBE / SMTPE, Audio, Design, Educational, Engineering, Transmission Line

Tags:

2 for 1 Sale

February 9th, 2012

My friend Jim is running a promotional sale on RF work, I’ll let him tell it -

Broadcasters,

I am initiating an rf system FIELD SERVICE STIMULUS PACKAGE to benefit both you and I! Beginning today through April 15 (no coincidence there!), I will offer new clients a SECOND DAY OF FIELD SERVICE AT NO ADDITIONAL CHARGE (labor only). You can now get that second transmission system checked out along with the first for the same amount of money.

Let’s face it, I know that $$$ are tight for many of you and you are just hoping that RF system stays together and operates as it has for all these years but why tempt fate and risk the potential of lost air time? I will come to your site and perform an RF network analyzer sweep of your complete system including from the TX output to the antenna or anything in between and furnish a detailed report on my findings and recommendations. I can also line up a tower crew to do a visual inspection at the same time if you would like.

For details on my services and company, visit my website at www.jsengineer.com. To obtain a quote for your particular site, email me at jim@jsengineer.com or call me at 207-632-8973.

I look forward to hearing from you and working together.

Regards,
Jim Stenberg

Be the first to like.
Unlike
Share

Comments Off »

Posted in Broadcasting, Engineering, Measurements, People in Engineering, Transmission Line, Transmitters

Tags:

Tracing Cables

December 8th, 2011

The Coax in question

The usual way to trace a coax cable is to find what you think are both ends and then short one out and measure the other end. This works pretty well if you think you know where the other end is. There were several coax cables that we wanted to use but were unsure which was which so we sent one of the riggers up with a little test cable I had made. It consisted of a BNC with a 100 Ω resistor from center to shield, we also gave him some adapters to go to N and DIN connectors. The reason for this is that a coax attached to an antenna many times

100Ω

looks like a short, but with 100 ohms across it I would know as soon as I saw the reading on the meter that I had the right one.

This worked well for most of the cables we were looking for until we came to the last two. In the room where I was with the meter these coaxs had  lightning arresters attached to them. I thought they would pass DC . I was wrong.

It's an Open Circuit!

With my little 100Ω on the other end I got nothing so we figured we had the wrong cables. Then I got the idea to test the lightning arrester, and found it was open to DC. Another test showed we had the correct cables.

Be the first to like.
Unlike
Share

Comments Off »

Posted in Engineering, Measurements, RF Microwave, Riggers, Tools, Tower, Transmission Line

Tags:

dBk in under 1KW

September 23rd, 2011

FCC form 302 Tech Box

Most transmitter engineers know what dBk is, its how the FCC likes to state RF power on it’s forms, like the 302 that you fill out when you put a new station on the air.

And most engineers, that is broadcast engineers are familiar with the audio level measurements in dBm, where zero dBm is equal to 1 mW into 600 ohms.

And I have dealt with dBK in the past in figuring out our ERP on our full power UHF station. Zero dBk equals 1KW of RF Power

Heres an example from our old analog transmitter -

Orignal General Electric UHF Transmitter  55KW TOP

ERP                     1320.0KW                31.2 dBK

Antenna Gain          28.5                       14.37 dB

Input to antenna       46.3KW                16.65 dBK

Total Line Loss         8.7KW                - 0.75 dB

TX Output Power    55.0KW          17.4 dBK

Actually I had to guess at the antenna gain as I could not find any paper work on it when I got there. It was an old RCA.

Since you can’t really measure the actual power out of the antenna you have to calculate it as you do many numbers in engineering. Normally you start with the ERP and work backwards. That’s the number the FCC is concerned with, you are licensed for a particular ERP level and you have to stick to it.

So in this case our ERP on our license was 1320 KW, so you get 31.2 dBk is equal to 1320KW. You just take the power level and perform 10•(Log10). Now you need the antenna gain which you get from the antenna manufacture, in this case I guessed it was 14.37dB (remember dB is an expression of a difference in power level). So now you subtract that  from the ERP dBk and add back in the loss from the transmission line. Once again you get this from the manufacture for the frequency your on, in our case it was 0.75dB. Once that’s added back on the dBk from the transmitter is 17.4dBK. That’s what it takes to produce the licensed 1320 KW from the antenna.

With a few guesses you can figure out that 17.4dBK equals 55.0KW.

Now this leads me to a resent experience where I was being asked to help with a new Low Power UHF station being put up in the mid west. I have never dealt with a low power transmitter before. But I did my best. They needed the FCC form 302 filled out and they asked me to do it, so I took the numbers they gave me and gave it a shot.

Well what happens when your power level falls below 1KW? You have to go into negative numbers for dBk is what happens. So what is the dBk of 800Watts, well it’s -0.96dBk is what it is. Now I went looking on the web for information on doing dBk calculations on power levels below 1KW but all I found was references to the FCC site telling you that they will be negative numbers. But I did find an application for a low power station that was filed by one of the big networks and I compared what I knew to their calculations and they were different.

Later I was talking to one of the engineers at Sutro Tower that works for that network and he told me how the guys at the home office had messed up (not his words) the application for his full power station and had gotten the power levels wrong. Just cuz their big don’t make them right.

Anyway, for power levels below 1kW you just enter the power as a decimal and that’s the only difference. Go ahead and try it – 1KW or 1.0 should give you an answer of 0.0, and plugging in 2 (twice the power) will result in 3dBK. Now enter 0.5 and you get -3.0dBk for half power. Now you know.

 

1 person likes this post.
Unlike
Share

1 comment »

Posted in Antenna, Broadcasting, FCC, Measurements, Transmission Line, Transmitters

Tags:

All Burned Up

September 22nd, 2011

A station resently installed an Aux antenna on their tower, now the 3″ Helix line had been installed sometime ago but they finally had the money to put up an antenna. They had the line & antenna swept, and all looked good, then they switched the transmitter over to it. Bang, bang, bang. VSWR trips but then all was well again, the transmitter ran into the new Aux antenna and line but they knew something was up.

So they swept the line and antenna again, and lo and behold there’s a blip where there was none before, about 140 feet up the line. They sent the rigging crew up to inspect the line but they could see nothing wrong. If this had been a rigid coax line they would have opened up the sections in the area and pulled the center out to inspect it, but this is Helix and the only connectors are at the ends of the line.

So to pin point the exact area they drilled small holes in the in the Helix and pushed a wire though to the center. On the Network Analyzer they can see this and it’s relationship to the blip (holes can be patched). When they zeroed in on the blip they cut the line open and put flanges on each end so they can add a section back in.

This is what they found -

All burned up!

The center conductor with melted Teflon

 

The prevailing thought is that the Teflon twisted spacer was folded at this point and allowed the center to come closer to the wall of the Helix and when power was applied it arced over, melting the Teflon. But if the center was not centered then they should have seen that when the line was first swepted. But even if they did see a little blip what could they have done? It’s Helix, you either take it or leave it. That’s the beauty of rigid coax, you have line sections that you can open up if there is any questions.

Well they put in a new section and they have a backup antenna that works.

 

 

 

1 person likes this post.
Unlike
Share

Comments Off »

Posted in Antenna, Broadcasting, Riggers, Tower, Transmission Line, Transmitters

Tags:

Low Power and Stairs

September 4th, 2011

As I mentioned before I’am involved in a low power station going up in the Midwest. They got the wrong T/L at first then the correct one and the rigger put it in with the antenna but left till after Labor day before the TX showed up.

Well it did arrive but the elevator stops one floor below where they need it, and it weights 600 lbs. I was called, I told them to get some big guys. Earlier today I was told they got it up the stairs. One guy on his butt scooting one step at a time with the hand truck, the other two pushing from below.

It’s in place but they want to test before the rigger comes back. I am being asked to walk them through attaching the T/L to the TX.

Then to turn on the TX the tech from the manufacture will talk them through powering it up. He’s in Europe. They want me in on that call too.

They are going to text me pics of the connectors and the TX so I have a reference.

Stay tuned to find out what happens next. By the way I am on vacation, with my family. Business as usual.

Be the first to like.
Unlike
Share

Comments Off »

Posted in Antenna, ATSC, Broadcasting, DTV, Riggers, Transmission Line, Transmitters

Tags:

What is it? Go on, Give it a try > Answer Below (Contest 01)

September 3rd, 2011

The wait is over, see below to find out what this thing is. This was a lot of fun, the answers given were interesting and creative. I think I’ll start another one with the next newsletter. We’ll see if we can come up with some sort of prize too. Of course you comments are always welcomed, let us know what you thought of our contest ———-

What is it, it’s a contest without a tangible prize, at least for now. See if you can figure out what the picture is of. Hint – it has to do with broadcasting.

If you think you know what it is add a comment, try as many times as you like.

So here we go, What is it?

Can you tell me what this is?

 

The original answer was – A Fine Match Tuner on a Coaxial Transmission Line, which it is. It’s located within the tower and the rust stains you see come from that clamp that was used to keep it from falling. A more complete answer comes from my friend Todd who knows more about it than I did, and was the first one with a correct answer. Paul also got it right and Steve was headed in the right direction  –

“(It’s a) Steatite tuner rod likely used for a VHF fine matching section. . .   Steatite is an insulator but has higher capacitance than Teflon.  They don’t use it much these days instead opting for capacitive paddles like you are accustomed to seeing.  At lower frequencies,they even penetrate through the inner to get enough capacitive reactance (or susceptance)

In the early days, Steatite was even used as the centreing insulator for the inner. . .but it was lossy and Teflon came along and helped that out so you didn’t need (a) huge inductive undercut to cancel out the insulator.”
From the pictures you can see the Marmon Clamp used to joint the Transmission Line sections together, this shows just how old this stuff is.

One of several Fine Match Tuners in a T/L

 

Here you can see how the tuners are used, they are spaced 1/8 Wavelength apart for the frequency you want to affect.

Tuners spaced 1/8 WL apart

Be the first to like.
Unlike
Share

15 comments »

Posted in Broadcasting, Transmission Line, What is it?

Tags:

The Difference between 7/8′s and 1-5/8″

August 31st, 2011
1-5/8" Foam CELLFLEX Cable

1-5/8" Helix

A low power station is going up in the mid-west and I got a call asking my opinion on a problem they have encountered there. Seems the rigger is on site and he’s telling them that the Helix they got is 1-5/8′s inch, not the 7/8′s inch he was told. Their only running a couple of kilowatts so thats okay either way. The guys they got the system from want to ship them some adapter cables and new hangers, their telling them that they can use the 1-5/8″ Helix, which of course they can.

The Rigger is telling them that he can’t bend the bigger stuff to make the turns they need to. So I was being asked who to listen to. Well, I told him, the 7/8″ is easier to use, hang and bend than 1-5/8″. And I am sure the seller does not want to pay for shipping of a new spool of 7/8″ Helix and the return of the old one.

I told him to talk to the Rigger and find out if he can use the 1-58″ at all, just to get them on the air, they are on a tight deadline with the FCC, then find out how long to get the correct 7/8″ Helix on site. Ultimately I told them to listen to the Rigger, as he is on site and has to deal with what ever goes in. I have been at the other end when I was told to install something I knew was wrong or was more difficult than it needed to be, so go with the Rigger.

Doing some checking I see that 7/8″ had a Min bend radius of 10 inches and 1-5/8″ is 20 inches, a big difference. In the long run the 1-5/8″ may be sticking out around the bends and get damaged over time where the 7/8″ will make for a better, cleaner install.

I also see that there is a smooth jacketed 1-5/8″ coax that has a bend radius of just 11 inches, wow!

2 people like this post.
Unlike
Share

Comments Off »

Posted in FCC, Riggers, Transmission Line

Tags: