50 ohm FM transmitter

[liradioprez]

I have a 15 Watt FM transmitter that is 50 ohms. Will it be okay if I run it into 10 feet of 75 ohm cable?

Thanks!

 

[rfry]

I have a 15 Watt FM transmitter that is 50 ohms. Will it be okay if I run it into 10 feet of 75 ohm cable?

To clarify, probably your transmitter is designed for a 50-ohm load, but that does not mean that its output (source) impedance is 50 ohms. It will be far less.

But given that, there are several possible answers to your question.

1) If the 10 foot cable has no termination, then the transmitter will see a very high SWR and either will not operate at full power, or this configuration will damage its output circuits.

2) If the cable is connected to either a 50 or 75 ohm load, then the SWR seen by the transmitter will not be as high as in 1) above but the output circuits still may be affected, and the transmitter may not be able to operate safely at its rated power.

3) If the load on the far end of the cable consists of an antenna, then this configuration will require an FCC station license to operate legally. An FM broadcast transmitter power of 15 watts is millions of times more than legally can be radiated under Part 15.

//

 

[liradioprez]

Sorry for the confusion, I am an intern engineer at a local radiostation, and I am working on a backup TX chain. So far I have a Ramsey 1 watt to feed the 15 watt PA and from the PA, it goes to a QEI 1Kw transmitter. Yes..I know..crappy setup but I'm working on a budget and using what we have laying around. Anyways, thank's for your help!

Kyle Kratoville

 

[Studio1]

Why would you even consider using 75 ohm cable for a broadcast transmission system?

50 ohm cable is cheap enough and easy enough to come by, even if you are on a budget. Running your TX into 75 ohm
cable is asking for trouble - potential damage of the TX output stage which will cost you a heap more than a few feet of 50 ohm cable.

In short, don't do it.

 

[radiorob2.0]

I can't find it on their website for all of the monster cables, but Rat Shack does sell a ten foot length of RG8-mini patch cable.

 

[rew]

I would not take a chance on using 75 ohm cable. You should be able to find 10 feet of RG8, RG58 or RG8X at Radio Shack or any real electronics part store. You may find it at a local truck stop. Same cable used on CB radios. Replacing the output transistor in that 15 watt amp would cost much more than the proper cable. Play it safe, don't try it.

 

[ironbear]

Correct answer: as a practical matter, you're unlikely to see a huge increase in VSWR if the mismatched cable section is very short, i.e. 10 feet. But why abuse equipment when you can make or buy a proper 50 ohm jumper for cheap?

Of more concern is the Ramsey 1 watt 'exciter'. Although Ramsey makes a couple FCC Type Approved broadcast items, I couldn't come up with a single 1 watt unit of theirs that isn't hobby (pirate?) grade. It's not approved for use as a regular backup as part of composite transmitter.

There are possible ways around this, I suppose, if the Ramsey unit can be proven to meet broadcast standards. The FCC will sometimes accept weird arrangements on a Special Temporary Authority (STA) application, IF a station is off the air and in dire need.

The real way to make a solid backup in this case is to find an inexpensive Type Approved exciter to use with the QEI. Try eBay.

The FCC is your friend! Here are their rules http://www.fcc.gov/mb/audio/bickel/amfmrule.html for broadcast stations, good reading for any rookie engineer.

 

[w9wi]

I would not take a chance on using 75 ohm cable. You should be able to find 10 feet of RG8, RG58 or RG8X at Radio Shack or any real electronics part store. You may find it at a local truck stop. Same cable used on CB radios. Replacing the output transistor in that 15 watt amp would cost much more than the proper cable. Play it safe, don't try it.

I'm a TV studio guy & don't do much transmitter work, but... is a 1.5:1 SWR really going damage a 15-watt PA???

That said, the other posters are right -- that 50-ohm cable is cheap and easy to find.

 

[Chuck]

I would not take a chance on using 75 ohm cable. You should be able to find 10 feet of RG8, RG58 or RG8X at Radio Shack or any real electronics part store. You may find it at a local truck stop. Same cable used on CB radios. Replacing the output transistor in that 15 watt amp would cost much more than the proper cable. Play it safe, don't try it.

I'm a TV studio guy & don't do much transmitter work, but... is a 1.5:1 SWR really going damage a 15-watt PA???

That said, the other posters are right -- that 50-ohm cable is cheap and easy to find.

It probably wouldn't make much difference if it is just a short jumper, but why not just do it right? We are not talking about a significant amount of money to get the right cable.

My bigger concern is the idea of connecting a Ramsey 1 watt transmitter to a 1000 watt power amplifier. Please don't do it! The Ramsey hobby grade transmitters are well known for throwing harmonics and spurs all over the FM band and well above it, including aircraft and TV frequencies. Unless you have a spectrum analyzer, you might never know what it is putting out. If you are anguishing over the cost of a piece of RG-8 cable, I think it is safe to assume that a spectrum analyzer isn't sitting on the shelf.

One of the reasons Ramsey stuff is so cheap is it lacks any real harmonic suppression circuitry. Those filters cost money. You'll note that even Ramsey's FCC Type Accepted transmitters cost a lot more than the 1 watt hobby versions. There is a reason. At the legal power levels the Ramsey's are intended to work at (milliwatts, not one watt) the harmonics are not a serious concern to you or your neighbors. At 1000 watts, they are very significant and can wreak havoc with a multitude of licensed radio services.

If you had to use a Ramsey in a dire emergency, at least use some external filters like the ones available from Progressive Concepts http://www.progressive-concepts.com/info/item.html?id=121 .
Even that would require a STA (Special Temporary Authorization) from the FCC to go on the air.

I understand your enthusiasm to use what you have, but a much better solution would be to find a used exciter that is FCC compliant. RVR makes an excellent one called the "Blues 30" that is marketed under a variety of brand names, including Bext, Armstrong and I think QEI. It even has a decent sounding stereo generator built in. Since it puts out 30 watts, you could use it to drive the transmitter directly. It is less to go wrong, and less to cause an unwanted visit from the FCC.

 

[XRQKFM]

My bigger concern is the idea of connecting a Ramsey 1 watt transmitter to a 1000 watt power amplifier

Chuck is right! You might as well call the FCC and turn yourself in. Ramsey's even at 25 milliwatts can trash your neighbors televisions for a block around. It seems like the majority of the micro power broadcasters that get busted have a Ramsey to blame, Ramsey's are not even safe for hobbyist.


Steve
www.RadioBrandy.com