Transmitter Reflected Power

On Nov 24, 9:26 am, Aibeetea wrote: > We have a 1kw Sirio FM transmitter on air with about 960 watts, and > the reflected power is between 0watt to 5 watts. > Another contractor supplied a 2KW Siel RFB2005 FM transmitter. The > reflected power while running at 1.3 kw is 60 watts. We are told the > max allowed is 180 watts. > We have had experiences with bad contracts in the past and we are a > bit wary. Is this normal? > Thank you.

Aren't you required to have a licensed engineer keep this stuff in spec? You're asking for help in Usenet? This IS an FCC licensed station, right? Good grief.

GG

The reflected power is a result of your antenna's standing wave ration and what's reflected obviously increases as the output power of your transmitter does. How are you reading the reflected watts? What is the SWR?

The ratio of forward to reflected has changed from your Sirio to your higher powered Siel. An explanation for this could be:

1. Your antenna is just able to handle 960 watts but starts to have power handling problems at 1.3 kw
2. The method for measuring the reflected power has changed. Are you using self supplied meters on each individual amp to get these readings rather than using the same separate meter for both amps?

Aibeetea-

Maximum allowed reflected power would be determined by the transmitter specifications. However, actual reflected power is a function of antenna matching, and is not due to a problem with the transmitter.

For 1300 watts, 60 watts of reflected power is just under 5 percent, which is probably acceptable.

Fred

I know of station here near me that employs a "technician" - "not" FCC licensed. I had the chance to meet with and talk to him and he scoffed at the License. Whatever floats your boat - buddy. Oddly enough - they were having a slew of issues and one Announcer - no longer with them (for obvious reasons) - every night for a week or so - blasted them over the air - about the crap they had at the station and the "Engineer" 's inability to resolve the issues. Yeah it was something to listen to... They eventually got a "new" station - building and all - and even with the new stuff - had a ton of issues. While they "seem" to have gotten most cleared up - they're not yet where they probably could be. It is hilarious to hear many of their announcers on air - talking of hitting switches and nothing working. They talk about being "Pioneers" in the radio industry - they probably should have stuck to the cans and string. The "Engineer" they "used" to have - was good - very good. He retired and has since passed away.

The FCC took the bite out of that license and now it is basically a free for all. Then they wonder why they have issues - such as they do.

Unless the transmitter is putting out a spurious frequency that the antenna rejects You might want to look at the transmitter output signal with a spectrum analyzer.

Bill K7NOM

On Sat, 24 Nov 2007 10:09:49 -0800 (PST), snipped-for-privacy@yahoo.com put finger to keyboard and composed:

The OP appears to be posting from Mauritius.

- Franc Zabkar

Below is a link to a VSWR calculator. 5 watts reflected to a 960 w transmitter appears normal. Bear in mind that the goal is to have a minimum VSWR meaning a maximum coupling between the transmitter and antenna. However it is possible to have a very poor grade transmission line and an apparently low VSWR. In that case the power is dissipated as heat. From your second example, even 60 watts reflected (1.55:1 VSWR) is a little bit high, and 180 watts may be unacceptable in practice. For example the manufacturer might be saying the transmitter is stable up to a 2:1 VSWR, however what happens if rain or ice detunes the antenna further? You don't want to be operating near the limit.

An antenna might have a specified VSWR of 1.2:1 and when used in the system only yield 1.4:1 based on the reflection coefficient of a number of components in the transmission line, such as connectors, and surge protectors. Operating the antenna outside its operating frequency, and/or bandwidth, or mounting too close to the tower can raise the VSWR. On the other hand a very lossey jumper at the transmitter end might disguise a very poor antenna VSWR by absorbing both forward and reflected power and re-radiating as heat.

Given this is an expensive high power transmitter, if you have any doubt, I would advise you to hire a licensed consulting engineer familiar with transmitters. He will look at the specified antenna system, the transmission line and calculate the expected VSWR, measure and confirm.

Aibeetea wrote:

Between 3% to 7% reflected power is usually acceptable. Depending on the installation and the manufacture specs, sometimes 10% reflected power is the very maximum that could be accepted. At above 7%, I would be spending a lot of time to verify why it cannot be less.

The readings can vary depending on the accuracy and methods of taking the readings. Reflected power is caused by the antenna matching, and capability of the antenna system.

Don't you have proper maintenance instrumentation, and employ a properly trained engineer at the facility to look after your equipment?

With RF transmission equipment, there are many safety issues involved, especially for radiation hazard! This is something that you may not feel, but you and your staff can be effected over time. This is why you need a properly trained and equipped engineer to look after your facility.

Thanks, but in Nigeria life is a bit different. There are too many opinions and honestly, I'm much more comfortable now that Usenet have responded. BIG THANKS.....See the response of the Sales Rep of the manufacturer. I was not comfortable.

Dear Ibrahim,

the readings you have written in the mail are absolutely correct; in regards to reflected power you can consider that the maximum allowed is 160W and therefore all it is under this value is a correct value. It is obvious that less is the reflected power, better it is. A recommended reflected power does not exist, but we can say that normal reflected power could be 70-80W.

We hope to have clarified all your doubts.

With kind regards,

Siel Broadcast SpA Fabrizio Tommasini Area Sales Manager

Tel. +39 02 45713300 Fax +39 02 45713351 Skype: fabrizio.siel

See You at NAB 2008 ( Las Vegas , April 14th -17th) Booth 9323 and Join Us at Broadcast Asia 2008 (Singapore ,June 17th-20th) Booth

Dear Sir,

We have a request to make on our newly installed FM Power Amplifier from your Company, the RFB 2005 FM Transmitter. We wand to know what to expect as the reflected power.

Here are the items installed at one of our stations, Rock-Side FM Kafanchan, Nigeria.

1. Exciter Model: EXC 25

1. Transmitter Model: RFB 2005

2. Size of Feeder Line: 7/8 inch

1. Length of Feeder Line: 60 metres

2. Antenna Model: Aldena ADE.01.02.220 - 8 dipoles

We need to know:

a) The RECOMMENDED REFLECTED POWER

b) The MAXIMUM ALLOWABLE REFLECTED POWER

We have these systems installed in two of our FM Stations. The present readings in one of the stations are:

Radiated Power: 1.6KW

Reflected Power: 47 Watts

Room Temperature: 23 degrees Centigrade

We need your comments on these readings as we have different opinions as to what the optimum reflected power ought to be.

Thank you very much,