SAFE REFLECTIVE POWER IN FM TRANSMITTER

[MwapeM]

I would like to find out. Is it possible to have a zero reflective power from a 1000 watts FM transmitter. If not what is the maximum limit of the reflective power for safe operations of the equipment.

 

[MikeMl]

Where is the point of reflection, at the antenna, or at the transmitter itself? A well engineered 1kW antenna system might have a reflected power of less than 20W...

The equipment will typically have a spec for max SWR on the antenna feedline. **broken link removed**...

 

[MwapeM]

Thanks MikeMl. I don't really know where the point of reflection. However what the transmitter is RVR TEX 1000 LIGHT. It shows on the reflective power on the display. When i set the transmitter to above 900 watts, the reflective power goes up to 48 watts. recently, the transmitter stopped showing forward power. When checked, the RF combiner resisters were burnt.

 

[MikeMl]

That is a SWR of ~1.6, which for a commercial FM stationtion is too high...

Was the reflected power lower than 48W previously? In other words, did the reflected power recently increase? If so, you should be checking the antenna/coax for water intrusion, wind, ice, or lightning damage.

 

[ronsimpson]

When i set the transmitter to above 900 watts, the reflective power goes up to 48 watts.

When I worked broadcast, my big transmitters used 3 inch coax. Once I had some water in the coax at the bottom of the tower. At full power it would start to spark over inside the cable. These are air dielectric piping.

I have also seen antennas glow at night where power is going where it should not. As Mike said; it is time for a good mechanical inspection.

 

[Tony Stewart]

For high power RF Tx's, conjugate impedance matching is critical sometimes by knowing how to choose the correct feedpoint. Static discharge can occur with dielectric contamination. (PD)

 

[Mikebits]

Having a good ground plane for the antenna is important. When I use to fix RF transmitters and antenna tuners in the Navy, I often encountered corroded connections for the grounding connection. At a non-shipboard site I once worked at, the ground plane was created by burying a braided wire mesh configuration, it was like a star configuration. Ugh that is not a very clear description is it? Basically it was a ground plane mesh buried about 1 foot in the dirt with braided wires going out away from the dipole. There was one attached wire that came out of the ground and was used to attach to the antenna ground connection.
Anyways, the connection points to this mesh would often be a big source of problems and really jack up the VSWR. I once had one so bad the VSWR was over 2.0, so with that long winded description, my point is, grounding is real important and can degrade system performance over time, and weathering can be a real culprit.
I can't remember what the ground mesh thing was called or I would try and locate a reference.

 

[Mikebits]

I was able to find a picture of the ground mesh I spoke of. https://www.am1000rangemaster.com/groundxmit.html

 

[MwapeM]

many thanks to all the responses. I will use the ideas once the transmitters are repaired. Check my other thread on faulty transmitters.