Correct way to measure FM radio antenna output?

An LPFM radio station I work with is having strength issues today. IT may be atmospheric, but the issue is they are having poor reception, areas they could be heard before they aren't up to a 1/2 mile away.

What I would like is for this case, and any other case involving fm radio antennas, (say FCC 100 watts up to perhaps 25,000 watts) is tool for measuring the effectiveness of the fm antenna. I want to make sure it is outputting in the air the said output power (eg 100 watts or 25,000 watts) it is suppossed to. And perhaps test the transmitter at the same time (if that is separate from the antenna).

also would that mean going out to the fringes of the antenna or can this test be completed at the transmitter/antenna location?


thanks for your time.

(p.s. if it needs to be 2 different tools, one for the lower powered LPFM's and one for higher powered 25k+ that is ok. I am more interested in the LPFM's as of now). I do know the lpfm uses bnc, but the 25k in question has a HUGE 500 foot wire/connector into a cabinet transmitter all the way up the 500 ft tower.

The first thing to do is to examine the aerial design, and figure out what it is meant to do. A high power aerial will normally be split into two arrays, so if an aerial fault occurs, the working part of the array can be used. This normally means that both halves of the array must be driven with both RF sources co-phased. So it is important that the phasing of both feeds is correct. Like wise any balancing load should be dissipating the minimum power.
Aerials are extremely reliable, water ingress is the big problem, this can act as a load, so the power is being dissipated in the water and not in the "ether". Aerial connectors can overheat and burn out,, but this usually results in an increased VSWR.
if the array has very sharp notches, it can be that it has been blown of alignment and is a, interfering with the area that is meant to be in the notch and b, the notch could then be in its main service area, so reducing coverage.
For band 2, a dipole feeding an attenuator and a receiver with a S meter would be good enough for comparative measurements but it will not mean anything unless compared to the original design specification.
Frank

HI and welcome to the EDA forums :)

OK lets just consider the low power system for a start, 100W

1) 100 W through BNC connectors is probably pushing the limits of the BNC a little bit. You should really be using N type connectors and low loss cable say LDF 4/50 specially with runs of ~ 500ft
2) you need to determine that the transmitter is actually producing its stated 100W ... a quality dummyload wattmeter or one that can have a dunny load attached to its output eg. the trusty Bird 43
3) Then with same meter inline with coax feed and antenna determine the forward and reflected power readings to determine if there may be a fault with either the feed or the antenna.
4) it may entail a further test with the meter on the far end ( antenna end) of the coax with its dummy load to see how much of your 100W is actually getting to the antenna feedpoint
5) inspect the coax visually and carefully looking for any damage
6) inspect the antenna system, checking for corrosion around the feedpoint and anywhere else ... also for physical damage ... brittle failure cracks etc
7) finally if all that checks out OK, you could look at doing some field strength reading around the region it is covering. These could be compared to field strength density maps that you can produce in programs like Radio Mobile

hope that helps some :)

cheers
Dave

what tools do I need to acquire the aerial design? this is an LPFM. 100 watts or so.




thanks! most definitely. i'll be looking into buying a bird. It's neat that it can show forward and reflected power . thanks much