Why are commercial FM antennas so big?

[djacobow]

I'm talking about large commercial transmitters, like the kind that broadcast a top-40 station in a big city. 250 kW or more is not uncommon, I understand.

I've never seen one of these transmitting antennas up close, but from afar I notice two things:

 1. they are up high, often on tall towers and buildings and
 2. they appear to be quite long

1, I get. Line of site, clear pattern without obstructions, etc.

2 is making me curious. They look like verticals. If so, I'd be expecting lamba/4 or lambda/2 length, but they appear to be much, much longer than that.  lambda is 3m at 100MHz. Why do some of these antennas appear to be 30m long, if not longer?

I'm sure these antennas are complex and sophisticated, nothing like the simple ones we ham use, but I feel like there's something fundamental I'm missing.

Best,
Dave
WE6EE

[Towger]

Multiple standard dipoles probably.

[Kire Pûdsje]

As towger said, probably multiple dipoles. Have a look at collinear antenna (array)s.
Stacking dipoles is used to generate directivity. A narrow beam (vertically) is created, to increase the range or lower the transmitter power depending on which point of view you take.

[vk6zgo]

As towger said, probably multiple dipoles. Have a look at collinear antenna (array)s.
Stacking dipoles is used to generate directivity. A narrow beam (vertically) is created, to increase the range or lower the transmitter power depending on which point of view you take.

Yes, the FM site has no interest in firing lots of RF up into the air, where it is of no interest to anyone except extraterrestrials.
In the horizontal axis the polar diagram is omnidirectional, but in the vertical axis, it is narrow & tilted downwards towards the service area.
Imagine it as an inverted shallow bowl.

Also, remember, these are gain antennas.
The EIRP of most FM Stations in Australia is 100kW, but the transmitters normally used are 5kW units.
A 13dB gain omnidirectional antenna at around 100MHz is a pretty tall order, so that is why they are large, composite antennas, made up of multiple dipoles.

 

[Cerebus]

Here you go. A whole bunch of FM mixed polarization dipoles on the BBC Wrotham transmitter.

[djacobow]

Very interesting, guys.

So, are these antennas typically fed in parallel? Does is there any adjustment to the phase of the signal arriving at each antenna's feedpoint?

With the aid of receivers at strategic locations, I imagine if the phase going to the antennas can be controlled, it would be possible to fine-tune the pattern and steer the lobes to the most economically important or difficult to cover areas.

[Cerebus]

At VHF frequencies it's usually cheaper and easier to use an antenna with the directivity you want (Yagi, log periodic etc.) rather than steering the beam by phased array type techniques.

One place where you do get a distinct phase difference between antennas is if horizontally and vertically polarized dipoles are used to create circularly polarized waves by feeding one polarization leading and the other lagging.

Normally these kinds of array are fed in parallel (possibly via multiple antenna tuning units) but the usual trade-offs apply so multiple transmitters versus single give you higher cost, higher reliability and so on. That said, I've never seen a commercial transmitter with anything more than a single transmitter (i.e. RF power amplifier).

[edpalmer42]

Does the large physical size help the antenna radiate that much power?

When you're pumping out 100 KW of power, you want it all to go up the feedline and leave the antenna.  Even a slight mismatch could result in melted cables!  At 100 MHz, 1/2 wavelength is only about 1.5 meters.  I don't want to be around when someone tries to slam 100 KW through a 1.5 meter long dipole!

Ed

[4CX35000]

Does the large physical size help the antenna radiate that much power?

When you're pumping out 100 KW of power, you want it all to go up the feedline and leave the antenna.  Even a slight mismatch could result in melted cables!  At 100 MHz, 1/2 wavelength is only about 1.5 meters.  I don't want to be around when someone tries to slam 100 KW through a 1.5 meter long dipole!

Ed

The power your seeing quoted is the ERP (Effective Radiated Power) which is a result of overall antenna/system gain x transmitter power. Therefore the transmitter in the building below is probably running 15 KW for a 100 KW ERP service. The BBC in the UK run 250 KW ERP services, but the transmitters are running around 30 KW and even then transmitters are duplicated, so running 15 KW each.

Antenna gain is related to antenna length in tiers relative to a dipole, a antenna panel is 1 tier.  So 8 antenna panels (8 tiers) on one side of a mast is approx 8dB gain. There is a calculation but I forget, it has been near 20 years since I worked in this field and I forgotten a great deal since.

Behind the antenna is a balun, whilst inside the mast there is a power dividing/matching network made up of specially cut lengths of solid section coaxial feeder, the result is the power is split across all the antenna panels. The power divider has impedance matching so the whole system see's the correct impedance at the working frequencies.

So as an example a 10 tier antenna on a 3 sided mast, the power is divided up so that each panel probably is only getting 1 KW from a 30 KW transmitter. Each panel can probably handle 5 KW, so a antenna can probably handle 5 services running 30 KW transmitters each.

Broadcasters tend to share antenna. Main reason is to save money as mast height gets expensive the higher you go. So transmitters are added together using large combiners. Devices made up of RF couplers and resonant filters.

I have heavily simplified the above as there are other factors such as power phasing/beam tilt involved, half antenna working and so on. Antenna engineering is a specialism in its own right.

[4CX35000]

Normally these kinds of array are fed in parallel (possibly via multiple antenna tuning units) but the usual trade-offs apply so multiple transmitters versus single give you higher cost, higher reliability and so on. That said, I've never seen a commercial transmitter with anything more than a single transmitter (i.e. RF power amplifier).

Modern transmitters use loads of LDMOS transistors in several module amplifiers running in parallel. So 10 KW transmitter might have 100 x 100 watt modules with 4 x LDMOS transistors on each module.

[vk6zgo]

Does the large physical size help the antenna radiate that much power?

When you're pumping out 100 KW of power, you want it all to go up the feedline and leave the antenna.  Even a slight mismatch could result in melted cables!  At 100 MHz, 1/2 wavelength is only about 1.5 meters.  I don't want to be around when someone tries to slam 100 KW through a 1.5 meter long dipole!

Ed

The power your seeing quoted is the ERP (Effective Radiated Power) which is a result of overall antenna/system gain x transmitter power. Therefore the transmitter in the building below is probably running 15 KW for a 100 KW ERP service. The BBC in the UK run 250 KW ERP services, but the transmitters are running around 30 KW and even then transmitters are duplicated, so running 15 KW each.

The transmitters I worked with were 5kW units with valve output stages, so the 13dB I quoted was for the antenna system, which, of course,includes.coax loss , so the antenna in isolation was a bit better than that.

The Australian TV Channel 7 transmitter I also worked with put 13kW sync tip power up the coax for 100kW Vision EIRP, so the antenna was 3dB or so worse in gain than the FM antennas.

In my earlier job, the Ch2 transmitters were 20kW, so the antennas were lower again in gain.
With a Vision carrier of 64.25MHz, those antennas were seriously big, so it was hard to get the same sort of antenna gain.

Antenna gain is related to antenna length in tiers relative to a dipole, a antenna panel is 1 tier.  So 8 antenna panels (8 tiers) on one side of a mast is approx 8dB gain. There is a calculation but I forget, it has been near 20 years since I worked in this field and I forgotten a great deal since.

Behind the antenna is a balun, whilst inside the mast there is a power dividing/matching network made up of specially cut lengths of solid section coaxial feeder, the result is the power is split across all the antenna panels. The power divider has impedance matching so the whole system see's the correct impedance at the working frequencies.

So as an example a 10 tier antenna on a 3 sided mast, the power is divided up so that each panel probably is only getting 1 KW from a 30 KW transmitter. Each panel can probably handle 5 KW, so a antenna can probably handle 5 services running 30 KW transmitters each.

Broadcasters tend to share antenna. Main reason is to save money as mast height gets expensive the higher you go. So transmitters are added together using large combiners. Devices made up of RF couplers and resonant filters.

I have heavily simplified the above as there are other factors such as power phasing/beam tilt involved, half antenna working and so on. Antenna engineering is a specialism in its own right.

It seems that lower transmitter power & more antenna gain is the normal method in Oz, as all of the high power stations I've seen do this.
I wonder if the use of higher Tx power in the UK for the same EIRP is because of problems with snow/ice buildup in cold countries?

[vk6zgo]

Does the large physical size help the antenna radiate that much power?

When you're pumping out 100 KW of power, you want it all to go up the feedline and leave the antenna.  Even a slight mismatch could result in melted cables!  At 100 MHz, 1/2 wavelength is only about 1.5 meters.  I don't want to be around when someone tries to slam 100 KW through a 1.5 meter long dipole!

Ed

Many people get confused by antenna gain.
A moment's thought will make it obvious that the only real power you have is that which your transmitter produces.
If 13dB of antenna gain, gave you a real power increase of 20times, you would have an "over-unity"device, & be able to solve the Earth's energy problems in one stroke!

Antenna gain is  a measure of how much power would need to be applied to an "Isotropic" antenna (effectively a point source) to produce the same field strength in the direction you are interested in.
Hams often quote gain w.r.t a dipole---- to convert that to EIRP add approx 2dB to the quoted figure.

From the above, you can see that all your antenna system has to handle is the transmitter output, in my quoted case, 5kW, so the requirements aren't as severe as you believed.
Furthermore, as 4CX35000 points out, there are multiple dipoles, each of which only need be rated for a fraction of the transmitter output power.

The large physical size comes down to several things:-

Thick elements are more wide banded than thin ones.
They are also more rugged, which is important in antennas which have to sit on top of high towers in all sorts of weather, for decades.

[Kire Pûdsje]

This is also why I specifically called it directivity in my mail.
For people that do not know it: Gain = Directivity - Loss (all in dB's). So for low-loss antenna's gain and directivity are close.

The total radiated power cannot change. We have to satisfy the conservation of energy.
What can be done is to direct the power into a direction to be most usefull. In this case towards the horizon. This 'additional power' at the horizon is taken from the undesired directions (zenith).
The balloon analogy gives a good impression of what happens. The balloon approximates an isotropic radiotor. If we press on the top and bottom of the balloon, it will bulge out at the equator.

[W2NAP]

Antenna gain is related to antenna length in tiers relative to a dipole, a antenna panel is 1 tier.  So 8 antenna panels (8 tiers) on one side of a mast is approx 8dB gain. There is a calculation but I forget, it has been near 20 years since I worked in this field and I forgotten a great deal since.

http://www.shively.com/ds-6822.pdf

8 bay antenna is going to be about 6.6DB gain. most FM broadcasters use CP a single CP bay has -3.4DB gain (yes that is minus 3.4DB) 2 bays will have 0DB apx and 3 bays 1.9DB

to get 8DB you will need 12 bay CP http://www.shively.com/ds-6828.pdf