Why do low frequency transmitters require so much power?

[lanbergld]

Hi folks, I've gotten into a days-long discussion with a coworker and we cannot find an answer. It began when I mentioned to him the massive size of longwave broadcast transmitters, and the gargantuan amount of power consumed by them. Examples not only including the European broadcast stations, but also the US Navy's NAA station.

My cohort (not into radios but does have a PhD in a related area) replied something to the effect of "That can't be. Long waves are lower energy so they must be easier to send, energy-wise. Higher frequency stations should require more power than low frequency ones."

I've proposed several reasons to my coworker, including the size of antenna required, but he keeps coming back with the same argument about the low energy in the waves themselves. So I've been trying to find the answer -- Why low frequency stations require the incredible amount of power that they do.

Does anybody know the answer?


Larry Lanberg

Richmond VA

[N_Jay]

Originally Posted by lanbergld

Why low frequency stations require the incredible amount of power that they do.

Does anybody know the answer?

Lower frequencies take do not take more power, however many low frequency systems are based on high power principles. (high loss paths, low efficiency receivers, extremely broad coverage, etc.)

The size of the antenna (at a given gain) has to do with the wavelength of the signal, not the "power".

[lanbergld]

"High loss path" looks like a reasonable answer. Signal loss is one aspect I didn't consider. I'd already looked into path differences (ground hugging of LF vs. ionospheric skip of HF), but that was shot down by my contentious coworker, who pointed out that low frequency waves go through buildings easier so less power should be required sending them.

Ok, so, in other words, the ground hugging signal dissipates more into nothing (?), and so more power is required by the station to keep the signal coming steady? Would that be correct, or close to correct? Thanks.

Ps Of course I'm aware of the wavelength relation to electrical size of the antenna, but I was thinking that since their size is so huge -- LW antennas are/were the tallest structures in Europe -- that, somewhere along the line, an increased need for power must be involved since we're considering transmitting. Is that totally wrong?


Larry Lanberg

Richmond VA

[N_Jay]

A certain path at low frequency signal may be 200 dB, where it is 500 dB at UHF.

But, if your receiver is fairly inefficient, you still need a very large signal to make the path work.

You have to look at the transmitter, path and receiver as a "system", not individual systems.

Using modern electronics it takes about the same amount of power to generate the same strength signal from LW through UHF (Higher frequencies get a little trickier, but we are getting better)

When paths become less than clear line-of-sight losses go up quickly, and generally go up more for higher frequencies. High gain antennas are easier at higher frequencies.

SO, in general higher frequencies are better for clear line-of-sight paths and as you rely on greater obstructed paths lower frequencies do better, but those paths may still be very high loss. And with lower gain antennas, raw power becomes more critical.

Class dismissed. (Tell your PhD friend there is more to system engineering than just the Physics)

[UPMan]

You should also note that the type of stations mentioned are intended to be received world-wide and not just locally. If you want sufficient signal to make it to a receiver 9,000 miles away when the signal will get there basically by bouncing around (off the upper atmosphere and earth), then you need lots of power.

VHF, UHF, and higher don't bounce off the atmosphere (except in relatively rare conditions) so are almost always intended to be received locally (i.e. within a 2- to 30-mile radius). You don't need as much power to have sufficient signal for reception at 30 miles as you do at 1,000+ miles.

[k9rzz]

Antenna efficiency is well below 1% in many cases. You can Google as well as me, but:

ON7YD, longwave, 136kHz, antennas

Assume we feed a short vertical antenna with a radiation resistance of 0.04 ohmsand a loss resistance of 60 ohms with a power of 200W. The efficiency of the antenna system is 0.067% or -31.8dB (0.04/60). This means that from the 200W transmitter power there will be 133mW radiated (EMRP). As the gain of a short vertical is 2.62dBd (x 1.83) the ERP is 244mW. This means that the antenna system and transmitter as described here will produce the same signal strength as a power of 244mW sent into a perfect 1/2 wave dipole. The EIRP of this station will be 400mW

[lanbergld]

Ok, so efficiency of the system as a whole. Easier to have high gain receiver antennas for the higher frequencies. Makes sense to me. Thanks guys.

But about area coverage...I would think that HF stations cover a larger area than LW broadcasters do. But in contrast to HF stations, the LW ones have their own (massive) power plants. I've got a book at home with pictures of those, including figures for the huge amount of power they consume. It just doesn't explain why they use that kind of power. So, after that and other searching, I decided to post my question here.

Thanks again.


Larry Lanberg

Richmond VA

[N_Jay]

Power In X Efficiency = Power Out

Is the question, "Why do they need such high power out?"
or "Why are they (you assume) less efficient?"
or, "Could the same be accomplished with less power in another band?"

If we don't know the question, you can't expect an answer.

[Apache_UK]

Not only, but also -

Generally (important word that!) LF systems 'tend' to be designed to be less directional that UHF or Microwave systems. Once you focus energy, you need less power from the Tx because you have gain in the antenna.

Slightly related, in EMC Testing it is MUCH easier to generate high field strenghts at microwave frequencies than it is at HF frequencies. ie - we struggle to acheive 200V/m at 30MHz, even with 5kW at our disposal, however, we can acheive 3000V/m at spot frequencies around 2.5MHz upwards with only a couple of hundred watts in our reverb chamber.

[wa8vzq]

The most common reason is low radiaton efficiency. A 1/4 wave vertical at 100 KHz is about 2460 ft tall. A station like NSS was in the 21 KHz range. So that antenna would have needed to be about 11,700 feet tall! When an antenna is made physically shorter the radiation resistance drops extremely quickly. The radiation efficiency of the antenna system = 100 * Rr/(Rr + Rloss). So the RF signal from the transmitter is divided between the radiation resistance of the antenna (that's the part that reaches the receiver) and the various loss resistances (that's the part that heats the ground). Even when the various losses are kept low, they are usually still much larger than the radiation resistance.

When in the military in the early 70's, I worked on a 100 KW LF transmitter operating below 60 KHz. The antenna in that case was a little different because it was buried but radiation efficiency was the reason for the higher power. We communicated between stations in Cutler, ME and Silver Creek, NE. The station in NE had an antenna that was in the vicinity of 1200 ft tall. I understand that it was decommisioned and torn down years ago.

Also stations operating MF and below rely of groundwave propagation. Ground waves work best when vertically polarized. When hroizontally polarized, the surface wave component of the ground wave is severely attenuated. Simply stated when horizontally polarized, the earth's surface shorts out the E field. The H field disappears too since they are mutually dependent. So the signal doesn't travel very far. That's the reason that AM broadcast stations use a vertical antenna; otherwise the coverage area would be a lot less. It has less to do with generating an omnidirectional signal, contrary to popular belief.

Dan

[gcgrotz]

Originally Posted by wa8vzq

The most common reason is low radiation efficiency.

....stations in Cutler, ME and Silver Creek, NE. The station in NE had an antenna that was in the vicinity of 1200 ft tall. I understand that it was decommisioned and torn down years ago.

Also stations operating MF and below rely on groundwave propagation.

Dan

I deleted a bunch of your quote Dan, but you said what I was going to say. It is all about efficiency, both transmit AND receive.

I think the tower in Cutler is still there, I read something about it recently and looked it up on Google earth. There are still a lot of towers at that site. Don't know about NE.

Groundwave propagation at LF eliminates depending on the ionosphere and is reliable day and night, and can also penetrate under ground and under water to some degree. I think at one time there was an underground antenna that the Navy operated in MI or MN someplace, I think it was 14KHz.

Finally, with the wavelengths in use, I would think that a directional antenna would be, for all practical purposes, impossible. I guess you could put up a trio of 2000 footers...

[wa8vzq]

Thanks George - I believe that you are thinking of the Clam Lake, WI site. The operating freq was much lower though, in the 76 Hz range. I remember reading an article about it in about 73 that stated that the antenna current was in the vicinity of 300A! The facility were I was at was at in south central PA just a whimperer in comparison. Here is an interesting article: http://www.fas.org/nuke/guide/usa/c3...ke_elf2003.pdf

[k9rzz]

Here's my chance to put a plug in for my Clam Lake video: YouTube - Shortwave DXpedition - Project: Clam Lake This was shot just north of Clam Lake in a little camp ground.

Longwaves also propagate off the ionosphere too though, E skip at least, because they fade in at sunset just like the other lower bands.

[nickcarr]

Originally Posted by lanbergld

I've proposed several reasons to my coworker, including the size of antenna required, but he keeps coming back with the same argument about the low energy in the waves themselves. So I've been trying to find the answer -- Why low frequency stations require the incredible amount of power that they do.

Sorry I didn't have time to read all of the replies but this is not hard to understand - it's just better to visualize it.

Think about ripples in a pond. If you drop in a large rock then it forms a large ripple wave. Drop in a small rock and a smaller (faster) ripple forms.

Larger waves (i.e. waves of longer wavelength) require more energy to reinforce the existing wave - thus to transmit farther distances. Higher frequencies have much smaller waves that don't need as much power to reinforce the existing wave as it's many times smaller.

[DaveNF2G]

I'm guessing that the original question applied to BROADCAST stations rather than utility stations. Given similar wavelengths, broadcasters will use more power than utilities. The main reason seems to be audio fidelity at the receiver. If you want your audience to hear music that sounds reasonably like music, then you need to ensure that their receivers get lots of signal.

Utility stations aren't as concerned about how "nice" the signal sounds at the other end.

Digital communications require even less power ceteris parabus, especially when error correction is included in the data being sent.

[kb2vxa]

There are two factors involved here that have barely been touched on correctly at least, propagation and antenna efficiency. First, distant LW and MW transmitters are only heard at night because of D layer absorption during and slightly after daylight hours. When the D layer dissipates absorption is much less so you can hear beyond the ground wave coverage area. When you get down into the SLF region it's entirely ground wave and don't forget a signal traveling along a boundary layer penetrates above and below it which allows communication with submarines and the like. Following the boundary layer between earth and sky signals can go around the world day or night quite independent of ionospheric effects.

Then there is antenna efficiency. The longer the wavelength the longer the antenna must be to resonate, LF and SLF antennas are MILES long. Being horizontal and looking like power lines that's not really a problem but there are other designs quite unlike the familiar broadcast towers although they may look alike. WWVB is a good example, that antenna is a variant of the Marconi T supported between several towers that only support the wires that comprise the antenna itself. Many other stations use this design that amounts to a giant capacitor. The umbrella is a similar design supported by a single tower with wires sloping downward to near ground level looking like its namesake. Often the resonant frequency is lowered by a series base inductor, this design goes way back to the beginning of wireless telegraphy. There are others such as loaded towers but I'm not writing a book. (;->) Getting to the point, radiating efficiency is sorely lacking and to overcome it severe power is used, the brute force method usually averaging 200KW or so. Taking an educated guess I'd say a quarter wave tower would fit the bill but I don't think the occupants of the Petronas Towers would enjoy somebody using them for antennas and then there is the nagging question; are they high enough? (;->)

Now we come to the receiving end where antennas are the same but separated from the transmitter by many miles although they're part of the same station. One good example was Tuckerton Wireless, an early ELF station with the transmitter on Tucker's Island in southern NJ and the receiving site on Long Island, NY. You need a huge antenna to receive too, that's why submarines use trailing wires miles long like early aviation antennas only really big. So in case you're wondering why you have reception problems there's your answer, you simply have an inadequate antenna. On the other hand LW broadcasting works well in Europe using ordinary car radios and portables because when you stand in the beam of a searchlight you can't miss it wearing welder's glasses. By the time the signal gets way over here you're trying to see a flashlight on the moon with night vision goggles.