50 watt radio range

October 12, 2006 12:46 AM

What is the maximum range of a 50watt VHF FM radio?

The specific model I am looking at is a Kenwood TM-241a. The specifications say that it is a 50 watt transmitter, but I cannot figure out what that means in total range. Is there an equation that calculates range (in km) from watts?

posted by afu to technology (10 comments total)

That rig puts out 50W at the antenna output, which is only a small part of overall range. How far that 50W will go is significantly shaped by your antenna system's design (The more gain your antenna has, the more effective power it will radiate, and the longer your range will be), the terrain around you (hills block VHF/UHF), your elevation (higher = better), and the sensitivity of the receiving system.

VHF FM is virtually always limited by line of sight (and 25W into a decent antenna is plenty to reach the end of LOS unless you're up a big hill or have a tall tower). The higher you can get your antenna, the longer your range will be (I used to frequently work 100+ miles with a 5 watt handheld -- from a 29th floor apartment balcony using a 3 element yagi).

In practice, 50W into a decent whip on a groundplane (you know, like a 1/4 wave spike on the roof of your car) will get you comfortably into most repeaters within 20-50 miles (assuming they're in good high locations). A multi-element, rotatable yagi up in the air (even on your chimney) will get you much further.

Can you describe where you're planning on putting your antenna, and perhaps how far you want to be able to transmit?
posted by toxic at 1:21 AM on October 12, 2006

That depends significantly on the gain of your antenna.

For example, using extremely high-gain directional antennas some people have got low power WiFi to transmit over many tens of miles whereas with an omnidirectional antenna you'd be lucky to get a few hundred metres.

Some more, technical, info here and here.
posted by wackybrit at 1:23 AM on October 12, 2006

It's much more complicated than that.

The effective range depends on more than just the transmitter's power amplifier rating. It depends on the characteristics of the transmitting antenna -- size, shape, directionality, construction; on the distance between the power amplifer and the antenna and the type of cabling used to feed the antenna; on the environmental factors, such as the presence of mountains, trees, tall city buildings; on the sensitivity of the receiving device's front-end as well as the size, directionality, et al. of its antenna; the selectivity of the receiving device's tuner; the presence or absence of interference; and so on.

There is a reason that the ARRL Handbook is over 1200 pages. I'm not a ham so I can't actually help in answering your question, but I just want to point out that much more information would be necessary in order to work out a range.
posted by Rhomboid at 1:24 AM on October 12, 2006

Here is an example of this in action - take standard Wi-Fi (802.11g) which has a "nominal" range of 100 feet indoors. But with a highly directional antenna, a sensitive tuner, and a little environmental luck you can extend that to literally miles.

The Wi-Pod has been used to connect line-of-sight to a single access point at a distance of 4.2 miles! This was a direct shot from one location to another, but is a simply amazing feat considering the router was a stock, off-the-shelf unit with the original rubber duck antenna installed!

posted by Rhomboid at 1:34 AM on October 12, 2006

The range of your FM radio is infinite. Any signal you broadcast will eventually reach the edge of the Universe.

The useful range is determined, as people are saying, by the frequency, the transmitting antenna, the receiving antenna, and the receiver's sensitivity. There is a noise floor in all receivers, and the signal must be enough louder to reliably determine its content.

Basically, you'll need spec sheets on your receiver and both antennas to get a first-order approximation, and then you can experiment from there.

With good antennas, I'd expect a 50w radio to carry for a great distance. Using highly directional antennas and careful aiming, they've taken 250mw 802.11 signals for 40+ miles. Your signal is 200 times stronger, so it ought to go something like 14 times further, based purely off the inverse-square law. There's probably about a billion reasons why it won't go that far, and any real radio engineer is likely doubled over laughing by now, but I'd guess 25 miles would be doable without overwhelming expense.
posted by Malor at 2:41 AM on October 12, 2006

Thanks for all the great information. I really have no idea about this, my boss just told me to find a radio that could had an effective range of 20km.

They will be used for communication between two boats on the yangtze river, that will be a maximum of 20km apart (yeah I know the boats should already have radios, this is china). The boat is about 20m tall.

The person at the store said the kenwood unit was the only one that they had that would communicate over distances like that, but from what you all are saying that might not be the case. Are their any handheld sets that would be able to reach 20km?
posted by afu at 6:43 AM on October 12, 2006

Handhelds usually run about 5w max. 20km is doable, again, depending on the antenna and terrain. Given that you probably want your antenna on your handheld and that you'll be navigating a river means no, 20km isn't likely. You'll have a little rubber antenna and hills, trees, curves in the river, etc to interfere.

You said the boat is about 20m tall. Is that a typo? I don't know much about the Yangtze, but that if I've heard of it, it must be a large waterway. But even so, 20m is about 65ft and that's one *very* tall craft. Nevertheless, if you have two boats with aerials that high, that 50w Kenwood just might do what you ask of it.
posted by kc0dxh at 8:13 AM on October 12, 2006

50 W, 12 Miles, VHF, more or less LOS? Piece of cake.

Deviate from Line Of Sight, and there are circumstances where it wouldn't. Rest assured, even the phone doesn't work all the time. Field conditions present unknowns and unpredictable circumstances.

Keep in mind... you are talking about two mobile units. Each one will confront specific local conditions that may impede radio performance. Do not expect 100%.

You may want to consider two sets of radios, (different bands) which would not be a bad idea anyway. Redundancy rocks, especially where a radio repair store is not handy.
posted by FauxScot at 8:46 AM on October 12, 2006

my boss just told me to find a radio that could had an effective range of 20km

Not sure what you're using it for, but the radio you link to is a ham radio, useful only if you've got a ham license. (I'm not sure how China handles this.) Although it can probably be modified out of band, this is illegal, at least here in the US. (If it's legal in China, though, it's probably easier to program than a normal land-mobile radio, which usually requires computer programming.)
posted by fogster at 1:59 PM on October 12, 2006

Not sure what you're using it for, but the radio you link to is a ham radio, useful only if you've got a ham license.

This particular chunk of spectrum is defined and protected by ITU regulation (i.e. International Treaty). There are marine VHF radios and marine HF radios which are designed (and legal) to use on boats -- but the Kenwood that you linked to is not one of them, unless all operators are licensed Hams and following local regulations (which in the US means non-commercial communications only).

The CSRA can give you lots of information about Ham activity in China.
posted by toxic at 4:57 PM on October 12, 2006