50KW at 540 vs. 1600

[dustintv]

I hope this is the right place to ask. Can someone explain to me why lower frequenices often travel farther than higher frequenices? Compare two 50kw stations, WFAN 660 and WQEW 1660 and notice WFAN's signal can travel further than WQEW's. Also look at http://www.radio-locator.com/cgi-bin/pat?call=KFYR&service=AM&status=L&hours=D]5kw KFYR 550's huge coverage area![/url] Why is this?

 

[jammerdave]

Lower frequencies always travel further for a given power level and a decent antenna system. The station on 1660 that you mention can only be 10kw day, though. Expanded band stations are 10kw day, 1kw night.

dave

 

[amfmsw]

WQEW is fifteen-sixty, not 1660. It is 50wk directional, which explains why it's "pulled in" over land. It does fly down the coast to here in South Jersey coast at night.

But dustin's right, the lower frequency is a longer wavelength, and travels further on same given wattage on similar frequency tuned antennas and ground. I'd rather have a 5kw at 560 than 50 kw at 1560 anyday.

There's less skywave in fringe as well. Remember, get past the end of the AM band, and you're in Short Wave territory! Skywave is king there.

Get below 530AM, and you're in the International Long Wave band, and old "Marine Band". A few hundred watts at 300Kc in Virginia or Cape May could be picked up by boats for navigaion from New Foundland to the Carribean. Google Loran.

 

[Info-warrior]

The ground wave of lower frequencies travels further because the longer wavelenghts hug the curvature of the earth better, thus their 'radio horizon' is much greater than say 2GHz which travels probably only as far as you can see from the antenna site.

 

[rfry]

Can someone explain to me why lower frequenices often travel farther than higher frequenices?
___

For the same radiated power and earth conductivity, groundwave signals on lower frequencies have less propagation loss than on higher frequencies.

For example, when 10 kW is radiated by a 1/4-wave broadcast monopole, the distance to the 5 mV/m groundwave contour over a terrain path conductivity of 4 mS/m is about 37.8 miles on 540 kHz, but only 11.9 miles on 1600 kHz.
//

 

[wkbam1690]

I hope this is the right place to ask. Can someone explain to me why lower frequenices often travel farther than higher frequenices? Compare two 50kw stations, WFAN 660 and WQEW 1660 and notice WFAN's signal can travel further than WQEW's. Also look at http://www.radio-locator.com/cgi-bin/pat?call=KFYR&service=AM&status=L&hours=D]5kw KFYR 550's huge coverage area![/url] Why is this?

KFYR has two things going for it...a low frequency and being located in some of the best ground conductivity in the country (30). Toward the west the conductivity drops to 8 (IIRC), resulting in the pattern not being circular, even though they are non-directional in the daytime. Just another reason the AM band is much more interesting than the FM band...

 

[LA_Guy]

I once saw a chart that compared power level vs frequency for the same signal strength at a certain distance. 250 watts on 540 and 25 Kw on 1600 were the same.

That's a 20 db diffference.

 

[rfry]

I once saw a chart that compared power level vs frequency for the same signal strength at a certain distance. 250 watts on 540 and 25 Kw on 1600 were the same. That's a 20 db diffference.

It is not quite that bad. A radiated power of 250 watts on 540 kHz produces a groundwave field of 5 mV/m at a radius of 14.6 miles, for a path conductivity of 8 mS/m.

It takes about 4.2 kW to do that on 1600 kHz (other parameters the same).

Radiating 25 kW on 1600 for these conditions moves the 5 mV/m contour out to 21.7 miles.

These values are based on the FCC's MW propagation curves for the frequencies and conductivity.
//

 

[jd]

Expanded band stations are 10kw day, 1kw night.

That's the way it started out, dave. The original plan for the expanded band also called for stations to operate non-directional, but exceptions have been made.

WWRU 1660 Jersey City NJ is 10kW unlimited and KDIA 1640 Vallejo CA has a construction permit to increase their nighttime power from 1kW to 10kW. In addition KHPY 1670 Moreno Valley CA runs 10kW day and 9kW at night. WWRU and KHPY use directional antennas and the KDIA CP calls for them to be directional as well. The reasoning behind all this centers on the fact that much of the signal for a coastal station might be lost at sea without an offsetting directional pattern. That being so, it has been argued, the stations could use higher power at night since they would already be affording protection to co-channel stations by being directional. In the case of KHPY, though, considering it to be a coastal station is pushing it since their transmitter site is a little over 45 miles inland.

There's one more exception to all this, and it's nowhere near a coastline: KFXY 1640 Enid OK runs the standard 10kW day/1kW night but has a directional pattern that throws a pretty good signal into Oklahoma City.

 

[greenboy]

I have a question relating to this subject. I am in the process of setting up my part-15 AM station, and I have always noticed that
the lower end of the band always travels farther than the upper band at lower power. I know that with 1/10Th of a watt and such
a small antenna length everyone says the upper band is is better, but I know of two TIS stations one on 530 and one on 1610, they
are both about 10-15 miles from my house and I can usually pick up the station on 530 and can never pick up the one on 1610.
I know both stations are 10 watts. So is it possible to do a part 15 on the lower end of the dial and get you're signal to travel
farther than at say 1660 AM?

 

[rfry]

So is it possible to do a part 15 on the lower end of the dial and get you're signal to travel farther than at say 1660 AM?

Assuming that the goal is to be a "community broadcaster," then the most important consideration is to maximize the far field. In that respect the upper frequencies on Part 15 AM have a definite advantage, as antenna system radiation efficiencies up there are higher. Here are some numbers to illustrate.

For a 3-meter, series-fed, base-loaded, vertical monopole with its base at earth level, 30 ohms total coil and r-f ground loss, and 50 mW of applied power:

At 540 kHz, ERP = 0.0202 mW, and the inverse distance groundwave field at 1 km is 0.043 mV/m.

At 1650 kHz, ERP = 0.1878 mW, and the field at 1 km is 0.130 mV/m.

The distance of 1 km was chosen to get well into the far field, yet be well within the coverage radius one might hope to achieve.

Ground conductivity begins to reduce the inverse distance field value of the groundwave at 1 km, and those losses increase with frequency. But for most such short-path situations that would not be enough to favor the 540 kHz system. A groundwave propagation path long enough for conductivity losses to favor the 540 kHz signal would mean that the amount of 540 kHz signal left would be too low to be useful, anyway.

So while neither frequency in the above comparison provides the robust signals probably wished for, it is clear that the system on 1650 kHz likely has the better performance for the purposes of Part 15 AM.

Another factor here is that many home AM receivers have better r-f sensitivity at high frequencies than low ones.
//

 

[CatFM]

Here is something you might find interesting from an old NRC Night Pattern book:

5000 watts on 550 kHz would have similar coverage to 50,000 on 1000 khz, or about two million watts on 1600 kHz.

A station with 50,000 watts on 1290 kHz would have slightly less coverage than 1000 watts on 550 kHz.

Using 50,000 watts on 1600 kHz would develop almost identical coverage to that of a 250 watt station on 550 kHz.

 

[rfry]

Here is something you might find interesting from an old NRC Night Pattern book: ... Using 50,000 watts on 1600 kHz would develop almost identical coverage to that of a 250 watt station on 550 kHz.

Sorry, CatFM, but with all conditions equal except for frequency and power, your statement is NOT supported by the relevant FCC MW propagation curves.

This conclusion is easy to verify with reference to those FCC curves, if you are able to access and use them accurately.

RF

 

[MarkAndrews]

I remember hearing those same comparisons that CatFM pointed out. Worked for a daytimer while still in high school that put out one of those comparisons in a sales piece for prospective clients.

Obviously, these measurements have been updated since 1970!

 

[littlejohn]

Back to the original question, regarding part 15. Read Mr. Fry's analysis... Part 15 severely restricts the ratdiator. That's going to make significantly more difference than the difference in propogation by frequency across the band. Could we see the figures for 1710KHz please? I bet it would be the weapon of choice for a Part 15 operation.

 

[rfry]

Could we see the figures for 1710KHz please?

For the same parameters as before:

ERP = 0.2016 mW
Field at 1 km = 0.135 mV/m

Of course a different set of Part 15 applies to the use of 1710 kHz (see below), so this system above would not be in compliance.

+ + +

Sec. 15.223 Operation in the band 1.705-10 MHz.

(a) The field strength of any emission within the band 1.705-10.0
MHz shall not exceed 100 microvolts/meter at a distance of 30 meters.
However, if the bandwidth of the emission is less than 10% of the center
frequency, the field strength shall not exceed 15 microvolts/meter or
(the bandwidth of the device in kHz) divided by (the center frequency of
the device in MHz) microvolts/meter at a distance of 30 meters,
whichever is the higher level. For the purposes of this section,
bandwidth is determined at the points 6 dB down from the modulated
carrier. The emission limits in this paragraph are based on measurement
instrumentation employing an average detector. The provisions in
Sec. 15.35(b) for limiting peak emissions apply.
(b) The field strength of emissions outside of the band 1.705-10.0
MHz shall not exceed the general radiated emission limits in
Sec. 15.209.

 

[menotti1]

What would be the best A M frequency for a 10 watt Travelers info station? thanks

 

[CatFM]

Here is something you might find interesting from an old NRC Night Pattern book: ... Using 50,000 watts on 1600 kHz would develop almost identical coverage to that of a 250 watt station on 550 kHz.

Sorry, CatFM, but with all conditions equal except for frequency and power, your statement is NOT supported by the relevant FCC MW propagation curves.

This conclusion is easy to verify with reference to those FCC curves, if you are able to access and use them accurately.

RF

As I mentioned in my posting, that information came from an old NRC Night Pattern book. The copyright date was 1975. I doubt that the authors and editors of their publications are engineers. Still, it seems like they would verify the accuracy of the material they publish. There are several graphs and tables with data, as well as text explaining the information in less technical terms, which is part of what I quoted. There is also a table that may be similar to what Mark Andrews mentioned. This one was used on the rate card for WMBS (590) in Uniontown, PA. It could be the same one he saw if someone distributed it for stations with the more desirable lower frequencies to use on their rate cards back then.

 

[xmtrland]

I just installed A TIS here on Vashon Island. We tested both 540Khz and 1650Khz. While the 540 did skip better 5-7 miles away. The 1650Khz was better able to provide a listenable signal within the 2-3 mile radius of a typical TIS service area. 540Khz was more susceptible to be interfered by overhead power line buzz. Since we are going to install two more TIS transmitters (all three would be synchronized using GPS) to cover the full length of the island (15 miles long). We felt using 1650 would give better control of each transmitters zone. We didn't have a field strength meter because all the field sets on Vashon (Home of seven am transmitters) were not able to go up to 1650Khz. So we relied on listening test using various car and portable radios. With that test we determined that 1650 did as well as 540 at ten watts in our area.
Just judging the signal on a field set does not take into account how a radio will react with the signal given other environmental variables such as power lines and 60Hz buzz. Or several 50KW blow torches just 3 miles away.

In you are in an open area that has no other transmitters within 20 miles and few overhead poer lines the 540 will probably go farther. But with overhead power lines (even in a rural area) 1650 serviced our needs better than 540. For open long stretches of highway the 540 signal would probably work better. But since our goal was to use 3 synchronized 10 watt transmitters to provide coverage. We didn't want the transmitter from the very north end of the island to skip and interfere with the synchronized transmitter at the south end, or middle for that matter. So I would say 540Khz if you have a real open area, 1650Khz if your in a city, town location or plan on using more than one ten watt transmitter to cover your area

Our TIS is installed on a 30 foot telephone pole with the Morad antenna (made here in Seattle) at the top. We did install a more robust ground system than is typically used for TIS installations.
Right now we just have one of the TIS transmitters running. In the next 6 months we hope to have the other two synchronized units running.

Unfortunately I did not include the TIS installation in my Vashon Tower Tour. Once you see six 50KW plants. The ten watt TIS just didn't seem that impressive.

If any one does travel to Vashon or Seattle bring your field set. We have KVI at 570Khz running 5kw ND, KIRO at 710Khz 50KW ND day, and KOMO 1000Khz 50KW ND day. All the transmitters are within 5 miles of each other. Oh yeah and a TIS at 1650khz.

 

[rfry]

What would be the best AM frequency for a 10 watt Travelers info station?

Part 15, Sec 90.242 gives the TIS rules (clip below), which state that a vertical monopole no taller than 15 meters may be used, that the tx output power cannot exceed 10 watts, and that the measured groundwave field at 1.5 km cannot exceed 2 mV/m.

With a 15-m monopole a TIS on 530 would just meet the inverse distance field strength of 2 mV/m at 1.5 km with 10 watts, if the total of the matching and r-f ground loss was not less than 30 ohms.

On 1700 kHz with 20 ohms of coil and ground loss and a 15-m monopole, the transmitter could not have more than 800 milliwatts of matched output power without exceeding the field strength limit at 1.5 km.

These calculations were made for a perfect ground plane. Typical ground conductivities will enter in, and the tx power for the 1700 kHz system would be a little higher without exceeding the 2 mV/m limit at 1.5 km. Or if the coil/ground loss was greater, then more tx power could be used to produce that 2 mV/m field.

The 530 kHz system already is maxed for legal tx power, but the 2 mV/m limit might be reached over real ground by reducing the loss in the coil and r-f ground.

So "best" depends on the design goal. Tradeoffs can be made within certain limits.

If best means greatest coverage area, then probably the systems at the low end of the band are best, as long as the higher r-f noise level down there is tolerable.

+ + +

(4) For a station employing a conventional radiating antenna(s) (ex.
vertical monopole, directional array) the following restrictions apply:
(i) The antenna height above ground level shall not exceed 15.0
meters (49.2 feet).
(ii) Only vertical polarization of antennas shall be permitted.
(iii) Transmitter RF output power shall not exceed 10 watts to
enable the user to comply with the specified field strength limit.
(iv) The field strength of the emission on the operating frequency
shall not exceed 2 mV/m when measured with a standard field strength
meter at a distance of 1.50 km (0.93 miles) from the transmitting
antenna system.