Choosing A Freq.

[bp37918]

I have noticed since I started reading the community radio board that most part 15 AM radio stations are on the upper band area. The reason given was the tx is more effecient at those freq's and lower noise on the upper band. I have always understood that you needed less power on the lower band to go the same distance. My question can you tune a tx to work well on the lower band and have good range? Where I live the lower band is someone quieter, especially at night,
than the upper band.

 

[carlvanorden]

> I have noticed since I started reading the community radio> board that most part 15 AM radio stations are on the upper> band area. The reason given was the tx is more effecient at> those freq's and lower noise on the upper band. I have> always understood that you needed less power on the lower> band to go the same distance. My question can you tune a tx> to work well on the lower band and have good range? Where I> live the lower band is someone quieter, especially at night,> > than the upper band.> Excellent question!.....the answer is actually simple....While it is true that you need less power at the left end of the dial (550kHz), the antenna required to produce any signal at all is way too long for part 15 purposes..........The smaller, legal antenna is better suited toward the "right" side of the dial (1500-1700kHz).......the shorter waves at this end of the dial benefit the part 15 broadcaster simply due to the lack of transmitter power.............most part 15 transmitters will tune every frequency on the commercial medium wave, but you will get less range the more you move away from 1500-1700kHz......hope that answers your [email protected]<P ID="signature">______________
[email protected], [email protected],[email protected]</P>

 

[pbolyn]

First, a 3 meter antenna will radiate 10 times less efficiently at 530kHz than at 1700kHz (radiation resistances: 530kHz=.01 ohm, 1700kHz=.1 ohm).

Second, the coil required to resonate with a 3 meter antenna needs to be very much larger at 530 than at 1700.

Here are coil sizes for both frequencies assuming a 106" long 1/2" dia. copper pipe antenna.:

1700 kHz
--------
Inductance: 295uH
Turns of #16 magnet wire on 3.5" dia. form: 71
Linear length of winding on form: 3.74"
Length of wire needed: 65.8 ft.

530 kHz
--------
Inductance: 3,045uH
Turns of #16 magnet wire on 3.5" dia. form: 539
Linear length of winding on form: 28.5"
Length of wire needed: 501.6 ft.

Since it is generally accepted that the linear length of the winding + antenna length + wires must be less than 3 meters, the 28.5" length of the 530 coil would require shortening the 106" antenna by about 25" which will reduce radiation and actually require an even larger coil. A multi-layer coil with the same wire size or a single-layer coil with smaller wire would reduce the length, but both solutions introduce some additional coil loss.

It is certainly possible to build a 3 meter antenna for 530, but the 10x reduction in radiation resistance and the huge coil make it less efficient and less practical to build.


> I have noticed since I started reading the community radio
> board that most part 15 AM radio stations are on the upper
> band area. The reason given was the tx is more effecient at
> those freq's and lower noise on the upper band. I have
> always understood that you needed less power on the lower
> band to go the same distance. My question can you tune a tx
> to work well on the lower band and have good range? Where I
> live the lower band is someone quieter, especially at night,
>
> than the upper band.
>
<P ID="signature">______________
Phil B
</P>

 

[Russty]

Its true about LPAM doing better on the upper frequencies for Part 15 setups, as well as Free radiate campus systems and even TIS stations. It is also interesting to note, that most of the Carrier Current stations utilize lower frequencies as the electric systems seem to be better suited for these wavelengths.

To Sum up if you are willing to explore the Carrier Current route, then you would be fine with the Lower part of the band. If you want to do a free standing antenna, then you are better off at the top.