Low Band AM

[rye]

I have often been curious as to why low band AM (lets say 540) travels so much further than higher-up frequencies (like 1700). The audio also seems to sound better, but i have not done anything more than causual listening tests.

I heard somewhere that the lower the band the more bandwidth available. Is this true? Why is low band better?

Lastly, is the a better side of the FM band as well or is every frequency pretty much equal?

 

[rbrucecarter]

> I have often been curious as to why low band AM (lets say
> 540) travels so much further than higher-up frequencies
> (like 1700). The audio also seems to sound better, but i
> have not done anything more than causual listening tests.

It is all atmospherics. Incidentally, you would be surprised how well the expanded band stations do in the daytime using DX equipment. 1000 mile plus is commonplace. The expanded band is getting close to shortwave propagation, which is probably why it was wise to limit daytime operation to 10kW,

> I heard somewhere that the lower the band the more bandwidth
> available. Is this true? Why is low band better?

No. The bandwidth of your receiver is determined by the IF strip, unless you are using a very old tuned RF radio from the 1930's, or a crystal set - in which case it is true.

> Lastly, is the a better side of the FM band as well or is
> every frequency pretty much equal?

When FM tuners were analog in architecture, the trim capacitors on the front end tended to be cranky, making the lower end of the band much better for coverage. Unless there is skip, all FM frequencies are pretty much equal these days. Skip tends to favor lower frequencies over higher.

 

[PhilJSmith67]

> > Lastly, is the a better side of the FM band as well or is
> > every frequency pretty much equal?
>
> When FM tuners were analog in architecture, the trim
> capacitors on the front end tended to be cranky, making the
> lower end of the band much better for coverage. Unless
> there is skip, all FM frequencies are pretty much equal
> these days. Skip tends to favor lower frequencies over
> higher.

Oh yeah, very true... I also remember those cheepies made in the 70s where the dial was tremendously alogarithmic, with the high end spread wide. I had a RatShack/Realistic radio where the slide rule looked something like this:

88--92---96---100----104----106----------108

Both sensitivity and selectivity were atrocious at the top end, even though the stations "appeared" to be spaced farther apart.

 

[xoanon]

Ground-wave losses are lower at the bottom of the band, so signals will propagate further during the day at 540 than at 1700. However, sky-wave is better at the top of the band, for nighttime DX.

 

[tpt]

The audio on the high band stations (back to talking about AM, of course) can often be better than a low band station because there is less change in reactance +/- 10 kc at the antenna on, let's say, a 1490 station than a 690 station.

On the other hand, one has to compare apples to apples, since many other factors affect the bandwidth of a given antenna system, not to mention the audio quality of the station up and out of the antenna terminals of the AM transmitter.

 

[pianoplayer88key]

> > > Lastly, is the a better side of the FM band as well or
> is
> > > every frequency pretty much equal?
> >
> > When FM tuners were analog in architecture, the trim
> > capacitors on the front end tended to be cranky, making
> the
> > lower end of the band much better for coverage. Unless
> > there is skip, all FM frequencies are pretty much equal
> > these days. Skip tends to favor lower frequencies over
> > higher.
>
> Oh yeah, very true... I also remember those cheepies made
> in the 70s where the dial was tremendously alogarithmic,
> with the high end spread wide. I had a RatShack/Realistic
> radio where the slide rule looked something like this:
>
> 88--92---96---100----104----106----------108
>
> Both sensitivity and selectivity were atrocious at the top
> end, even though the stations "appeared" to be spaced
> farther apart.
>


You think FM was skewed... check out AM...

53------------60----------80-------100----130---170

 

[PhilJSmith67]

> > radio where the slide rule looked something like this:
> >
> > 88--92---96---100----104----106----------108
> >
> > Both sensitivity and selectivity were atrocious at the top
>
> > end, even though the stations "appeared" to be spaced
> > farther apart.
> >
>
>
> You think FM was skewed... check out AM...
>
> 53------------60----------80-------100----130---170
>

True... Although, if you replaced those numbers with wavelengths, it would be considerably more linear. The FM scale would still be skewed even if marked with wavelengths.

 

[CW]

> > > radio where the slide rule looked something like this:
> > >
> > > 88--92---96---100----104----106----------108
> > >
> > > Both sensitivity and selectivity were atrocious at the
> top
> >
> > > end, even though the stations "appeared" to be spaced
> > > farther apart.
> > >
> >
> >
> > You think FM was skewed... check out AM...
> >
> > 53------------60----------80-------100----130---170
> >
>
> True... Although, if you replaced those numbers with
> wavelengths, it would be considerably more linear. The FM
> scale would still be skewed even if marked with wavelengths.
>
Nope...it is skewed because of the tuning circuits and the % of change of freq vs the % change of the component values....(capacitors mostly in this case).
Wavelength is merely 300/freq in MHz so the number would be different but the scale would remain the same...

 

[PhilJSmith67]

> > > > radio where the slide rule looked something like this:
>
> > > >
> > > > 88--92---96---100----104----106----------108
> > > >
> > > > Both sensitivity and selectivity were atrocious at the
>
> > top
> > >
> > > > end, even though the stations "appeared" to be spaced
> > > > farther apart.
> > > >
> > >
> > >
> > > You think FM was skewed... check out AM...
> > >
> > > 53------------60----------80-------100----130---170
> > >
> >
> > True... Although, if you replaced those numbers with
> > wavelengths, it would be considerably more linear. The FM
>
> > scale would still be skewed even if marked with
> wavelengths.
> >
> Nope...it is skewed because of the tuning circuits and the %
> of change of freq vs the % change of the component
> values....(capacitors mostly in this case).
> Wavelength is merely 300/freq in MHz so the number would be
> different but the scale would remain the same...

That's exactly right, which is why that old radio had such an alinear scale... What I meant to say was, the FM scale on *that* radio (above) would still be whacked, even if it were marked in wavelengths. I have a couple newer analog receivers where the frequencies on FM are spaced more uniformly.