[Lowfer] MP 137.780 ON

[lowfer@mailman.qth.net]

Hello Folks,

W1VLF weighing in here with a comment relating to but not exactly about the
topic.

A broadcast engineer told me that he had a rule of thumb that he applied
that said,  All other things being equal a 5 Kw AM braodcast station on 540
Khz would be as well recieved as a 50 KW station at 1600 Khz.
Antennas of course being equal in height in terms of degrees.

This is very unscientific I,  realize but the point was the that surface
wave attenuation at 540 Khz was significantly less than that of 1600 Khz.
It seems then that although it may be more difficult to radiate, and the
same size antenna is less effective, the surface wave losses between to
points(daytime) may make up for some of this?

Any ideas on how much difference this might be?

Just wanted to throw in my 2 cents worth.

Question:  Does Mitch describe his antenna height and top hat arrangement
somewhere?
I have been busy and missed this.

Paul C

W1VLF



----- Original Message -----
From: "Lyle Koehler" <lyle@mlecmn.net>
To: <lowfer@mailman.qth.net>
Sent: Monday, March 31, 2003 11:35 AM
Subject: Re: [Lowfer] MP 137.780 ON


> I've gone through the same exercises as John Andrews to calculate the
> frequency dependence of loop and vertical antennas. Like John, I seem to
> have discarded the scraps of paper on which I made the calculations.
Rather
> than starting over from scratch, I got lazy and used the antenna modeling
> software. Here are the results comparing gains at 8 degree elevation
angles.
>
> 1) LEK vertical with coil Q of 500 and with ground losses of 20 ohms;
> loading coil inductance adjusted for resonance at each frequency.
>
> Gain at 185 kHz = -23.4 dBi
>
> Gain at 137 kHz = -26.1 dBi
>
> Difference = 2.7 dB
>
> 2) 15 meter square loop of 0.5" copper with additional 0.22 ohm fixed
series
> resistance. The fixed resistance is added to account for differences
between
> Bill Ashlock's measured antenna current at 1 watt, and the predicted
current
> based on copper losses in the loop.
>
> Gain at 185 kHz = -26.4 dBi
>
> Gain at 137 kHz = -31.1 dBi
>
> Difference = 4.7 dB
>
> You can plug in different assumptions for loss resistances and get
> considerably different results, but I think these are in the right
ballpark
> for practical antennas. The net result is that both kinds of antenna will
> have lower efficiencies (for a given size) at 137 kHz than at 185 kHz.
>
> As for why Mitch isn't burning up receivers with 75 watts when 1-watt
> signals can be copied at such great distances, welcome to the world of RF.
A
> power ratio of 75 (18.75 dB) sounds like a huge amount. However, noise
> levels and propagation path losses can vary by much more than that between
> winter and summer, or even from day to day. At a distance of a few hundred
> miles, 100 watt signals will be readable more often than 1 watt signals,
but
> not all the time!
>
> Lyle, K0LR
>
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