[AMRadio] FCC's AM power


Jim Candela jcandela at prodigy.net
Wed Sep 28 07:45:31 EDT 2016


As a Novice back in the early 70's I was always thinking that if I dipped the final at > 75 watts DC input, I'd have a knock on the door from the FCC, and they'd confiscate my equipment and then arrest me. 

Looking at the issue today, it seems to me that a few DB in power to the antenna is offset with the variability of propagation which is in the 10's of DB; maybe 50 db on 75m between 5 PM and 8 PM. Therefore whether you are running 250 watts or 750 watts AM, the resultant difference is hardly noticeable unless your signal at the lower power is being received at the receiver threshold.
With the advancement of SDR receivers, one can tell at a glance relative signal strength, and spectrum purity over the frequency width of the pan Adapter display. The guys causing trouble are the ones that have a dirty signal which spread out both sides of the center frequency due to harmonic distortion and intermodulation distortion products. A 100 watt rig over driven can cause far more harm than a clean rig running greater than the legal limit.
I am always amazed when I see Don K4KYV on my SDR Pan Adapter. Don is often one of the stronger signals seen on the band, and although strong, his AM signal is very clean...in fact, narrower than many SSB signals seen on the band. The main reason he is strong is because of his antenna and radial system.
To conclude, if your are running QRO, have a nice clean signal, and you will do little harm to the adjacent signals on the band. By clean, limit the audio bandwidth to something like 3.8 Khz, but more importantly, make sure the audio/RF chain are not overdriven. Relying on ALC from a linear back to the exciter is pretty much a guarantee transmitting a broad signal. The ALC during the attack time constant cannot throttle back the exciter fast enough.
JimWd5JKO
 

    On Wednesday, September 28, 2016 5:01 AM, Rob Atkinson <ranchorobbo at gmail.com> wrote:
 

 I have a Bird 43 with the add on circuit for the peak reading function
with the 9 v. battery inside.  Not long after I got it (at a hamfest)
I removed the battery and put it back into regular Bird 43 mode
because I thought the peak reading thing was a waste of time and I
didn't want to forget about the battery and find it having leaked all
over 10 years or more later on.

I also measure power when I need to, with RF amp meters into known
measured resistances.  But RF amp meters are not always accurate.

The more you get into power measurement the more you realize you are
chasing an illusion if you are trying to get some exact measurement.
Unless you have costly scientific lab apparatus you mostly have to
take readings from a few sources and average them to get the best
approximation.  Nowadays my concern with output power is limited to
guestimating the efficiency of the final PA.

73

Rob
K5UJ
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