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Using a 320' radio tower for *legal* Part 15 AM broadcasting?

J

JasonW

Guest
Hello All,

There is a 320 foot tall AM radio tower (once used by a now-defunct radio station) near me.

The Part 15.209 AM rules, unlike the alternative Part 15.219 AM rules, do not specify a maximum antenna length, but go by Field Strength. For 490 kHz - 1705 kHz, the maximum allowed Field Strength (in microvolts/meter) is 24,000/F(kHz) as measured 30 meters from the antenna.

The tower is a half-wavelength tall at about 1460 kHz and would radiate at over 90% efficiency at that frequency and at nearby frequencies. The tower's ground radial system is largely intact, although radials are less important for voltage-fed, end-fed high-impedance half-wavelength antennas.

Has anyone ever tried using a full-size tower at Part 15 AM output power? (Some AM stations effectively broadcast with as little as 1 watt at night.) Also, what Field Strength Meter(s) could be used to determine Part 15.209 rules compliance (most FSMs won't give accurate readings at such low field intensities)?

Operated as a half-wavelength vertical antenna, the tower would have a low angle of radiation and its signal should reach several miles even at Part 15 AM power levels. There's plenty of room in the tower's ATU "doghouse" for a Talking House or TalkingSign Part 15 AM transmitter and a parallel-tuned LC matching network to match the transmitter's output impedance to the tower. :) -- Jason
 
My guess is that you will have to cut the power back so far to keep the field strength legal that you would get much better coverage operating under the 100mW and 3m antenna section.

You would need a calibrated Field Intensity Meter to determine the power output that you could run legally, and these devices are extremely expensive, $6000+ although if you could find a friendly local engineer to borrow one from that would do it.

Of course, you would have to determine if using something other than the manufactuer's supplied antenna is legal, which in most cases is not the case.





> Hello All,
>
> There is a 320 foot tall AM radio tower (once used by a
> now-defunct radio station) near me.
>
> The Part 15.209 AM rules, unlike the alternative Part 15.219
> AM rules, do not specify a maximum antenna length, but go by
> Field Strength. For 490 kHz - 1705 kHz, the maximum allowed
> Field Strength (in microvolts/meter) is 24,000/F(kHz) as
> measured 30 meters from the antenna.
>
> The tower is a half-wavelength tall at about 1460 kHz and
> would radiate at over 90% efficiency at that frequency and
> at nearby frequencies. The tower's ground radial system is
> largely intact, although radials are less important for
> voltage-fed, end-fed high-impedance half-wavelength
> antennas.
>
> Has anyone ever tried using a full-size tower at Part 15 AM
> output power? (Some AM stations effectively broadcast with
> as little as 1 watt at night.) Also, what Field Strength
> Meter(s) could be used to determine Part 15.209 rules
> compliance (most FSMs won't give accurate readings at such
> low field intensities)?
>
> Operated as a half-wavelength vertical antenna, the tower
> would have a low angle of radiation and its signal should
> reach several miles even at Part 15 AM power levels.
> There's plenty of room in the tower's ATU "doghouse" for a
> Talking House or TalkingSign Part 15 AM transmitter and a
> parallel-tuned LC matching network to match the
> transmitter's output impedance to the tower. :) -- Jason
>
 
> JasonW:
> There is a 320 foot tall AM radio tower (once used by a
> now-defunct radio station) near me.
>
> The Part 15.209 AM rules, unlike the alternative Part 15.219
> AM rules, do not specify a maximum antenna length, but go by
> Field Strength. For 490 kHz - 1705 kHz, the maximum allowed
> Field Strength (in microvolts/meter) is 24,000/F(kHz) as
> measured 30 meters from the antenna. (etc)
______________

Here are some numbers for a Part 15 AM operating under 15.219 (100 mW tx input power, 3-meter radiating structure).

DATA:
Frequency = 1700 kHz
Applied Power = 80 milliwatts (~output power of Part 15 AM tx)
Radiator = 3-meter total length including the conducting path from the tx chassis to the ground plane (antenna is ground-mounted)
Antenna System RF Resistance, Loading Coil plus Ground = 10 ohms
Ground Conductivity = 8.0 mS/m (typical value)
Radiation System FCC Efficiency = 21.12 mV/m at 1 mile for 1 kW applied

RESULTS:
Field Strength > Distance
2 mV/m > 0.09 miles (good signal)
1 mV/m > 0.18 miles (fair)
0.5 mV/m > 0.34 miles (noisy)
0.05 mV/m > 2.5 miles (very noisy)

Using 15.209 the 1,700 kHz field strength allowed at 30 meters is about 14 uV/m, which is far below the values shown above even at the greater distances.

A 1/2-wave radiator on 1,700 kHz produces a field about 20 dB above a 3-meter radiator, other things equal.

Doing the number-crunching on this, the maximum power that could be radiated by your defunct 1/2-wave vertical to just meet the 15.209 limit of 14 uV/m at 30 meters is about 1.79 nanowatts.
 
I thank you both for your replies. Using the tower as a radiator, dropping the frequency to 540 kHz would allow a higher field strength (that's why AM Carrier Current stations prefer the lower frequencies), although the tower would be a less efficient radiator at this lower frequency.

If I can get permission from the tower owner, I may side-mount a stock Talking House or TalkingSign ATU/whip on the tower (although nowhere near the top). It would provide *way* more height than I'd ever need, and as the RF ground the tower would still radiate some (as Bill Blew's tower did with his Rangemaster on it). I'd just connect a copper strap across the tower's "Johnnny Balls" lightning arrestor to DC ground it. -- Jason
 
What you want to do is unhook the tower from any tuning equipment. set the Part 15 unit right at the base of tower and hook to sites ground skirt. place the 3m radiator parralell to tower while not actually touching tower, but close enough to get inductive signal into tower. I have heard of this being done.
 
Dr_Johnnie_Fever said:
What you want to do is unhook the tower from any tuning equipment. set the Part 15 unit right at the base of tower and hook to sites ground skirt. place the 3m radiator parralell to tower while not actually touching tower, but close enough to get inductive signal into tower. I have heard of this being done.
Click to expand...
Using the tower's ground radials for the Part 15 AM transmitter, even without the tower, would certainly help a lot.I've heard of this parasitic tower excitation technique as well. I've never tried it, but it would seem that it might work better if the tower were connected to its ground radial system through either a series-tuned or parallel-tuned LC network to tune the tower/radial system to resonance at the chosen frequency.Mounting the transmitter's 3 meter antenna next to the tower would be like placing a small vibrating tuning fork next to a larger tuning fork of the same musical note so that the larger one would vibrate in sympathy with the small one. -- Jason
 
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