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Code of Federal Regulation Title 47, Part 90 Private Land Mobile
Paragraph 90.205 Power and antenna height limits.
VHF and UHF ERP Limitations
Part 90
extract of this
rule paragraph (pdf)
A local html version of the
paragraph.
This rule part lays out the allowable relationship between the height of
your transmit antenna, the amount of ERP (power) of your system, and the
authorized service area radius, for VHF and UHF systems.
There are three parts to this rule that are important
1.
Table 1 VHF
2. Table 2 UHF
3. ERP Formula: ERPallow = ERPmax x (HAATref / HAATactual)2
Generally, the maximum ERP you are allowed for a base station or a repeater
is based on the service area radius that you request, and the height of your
antenna. The "standard maximums" for VHF is 40 kilometers, and 500 watts ERP,
but that is adjusted if you request a service area of less (or more) than 40
kilometers, or if the HAAT
(Height Above Average Terrain) for your antenna is higher than the
"reference HAAT".
For UHF, the "standard maximums" are 32 kilometer service
area, and 500 watts ERP.
Below is a table that summarizes the rules for VHF.
The numbers vary slightly for UHF, but the concept and methods of
calculations remain the same.
| |
Requested Service Area |
Max ERP |
Max (Ref) HAAT |
|
| formula |
|
ERPmax |
HAATref |
|
| |
3 kilometers |
1 watts |
15 |
|
| |
8 kilometers |
28 watts |
15 |
|
| |
13 kilometers |
178 watts |
15 |
|
| |
16 kilometers |
500 watts |
15 |
|
| |
24 kilometers |
500 watts |
33 |
|
| |
32 kilometers |
500 watts |
65 |
|
| |
40 kilometers |
500 watts |
110 |
|
| |
48 kilometers |
500 watts |
160 |
|
| |
64 kilometers |
500 watts |
380 |
|
| |
80 kilometers |
500 watts |
670 |
|
Using the formula from line 3, and the information about
your maximum allowances in Table 1, to determine your max ERP, there are two
things you must figure out first. 1) the HAAT of your antenna and 2) your
requested service area. After you have the answers, you simply plug
them into the formula and it will tell you what your maximum ERP is for any
site configuration.
Examples:
1. You request a service area of 2 kilometers, your
HAAT is 14 meters, the maximum allowable ERP from the Table is 1 watt.
Service Area = less than 3 kilometers
ERPmax = 1 watt (from the Table)
HAATref = 15 meters (from the Table)
HAATactual = 14 meters from your calculations.
When you plug all the figures into your formula you get:
| a. |
ERPallow |
= |
ERPmax |
x |
(HAATref |
/ |
HAATactual)2 |
|
| b. |
|
= |
1 watt |
x |
(15 meters |
/ |
14 meters)2 |
|
| c. |
|
= |
1 watt |
x |
1.072 |
|
|
|
| d. |
1.1449 watts |
= |
1 watt |
x |
1.1449 |
|
|
|
Your calculations say that you should be able more than 1
watt. However, the Table limits you to a maximum of 1 watt.
Therefore ... your ERPmax is 1 watt
2. You request a service area of 7 kilometers and
an ERP of 100 watts. Your HAAT is 13 meters.
Service Area = less than 8 kilometers
ERPmax = 28 watts (from the Table)
HAATref = 15 meters (from the Table)
HAATactual = 10 meters from your calculations.
| a. |
ERPallow= |
= |
ERPmax |
x |
(HAATref |
/ |
HAATactual)2 |
|
| b. |
|
= |
28 watt |
x |
(15 meters |
/ |
10 meters)2 |
|
| c. |
|
= |
28 watt |
x |
1.52 |
|
|
|
| d. |
63 watts |
= |
28 watts |
x |
2.25 |
|
|
|
Your calculations say that you should be able to have
63 watts ERP. However, the Table limits you to a maximum of 28 watt.
Therefore ... your ERPmax is 28 watts
3. You request a service area of 40 kilometers and
an ERP of 200 watts. Your HAAT is 98 meters.
Service Area = 40 kilometers
ERPmax = 500 watts (from the Table)
HAATref = 110 meters (from the Table)
HAATactual = 98 meters from your calculations.
| a. |
ERPallow |
= |
ERPmax |
x |
(HAATref |
/ |
HAATactual)2 |
|
| b. |
|
= |
500 watt |
x |
(110 meters |
/ |
98 meters)2 |
|
| c. |
|
= |
500 watt |
x |
1.12242 |
|
|
|
| d. |
629.9 watts |
= |
500 watt |
x |
1.25978 |
|
|
|
| |
|
|
|
|
|
|
|
|
Your calculations say that you should be able to have an
ERP of 629.9 watts, but your limit is 500 watts. Since you are only
requesting 200 watts, you have no problem getting it.
4. You request a service area of 40 kilometers and an ERP
of 600 watts. Your HAAT is 125 meters.
Service Area = 40 kilometers
ERPmax = 500 watts (from the Table)
HAATref = 110 meters (from the Table)
HAATactual = 125 meters from your calculations.
| a. |
ERPallow |
= |
ERPmax |
x |
(HAATref |
/ |
HAATactual)2 |
|
| b. |
|
= |
500 watt |
x |
(110 meters |
/ |
125 meters)2 |
|
| c. |
|
= |
500 watt |
x |
.882 |
|
|
|
| d. |
387.2 watts |
= |
500 watt |
|
.7744 |
|
|
|
While you requested 600 watts, the maximum ERP that you
are authorized is 387.2 watts.
It is possible to get a higher ERP than allowed by the
table, but it depends on several factors, and you must present a
special ERP study and justification,
and the blessings of an authorized frequency coordinator before the FCC will
consider granting it.
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