(Article courtesy of Author)
Sound twice as loud!
If you wanted your signal to sound twice as loud, you must increase your power to 800 watts from 100 watts (9 db = 3 db + 3 db + 3 db = x2 x2 x2 = x8)!
Clearly, increasing power by 20 watts, say from 100 to 120 watts, is not going to make any difference at all to the person receiving your signal. On the other hand, if you cut your power in half from 100 watts to 50 (a 3 db decrease in power), the other operator will hardly notice any drop at all in your signal strength.
So why beat your transceiver into the ground by running it at full power?
If you run at 75 watts instead of 100, your transceiver will run cooler and no one that you talk to will know the difference. There is someone who may notice the difference however, your neighbors. If you are having interference problems, cutting your power level in half could solve those problems without having any noticeable affect on your ability to make contacts.
For example, when I operated on 10 meters at 100 watts, my lawn sprinklers would turn on whenever I keyed my transceiver. When I dropped to 50 watts, the problem went away. Running at 50 watts turned out to be a great water conservation technique.
The table below puts antenna cost verses performance gain somewhat into perspective. This table compares various yagi beam configurations to the performance of a dipole. The table shows the db gain, relative to a dipole, achieved by each of the antennas. The antennas get more expensive as you go down the table. The table also indicates the increase in signal strength observed by the S-meter on a distant transceiver that is receiving your signal.
As an illustration, 100 feet of LMR 400 coax used to connect a transceiver with a 10 meter antenna will produce a loss of 0.7 db. If standard RG-8/X coax is used instead, the loss will be 2.0 db. The difference in loss between the two types of coax is 1.3 db. Is it worth buying the more expensive LMR 400 coax to reduce loss by 1.3 db? Probably not.
The strength of your signal in this example will sound the same to other hams regardless of which type of coax you use. Notice in making a comparison between two types of coax (or two types of antennas, etc.)
it is the difference in loss (or gain) that is important, not the actual loss (or gain).
At UHF frequencies, the differences in loss will be greater. 100 feet of LMR 400 coax at 440 MHz has a loss of 2.7 db. In comparison, RG-8/X has a loss of 8.1 db. The difference in loss is 5.4 db. In this case the more expensive LMR 400 coax may be worth the money. LMR 400 coax is relatively thick, stiff, and difficult to work with compared to RG-8/X, particularly inside the radio shack.
Suppose that you use 75 feet of LMR 400 to get from your 440 MHz antenna to the wall outside your radio shack. Then you use a 25 foot length of RG-8/X to come through the wall and into the radio shack because RG-8/X is smaller and easier to handle in the shack. What performance penalty will you pay for doing this? The loss of 25 feet of RG-8/X is about 2.03 db. If you brought the LMR 400 all the way into the shack, the loss associated with the additional 25 feet of LMR 400 would be 0.68 db. The difference in loss is approximately 1.36 db, a negligible amount. Using RG-8/X within the radio shack is thus a good choice since it simplifies cable management within the shack and provides negligible additional loss.
The total system gain of 5.3 db probably is worth the effort, even thought the gains between the individual components was not that attractive.
The system trade-off can easily go the other way as well. At 440 MHz, 100 feet of LMR 400 coax has a 5.4 db performance gain over RG-8/X coax and is clearly better. However, if your transceiver has power settings of 5, 10, and 50 watts, and you can hit all of the area repeaters at 10 watts using RG-8/X coax, why upgrade to LMR 400? Unless you are running off of batteries, using LMR 400 coax so that you can drop your transmit power to 5 watts probably is not worth the trouble or cost.
A difference of 3 db will not be apparent to the hams that you are communicating with. They will hardly notice the difference if you run your transmitter at 50 watts instead of its maximum 100 watt output power.
A difference of 3 db or less between two antennas, two types of coax, or two system implementations is usually not sufficient to justify higher costs.
However, a difference of 6 db may justify the more expensive approach.
Editors note: To see the relationship between this great article by Ken Larson, KJ6RZ and ERP, Effective Radiated Power, see this article!
(Article provided with permission by Ken Larson KJ6RZ)
|Antenna||db Gain||S-unit Increase||Comment|
|2-element Yagi Beam||
|Marginal performance increase|
|3-element Yagi Beam||
|Good performance increase|
|10-element Yagi Beam||
|Excellent performance increase|