Transmitter Power, Antenna Gain, and Coax Loss Trade-offs

By Ken Larson KJ6RZ


In the 1950s and 60s many hams built their own transmitters for the simple reason that commercial transmitters were too expensive. For example, a Johnson Viking II transmitter cost $300, which doesnít sound too bad until you stop to consider that a new Ford or Chevy cost $1,000. The alternative was to buy cheep war surplus radios and use the parts to build one of the transmitters shown in the Radio Amateurís Handbook. In a way, that was more fun. As far as power was concerned, you had control! You could push your transmitter as hard as you dared, to squeeze every bit of power out of it, even to the point where the plates of the transmitterís output vacuum tubes glowed cherry red.

I was convinced in those days that if I could just get another 20 watts of output from my transmitter that it would make all the difference in the world at the receiving end. If I could just get those extra 20 watts that rare DX operator in a distance land would see my signal jump from a pitifully weak whisper to a loud boom that he could not ignore, and I would get that contact. Today I know that little extra power would not have made any difference at all. However, I still have an intense desire to push my transceiver to its maximum power output to get a DX contact. But it doesnít stop there. I want every db of gain that I can possibly get out of my antenna. As far as coax is concerned, I want that big, heavy, hard to handle, expensive coax because I donít want to loose any of my valuable watts getting from my transmitter to the antenna. Does all of this pushing, shoving, and optimization really make a difference? Probably not!

It turns out that you must increase the output power of your transceiver by at least 3 db in order for the person you are talking with to notice any change in your signal strength. For your signal to sound twice as loud, you must increase your power out by about 9 db.

How much is a 3 db increase in power? A 3 db power gain is equal to a times 2 increase in power (3 db = x2). So, if your transceiver is running 100 watts, you must increase your transceiverís output to 200 watts in order for the person you are talking with to notice any increase in your power. If you wanted your signal to sound twice as loud, you must increase your power to 800 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.

What about antennas? The same 3 db rule applies. You can go to a lot of trouble and expense on 40 and 80 meters putting up phased vertical arrays to achieve 2 or 3 db of gain. But 3 db of gain will hardly be noticeable to anyone listening to your signal, so why bother? The threshold in antenna cost verses performance gain is around 6 db. If your antenna provides 6 db of gain, operators listening to your signal will notice a difference. Your signal will not be twice as loud, remember you have to get 9 db of gain for that to happen, but at 6 db the gain will be noticeable. 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.



Antenna db Gain S-unit Increase Comment
Dipole

0

0

Baseline
2-element Yagi Beam

4

0.6

Marginal performance increase
3-element Yagi Beam

6

1.0

Good performance increase
10-element Yagi Beam

12

2.0

Excellent performance increase


The cost verse performance trade-off for the transmission line connecting a transceiver to an antenna is similar to the antenna cost trade-off. However, this time the trade-off relates to the difference in loss between two types of transmission lines, for example, between two different grades of coax cable. 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.

In making trade-off comparisons, you have to look at the total system as well as the individual components. For example, a 2-element 10 meter yagi antenna (4 db gain over a dipole) feed by LMR 400 coax (1.3 db gain over RG-8/X coax) produces a total system gain of 5.3 db compared to a 10 meter dipole feed with RG-8/X coax. 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.

In conclusion, when making trade-offs between transmitter power, antenna gain, coax loss, and total system performance, it is the db difference between the options available to you that is important. 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.




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