Hidden Losses in the Antenna Circuit
So, it is time for a discussion of something that seems to have been lost to general knowledge, SKIN EFFECT. “What is this skin effect, something that causes freckles on my rig?? A new disease that affects tomato skin? Some new taxes?” No, none of the above or any combination of the above. Nor does it cause cancer in mice.
Skin effect is the phenomena where rf energy flows on the surface of a conductor rather than throughout the entire conductor. Dc flows uniformly through a conductor from one end to the other. If there is one amp flowing in a length of #10 wire, then there is one amp of current to be found all through the entire wire. The current will be evenly dispersed through the wire, from the center to the outside surface of the wire. Rf is a little different in the way it flows through a conductor because it doesn’t. Rf flows on a conductor.
That is right, rf flows on the surface of a conductor, and not too deep into the main portion of the conductor. If you look at the diagrams, the Diagram A represents a cross section of a wire carrying dc. (Keep in mind rf is alternating current.)
Diagram B is a cross section of the same wire carrying low frequency rf. The rf has not gotten to the center of the wire before it has to change polarity and the direction of the current has to reverse direction. It seems funny, but this is what actually happens. Due to the combined resistance and impedance of the wire, the current flow does not have time to completely permeate the conductor before there is a current flow reversal.
The higher the frequency, the more pronounced the affect. There is a very complicated formula for figuring out how deep a given frequency will flow on a conductor, but it is not necessary for us worry about how deep the rf is or is not flowing on anything we deal with. What we need to be aware of is the effect of skin effect losses on power transference from our transmitters to the antennas.
Diagrams C to F show how the rf current moves from the inside to the outside of the conductor as the frequency goes up. “How does this affect my signal?” you are wondering. Well, like this. Since your antenna, regardless of the type, has a connection to a feedline and beams have elements that fit together. The elements are clamped together and corrosion enters the picture. Corrosion is usually high resistance stuff, and since it is on the surface of the conductor it is going to eat up the rf that flows there. So you lose power due to the old law of “I squared R”, in other words, heat loss due to the rf current being absorbed by the resistance of the corrosion.
The way to cut down on these losses is to weatherproof all connections, solder all connections where you can, and frequently clean the connections that you can. Some hams periodically take down their beams and clean all connections, element joints and all other rf connecting points. This is a good practice in areas of high corrosion, such as saltwater coastlines, and areas where there is a lot of air pollution. It helps to prevent beams falling apart and shedding their elements, which could possibly cause some harm to something or someone. Having an antenna element impale the next door dog or cat is not good PR.
It is unfortunate we have to deal with corrosion, but that is nature’s way to get rid of old cars and washing machines. The fact of the matter is the metals we use in our antennas are going to corrode no matter what we do about it, so steps must be taken to lessen the losses from this cause. Another problem that can arise due to corrosion is called nonlinear diode rectification.
This occurs where two different metals contact each other and the resultant electrolysis causes the connection to act like a diode mixer in the presence of strong rf fields. It will reradiate your transmitted signal mixed with whatever else is out there and cause TVI and BCI.
This sort of action is sometimes found on power line connections and anywhere such dissimilar metals are found. Getting blamed for causing a problem that is not due to anything we did, other than be there, is certainly disgusting, but that is the way it is sometimes.
An antenna that is a high current antenna, such as a 75-meter mobile whip is going to be particularly sensitive to losses due to skin effect. An 8 ft. 10 meter mobile whip will not be as sensitive to skin effect losses since there is no loading coil and the impedance is greater than the 75-meter antenna.
For example, consider a 75-meter mobile whip that consists of a center-loading coil wound on a 2 inch diameter coil with #22 wire, and a top section consisting of a 3 ft. chrome-plated telescoping whip section and a bottom section of 1/2 inch chrome-plated steel rod. This is a typical mobile whip for 75 meters, now to make matters worse, mount the thing on a chain type bumper mount.
Starting at the bottom and working up, the chain mount, be it single or double chain will not make a good ground connection to the bumper or the rest of the car body for very long. There is no real solid electrical connection to either the bumper or the body of the car. The mechanical contact the chain mount makes is going to be infiltrated by road grime and moisture, then corrosion will set in, which will increase contact resistance. Then the connections the coax makes to both the ground connection and the antenna connection on the mount will suffer the same fate.
So, there is going to be loss due to the surface resistance at each connection and contact point on the bumper mount where it touches the bumper. Remember, corrosion increases surface resistance, and then loss occurs due to resistance heating, due to rf current flow on the surface of the conductor.
Next on the loss list is the bottom section of the antenna. Chrome has a much higher resistively than copper, but it makes a mast section look good, and resistant to corrosion. At 0 degrees C chrome has 2.6 ohms-cm x 10^-6. Annealed copper is rated at 1.74 ohms-cm x10^-6 at 20 degrees C, so there is a great difference in conductivity. This is assuming the copper is corrosion free. Copper oxide has a higher loss than shiny clean copper. Now most mast sections have a hinge some where so the antenna can be folded when the car is put in the garage, or the antenna is taken off and put in the trunk. Add a little more loss at the joint due to road grime and corrosion.
Now, to the coil. Here within lies the sticker. To make the point clear, assume the total rf resistance has gotten to 40 ohms and the antenna impedance is 16 ohms. So the antenna looks like 56 ohms. The swr bridge shows somewhere near 1.6:1 and that is close enough to 50 ohms to make the transmitter happy. So it is putting out 100 watts and not complaining about it.
Unfortunately, all is not power that ends well at the antenna, or at the receiving end. Some of it is going up in heat. Heat that cannot be felt since it is dissipated all over the system. Now, for the bad news. George Simon Ohm had this neat set of formulas now called Ohms Law. One states that the current in a circuit is equal to the square root of the power in the circuit divided by the resistance. Fine, we have a combination of 40 ohms rf resistance plus 16 ohms antenna impedance. The transmitter is putting out 100 watts into the coax and that is what is going into the antenna connection at the mount.
So, if you take the square root of 100 divided by 56, you get 1.33 amps flowing in the antenna circuit. BUT, let us break it down into where it is flowing. Remember, 40 ohms of rf resistance is going to get a certain amount of that current, and dissipate it as heat, so I squared R steps in. 1.33 squared is equal to 1.7689, X 40 which equals 70.756 watts going up as heat losses. Only 28.3024 watts are getting into the antenna.
Now, for the coil. Since the coil is wound with #22 wire, with a diameter of 28.46 circular mills, there is not a lot of surface area for 1.33 amps worth of rf amps to flow on. So it gets hot. More loss due to I^R heating. To prove this, just put 100 watts of carrier into your mobile antenna for about 30 seconds to one minute then see if you can hold the coil in your hand. One company has a 1 KW rated mobile coil that fails this test. The 100-watt versions are worse. Under normal ssb operating conditions, the coils do not get noticeably hot, but the loss is still there.
The losses in drastically shortened antennas such as a 75 meter mobile is what causes the antennas to be only 2 to 3 % efficient. The very short magnet mount center loaded cb whips that are only about two feet long have the same type losses. The numbers are slightly better for the cb antenna, as the 8 ft. 75 meter antenna is only 3.25 % of a wavelength, and the cb antenna is 5.8 % of a wavelength.
Now, to the 8 ft. ten-meter whip. The 8 ft. whip is 24.3 % of a wavelength, so close we can call it a 1/4 wave whip without being out of line. A 1/4 wave vertical has an impedance of 36 ohms and putting it on the same bumper mount as the 75 meter antenna will show some interesting differences. So, a look at the differences is in order.
75 meter antenna:
In three sections, base section, loading coil, top section, 3.5% of a wavelength
10 meter antenna:
One 8 ft. whip, 25% of a wavelength.
Loosening the coil, and having the antenna in one section instead of three will cut the losses down to that found in the bumper chain mount. Coupled with the improved efficiency of the 1/4 wave antenna will cause a drastic improvement in overall system efficiency. If for the sake of figures we assume the losses in rf resistance were cut in half to 20 ohms, then the rf power lost would drop by a corresponding amount. So, if the antenna impedance is 36 ohms and the rf resistance losses are 20 ohms, the total impedance is still 56 ohms, and the transmitter remains happy and putting 100 watts into the coax. BUT, the antenna is seeing more power of 65 watts. The power being lost in the rest of the circuit is down to 35 watts.
This is an improvement of noticeable proportions. We will never be able to get 100% out of our antennas, but we can cut them a lot by improving the overall system conductivity. This means using larger conductors in coils and possibly the antennas as well. There are some limitations though. A 75 meter dipole of 3 inch copper tubing would pose some problems in suspension, as well as finances. It would work quite well, and be rather broadband it would seem.
We must do the next best thing and that is make all the connections to the antenna as solid and weatherproof as possible. Next, where we cannot get out of using coils, such as matching networks for antennas, the largest conductors that will fit should be used. Broadcast stations at the 1 kw level use silver-plated copper tubing anywhere from 1/2 to 3/4 inch in antenna matching circuits coils and in the final tank circuit. Edge wound silver-plated copper strap is used for rotary coils. Silver-plated copper strap 1/2 to 1 inch is used for all of the connections to the tapped coils. Every rf path is via large surface area conductors with very low rf resistance.
“Yes, but I am not running that kind of power and I don’t operate AM.” you say. Well, it does not matter. The same losses are there and even worse at 10 meters and higher. Losses don’t cease to exist just because we are hams and do not run the kind of power as the commercial stations.
A frequency of 650 kHz results in a wavelength of 1440 feet. And, if the station is located where it cannot put up a 1/4 wave vertical which would be 285 feet tall, the engineers designing the antenna system are going to have to come up with a cost-efficient antenna that can radiate efficiently. If the antenna cannot be any taller than 150 feet due to proximity to an airport, the antenna is going to be only 10.4 % of the wavelength, with a corresponding reduction in antenna impedance. You can bet the engineers are going to pull every trick out of the antenna efficiency bag they can find. Using large conductors in the matching network is one of them.
Remember the surface area of the conductor is going to determine the total rf resistance of the circuit. So this needs to be kept in mind when coils are being wound for antenna couplers and loading coils for your antennas, particularly in the case of short antennas.
A good example of this can be found in the experiences of one local ham 30 years ago. His job took him all over Texas, New Mexico, Oklahoma and Louisiana. So he had a good multi-band mobile that ran 60 watts AM, the good old ELMAC AF-67. He was using a ball mount mounted on the left rear fender. The antenna was am 8 ft. whip on top of a homemade bugcatcher coil which was on top of a 5 ft. commercial mast section. This gave him a very good signal all over on 75 meters at night, and a good 40-meter signal in the day.
One day, as he was traversing the East Texas Piney Woods region, his bugcatcher was destroyed by a low flying UFBB, that came flying out of the dark, (Unidentified Flying Beer Bottle,) possibly from a passing pickup. Unable to find a replacement coil, he was forced to rewind his coil out of 1/8 inch copper tubing on a two-foot long three inch diameter form. The use of the copper tubing cut the losses in the coil and his signal went up by a significant amount.
Later on, the 1/2 inch diameter commercial mast managed to get bent at an angle somewhere between 60 and 90 degrees. Undaunted, our hero went to a local plumbing store and bought a 6 ft. length of 2 inch hard drawn copper pipe and some pipe caps to match. After a trip to a hardware store for some bolts, he constructed a lower mast section. This resulted in a 16-ft. mobile whip and a tremendous signal.
He used this antenna until he got married, and his new wife refused to ride in any car with such a monstrosity on it. Later on, he got her a car and went back to the monster. This is an example of loss cutting that brought immediate results.
Not all changes will make as great a change as this, but the cumulative result of changes to any system are not always as apparent. Sometimes, the results are only that the transmitter runs cooler or there is less noise on receive. Either way removal of rf resistance in your antenna circuit should be of the utmost importance. The importance of the skin effect phenomena is not stressed now as it was a few years ago, and is generally overlooked by most hams.
It should be noted here that any of the techniques used by the commercial station can be effectively used by amateurs in our hobby. It is important for the reasons of using various techniques are well understood to be used effectively.
Originally posted on the AntennaX Online Magazine by Richard Morrow, K5CNF
Last Updated : 9th March 2024